Photoelectron spectroscopy of coronene molecules embedded in helium nanodroplets
L. Ben Ltaief, M. Shcherbinin, S. Mandal, S. R. Krishnan, R. Richter, S. Turchini, N. Zema, M. Mudrich
PPhotoelectron spectroscopy of coronene molecules embedded in helium nanodroplets
L. Ben Ltaief, M. Shcherbinin, S. Mandal, S. R. Krishnan, R. Richter, S. Turchini, N. Zema, and M. Mudrich
1, 3, ∗ Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark Indian Institute of Science Education and Research, Pune 411008, India Department of Physics, Indian Institute of Technology, Madras, Chennai 600 036, India Elettra-Sincrotrone Trieste, 34149 Basovizza, Trieste, Italy Istituto Struttura della Materia-CNR (ISM-CNR), 00133 Roma, Italy (Dated: July 9, 2020)We present the first measurement of a one-photon extreme-ultraviolet photoelectron spectrum(PES) of molecules embedded in superfluid helium nanodroplets. The PES of coronene is comparedto gas phase and the solid phase PES, and to electron spectra of embedded coronene generated bycharge transfer and Penning ionization through ionized or excited helium. The resemblence of theHe-droplet PES to the one of the solid phase indicates that mostly Cor clusters are photoionized.In contrast, the He-droplet Penning-ionization electron spectrum is nearly structureless, indicatingstrong perturbation of the ionization process by the He droplet. These results pave the way toextreme ultraviolet photoelectron spectroscopy (UPS) of clusters and molecular complexes embeddedin helium nanodroplets.
I. INTRODUCTION
Over the past decades, helium (He) nanodroplets haveestablished themselves as flying nano-cryo-laboratories toinvestigate cold molecular reactions [1–3], novel nanos-tructures [4–8], and high-resolution molecular spectra [9–12]. The main benefits of He nanodroplets are that indi-vidual molecules or small clusters of atoms or moleculescan be isolated in an ultracold (0.37 K), highly dissipa-tive but weakly perturbing environment (He nanodropletisolation, HENDI). High-resolution absorption and emis-sion spectra of the embedded species can be recorded inthe infrared, visible, up to ultraviolet spectral regions.Extending HENDI spectroscopy to the extreme-ultraviolet and x-ray range to probe valence and inner-shell spectra of embedded (“dopant”) molecules is morechallenging, though. When the dopants or the Hedroplets are photoionized, strong interactions of the pro-duced photoions and electrons with the local environ-ment tend to massively shift and broaden spectral linesand to alter the fragmentation dynamics compared tothe gas phase [12]. Moreover, whenever the photon en-ergy hν is high enough to even excite or ionize the Hedroplets, i. e. hν >
21 eV, indirect Penning ionization ofthe dopants through excited He or charge transfer (CT)ionization through ionized He usually predominates overdirect dopant photoionization. Therefore, only few pho-toelectron spectroscopic studies of dopants in He dropletshave been reported, all of which employed resonant multi-photon ionization schemes [13–20]. In resonant multi-photon ionization using nanosecond laser pulses, nuclearrearrangement following excitation occurs on the picosec-ond time scale, such that photoelectrons are emitted froma transient state of the system. ∗ [email protected] Only recently, we have reported the first directone-photon ionization electron spectra of He droplet-embedded ground state species – small clusters of xenon(Xe) atoms ionized by soft x-ray synchrotron radia-tion [21]. In that experiment, electron emission from theXe dopants was enhanced by tuning the photon energyto the absorption ‘giant resonance’ (4d-shell ionization)around hν = 100 eV. As the absorption cross section ofHe is much smaller at that photon energies, direct pho-toionization of the Xe dopants was comparable or evenhigher in intensity to CT ionization through the pho-toionized He droplets.Here, we report the first measurements of a photo-electron spectrum (PES) of a molecule, the polycyclicaromatic hydrocarbon coronene (Cor), embedded in Henanodroplets. We suppress photoionization of the Hedroplets and subsequent CT ionization of the dopants bytuning the photon energy below the lowest absorptionbands of He droplets, hν <
