Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics
Suddhasattwa Mandal, Ram Gopal, Hemkumar Srinivas, Alessandro D'Elia, Arnab Sen, Sanket Sen, Robert Richter, Marcello Coreno, Bhas Bapat, Marcel Mudrich, Vandana Sharma, Sivarama Krishnan
JJournal Name
Coincident angle-resolved state-selective photoelectronspectroscopy of acetylene molecules: a candidate systemfor time-resolved dynamics
S. Mandal a , R. Gopal b , H. Srinivas c , A. D’Elia d , A. Sen a , S Sen e , R. Richter f , M. Coreno g , h ,B. Bapat a , M. Mudrich i , j , V. Sharma e † and S. R. Krishnan j ∗ The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamicalprocesses where the coupling of the electronic and nuclear degrees of freedom provides a fertileplayground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet(EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailedinvestigations of this molecular system in the photon energy range ... eV where EUV pulsescan probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spec-troscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excitedstates of the C H +2 cation, the primary photoion, is specifically addressed. From photoelectronenergy spectra and angular distributions, the nature of the dissociation and isomerization channels isdiscerned. Exploiting the π -collection geometry of velocity map imaging spectrometer, we not onlyprobe pathways where the efficiency of photoionization is inherently high but also perform PEPICOspectroscopy on relatively weak channels. One of the outstanding problems of interest in time-resolved spec-troscopy and quantum dynamics of molecular systems is phenom-ena involving the interplay between nuclear motion and electrondynamics . In femto- and sub-femto-second timescales, a deepunderstanding of these scenarios is intimately related to realiz-ing the grand challenge of making molecular movies; "watch-ing" chemical reactions take place . Among important aspectsof the physics of systems beyond the Born-Oppenheimer approx-imation , decoupling nuclear and electronic dynamics, the roleof conical intersections , shape resonances , and fast rearrange-ments within molecules are of particular interest. Proton mi-gration ensuing in the rearrangement of photoexcited molecularsystems has a prominent place not only owing to the intriguing a Indian Institute of Science Education and Research, Pune 411008, India b Tata Institute of Fundamental Research, Hyderabad 500107, India c Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany d IOM-CNR, Laboratorio TASC, Basovizza SS-14, km 163.5, 34149 Trieste, Italy e Indian Institute of Technology Hyderabad, Kandi 502285, India; E-mail:[email protected] f Elettra-Sincrotrone Trieste, 34149 Basovizza, Italy g Istituto di Struttura della Materia - Consiglio Nazionale delle Ricerche, 34149 Trieste,Italy h INFN – LNF, via Enrico Fermi 54, 00044 Frascati, Italy i Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark j Department of Physics and QuCenDiEm-Group, Indian Institute of Technology Madras,Chennai 600036, India; E-mail: [email protected] physics, but also due to its importance in biological systems; thisplays a key role in processes underlying human vision , photo-synthesis , proton tunneling in DNA and radiation damage ,to name a few.The acetylene-vinylidene system has long served as the bench-mark for investigations of isomerization especially on ultrafasttimescales as well as in static spectroscopy and theoreti-cal investigations . Both the photoexcitation of outer-valenceelectrons in the extreme-ultraviolet as well as core-shell elec-trons in the hard-xray regimes can effect isomerization . Under-standing this system paves way for investigating the dynamics ofproton migration in larger systems such as benzene , and protonconduction in covalently bonded molecules and weakly boundaggregates such as bio-interfaces . In order to perform time-resolved spectroscopy of the acetylene-vinylidene system, an in-timate knowledge of not only the neutral molecule but also theresidual ion and more importantly, the details of photoelectronenergies and angular distributions is essential: For example, tran-sient absorption, laser-induced fluorescence or resonant multi-photon ionization methods which are popular in this context areeffective when the system is spectroscopically well-characterized.The dynamics of wavepacket resulting from the finite bandwidthof interrogating pulses can be traced effectively when the statesinvolved are known a priori.In this article, we use photoelectron imaging in coincidencewith photoion spectrometry to uncover the details of this bench- Journal Name, [year], [vol.] , a r X i v : . [ phy s i c s . a t o m - ph ] F e b ark system in the spirit of preparing the ground for further in-vestigations of this system using table-top as well as free-electronpulsed laser sources. While, reports on the transient dynamics ofthis system have been published, our recent investigations of thismolecular system embedded in He nanodroplet-environment mo-tivate further time-resolved studies to estimate the time scaleof environment assisted Penning ionization of C H from higherlying states of He ∗ ( n = ) band. One of the key advantages ofphotoelectron spectroscopy is that it can be readily applied intime-resolved studies bringing with it the advantage of accessingthe entire reaction coordinate even when the electronic and vibra-tional states evolve in time. Thus, photon energy dependent studyof partial cross sections and photoelectron angular distributionshas proved to be a useful tool to probe different resonant autoion-ization processes and shape resonance phenomena in molecularspecies .In these investigations of acetylene (C H ) photoionizationby the photoelectron-photoion coincidence technique, we reportthe photoelectron energy spectra (PES) corresponding to dif-ferent ionization channels of C H +2 along with their accom-panying photoelectron angular distributions (PADs). This al-lows us to discern PES and PADs for each C H fragment ionas a function of photon energy; this includes the primary pho-toion C H +2 as well as those resulting from further dissocia-tion and isomerization. The choice of the velocity-map-imagingscheme for photoelectrons is deliberate. This technique is a pre-ferred method for studying molecules and clusters with extreme-ultraviolet pulses and high-harmonic generation methodology owing to the inherently high-collection efficiency over the entiresolid angle . Thus, our results can be immediately carriedforward and applied to these scenarios.The key findings of this work are as follows: Firstly, we pre-cisely characterize all the fragmentation channels and determinethe electronic states responsible for producing each of these frag-ments. These are validated by the fact that the contributions ofthe highest occupied molecular orbital (HOMO) to the photoelec-tron spectra are in good agreement with prior theoretical work .However, the photoelectron angular distribution measurementsdo not always agree with reported theory; nor do they evidenceautoionizing resonances when correlated to particular ionic frag-ments. But it is noteworthy that earlier computations do notmatch unanimously, either . Thus, our work provides perti-nent inputs for revisions over and above the existing work. Owingto the merits of the experimental technique, we employ, we couldascertain that the less abundant ionic fragments which result fromsingle ionization including, C +2 , CH +2 , CH + and C + arise fromthe higher-excited ionic states. The hallmark of this article is that,to the best of our knowledge, we have for the first time measuredstate-selective branching ratios, photoelectron angular distribu-tions and asymmetry parameters, as a function of photon energy,for all the relevant cationic states of primary photoion, C H +2 ,for different photoionization pathways both below and above thedouble ionization energy of this paradigmatic molecular system. Fig. 1
Schematic of the experimental setup: Acetylene gas is effusedthrough a leak valve in the source chamber adjacent to the spectrom-eter chamber which is connected by a conical skimmer. Subsequently,through the skimmer, C H is flooded into the spectrometer chamberwhere the electron-VMI and the ion-ToF spectrometers are situated. Inthe spectrometer chamber, C H is ionized by linearly polarized EUVphoton beam. The experiments reported here were carried out at the Gasphasebeamline of the Elettra Sincrotrone, Trieste. Fig.1 shows theschematic diagram of the experimental setup, whose details havebeen published earlier . Here, high-purity C H gas was effusedinto the source chamber through a dosing valve. The C H gaswas distilled before entering this valve to remove acetone contam-ination. In the distillation process, the gas mixture was passedthrough a slurry of ethanol and liquid N maintained at − ◦ C . The source chamber is connected to spectrometer chamberthrough a conical skimmer which maintains a differential pres-sure; C H gas effuses into spectrometer chamber which is main-tained at ∼ − mbar, while the source chamber remains at ∼ − mbar.The spectrometer chamber holds two co-axial spectrometers -a velocity map imaging (VMI) spectrometer and a time-of-flight(ToF) spectrometer (cf. fig.1). A focused beam of linearly polar-ized extreme-ultraviolet (EUV) photons passes through the geo-metric centre of the two spectrometers at right angle to the spec-trometer axis which is also perpendicular to its polarization axis( ε ). Photon energies in the range between and eV wereused in our study. We exploited the excellent photon energy defi-nition possible at this beamline quantified by the resolving powerof the monochromator upstream, ∆ E / E ≤ − ; using a set ofgratings, high-quality photon beams in the energy range − eV are accessible here. The synchrotron ring delivers the photonbeam in this case in the form of ∼ ps pulses with typical peakintensity of ∼ W/m and repetition rate of MHz. Here,
Journal Name, [year], [vol.][vol.]
