Penning Spectroscopy and Structure of Acetylene Oligomers in He Nanodroplets
S. Mandal, R. Gopal, M. Shcherbinin, A. D'Elia, H. Srinivas, R. Richter, M. Coreno, B. Bapat, M. Mudrich, S. R. Krishnan, V. Sharma
PPenning Spectroscopy and Structure of Acetylene Oligomers in He Nanodroplets
S. Mandal, R. Gopal, M. Shcherbinin, A. DElia, H. Srinivas, R. Richter, M.Coreno,
6, 7
B. Bapat, M. Mudrich,
3, 8
S. R. Krishnan, a) and V. Sharma b)1) Indian Institute of Science Education and Research, Pune 411008,India Tata Institute of Fundamental Research, Hyderabad 500107,India Aarhus University, 8000 Aarhus C, Denmark Department of Physics, University of Trieste, Via A. Valerio 2, 34127 Trieste,Italy Max-Planck-Institute f¨ur Kernphysik, 69117 Heidelberg,Germany Elettra-Sincrotrone Trieste, 34149 Basovizza, Italy Consiglio Nazionale delle Ricerche Istituto di Struttura della Materia,34149 Trieste, Italy Indian Institute of Technology Madras, Chennai 600036,India Indian Institute of Technology Hyderabad, Kandi 502285,India (Dated: 13-April-2020) a r X i v : . [ phy s i c s . a t m - c l u s ] A p r mbedded atoms or molecules in a photoexcited He nanodroplet are well-known tobe ionized through inter-atomic relaxation in a Penning process. In this work, weinvestigate the Penning ionization of acetylene oligomers occurring from the photoex-citation bands of He nanodroplets. In close analogy to conventional Penning electronspectroscopy by thermal atomic collisions, the n = 2 photoexcitation band plays therole of the metastable atomic 1 s s , S He ∗ . This facilitates electron spectroscopyof acetylene aggregates in the sub-kelvin He environment, providing the followinginsight into their structure: The molecules in the dopant cluster are loosely boundvan der Waals complexes rather than forming covalent compounds. In addition, thiswork reveals a Penning process stemming from the n = 4 band where charge-transferfrom autoionized He in the droplets is known to be the dominant relaxation channel.This allows for excited states of the remnant dopant oligomer Penning-ions to bestudied. Hence, we demonstrate Penning ionization electron spectroscopy of dopeddroplets as an effective technique for investigating dopant oligomers which are easilyformed by attachment to the host cluster. a) [email protected] b) [email protected] . INTRODUCTION He nanodroplets have been regarded as an ideal host environment for spectroscopic studiesof embedded atoms and molecules over a vast spectral range spanning from the infrared tothe vacuum ultraviolet due to their ability to ro-vibronically cool these dopants without anychemical modification. However, this seemingly passive He host environment proves to befertile ground for studying a rich class of intermolecular relaxation processes between theexcited host and the attached dopants when being photoexcited . A potent observablefor obtaining insights into these processes is the energy distribution of ejected electronstagged to particular ions arising out of these multi-atomic processes. This observable can beapplied specifically to study the indirect Penning ionization of dopant aggregates interactingwith photoexcited He ∗ in the droplets whereby the participating quantum states of both theembedded species and the droplets can be discerned. Although recent report suggestedthat the scattering of electrons following Penning ionization may obscure the molecularfeatures, we were able to resolve the Penning ionization electron spectra (PIES) in the caseof acetylene (C H ) oligomers. This motivates the development of ion-correlated energy-resolved electron detection in combination with the Penning process as a spectroscopic toolto study the electronic structure of weakly-bound quantum aggregates. These atomic andmolecular complexes can be aggregated with relative ease by employing He nanodropletsas a nanoscale sub-Kelvin container . The application of Penning spectroscopy aidedby coincident electron-ion detection to small acetylene clusters in nanodroplets indicates aloosely bound van der Waals aggregate of C H molecules in the sub-Kelvin He nanodropletenvironment.Our investigation includes a series of electron-ion coincidence measurements detailed inthe next section. Aided with information about the prominent fragmentation productsthrough 20 ...