21 eV. Cor is particularlywell-suited for this purpose owing to its high photosta-bility and its large absorption cross section of about 500Mbarn at hν = 18 . II. EXPERIMENTAL METHODS
The experiments are performed using a He nan-odroplet apparatus combined with a velocity-map imag-ing photoelectron-photoion coincidence (VMI-PEPICO)detector at the CiPo beamline of Elettra-Sincrotrone Tri-este, Italy. The use of a gold coated normal incidence a r X i v : . [ phy s i c s . a t m - c l u s ] J u l monochromator (2400 l/mm) ensures a high spectral res-olution and the suppression of higher-order radiation toa high degree.The apparatus has been described in detail else-where [24, 28]. Briefly, a beam of He nanodroplets is pro-duced by continuously expanding pressurized He (50 bar)of high purity out of a cold nozzle (14 K) with a diameterof 5 µ m into vacuum, resulting in a mean droplet size of¯ N He = 2 . × He atoms per droplet. Further down-stream, the beam passes a mechanical beam chopper usedfor discriminating droplet-beam correlated signals fromthe background. The He droplets are doped with Cormolecules by passing through a vapor cell containing ele-mentary Cor crystalline powder heated to 150 ◦ C. At thistemperature, the He nanodroplets pick up 1 Cor moleculewith highest probability, 37 %. As the pick-up processusually obeys the Poissonian statistics [29], fractions ofthe He droplets pick up 2 molecules (18 %) or more (8 %).In the detector chamber, the He droplet beam crossesthe synchrotron beam in the center of the VMI-PEPICOdetector at right angles. By detecting either electronsor ions using the VMI detector in coincidence with thecorresponding particles of opposite charge with the TOFdetector, we obtain either ion mass-correlated electronimages or mass-selected ion images. Kinetic energy dis-tributions of electrons and ions are obtained by Abel in-version of the images [30]. The energy resolution of theelectron spectra obtained in this way is ∆
E/E = 6%.
III. RESULTS AND DISCUSSION
Figure 1 b) shows three typical mass spectra recordedfor Cor molecules embedded in He nanodroplets at hν =18 .
5, 21 .
6, and 26 eV. Contributions from gasphase Corin the background gas were subtracted. For reference,we show in a) the electron impact ionization mass spec-trum of gas phase Cor from the NIST Mass Spectrom-etry Data Center [31]. Clearly, the most abundant ionin all three mass spectra is singly charged Cor, C H +12 ,at mass 300 amu. The mass peak at 276.4 amu (peak I)corresponds to C H +12 which is an impurity in the Corsamples. The one at 316.0 amu (peak II) is C H O + due to oxidization of Cor ions in the reaction with H Omolecules which are also present in the He nanodropletsin small amounts. On the larger mass scale, small un-fragmented Cor clusters, Cor n with n = 2-5 are clearlyvisible, see panel (c). The abundance of the Cor clustersreflects the Poissonian distribution of oligomers formedby doping with multiple Cor molecules [29]. Thus, Coris very stable against fragmention upon ionization, andmostly unfragmented Cor molecules and Cor n are de-tected.The formation of Cor n ions at hν = 26 eV is due tocharge transfer ionization through the ionized He nan-odroplet [32], whereas at hν = 21 . hν = 21 . H ) ( C H ) C H G a s P h a s e f r o m D a t a N i s t
Rel. intensity (arb. u.) ( a ) C H I I 2 6 e V 2 1 . 6 e V 1 8 . 5 e VM a s s ( a m u )( b ) II I II I 2 6 e V 2 1 . 6 e V1 4 K1 5 0 ° C 1 4 K1 5 0 ° C
Rel. intensity (cps)
M a s s ( a m u ) ( C H ) +5 ( C H ) +4 ( C H ) +3 ( C H ) +2 C H ( c ) I Figure 1. (a) Reference mass spectrum of coronene recordedby electron impact ionization [31]. (b), (c) Mass spec-tra recorded by photoionization of coronene-doped He nan-odroplets at different photon energies.