Journal Name, [year], [vol.][vol.] , andomly oriented C H molecules are photoionized by the EUVlight and the resultant photoelectrons and photoions are detectedin coincidence with the VMI and ToF spectrometers, respectively.The charged particle count rate was maintained at ∼ kHz byadjusting two slits on the photon beam path. This synchronousdetection scheme of photoelectrons and photoions enables us tomeasure the kinetic energies and angular distributions for pho-toelectrons correlated to different photoions formed due to C H photoionization. Therefore, unlike previous studies , notonly do we get the photoelectron energy distributions of C H photoionization, but also this provides photoelectron energy spec-tra and angular distributions correlated to specific photoions andphotoionization channels.We implemented Abel inversions using the well-establishedprogram, MEVELER , to obtain the full 3D velocity distributionof photoelectrons from 2D projection images captured by the VMIspectrometer. We used known photoelectron energy distributionof He at different photon energies above the atomic He ioniza-tion energy ( E i = . eV) to calibrate VMI spectrometer. Theaverage energy resolution ( ∆ E / E ) achieved by the spectrometeris about . For one-photon ionization by linearly polarized light,under dipole approximation, the differential cross section can beexpressed as: d σ d Ω = σ total π [ + β P ( cos θ )] . (1)Since, the photoelectron velocity ( (cid:126) v ) has the cylindrical symme-try along the polarization axis ( ε ), the differential cross sectionhas no azimuthal ( φ ) dependence. P ( cos θ ) is the second orderLegendre polynomial and θ is the angle between (cid:126) v and ε . Thephotoelectron angular distribution (PAD) is characterized by theasymmetry parameter, β .In the current study, to obtain the value of β specific to differ-ent ionic states, we used the following scheme: Multiple Gaussianfunctions are fitted to the PES to determine different ionic statesand their full-width-at-half-maximum (FWHM). Then, we obtainthe PAD for each state by integrating the angular photoelectroncounts over the FWHM limit of each state from the Abel-inverteddistribution. Finally, we fitted eq.(1) on the PAD to get the asym-metry parameter, β . For example, fig.2 a), b) show the experi-mental VMI distribution and the Abel-inverted distribution of thephotoelectron emitted due to photoionization of effusive He at eV, respectively. Fig.2 c) presents the PAD of the observed He s ionization, where the value of β obtained from fitting eq.(1)is . ± . which correctly correlates to the PAD of p - partialwave resulting from one-photon ionization . Acetylene in its neutral ground state ( Σ + g ) has the following elec-tronic configuration: ( σ g ) ( σ u ) ( σ g ) ( σ u ) ( σ g ) ( π u ) , with π g , σ u , σ g and σ u being the lowest lying unoccupiedorbitals. In the spectral range from . eV to below double ion-ization energy ( E di ∼ eV), electrons are predominantly excitedor ionized from the valence orbitals, π u , σ g , σ u and σ g . Con- a) b) Abel Inverted image
HighLow
VMI Image c) Y ( mm ) X (mm) PAD Fit
Fig. 2 a) VMI distribution and b) Abel-inverted distribution of thephotoelectrons, in a logarithmic color scale, due to photoionization ofeffusive He at eV and (c) the photoelectron angular distribution (PAD)obtained from b). The red line shows the fitting of the PAD for β = . ± . , demonstrating the performance of the spectrometer. sidering the independent particle model, ionization from the π u , σ g , σ u and σ g orbitals leads to X Π u , A Σ + g , B Σ + u , and C Σ + g states in C H +2 , respectively. Along with these direct ionizationchannels, there exists several indirect autoionizing resonances inC H , where electrons are excited from the valence orbitals to thevirtual orbitals upon photoabsorption. As these excitations decayto the ionic states, X , A , B and C , the corresponding kinetic ener-gies of photoelectrons remain same irrespective of the ionizationmechanism. However, the ionization cross sections of these statesand the associated photoelectron angular distributions are greatlyinfluenced by the involved ionization processes .Here, we will discuss the photon energy dependent photoioniza-tion cross sections and the photoelectron angular distributionsassociated with different cationic states of C H +2 both for thephotoionization and for different photodissociation channels. Theremainder of this article is organized as follows: First we discussphotoion mass spectra which enable us to identify distinct ioniza-tion channels characterized by the dissociation pathways of theC H +2 ion. The mainstay of this article, photoelectron energyspectra (PES) specific to these ionization channels, as well as thephotoelectron angular distributions (PADs) and the asymmetryparameters ( β ) particular to each ionization channel and ionicstate are presented. We compare our work with existing studieswherever it is relevant to underscore new findings. To identify C H photoionization channels, we recorded the pho-toion ToF mass spectra, presented in fig.3, at different photon en-ergies. We observe several fragmented ions, C H + , C +2 , CH +2 ,CH + and C + as well as unfragmented parent molecular ion,C H +2 . Each of these fragmented ions represents a