26 eV photon energy, the kinetic energy distributions of electrons in coinci-dence with these ions are measured. To initiate the Penning ionization we chose 21 . n = 2 droplet band. Typically, dopedalkali atoms which reside on the droplet surface are known to be preferentially Penning ion-ized by this excitation of the complex . Here, we measured electron kinetic energy spectrain coincidence with the most abundant dopant cluster ions, C H +2 , [C H ] +2 and [C H ] +3 .In contrast to studies hitherto , we were also able to use the autoionizing n = 4 droplet3and for PIES of acetylene doped He nanodroplets which can reveal acetylene cluster ionsleft in excited states higher than those possible in the case of the n = 2 excitation. Not onlydoes PIES reveal details about the dopant oligomers, the converse, the relaxation behaviorof excited He ∗ in the droplet containing the dopant is also a subject of current studies. TheHe droplet is expected to internally relax from the dipole allowed 1 s p P to 1 s s , S He ∗ states before the Penning ionization ensues .This work on the Penning ionization of acetylene oligomers mediated by different photo-excitation bands of He nanodroplets is revealing in many respects:First, we gain insights in the relaxation dynamics of excited He nanodroplets and inthe electronic states of acetylene oligomers involved in the Penning ionization process. Inaddition to the n = 2 states of He ∗ , higher states of He ∗ are found to induce Penningionization of acetylene thereby accessing higher-lying states of the acetylene product ion.Second, PIES reveals a dominant monomer-like feature even for Penning electrons taggedto the acetylene dimer and trimer ions pointing to a weakly bound van der Waals system ofthe aggregate inside the droplet. This is reminiscent of the foam-like structure evidenced inthe case of Mg doped into He nanodroplets . II. EXPERIMENTAL DETAILS
Our investigations are the result of a beamtime at the Gas-Phase (GAPH) beamline atthe Elettra Synchrotron Trieste, Italy. The schematic of the experiment is depicted in fig.1while the details have been presented elsewhere . In brief, the implementation consistsof three sections. The first one is the source chamber where He nanodroplets are generatedby supersonic expansion from a cryogenically cooled nozzle. This is attached to a dopingchamber where the droplet jet picks up dopant molecules from the doping chamber. Thestream of doped droplets then enters the interaction chamber where the doped dropletsare intercepted by the synchrotron radiation. To produce He nanodroplets, in the sourcechamber pressurized ( ∼
50 bar) high-purity helium gas (He 6 .
0) is supersonically expandedthrough a cryogenically cooled nozzle with a 5 . µ m orifice. The jet is extracted using atrumpet-shaped skimmer with a 0 . T noz ) serves to control the mean size of the droplets; typically varying T noz between 16 and14 K allows a control of droplet sizes between 8800 and 23000 He atoms per droplet on the4 IG. 1. Schematic diagram of the experimental setup. He droplets are generated in the sourcechamber by supersonic expansion of the He gas through the cryogenic nozzle and extracted intothe next region by a skimmer. The jet of droplets is doped by picking up C H molecules whichare effused in the doping chamber downstream. Subsequently, in the interaction chamber, thedoped He droplet jet is ionized by EUV synchrotron radiation. The resultant electrons and ionsare measured in coincidence by the VMI and TOF spectrometers operating in tandem. average .The skimmed jet of He nanodroplets exits the source chamber to pick up C H moleculeswhich were effused into the doping chamber by a controlled leak through a dosing valve.This variation of the partial acetylene pressure ( P d ) in this region, 6 × − mbar to 4 × − mbar, offers a direct control over the pick up of the dopant molecules which follows Poisso-nian statistics. The number of dopant molecules per droplet can be varied between at themost one dopant molecule per droplet, to several molecules captured into a typical dropletin the jet. Before doping, it is important to distil acetylene gas to remove inevitable ace-tone contamination. We passed the precursor through a coiled copper tube immersed ina bath with ethanol and liquid N slurry maintained at 173 K. Furthermore, a mechanicalchopper operating between the source and the doping chambers to periodically interceptthe nanodroplet jet enables us to record distinct background signals arising out of the ef-5usive residual gas molecules in addition to acquiring signals from doped droplets. Thesemeasurements performed in quick succession, typically switching at ∼