He nanodroplets are resonantly excited to their 1s2p Pstate, and Penning ionization occurs after relaxation tothe 1s2s , S state [23, 26, 33]. In the low-mass range (1-130 amu) of the mass spectrum recorded at hν = 26 eV, aseries of He + k complexes is present with intensities reach-ing up to that of Cor + . The shoulders extending theCor + n peaks to higher masses are due to charged Cor-Hecomplexes, so-called snowballs. The detailed structuresof [Cor n He k ] + snowballs have recently been studied us-ing electron impact ionization mass spectrometry [34, 35].Peak III at mass 446.4 amu nearly matches the mass ofthe doubly charged Cor trimer cation, [C H ] , whichwas previously observed in Ref. [36].The doubly charged Cor cation, present in the electron-impact mass spectrum, shown for reference in panel a),has an appearance energy about 18 . H +3 fragment ion [37]. Possibly,fragmentation is suppressed due to the cold environmentprovided by the He nanodroplets as it was previously ob-served [38].Indirect ionization of molecules doped in He nan-odroplets by charge transfer or Penning ionizationthrough ionized or excited He nanodroplets, respectively,has been reported multiple times. Here we report for thefirst time direct ionization of molecules embedded in Henanodroplets by a one-photon process. The mass spec-trum recorded at hν = 18 . and 10 . Once ionized, theHe droplet transfers its charge to the dopant with high ef-ficiency [32], thereby creating dopant ions in coincidencewith an electron emitted by He.However, in the present experiment the higher-ordercontent is low owing to the use of a normal incidencemonochromator. Our claim that at hν = 18 . + is predominantly formed by direct photoionization, issupported by the fact that the mass spectrum signifi-cantly differs from the one measured in the regime ofHe droplet photoionization at hν = 26 eV: The seriesof He + k mass peaks is entirely missing. However, thecount rate of Cor + is lower compared to the measure-ment at hν = 26 eV by about 2 orders of magnitude.This factor roughly matches the reduction of the absorp-tion cross section of Cor (500 Mbarn) compared to Hedroplets (23 . × . hν = 18 . × hν = 37 eV)still present to a small extent. The area of this peakamounts to (cid:46)
6% of the total coincident electron counts,which confirms that the electron spectrum is mainlygiven by electrons emitted directly from the Cor in-side the droplets. For comparison, PES of gasphaseCor (black) [41] and of solid Cor are also shown (greycurve) [42, 43]. To take into account the finite resolu-tion of our VMI spectrometer, we convolve the gasphasespectrum with a gaussian function with at full width athalf maximum (FWHM) of 6 % of the peak energy, seethe light blue line in Figure 2. This spectrum clearlyresembles the one measured in He droplets in that the
Electron counts (arb. u.)
E l e c t r o n k i n e t i c e n e r g y ( e V )
S o l i dG a s p h a s eC o n v . S i m u l .ABCDEF G H E x p .
Figure 2. He nanodroplet-correlated photoelectron spec-trum of coronene (red) recorded at hν = 18 . × He atoms per droplet. triplet of peaks labeled E-G is the dominant feature andpeaks A-D are also visible. However, peaks A-D are sig-nificantly less pronounced in proportion to peaks E-G.In this respect, the droplet PES more closely follows thePES measured for solid Cor, in which peaks E-G are thedominant features.To test whether elastic scattering of the photoelectronsupon He atoms on their way out of the He droplets couldaccount for the redistribution of peak intensity frompeaks A-C to E-G we have also carried out a classical 3-Dscattering simulation based on the differential electron-He scattering cross sections. A detailed description ofthe simulation can be found in our previous work [25].We find that the result of the simulation (blue curve)resembles the PES of solid Cor and to some extent ourmeasured PES in He droplets. The effect of elastic scat-tering is to shift peak positions down in energy by afew 100 meV and to transfer the overall intensity to-wards the low-energy part of the spectrum. However,features A-C remain more pronounced in the simulationcompared to the He droplet spectrum. The close re-semblance with the solid-phase Cor spectrum indicatesthat mostly Cor clusters contribute to the He-dropletPES, as clusters are an intermediate state of matterbetween individual molecules and the condensed phase.Thus, while He-droplet Penning ionization electron spec-tra (PIES) seem to be strongly perturbed and not to con-tain much useful information about the dopant’s electronspectrum [25, 27], PES may still reveal the main featuresof the electron binding energies. This opens new op-portunities for studying molecular complexes and nanos-tructures formed by He-nanodroplet aggregation at lowtemperature [2].
Electron counts (arb. u.)