distinct pho-todissociation channel, where the respective ionic fragment is ac-companied by undetected neutrals. Among these ionic products,C H +2 and C H + ions are the most abundant ionic species, con-stituting ∼ of total ion-yield, while the other fragments com-prise the remaining fraction. Notably, in fig.3 the ion-yields ofthe photoions ( x ), where x represents C H +2 , C H + , C +2 , CH +2 ,CH + and C + , vary with photon energy, evidencing the corre-sponding dependence of the relative ionization efficiencies ( η x ) Journal Name, [year], [vol.] , able 1 Comparison of relative ionization efficiencies ( η x ) of the photoions ( x ) as a function of photon energy ( h ν ) with the results obtained byHayaishi et al. h ν (eV) η x (arb. u.) η (arb. u.) x = C H +2 x = C H + x = C +2 x = CH +2 x = CH + Current Previous Current Previous Current Previous Current Previous Current Previous Current19.0 0.777 0.777 0.105 0.112 0.006 0.004 0.004 0.002 0.007 0.000 0.90421.6 0.739 0.734 0.183 0.191 0.014 0.008 0.008 0.011 0.029 0.004 0.98823.9 0.651 0.628 0.148 0.149 0.016 0.024 0.008 0.026 0.028 0.019 0.86426.0 0.588 0.511 0.137 0.119 0.019 0.025 0.007 0.016 0.024 0.025 0.78428.0 0.377 0.412 0.090 0.087 0.014 0.021 0.005 0.012 0.017 0.023 0.50936.0 0.283 — 0.073 — 0.015 — 0.005 — 0.027 — 0.41640.0 0.209 — 0.061 — 0.014 — 0.005 — 0.029 — 0.331 of the channels involved on the same parameter.Hayaishi et al. extensively studied the photoionization dy-namics of C H by measuring the photoion-yields of C H +2 ,C H + , C +2 , CH +2 and CH + as a function of photon energy. Theydiscussed the appearance energies of these ions as well as as-signed electronic excitations that result in these photoions from ab initio theoretical calculations. Here, the relative ionization ef-ficiencies ( η x ) are calculated from the integral area of differentphotoion ( x ) peaks in the TOF mass spectra as a function of pho-ton energy, shown in fig.3. Herein, the photoion ToF mass spectraare normalized such that the total ion-yields of background N +2 ions at different photon energies are proportional to the respec-tive partial ionization cross section of N +2 from N photoioniza-tion . Since, we kept the data acquisition time and the spec-trometer chamber pressure at the same values for all the mea-surements at different photon energies, assuming the identicaldetection efficiencies of different photoions, the relative ioniza-tion efficiencies calculated here are proportional to the respectivepartial photoionization cross sections of C H photoionization .Table.1 shows the comparison of the observed relative ionizationefficiencies ( η x ) of C H +2 , C H + , C +2 , CH +2 and CH + ions withthe results obtained by Hayaishi et al. . The reported relativeionization efficiencies ( η x ) scaled suitably so that the ionizationefficiency of C H +2 at . eV photon energy, are matched withthat of Hayaishi et al. . In this context, it is important to notethat, in the work of Hayaishi et al. at h ν = . eV the relativeionization efficiency of C H +2 is equal to arb. u.. In table.1, η represents the total relative ionization efficiency of cumulative allC H photoions shown in fig.3.It is encouraging to note that there is a good agreement be-tween our results and corresponding values from earlier studiesfor C H +2 and C H + ions: The reported photoionization thresh-old of C H is . eV and appearance energy of C H + is . eV which can be associated with the C H +2 states, X Π u and A Σ + g , respectively. For photoions whose appearance energies arehigher than that of C H +2 and C H + , > eV, their yield andphotoionization efficiencies are also relatively lower (cf. table.1).We identify C +2 ion originating from two photodissociation chan-nels: C H + h ν → C + + H + e − (2), and C H + h ν → C + + + e − (3)with appearance energies of . eV and . eV, respectively .
11 12 13 14 23 24 25 26 27 28 29 300.00.20.40.60.81.0 I on - y i e l d ( a r b . u . ) M/q 19.0 eV 21.6 eV 23.9 eV 26.0 eV 28.0 eV 36.0 eV 40.0 eVC + CH +
10 CH +2 C H + C +2 C H +2 N +2 Fig. 3
Photoion ToF mass spectra at different photon energies. Thex-axis represents mass to charge ratio ( M / q ) of the photoions in atomicunits. The shaded portion from M / q = to and from M / q = . to are magnified times. Throughout, peaks are labeled by the single- anddouble-ionization ionic fragments from acetylene - C H + , C +2 , CH +2 ,CH + and C + , along with the singly ionized parent molecular ion, C H +2 .However, at h ν = and eV , non-dissociative double-ionization prod-uct, C H , make small contribution to the peak at M / q = . Whilethe contribution to the M / q = peak from N + due to residual nitrogen(N ) ionization is tiny at lower photon energies, at and eV this canbe significant . The peak at M / q = corresponds to N +2 ions. Thephotoion ToF mass spectra are normalized such that the background N +2 ion yields are proportional to the partial photoionization cross-sections ofN producing N +2 ions at the respective photon energies . The rel-ative ionization efficiencies ( η x ) for different photoions ( x ) are directlycalculated from the integral-area of the respective photoion peak at dif-ferent photon energies which are proportional to the respective partialphotoionization cross sections. Journal Name, [year], [vol.][vol.]