70 Hz, allow us to re-liably subtract the background due to effusive gases enabling low-noise acquisition of dropletspecific signals.Downstream of the doping chamber, the doped droplet jet passes through a second skim-mer to enter into the interaction chamber (cf. fig.1). This chamber, maintained at ∼ − mbar houses a velocity map imaging (VMI) spectrometer for electrons along with a time-of-flight (TOF) spectrometer for ions operating in tandem. At the geometric center of theinteraction chamber the doped droplets interact with the focused beam of linearly polarizedEUV photons from the synchrotron. The photon beam has a typical peak intensity of ∼ − at a repetition rate of 500 MHz in the form of ∼
150 ps pulses. We have used pho-ton energies between 20 and 26 eV for electronic excitation and ionization of the host Hematrix. Two slits in the photon beam path were adjusted to maintain moderate count-ratesin the range of 10 −
20 kHz on the charged particle detectors. To suppress the higher orderharmonics of the synchrotron radiation, a Sn filter was used for measurements at 21 . E/E ≤ − over thewhole photon energy range.The VMI and TOF spectrometers operating synchronously in the interaction chamberenable electron-ion coincidence measurements. Both the single and double ion coincidenceswith electrons were implemented in these experiments. From sufficiently long acquisitions,time-of-flight ion-ion correlation maps were obtained along with the corresponding electron-VMI distributions. However, these ion-ion coincidence maps did not evidence any doubleionization of C H doped He nanodroplets in the studied photon energy range. The kineticenergy distributions of the electrons are derived from the velocity-map-images recorded ona position sensitive delay-line detector of the VMI spectrometer. These were Abel invertedusing well-established protocols - we employed B. Dick’s MEVELER for inversion. Tocalibrate the kinetic energy of electrons for a given configuration of the VMI spectrometer,we referenced photoionization of atomic He over a few photon energies in the range of 25 eVto 40 eV. The average energy resolution (∆ E/E ) of the VMI spectrometer is typically ∼ II. RESULTS AND DISCUSSIONA. Ion yield
Droplet induced ionization of doped acetylene can be readily observed by recording theion yield of C H +2 as a function of photon energy in the energy range from 20 eV to 26eV (cf. fig.2). Fig.2 also shows the measured He +2 yield from doped droplets in the samephoton energy range for comparison as this is known to be the most prominent ion arisingout of the host. This reveals two important aspects of the induced ionization of acetylenedoped droplets. The feature at the n = 2 droplet band centered at 21 . H +2 ions (red) are seen without any significant yield of He +2 is characteristic ofthe Penning process. At higher photon energies, beyond the autoionization threshold ( ∼ H +2 and He +2 yields follow very similar trends.It is well known that, at a photon energy of 21 . n = 2 band derived from the 1 s p atomic He level .Following this photoexcitation, due to repulsive interaction with the droplet environment,the excited He ∗ usually migrates to the surface of the droplet and Penning ionization is foundto be particularly efficient for surface-bound alkali atoms . However, more recently, Penningionization of immersed molecules was also clearly observed . As we observe a peak in theC H +2 yield from the droplet at the same photon energy, we expect the doped acetylenemolecules, which are believed to stay at the interior of the droplet, to be ionized by thefollowing Penning process:He m + [C H ] n + hν −→ He ∗ m + [C H ] n −→ He m + [C H ] + n + e − Penning (1)The peak structures below the ionization energy of atomic He ( E He i = 24 .
58 eV) in theHe +2 yield correspond to the autoionization of the He nanodroplets. This occurs via dropletphotoexcitation to Rydberg states of He ∗ which are derived from atomic 1 s np P , n > < hν < E He i ). For hν ≥ E He i , direct ionization of He atom in the nanodropletsoccurs. Common to both these ionization regimes, the He + or He +2 ion formed in thedroplet usually migrates to its interior due to the net attractive interaction with rest of thedroplet enabled by fast charge hopping . In the case of rare gas dopants, which reside in the7 I on Y i e l d ( a r b . u . ) Photon Energy (eV) He +2 C H +2 ( 5) 1s2p P 3p P 4,5p P E
Hei
FIG. 2. Ion yields of He +2 and C H +2 from the acetylene doped He droplet as function of photonenergy. The nozzle temperature and doping chamber pressure are maintained at 16 K and 6 . × − mbar, respectively. The blue vertical dashed line represents the ionization energy of atomic He( E He i ) and the vertical green dashed lines at 21 .
21 eV, 23 .
09 eV, 23 .
78 eV, and 24 .
04 eV representatomic He ∗ s p P , 1 s p P , 1 s p P and 1 s p P energy levels, respectively. The yellow shadedregion shows the droplet photoexcitation band correlated to the 1 s p atomic He level. droplet interior, ionization by charge-transfer is the dominant dopant ionization process .Thus, both these regimes, autoionization and direct ionization, are expected to contributeto dopant ionization. This is convincingly evidenced by the yield of C H +2 ions followingthe trend of the host He +2 ion for hν >
23 eV, cf. fig.2, due to a charge-transfer process:8e m + [C H ] n + hν −→ He + m + [C H ] n + e − He −→ He m + [C H ] + n + e − He . (2)However, in the autoionization regime (23 eV < hν < E He i ), previous studies with Li andAr doped He nanodroplets reported significantly low contributions of Penning ionizationfrom He ∗ (1 s s ) excited states which competes with the dominant charge-transfer channel .Further, in comparison to the He ∗ s s states, the contribution to dopant Penning ionizationfrom higher excited states He ∗ (1 s np , n >
2) in Li doped droplets is rather small . Theenergy distributions of the emitted electrons measured in coincidence with ions for boththe Penning (1) and charge-transfer (2) ionization processes are expected to be distinct.The corresponding electron spectra would also enable the identifications of the ionizationprocesses and of the participating electronic states corresponding to the doped C H oligomerand the He host.Fig.3 shows the ion mass spectra at two different photon energies: a) 21 . . +2 yields are proportional to thephotoionization cross sections of N for producing N +2 at the respective photon energies .We observe acetylene oligomer ions including the monomer (C H +2 ), the dimer ([C H ] +2 )and the trimer ([C H ] +3 ) from doped droplets. At 23 . + m , m = 1 −
3) originating from droplet autoionization. We also observe more extensivefragmentation of acetylene oligomer ions around the corresponding dimer and trimer ionpeaks at this photon energy which is significantly different from the Penning ionization at hν = 21 . + ions with C H molecule .In the Penning ionization process, at hν = 21 . H +2 ions with significant contrast of droplet specific signal over residual gas background wasextremely low. Both for single doping ( P d = 6 . × − mbar) and for conditions optimizedto multiply dope the He droplet with C H molecules with relatively high acetylene pressures9 P d = 4 . × − mbar) in the doping chamber, the signal of droplet specific monomer C H +2 ions was low. Note that, even for multiple doping conditions, only acetylene monomers aredoped into the He droplet. This is evident from the residual background ion signal whereonly C H monomer signal is present. We ascribe two reasons for the low detection ofC H monomer ions compared to C H oligomer ions in this Penning ionization process: a)Suppression of the escape of the smaller ion (C H +2 ) from the droplet following the Penningionization; b) In the case of multiply doped droplets, the formation of larger oligomer ionsby the association of first Penning ionized C H +2 with other doped neutral C H moleculesin the droplet. We will discuss this further in the context of the PIES in the next section.The energy released in the association process evaporates several He atoms from the dropletleading to a disintegration of the complex which enables the escape and eventual detectionof C H oligomer ions. B. Electron Energy Spectra
1. At n = 2 droplet excitation band Fig.4 presents the PIES correlated to (a) acetylene dimer ions ([C H ] +2 ) and (b) acety-lene trimer ions ([C H ] +3 ) originating from dopant Penning ionization when the dopeddroplet is photoexcited at 21 . . . . −