Photoion ToF mass spectra at different photon energies. Thex-axis represents mass to charge ratio ( M / q ) of the photoions in atomicunits. The shaded portion from M / q = to and from M / q = . to are magnified times. Throughout, peaks are labeled by the single- anddouble-ionization ionic fragments from acetylene - C H + , C +2 , CH +2 ,CH + and C + , along with the singly ionized parent molecular ion, C H +2 .However, at h ν = and eV , non-dissociative double-ionization prod-uct, C H , make small contribution to the peak at M / q = . Whilethe contribution to the M / q = peak from N + due to residual nitrogen(N ) ionization is tiny at lower photon energies, at and eV this canbe significant . The peak at M / q = corresponds to N +2 ions. Thephotoion ToF mass spectra are normalized such that the background N +2 ion yields are proportional to the partial photoionization cross-sections ofN producing N +2 ions at the respective photon energies . The rel-ative ionization efficiencies ( η x ) for different photoions ( x ) are directlycalculated from the integral-area of the respective photoion peak at dif-ferent photon energies which are proportional to the respective partialphotoionization cross sections. Journal Name, [year], [vol.][vol.] , e infer these mechanisms noting that the observed differencebetween these two C +2 appearance energies matches the dissoci-ation energy of the H molecule . Similarly, for CH + ion, thereare two distinct appearance energies at . eV and . eV arisingdue to the following photoionization channels: C H + h ν → CH + + CH + e − (4), and C H + h ν → CH + + C + H + e − (5), respectively . The appearance energies of the CH +2 + C andC + + CH photodissociation channels are . eV and . eV , respectively. However, previous electron impact ioniza-tion study on C H reported the appearance energy of C + ionat a lower energy of . eV . Table.1 is a concise summaryof the measured relative ionization efficiencies ( η x ) of these pho-toion compared with literature , corresponding to the aforemen-tioned ionization channels, affirming the reliability of our mea-surements. We are now in a position to obtain insights into theseprocesses taking advantage of the photoelectron imaging corre-lated to each of these photoions. This enables us to derive insightsinto state-selective photo-fragmentation dynamics. To understand the mechanisms underlying the photoionization ofC H , here we present the photoelectron energy spectra (PES)and photoelectron angular distributions (PADs) of the photoelec-trons correlated to all the ionic products of C H photoionization.Then, PES and PADs in coincidence with each of the product ionsare presented to investigate the ionization channels leading tothese product ions. This allows us to compare our work with ear-lier reports, whereas the ionization channel specific investigationis a particular specialty of this work. We first discuss the C H photoionization and the ionization channels that produce mostabundant ions, C H +2 and C H + . In the latter part, we discussrest of the photodissociation channels.In fig.4, panels a) and c) show the photoelectron VMI distribu-tions and panels b) and d) show the Abel-inverted distributionscorrelated to C H +2 and C H + ions, respectively, at eV pho-ton energy. Fig.4 e) shows the cumulative PES summed over allthe photoelectrons associated to all the photoions resulting fromC H photoionization. There are four distinct peak-structures inthe PES centered at . , . , . and . eV representing dif-ferent ionized states of C H . The vertical green dashed lines infig.4 e) present the known ionization energies of first five cationicstates X Π u , A Σ + g , B Σ + u , C Σ + g and D Σ + u at . , . , . , . and . eV, respectively . Therefore, in this photon en-ergy range ( − eV), these five states are mainly populatedupon photoionization of C H . Interestingly, PES correlated tothe unfragmented C H +2 ion (cf. fig.4 f)) do not have the fourthpeak corresponding to the C and D states, evidencing that the un-fragmented C H +2 ion is only produced from first three states.In contrast, PES correlated to C H + ion (cf. fig.4 g)) indicatethat only the higher excited states, excluding X , lead to the C H + C H + d)c)b) E l e c t r on S i gna l ( a r b . u . ) H photoionsX A B C D a)C H +2 E l e c t r on S i gna l ( a r b . u . ) H +2 E l e c t r on S i gna l ( a r b . u . ) Binding Energy (eV) 19.0 eV H + Fig. 4
Photoelectron a) & c) VMI distribution, and b) & d) Abel-inverteddistribution correlated to C H +2 & C H + ions at h ν = eV , in loga-rithmic color scale. Photoelectron energy spectra (PES): e) cumulativespectra of electrons summed over all photoions from C H ionization, f)correlated specifically to the C H +2 ion and g) the C H + ion, respec-tively. The vertical green dashed lines denote the ionization energies ofC H +2 states. Journal Name, [year], [vol.] , able 2 State-specific binding energies (BE) correlated to all C H photoions and in coincidence with specific ions at different photon energies ( h ν ) h ν (eV) BE (eV)All C H photoions C H +2 C H + X Π u A Σ + g B Σ + u C Σ + g , D Σ + u X Π u A Σ + g B Σ + u A Σ + g B Σ + u C Σ + g , D Σ + u ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Table 3
Comparison of equivalent state-selective ionization cross sections ( σ i ) of different states ( i ) correlated to cumulative all C H photoions withTDDFT calculation h ν (eV) σ i (Mb) i = X Π u i = A Σ + g i = B Σ + u i = C Σ + g , D Σ + u Current TDDFT Current TDDFT Current TDDFT Current TDDFT ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± fragment, which in addition release a neutral H. This observa-tion reveals the mechanism underlying the previously reportedappearance energy of C H + at . eV . Coincident photo-electron imaging in forthcoming discussions will reveal furtherdetails of the dynamics in this channel and others.In order to determine the binding energies (BE), multiple Gaus-sian functions are fitted to the PES; peak positions and relativeintensities associated with different maxima are determined. Ow-ing to the finite energy resolution of the VMI spectrometer, we arenot able to distinguish between the closely spaced C and D states.Therefore, we address the properties of this peak by labelling it as C , D . Table.2 presents the binding energies corresponding to dif-ferent C H +2 states obtained from the fitting of PES correlated tocumulative all C H photoions as well as spectra in coincidencewith specific photoions, C H +2 and C H + , respectively.In fig. 4 e)- g), we note upward shifts in PES peaks correspond-ing to the ionic state, X , compared to its ground vibrational levelshown by the vertical green dashed line at . eV. This upwardshift in BE of the X state can be attributed to the photoionizationof C H into the higher vibrational levels, ν = and , belongingto the ground ionized state ( X ) with energies . and . eV,respectively . However, it should be noted that, the finite energyresolution of the spectrometer leads to significant widths in thereported binding energies, cf. table.2. From table.2, we see thatthe PES peaks in the spectra in coincidence with cumulative allC H photoions, and those correlated to C H +2 are nearly at thesame positions for X and A states, whereas maxima in spectra cor-related to A and B states in C H + are significantly shifted towardshigher binding energies by ∼ . eV and ∼ . eV, respectively,as compared to the same in C H +2 , also evident in panel g) of fig.4. Thus, the additional binding energy is expended in climbing upthe vibrational manifold of the A and B states of the C H +2 ionleading up to the dissociation into the C H + ion and the neutralH . For this dissociative ionization channel, the contributions ofthe higher excited states ( C , D ) are also significantly enhanced.Furthermore, even though we are able to decipher the elec- tronic states resulting from C H photoionization, particularly inthe case of C H +2 and C H + product ions, it is difficult to discernthe exact ionization process for the following reasons without ad-ditional knowledge: Both the direct photoionization and indirectautoionization processes lead to the same final electronic state;photoelectron kinetic energies emerging from the final state re-main identical irrespective of the ionization mechanisms. How-ever, the partial ionization cross sections of the final states andthe associated photoelectron angular distributions will dependon the details of the ionization processes. At photon energiesnear the autoionization resonances, we may expect to see the im-pact of resonances both in the partial ionization cross sections ofthese states and in the associated photoelectron angular distri-butions (PADs). Therefore, to discern the C H photoionizationprocesses, we will discuss photon energy dependent partial ion-ization cross sections in terms of state-selective branching ratiosand photoelectron asymmetry parameter of these states in detail.To the best of our knowledge, state-selective branching ratios andphotoelectron asymmetries for different photoionization channelsof C H as a function of photon energy are reported for the firsttime.To determine the state-selective branching ratios ( R i ) as a func-tion of photon energy for different photoionization channels, weuse the following method: First, the PES correlated to a specificphotoionization channel for a given photon energy, h ν , is fittedwith the following multiple Gaussian functions, F ( E ) , of the form: F ( E ) = ∑ i C i √ πσ (cid:48) i e − ( E − BE i σ (cid:48) i ) (6)where, BE i , σ (cid:48) i and C i represent the binding energy (BE), standarddeviation and intensity of the i th state in the fitted PES, respec-tively. Now, the branching ratios ( R i ) of different ionic states ( i )for the concerned photoionization channel at the photon energy, h ν , can be written as: R i = C i ∑ i C i (7) Journal Name, [year], [vol.][vol.]