11 eV.Earlier reports on rare gas and alkali metal doped droplets , provided evidence thatupon photoexcitation at 21 . ∗ s p P dipole excited state but also prominently from long-lived He ∗ s s , S states whichare populated upon fast relaxation. We interpret the measured PIES correlated to acetylenedimer and trimer ions using PIES arising out of all these channels. (author?) studied Penning ionization of C H by metastable He ∗ (1 s s S ) in slowatomic collisions and reported the corresponding PIES. To obtain the PIES of acetylenemonomer due to Penning ionization by He ∗ in the 1 s s S and the dipole allowed 1 s p P states, we shifted the reported PIES by the corresponding energy differences, +0 . . .00.20.40.60.81.01.82.0 2 10 18 26 34 42 50 58 66 74 820.00.20.40.60.81.01.82.0 I on C oun t ( a r b . u . ) Droplet+Effusive Background Effusive Background(a) h = 21.6eV(b) h = 23.9eVHe +2 C H +2 [C H ] +2 [C H ] +3 I on C oun t ( a r b . u . ) M/q Droplet+Effusive Background Effusive Background [C H ] +3 [C H ] +2 C H +2 FIG. 3. Ion mass spectra at photon energies of (a) 21 . . T noz = 14 K and P d = 4 . × − mbar. The blue line represents droplet and effusive background signal and red linerepresents only the effusive background signal. The horizontal axis shows the mass ( M ) to charge( q ) ratio of the ionic fragments. the fitting of high energy feature of the measured PIES, though. Noting that the ionizationthreshold of C H dimer lies 0 .
96 eV below the ionization threshold of its monomer , thePIES of the C H dimer is derived from the corresponding PIES of the monomer by a furthershift of +0 .
96 eV. For the sake of keeping the number of free parameters within a reasonablelimit, the relative amplitudes of the three components in the model of the dimer spectrumare kept fixed at the values obtained from the fit using the monomer fit function. Dueto finite energy resolution of the VMI spectrometer the derived PIES are convoluted withthe spectrometer instrument function. The resulting convoluted PIES for C H monomerPenning ionization from He ∗ s s S , 1 s s S and 1 s p P states are represented as red,11 He * ( P) [C H ] S S P FIG. 4. PIES correlated to (a) [C H ] +2 and (b) [C H ] +3 from acetylene doped He dropletionization at photon energy of 21 . P d = 4 . × − mbar and T noz = 14 K. The red, blueand green spectra with hatched shading are the convoluted PIES for C H monomer from He ∗ s s S , 1 s s S and 1 s p P states, respectively, while the convoluted PIES of C H dimer from theseHe ∗ states are represented by the red, blue and green spectra with filled shading, respectively. Thesolid black line in each panel shows the total fit performed over the high energy feature from 7 . H monomer and dimer correlated toC H +2 X Π u state. The parameters for the fit were determined only using experimental data inthe region 7 . ≤ . A Σ + g and B Σ + u states. In panel (b), the red,blue and green vertical lines denote the PIES peak positions of C H trimer Penning ionizationfrom He ∗ s s S , 1 s s S and 1 s p P states, respectively. H dimerPenning ionization from these three states of He ∗ are shown as red, blue and green lines withfilled shading, respectively.The high energy feature (7 . −
11 eV) correlated to [C H ] +2 (cf. fig.4a) fits quite nicelyto the modeled sum of PIES of the acetylene monomer and dimer. The contribution ofthe 1 s s S state of He ∗ to the experimental PIES is relatively large ( ∼ . While the 1 s s S state is not expected to be efficiently populated bydroplet-induced relaxation, other factors may enhance the corresponding measured electronsignal in the PIES. The Penning ionization cross section as well as the ejection mechanismout of the droplet leading to bare [C H ] +2 Penning ions also determine the amplitude ofthe 1 s s S -state contribution.The predominant contribution of this well-matched fit comes from the three channelscorresponding to the Penning ionization of the acetylene monomer which is significantlyhigher than that from the ionization of the dimer. Likewise, applying this procedure toPIES correlated with [C H ] +3 (cf. fig.4b), using the same relative amplitudes of Penningionization channels from He ∗ (1 s s S , 1 s s S , and 1 s p P ) states obtained from the model-fitting of [C H ] +2 PIES, results in a higher contribution of the monomer ionization thanthe dimer ionization to match with the observed spectrum. If Penning ionization occurreddirectly from these trimers, we would have expected the corresponding peak structures inthe PIES beginning from ∼
10 eV rather than ∼ .
83 eV . This would result in maximumkinetic energies of Penning electrons extending up to ∼ ∼ H trimer Penning ionization from He ∗ s s S , 1 s s S and 1 s p P states, respectively.Assuming again a fixed ratio of peak amplitudes, the contribution of these peaks to the fitof the overall spectrum would be negligible, though.The ratios of dimer to monomer ionization channels for the PIES correlated to [C H ] +3 and [C H ] +2 are 0 .
92 and 0 .
27, respectively. In both cases, the major contributions toPenning ionization are from the monomer ion rather than from larger oligomers. Thisindicates that acetylene molecules form loosely bound oligomers in He nanodroplets, whichlargely retain the electronic structure of the acetylene monomer. This makes a strong casefor processes alternative to the direct Penning ionization of acetylene dimers and trimers as13he underlying mechanism for dopant ionization. This finding is in line with recent resultsfrom infrared spectroscopic studies of acetylene dimer in He nanodroplets . The followingpicture emerges consistent with the salient features of the observed PIES correlated toacetylene dimer and trimer Penning ions: Following Penning ionization from an excitedHe ∗ in the droplet, the acetylene monomer and dimer ions associate with additional dopantmolecules in the droplet to form larger acetylene oligomer ions. The energy of formationreleased into the droplet aids ejection out of the He nanodroplets and the detection of thereleased bare dimer and trimer ions. Free jet studies indicate that the [C H ] +2 and [C H ] +3 are covalently bound with substantial binding energies > . We may also note that insingly doped droplets a monomer C H +2 ion does not have the benefit of further associationand may not escape the droplet at all due to electrostrictive forces which bind it stronglyto the He host. Thus, the structure of an acetylene cluster formed in He nanodroplets isthat of a loosely bound van der Waals molecular aggregate rather than a covalently bondedsystem. This is reminiscent of a foam-like structure consisting of discrete units of dopantmonomers and dimers rather than entire [C H ] n clusters, for n ≥
3. Nonetheless, thisvan der Waals aggregate collapses into a larger bound cluster [C H ] + n ion upon Penningionization. This is very similar to the photoinduced collapse observed in the case of Mg-doped He nanodroplets . Likewise, ionization induced recombination has been observed forCr atoms attached to He nanodroplets . This is a central result of this work demonstratingthe implementation of the Penning spectroscopy in droplets to elucidate the structure ofdopant aggregates.In the remainder of this article we discuss the low-energy feature (0 . − . . (author?) , measured PIESof the dopants arising from the He ∗ s p P and 1 s s S states in the droplet. Theseelectrons undergo inelastic scattering inside the droplet and lose energy which leads to alow-energy feature in the PIES. Recently, PIES of acene doped He nanodroplet were foundto be massively broadened presumably due to the scattering and many-body interaction ofthe emitted Penning electron with the surrounding He. Similar to that work, we account forthe scattering effects on the Penning electrons in the droplet by performing a Monte Carlosimulation. However, despite incorporating all the features of electron-He scattering in oursimulations we found that even for large droplet radius of 30 nm (compared to 6 . T noz = 14 K and He expansion pressure of 50 bar) the simulated PIESdo not agree with the low-energy part of the photoelectron energy spectra. While the highenergy feature (7 . −
11 eV) is result of Penning ionization of C H to C H +2 ( X Π u ), thislow-energy feature (0 . − . H leavingit behind in higher excited A Σ + g and B Σ + u states whose ionization energies are 16 . . . This large Penning ionization signal involving the A and B statesas compared to that in earlier molecular beams experiments may be related to the steepdependence of the corresponding effective cross-sections on the collision energy .