Comparison of equivalent state-selective ionization cross sections ( σ i ) of different states ( i ) correlated to cumulative all C H photoions withTDDFT calculation h ν (eV) σ i (Mb) i = X Π u i = A Σ + g i = B Σ + u i = C Σ + g , D Σ + u Current TDDFT Current TDDFT Current TDDFT Current TDDFT ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± fragment, which in addition release a neutral H. This observa-tion reveals the mechanism underlying the previously reportedappearance energy of C H + at . eV . Coincident photo-electron imaging in forthcoming discussions will reveal furtherdetails of the dynamics in this channel and others.In order to determine the binding energies (BE), multiple Gaus-sian functions are fitted to the PES; peak positions and relativeintensities associated with different maxima are determined. Ow-ing to the finite energy resolution of the VMI spectrometer, we arenot able to distinguish between the closely spaced C and D states.Therefore, we address the properties of this peak by labelling it as C , D . Table.2 presents the binding energies corresponding to dif-ferent C H +2 states obtained from the fitting of PES correlated tocumulative all C H photoions as well as spectra in coincidencewith specific photoions, C H +2 and C H + , respectively.In fig. 4 e)- g), we note upward shifts in PES peaks correspond-ing to the ionic state, X , compared to its ground vibrational levelshown by the vertical green dashed line at . eV. This upwardshift in BE of the X state can be attributed to the photoionizationof C H into the higher vibrational levels, ν = and , belongingto the ground ionized state ( X ) with energies . and . eV,respectively . However, it should be noted that, the finite energyresolution of the spectrometer leads to significant widths in thereported binding energies, cf. table.2. From table.2, we see thatthe PES peaks in the spectra in coincidence with cumulative allC H photoions, and those correlated to C H +2 are nearly at thesame positions for X and A states, whereas maxima in spectra cor-related to A and B states in C H + are significantly shifted towardshigher binding energies by ∼ . eV and ∼ . eV, respectively,as compared to the same in C H +2 , also evident in panel g) of fig.4. Thus, the additional binding energy is expended in climbing upthe vibrational manifold of the A and B states of the C H +2 ionleading up to the dissociation into the C H + ion and the neutralH . For this dissociative ionization channel, the contributions ofthe higher excited states ( C , D ) are also significantly enhanced.Furthermore, even though we are able to decipher the elec- tronic states resulting from C H photoionization, particularly inthe case of C H +2 and C H + product ions, it is difficult to discernthe exact ionization process for the following reasons without ad-ditional knowledge: Both the direct photoionization and indirectautoionization processes lead to the same final electronic state;photoelectron kinetic energies emerging from the final state re-main identical irrespective of the ionization mechanisms. How-ever, the partial ionization cross sections of the final states andthe associated photoelectron angular distributions will dependon the details of the ionization processes. At photon energiesnear the autoionization resonances, we may expect to see the im-pact of resonances both in the partial ionization cross sections ofthese states and in the associated photoelectron angular distri-butions (PADs). Therefore, to discern the C H photoionizationprocesses, we will discuss photon energy dependent partial ion-ization cross sections in terms of state-selective branching ratiosand photoelectron asymmetry parameter of these states in detail.To the best of our knowledge, state-selective branching ratios andphotoelectron asymmetries for different photoionization channelsof C H as a function of photon energy are reported for the firsttime.To determine the state-selective branching ratios ( R i ) as a func-tion of photon energy for different photoionization channels, weuse the following method: First, the PES correlated to a specificphotoionization channel for a given photon energy, h ν , is fittedwith the following multiple Gaussian functions, F ( E ) , of the form: F ( E ) = ∑ i C i √ πσ (cid:48) i e − ( E − BE i σ (cid:48) i ) (6)where, BE i , σ (cid:48) i and C i represent the binding energy (BE), standarddeviation and intensity of the i th state in the fitted PES, respec-tively. Now, the branching ratios ( R i ) of different ionic states ( i )for the concerned photoionization channel at the photon energy, h ν , can be written as: R i = C i ∑ i C i (7) Journal Name, [year], [vol.][vol.] , .00.20.40.60.00.20.40.6 18 19 20 21 22 23 24 25 26 27 28 290.00.20.40.60.8 B r an c h i ng R a t i o ( R i ) i = X A B C , D (a) All C H photoions B r an c h i ng R a t i o ( R i ) i = X A B (b) C H + 2 B r an c h i ng R a t i o ( R i ) Photon Energy (eV) i = A B C , D (c) C H + Fig. 5
State-selective branching ratios ( R i ) as a function of photon en-ergy defined by equations 6 and 7 for different photoionization channelscorrelated to a) cumulative all C H photoion, b) C H +2 and c) C H + ions. The state-selective branching ratios, R i , are labelled by i which cor-respond to the states, X , A , B and C , D of the primary photoion C H +2 fordifferent photoionization channels producing C H +2 , C H + , C +2 , CH +2 ,CH + and C + ions. The dependence of state-selective branching ratios( R i ) on photon energy reveal the role of possible resonances involved, asdiscussed in the text. These state-selective branching ratios ( R i ) provide a normalizedscaling for the relative intensities of individual ionic states ( i ) ina specific photoionization channel at a particular incident photonenergy, where the total intensity summed over all the ionic states, ∑ i R i , is equal to .The state-selective branching ratios as a function of photon en-ergy producing all C H photoions are presented in fig.5 a). Asa trend, with increasing photon energy, the population of X de-creases while that of higher excited C , D states increase. And,these ratios in the case of A and B states, respectively, show mod-erate photon energy dependence. These decreasing nature of rel-ative intensities of X and A states can be attributed to the involved p atomic orbital. On the other hand, increasing state-selectivebranching ratios of B and C , D states are due to the involved s atomic orbital . In addition, a weak local maximum is observedin the A state around . eV. A comparison between the equiva-lent state-selective ionization cross sections ( σ i ) of different states( i ) obtained from our study with the same from theoretical time dependent density functional (TDDFT) calculation as a func-tion of photon energy is reported in table.3. To calculate theequivalent state-selective ionization cross sections, we implementthe following method: First, the state-selective relative ionizationefficiencies ( η i ) of different ionic states ( i ) for C H photoioniza-tion at a particular photon energy are calculated as, η i = η × R i (8)where, η and R i are the total relative ionization efficiency and thebranching ratio of the corresponding ionic state ( i ). η is related tocumulative efficiency of all photoions resulting from C H ioniza-tion at the corresponding photon energy (cf. table.1). We take ad-vantage of previously reported partial photoionization cross sec-tions of C H in the work