2. At n = 4 droplet excitation band To learn about the ionization mechanisms in the autoionization regime (23 eV < hν 500 meV) electrons which is a signature of He nanodropletautoionization . Interestingly, the electron spectra correlated to C H +2 and [C H ] +2 (cf.fig.5b1,b2,c1,c2) at 23 . . +2 . This indicates that conventional charge-transfer from the He +2 in the dropletformed due to autoionization (2) cannot be the only mechanism for the underlying forma-tion of the acetylene oligomer ions. The additional low energy peak around 0 . H , leading to highly excited C H +2 . We mea-sured photoelectron spectra (PES) from effusive C H molecule at the two relevant photonenergies (cf. fig.5d1,d2). Simulated PES from the higher-lying C Σ + g and D Σ + u states ofC H +2 whose ionization energies are 23 . 33 eV and 23 . 53 eV , respectively, fit the observedPES quite well.To confirm the additional Penning ionization of acetylene from 1 s p P state of He ∗ resulting in C H +2 in C Σ + g and D Σ + u states, we used the following two-function fittingprocedure to analyse the electron spectra correlated to C H +2 and [C H ] +2 from droplet15 C H Fit PES (C ) PES (D )D B A X FIG. 5. Electron energy spectra at two photon energies, (1) 23 . . +2 , (b1,b2) C H +2 , (c1,c2) [C H ] +2 from the acetylene doped dropletionization at P d = 4 . × − mbar and T noz = 14 K. The conventional charge-transfer (CT)ionization processes are shown by red curves whereas the new Penning processes via He ∗ (1 s p P )state leading to C H +2 C Σ + g and D Σ + u states are shown by blue and green curves respectively.Black curves are the sum of CT and Penning processes. The insets in panel(c1) and (c2) showthe zoomed out electron spectra correlated to [C H ] +2 ion at hν = 23 . hν = 24 . ∗ (1 s s , 1 s p )states represented by brown lines in (c1,c2) observed at hν = 21 . H photoionization at (d1) 23 . . H +2 C Σ + g and D Σ + u states, respectively. The insetsin panel(d1) and (d2), PES peaks from C H +2 X Π u , A Σ + g , B Σ + u , C Σ + g and D Σ + u states canbe observed. F ( E ) = C MB ( k B T ) / √ Ee − EkBT + (cid:88) i =1 , C Gi σ e − ( E − Epi )22 σ where the Maxwell-Boltzmann distribution function (first term) represents the charge-transfer ionization component and the Gaussian functions (second term) represent the newPenning ionization channels. Using the value of T , obtained from the fitting of the elec-tron spectra in coincidence with He +2 , we fit the electron spectra correlated to C H +2 and[C H ] +2 varying the coefficients C MB and C Gi . Since Penning ionization could occur fromatomic He ∗ (1 s p P ), the Gaussian peak positions ( E Pi ) are fixed at the energy differencebetween He ∗ (1 s p P ) and C H +2 ( C Σ + g , D Σ + u ) states while the standard deviation ( σ ) isfixed from the fitting of the effusive acetylene. As the energy distribution of the autoionizedelectrons which are detected in coincidence with He +2 fits the Maxwell-Boltzmann distribu-tion quite well, we used the same Maxwell-Boltzmann distribution to fit the charge-transferionization component. Note that, the observed energy distribution of the autoionized elec-trons are relatively high compared to previous report by (author?) due to the finiteenergy resolution of the VMI.The charge-transfer components are marked by red curves whereas blue and green linesrepresent the new Penning ionization components from C H +2 C Σ + g and D Σ + u states,respectively, in fig.5 b1), b2), c1) and c2). Thus, we identify a prominent Penning mechanismleading to higher-lying C Σ + g and D Σ + u states of C H +2 via He ∗ s p P .Whereas at hν = 23 . hν = 24 . H ] +2 at these energies (insets of fig.5c1, c2) also have weak long tail, extending upto nearly 11 eV. This tail is possibly due to Penning ionization from He ∗ s s , 1 s p statesarising from internal relaxation, as observed in PIES at hν = 21 . n = 2 droplet excitation band at 21 . H +2 in X , A , and B states. However,when the droplet is photoexcited to even higher 1 s p state, Penning ionization channelsleading to higher-lying C H +2 states such as C and D states become energetically accessibleand are observed here in this autoioization regime along with Penning ionizations leading tolower-lying C H +2 states ( X, A, B ). The enhanced Penning ionization cross-section leadingto C and D states by He ∗ s p P could be related to the near degeneracy of these C and17 states with the 1 s p P state.This interpretation is further supported by PIES recorded for He nanodroplets doped withLi atoms presented previously by (author?) . The spectrum recorded at hν = 21 . s p P absorption band) is dominated by electrons emitted by Penningionization of Li interacting with excited He in the 1 s s S and S states. In the coincidentelectron energy spectra recorded above the droplet ionization threshold (23 eV), the peaksbetween 13 and 16 eV reflects Penning ionization of Li after electronic relaxation of theexcited He droplet into the 1 s s S and S states , and the low-intensity feature around18 eV is due to Penning ionization involving the 1 s s , 1 s p and 1 s s , 1 s p states of He ∗ .The fact that the contribution to the Li Penning ionization signal arising from these higherexcited states is significantly lower than in the case of acetylene may be due to the differinglocation of the dopants in the droplet: Li atoms are on the surface, whereas C H moleculesare expected to be located in the He droplet interior. Consequently, a He ∗ excitation initiallylocalized within the He droplet migrates over a significant distance until it reaches the Lidopant which affords the relaxation into the metastable 1 s s S and S states before Penningionization occurs. In comparison, the distance between the He ∗ and the acetylene dopant isshorter on average thereby facilitating direct Penning ionization prior to He ∗ relaxation. IV. CONCLUSION Scattering of electrons following the Penning ionization of dopants in He nanodropletsis thought to obscure molecular electron spectra, as demonstrated earlier in the case ofacene molecules used as dopants . In this work we show that this is not always the case.Penning ionization can indeed be used as spectroscopic tool to study atomic and molecularquantum aggregates formed in He nanodroplets by exciting the host matrix. By studyingthe electrons and ions for the host and dopant in coincidence, we identify relevant excitedstates of the host and the dopant. Furthermore, we demonstrate that this generic Penningionization electron spectroscopy scheme is not limited only to the n = 2 droplet excitationbut can be extended to perform spectroscopy employing higher droplet excitation bandssuch as n = 4 to probe the excited states of the dopant cluster e.g., the C and D statesin acetylene ion. Employing this technique we uncover the structure of acetylene clustersformed inside nanodroplets. They coalesce in the form of loosely bound van der Waals18ggregates, akin to a foam as observed for magnesium, rather than as a covalently boundsystem. This structure collapses into a composite oligomer ion following Penning ionization.Our work motivates further investigation of the structure of molecular aggregates in Henanodroplets. This study establishes Penning ionization electron spectroscopy as a widelyapplicable technique to probe mesoscopic quantum aggregates in nanodroplets which behavelike nano-cryostats even when they are photoactivated. 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