of Cooper et al. , to determine thestate-selective ionization cross sections, which are a fraction ofthe total cross section. Implementing this scheme, we calculatedthese state-selective ionization cross-sections ( σ i ) from η i usingthe relation, σ i (in Mb) = ( . ± . ) × η i (in arb. u.). The suc-cess of the measurements and this procedure is vindicated by theagreement of these experimental state-selective ionization crosssections with the theoretical calculation.To our advantage, the PEPICO technique enables the measure-ment of state-selective branching ratios associated with the pho-toionization channels producing C H +2 and C H + ions as a func-tion of photon energy, see fig.5 b) and c). This leads to a com-prehensive picture of ionization and dissociation in the photo-fragmentation process examining the PES correlated to C H +2 and C H + ions. As observed from the PES (cf. fig.4) the higherexcited ionic states, ( C , D ) , do not leave behind unfragmentedC H +2 ions, the lowest ionized state, X , does not participate inthe dissociation process to produce C H + . For the C H +2 ion,the state-selective branching ratio of X state dominates over thesame of A and B states. The state-selective branching ratios ofthe X and A states slightly decrease with increasing photon en-ergy and are significantly higher than that of the B state whichslightly increases with increasing photon energy. On the otherhand, for C H + ion, the contributions of B states are dominantover the A and C , D states. Beyond the photon energy of ∼ eV, the state-selective branching ratio of A largely remain inde-pendent of photon energy. On the other hand, opposite behaviorsare seen for B and C , D states, in which the branching ratio of B decreases and the same of C , D increases with increasing photonenergy. The most significant aspect of correlated photoelectron imagingis the opportunity it provides to examine photoion- and state-specific PADs. This immediately reveals the variations of theasymmetry parameters ( β ) of the photoelectron angular distri-butions correlated to different electronic states. Fig.6 a), b) andc) depict β as a function of photon energy; these are determinedfor the cases of photoelectrons in coincidence with cumulative allC H photoions as well as PADs correlated to C H +2 and C H + ions, respectively. Since the π -electron usually leads to a higher Journal Name, [year], [vol.] , .00.51.01.52.00.00.51.01.52.0 18 19 20 21 22 23 24 25 26 27 280.00.51.01.52.0 X A B C , D a) All C H photoions X A B b) C H +2 Photon Energy (eV) A B C , D c) C H + Fig. 6
Photoelectron asymmetry parameter ( β ) of different states cor-related to a) cumulative all C H photoions b) C H +2 ion and c) C H + ion, respectively, as a function of photon energy. degree of asymmetry than that of σ electron ejection , the ob-served large value of β for X state originating from the ionizationof the HOMO ( π u ) is justified. Considering the low relative pho-toionization efficiencies of some of the channels, the VMI tech-nique plays an important role in the measurement of β param-eters in which photoelectrons are collected over the entire solidangle. In this case, the measured β ( ∼ . ) for X states is higherthan the corresponding values obtained from the previous experi-mental studies where angle-resolved spectra were recorded usinghemispherical electron analyzers .Two significant trends underscore the behaviour and physics ofthe dependence of the asymmetry parameter ( β ) as a functionof photon energy: i) For the X and A states, the absence of au-toionizing resonances in the chosen photon energy region, ... eV, underlies the observation of a weak dependence of the β , theasymmetry parameter on h ν , cf. the black and red lines in fig.6. Itis well known that autoionizing resonances influence PADs; thereare no such channels decaying to the lower ionized states, X and A , for h ν > eV , consistent with earlier results . ii) Thehigher ionized B state shows considerable variations in the β pa-rameter with photon energy in the PADs of the cumulative all pho-toion distribution and those correlated with the C H +2 ion, cf. theblue line in fig.6 panels a) and b). We observe a local maximum around . eV in the β vs. h ν curve for photoelectrons in coin-cidence with cumulative all photoions and a minimum at ∼ . eV in the photon energy dependence of β correlated to the C H +2 ion. In previous works, a minimum was observed at h ν = eV forthe B state and reasoned as occurring due to interplay between σ u → k σ g and σ u → k π g transitions, where k represents a state inthe continuum . Therefore, it is surprising that we do notobserve the minimum corresponding to the B state curve for thedissociation product, C H + ion; there is no such structure in the β vs. h ν curve. Rather, we only observe a nearly constant β withincreasing photon energy, cf. the blue line in fig.6 c). This leadsus to the conclusion that the observed local minimum around eV in the β vs. h ν curve for C H +2 ion is related σ u → k σ g , k π g autoionizing resonances which play a prominent role in the for-mation of C H +2 upon photoionization of the parent molecule.While C H + is formed by the dissociation of C H +2 , the absenceof this minimum (at ∼ eV) in the β associated with the for-mer indicates formation C H + ion from non-autoionizing states.A fraction of the population of C H participates in the aforemen-tioned autoionizing resonance which is left undissociated when itdecays, very likely, to lower vibrational states. However, C H + isformed by dissociation from a competing channel which proceedsthrough a population of the higher vibrational states of C H +2 upon direct ionization. This observation motivates further theo-retical investigations including multichannel interactions consid-ering autoionizing states and nuclear dynamics. Finally, for the C , D states, the value of β increases steadily with increasing pho-ton energy for cumulative all ions and C H + ions (green line infig.6 a), c)). Since these states do not produce unfragmentedC H +2 , autoionizing channels play no role.Several theoretical studies performed hitherto addressedthe variation of electronic state specific asymmetry parameter byconsidering multichannel interaction of electronic excitations inthis photon energy range. Wells and Lucchese implementedmultichannel scattering methodology (MCSCF) where the par-tial cross sections and corresponding β parameters of differentionic final states were calculated for different autoionization reso-nances. Fronzoni et al. used time dependent density functionalmethod on a fixed nuclei geometry to calculated the autoioniza-tion channels and the asymmetry parameters. Table.4 shows thecomparison of the β parameter obtained in our experiment withthe previous theoretical studies . Only for the B state, reason-able agreement between our experimental result and the theoret-ical calculation is observed. While for other ionized states our β values are quite different from the same calculated from theory.It should be noted that both the theoretical studies estimate dif-ferent values of β for the X , A and C , D states, with reasonableagreement only for B state. Before concluding this article, we discuss the photoionizationchannels that produce low ion-yields at M / q = , , and in the photoion ToF mass spectra. Here, both the single and dou-ble ionization regimes of C H are covered in the photon energy Journal Name, [year], [vol.][vol.]
Photoelectron asymmetry parameter ( β ) of different states cor-related to a) cumulative all C H photoions b) C H +2 ion and c) C H + ion, respectively, as a function of photon energy. degree of asymmetry than that of σ electron ejection , the ob-served large value of β for X state originating from the ionizationof the HOMO ( π u ) is justified. Considering the low relative pho-toionization efficiencies of some of the channels, the VMI tech-nique plays an important role in the measurement of β param-eters in which photoelectrons are collected over the entire solidangle. In this case, the measured β ( ∼ . ) for X states is higherthan the corresponding values obtained from the previous experi-mental studies where angle-resolved spectra were recorded usinghemispherical electron analyzers .Two significant trends underscore the behaviour and physics ofthe dependence of the asymmetry parameter ( β ) as a functionof photon energy: i) For the X and A states, the absence of au-toionizing resonances in the chosen photon energy region, ... eV, underlies the observation of a weak dependence of the β , theasymmetry parameter on h ν , cf. the black and red lines in fig.6. Itis well known that autoionizing resonances influence PADs; thereare no such channels decaying to the lower ionized states, X and A , for h ν > eV , consistent with earlier results . ii) Thehigher ionized B state shows considerable variations in the β pa-rameter with photon energy in the PADs of the cumulative all pho-toion distribution and those correlated with the C H +2 ion, cf. theblue line in fig.6 panels a) and b). We observe a local maximum around . eV in the β vs. h ν curve for photoelectrons in coin-cidence with cumulative all photoions and a minimum at ∼ . eV in the photon energy dependence of β correlated to the C H +2 ion. In previous works, a minimum was observed at h ν = eV forthe B state and reasoned as occurring due to interplay between σ u → k σ g and σ u → k π g transitions, where k represents a state inthe continuum . Therefore, it is surprising that we do notobserve the minimum corresponding to the B state curve for thedissociation product, C H + ion; there is no such structure in the β vs. h ν curve. Rather, we only observe a nearly constant β withincreasing photon energy, cf. the blue line in fig.6 c). This leadsus to the conclusion that the observed local minimum around eV in the β vs. h ν curve for C H +2 ion is related σ u → k σ g , k π g autoionizing resonances which play a prominent role in the for-mation of C H +2 upon photoionization of the parent molecule.While C H + is formed by the dissociation of C H +2 , the absenceof this minimum (at ∼ eV) in the β associated with the for-mer indicates formation C H + ion from non-autoionizing states.A fraction of the population of C H participates in the aforemen-tioned autoionizing resonance which is left undissociated when itdecays, very likely, to lower vibrational states. However, C H + isformed by dissociation from a competing channel which proceedsthrough a population of the higher vibrational states of C H +2 upon direct ionization. This observation motivates further theo-retical investigations including multichannel interactions consid-ering autoionizing states and nuclear dynamics. Finally, for the C , D states, the value of β increases steadily with increasing pho-ton energy for cumulative all ions and C H + ions (green line infig.6 a), c)). Since these states do not produce unfragmentedC H +2 , autoionizing channels play no role.Several theoretical studies performed hitherto addressedthe variation of electronic state specific asymmetry parameter byconsidering multichannel interaction of electronic excitations inthis photon energy range. Wells and Lucchese implementedmultichannel scattering methodology (MCSCF) where the par-tial cross sections and corresponding β parameters of differentionic final states were calculated for different autoionization reso-nances. Fronzoni et al. used time dependent density functionalmethod on a fixed nuclei geometry to calculated the autoioniza-tion channels and the asymmetry parameters. Table.4 shows thecomparison of the β parameter obtained in our experiment withthe previous theoretical studies . Only for the B state, reason-able agreement between our experimental result and the theoret-ical calculation is observed. While for other ionized states our β values are quite different from the same calculated from theory.It should be noted that both the theoretical studies estimate dif-ferent values of β for the X , A and C , D states, with reasonableagreement only for B state. Before concluding this article, we discuss the photoionizationchannels that produce low ion-yields at M / q = , , and in the photoion ToF mass spectra. Here, both the single and dou-ble ionization regimes of C H are covered in the photon energy Journal Name, [year], [vol.][vol.] , able 4 Comparison of asymmetry parameters ( β ) correlated to the photoelectrons in coincidence with cumulative all C H photoions with previoustheoretical studies at different photon energies ( h ν ) h ν (eV) β X Π u A Σ + g B Σ + u C Σ + g , D Σ + u Current TDDFT MCSCF Current TDDFT MCSCF Current TDDFT MCSCF Current TDDFT MCSCF X A B DC E di a) C +2 b) CH +2 E l e c t r on S i gna l ( a r b . u . ) c) CH + , C H d) C + Binding Energy (eV)
Fig. 7
The photoelectron energy spectra correlated to a) C +2 , b) CH +2 ,c) CH + , C H and d) C + ions, respectively, at different photon ener-gies. The vertical green dashed lines show the binding energies of singlyionized states of C H . Whereas the blue dashed line shows the dou-ble ionization energy ( E di ) of acetylene. The red vertical dashed linesshow the new peaks centred at . and . eV . The integral areas ofthe PES correlated to these ions at different photon energies are propor-tional to the relative ionization efficiencies ( η x ) of these photoions ( x ) atthe respective photon energy, shown in table.1. range, ... eV, to access the higher excited electronic states ofC H +2 leading to these low-yield ions. However, the previous sec-tions, only the single ionization pathways of C H were discussedby presenting the PES and PADs correlated to cumulative all C H photoions and the high yield ions, C H +2 and C H + . PES corre-sponding to these photoions are plotted in fig.7 as a function ofbinding energy (BE), where BE is calculated by subtracting thekinetic energy of the detected electron from the incident photonenergy. Since we detect only one emitted electron both for singleand double ionization events, a correct assignment of electronicstates is only possible for the cationic states (BE < E di ) which re-sult from single ionization events. To assign dicationic states (BE ≥ E di ) relevant to double ionization, it would be necessary to takeinto account the total kinetic energy carried by both the emittedelectrons. In fig.7, the vertical green dashed lines show cationicstates leading up to the blue dashed line showing the double ion-ization energy ( E di ∼ eV) of C H .Since, the studied photon energies cover the spectral rangeboth below and above the E di , we discuss these two regimes sep-arately. For h ν < E di , only single ionization of C H molecule ispossible. Therefore, photoion ToF mass peaks at M / q = , , and correspond to C +2 , CH +2 , CH + and C + ions, respectively,which result from different fragmentation channels of C H +2 ion.In fig.7, the PES correlated to all these ions have onset around the B Σ + u which show intense peak structures around . eV BE. Thisimplies that the low-yield ions are produced from the higher ex-cited states, B and beyond, whereas the high-yield C H +2 andC H + ions are found to be predominantly produced from thelower-lying X , A and B states. For h ν = . eV, we observe smallPES peaks at BE around B state. For C +2 , CH +2 and C + ions, thecorresponding PES peaks are centered at BE = . eV, whereasfor CH + ion, the associated peak is coinciding with B state (cf.fig.7 c)). At h ν = . eV, PES correlated to all these photoions(red line in fig.7) have similar maxima around a binding energyof . eV corresponding to the C state. However, the associatedphotoelectron asymmetry parameters ( β ) for these ions are quitedifferent (cf. table.5). The observed β decreases from . forC +2 ion to − . for C + ion. Similar peaks around . eV areobserved for photoionization at . and . eV (blue and greenline in fig.7), along with two additional peak structures centeredaround Π u (BE = . eV) and Σ + g (BE = . eV) states for h ν = . eV. The peak around . eV (vertical red dashed linein fig.7) cannot be associated with any reported cationic state,though. For C +2 and CH +2 ions, the . eV peak dominates overthe other two peaks assigned to Π u and Σ + g states, while for Journal Name, [year], [vol.] , able 5 State-selective photoelectron asymmetry parameters ( β ) correlated to C +2 , CH +2 and CH + , and C + ions at different photon energies ( h ν ) C H +2 state β h ν = eV h ν = eV h ν = eV h ν = eV h ν = . eVstate BI (eV) C +2 CH +2 CH + C + C +2 CH +2 CH + C + C +2 CH +2 CH + C + C +2 CH +2 CH + C + C +2 CH +2 CH + C + C Σ + g Π u Σ + g Σ + g Σ + u Σ + g Σ + g CH + and C + ion all the three peaks are almost at equal intensity.For Π u (BE = . eV) state, the observed photoelectron angu-lar distributions are isotropic, which resulted in β values close tozero. This is in contrast to the observed asymmetry ( β ) for X Π u state where we observe higher degree of asymmetry ( β ∼ . ).However, the new peak around . eV shows higher degree ofasymmetry which increases from . for C +2 to . for C + at h ν = . eV.For h ν > E di , both single and double ionization of C H arepossible. Therefore, the mass peaks at M / q = , , and correspond to fragment ions from both single and double ioniza-tion events. However, the contributions of single and double ion-ization events can be distinguished from the BE scale in the PES.For BE < E di , all the events are from single ionization processeswhich are discussed earlier. The relevant state-selective asymme-try parameters ( β ) are shown in table.5 for h ν = and eV. ForBE > E di , the PES correspond to the detection of one of the twoemitted electrons from double ionization of C H . The assign-ment of electronic states is not feasible, as stated earlier. Fig.7 a)shows the PES correlated to C +2 ion produced from the C H → C +2 + H + + H dissociation process . Fig.7 b) and d) showthe PES corresponding to CH +2 and C + which result from thesame photo-fragmentation channel C H → C + + CH +2 ; thischannel involves the characteristic isomerization of acetylene .For C H and CH + photoions produced by non-dissociative andthe dissociative (C H → CH + + CH + ) double ionization chan-nels, respectively, the corresponding PES are shown in fig.7 c) .For all the cationic states, we determine fragmentation channelspecific photoelectron asymmetry parameters ( β ) from the an-gular distributions obtained by integrating photoelectron countsin Abel-inverted distributions considering a eV energy-windowcentered at the BE of each state. Table.5 presents the details ofthe asymmetry parameter along with the BE of each state. This in-cludes state-selective β values which are distinct to photoioniza-tion channels with relatively low cross sections, producing C +2 ,CH +2 , CH + and C + ions, contributing new knowledge about thisimportant molecular system. Several intriguing dynamics of the acetylene-vinylidene systemwhich play a central role in our understanding of proton mi-gration and isomerization in the extreme ultraviolet, ... eVare uncovered. State-selective ionization pathways are identifiedfor C H photoionization. We observe that the unfragemented C H +2 ion mainly results from the lower-lying X , A and B states,while photodissociation of C H +2 from A , B , C and D states leadsto C H + ion and neutral H. Less abundant ions (C +2 , CH +2 ,CH + and C + ) are predominantly produced from even higher ex-cited states, B and beyond. For photoionization above the doubleionization energy ( E di ), these ions are produced due to fragmen-tation of C H . Below E di , the isomerization of acetylene is ad-dressed by presenting the PES and PAD in coincidence with CH ion. State-selective branching ratios and photoelectron asymme-try parameters ( β ) correlated to the relevant cationic states arereported as a function of photon energy for all the C H ion-ization channels. Photon energy dependent photoelectron asym-metry parameter shows distinct patterns for the photoionizationsleading to C H +2 and C H + ions. Previously reported autoion-izing resonance around eV decaying to B state is found tobe selective to the ionization pathway it proceeds. We observethis autoionization signature in the β parameter only for unfrag-mented C H +2 ion. Whereas photo-fragmentation channel pro-ducing C H + does not indicate such autoionization in the varia-tion of its photoelectron angular distribution with photon energy.To understand the photoelectron dynamics in this system, par-ticularly theoretical explorations combining both the nuclear andthe electron dynamics are required. Finally, these results openavenues urging time-resolved studies of this important molecu-lar system using table-top high-harmonic and free-electron laserpulses. Conflicts of interest
Authors confirm that there are no conflicts of interest to declare.
Author contributions
VS, RG and SRK proposed and designed this research. SM, RG,HS, AD, RR, MC, MM, VS and SRK performed the experiment.SM, RG, RR, MM, VS and SRK contributed to analysis of theexperimental data. SM, RG, RR, BB, AS, SS, MM, VS and SRKworked on the interpretation and phenomenology. SM, RG, RR,MC, SS, BB, MM, VS and SRK were involved in preparing themanuscript. BB, MM, RR, MC, VS and SRK contributed with sci-entific resources and funding towards the experimental realiza-tion and beamtime.
Acknowledgements
VS, RG and SRK are grateful to DST, India and ICTP, Trieste, forsupport (proposal
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Journal Name, [year], [vol.][vol.] , lettra Synchrotron facility. VS and SRK acknowledge financialsupport from the IMPRINT and DAE-BRNS scheme. SRK thanksthe Max Planck Society for supporting this research via the Part-ner group. MM acknowledges support from the Carlsberg Foun-dation, and with SRK and VS for the funding from the SPARCprogramme, MHRD, India. Notes and references
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