E l e c t r o n k i n e t i c e n e r g y ( e V )x 1 5 0
Figure 3. He nanodroplet-correlated Penning ionization elec-tron spectrum recorded in coincidence with coronene ions at hν = 21 . For comparison, we have also measured the PIES incoincidence with Cor + for Cor-doped He nanodroplets,see the red line in Fig. 3. In contrast to the PES, this spectrum is nearly structureless and peaks at low ener-gies, around 1.2 eV. It largely resembles PIES measuredfor other PAH molecules doped in He nanodroplets [25].Note that the PIES measured for a solid Cor surface(grey curve in Fig. 3) is also dominated by a peak at1.5 eV [44]. However, part of the spectrum between 4and 14 eV still reveals some structure reminiscent of theelectron binding-energy spectrum. In that experiment,the contribution of both 1s2s S and S excited He atomslead to the congestion of the PIES.The signal minimum at electron energies between 0and 1 eV in the He-droplet PIES, which was previouslyobserved for other dopants, is likely related to the en-ergy gap between the ionization energy of He atoms andthe lower conduction-band edge of electrons in liquid Hewhich causes low-energy electrons to be trapped and torecombine with the ion [2, 23, 25]. Merely the onset of theelectron signal at 14.6 eV is reminiscent of the PES whenaccounting for the lower photon energy hν = 18 . S-excited He(20.6 eV) inducing Penning ionization. The large differ-ence between the PES (Fig. 2) and the PIES (Fig. 3)shows that perturbations of the electron energies cannotsolely be due to the interaction of the emitted electronwith the He droplet which should be the same for bothcases, He-droplet PES and PIES. Apparently, the Pen-ning ionization process itself is more strongly affected bythe He shell surrounding the dopant than the photoemis-sion process. This finding should be further investigatedboth experimentally and theoretically.In addition to the He-droplet PIES measured in coinci-dence with Cor + , Fig. 3 shows the PIES measured whenrecording all emitted electrons (black line). To elimi-nate contributions from ionization of the residual gas, wesubtracted the corresponding spectrum measured whenthe He droplet beam is blocked. The resulting electronspectrum appears to contain an additional componentaround 5 eV, which we tentatively inferred by subtract-ing the coincidence PIES weighted by factor 1/70, seethe blue line. This electron component must stem fromionization events where the ion eludes its detection as itis not present in the coincidence PIES. Most likely it isdue to Penning ionization of larger Cor clusters whoseions remain bound inside the He droplets, as it occursfor xenon atoms [21, 23]. Differences between the coin-cidence and total electron PIES of alkali metal dopantshave recently revealed details about the Penning ioniza-tion process which we dicussed in the context long-rangeand short-range interatomic Coulombic decay [26]. How-ever, this concept does not seem to apply in the presentcase as neither of the PIES reproduce the expected elec-tron binding energy spectrum.For completeness, we also show electron spectrarecorded at hν = 26 eV, i. e. in the regime of directphotoionization of the He nanodroplets, see the red linesin Fig. 4. Panel a) displays the spectra of all emitted elec-trons and panel b) shows the electron spectra recordedin coincidence with the dopant ions. It is interesting to - 3 - 2 - 1 C o r L i
Electron counts (cps) ( a ) T o t a l e l e c t r o n s , h n = 2 6 e V C o r L i( b ) C o i n c i d e n c e e l e c t r o n s , h n = 2 6 e VE l e c t r o n k i n e t i c e n e r g y ( e V ) Figure 4. Comparison between electron spectra recordedfor He nanodroplets doped with coronene and with lithiumatoms in the regime of droplet photoionization ( hν = 26 eV).(a) Spectra of all emitted electrons. (b) Spectra of electronsdetected in coincidence with coronene and lithium ions. compare with previously measured spectra of He dropletsdoped with lithium (Li) atoms as the He-droplet PIES ofalkali metals are very well resolved [24, 26]. The domi-nant feature is the He photoline at 1.4 eV, even in thespectra recorded in coincidence with the dopant ions.This indicates efficient charge-transfer ionization of thedopants.Surprisingly, the spectra contain a broad shoulder ex-tending to 18 eV at a lower signal level by nearly 3 ordersof magnitude. This signal appears to be due to Penningionization as its onset roughly matches the electron ener-gies one expects for Penning ionization, which are in theranges 13.2-17.2 eV for Cor and 15.2-19.2 eV for Li forthe case of He in the 1s2s S state up to high Rydbergstates. The total-electron spectrum recorded for Li dop- ing [grey line in Fig. 4 a)] even features a clear maximumaround 16 eV. This shows that even at a photon energy1.4 eV above the ionization threshold of He, a small frac-tion of the electrons remain bound to the droplets, partlyrelax to lower He excited states, and undergo Penningionization. Previously, this effect was only observed for hν ≤
25 eV, that is below the lower conduction band edgeof liquid He [24, 27]. This highlights the strong couplingof quasi-free electrons to He nanodroplets and the ca-pability of He droplets to efficiently dissipate energy byinducing electronic relaxation and Penning ionization.
IV. CONCLUSION
In summary, we have presented the first one-photonextreme-ultraviolet photoelectron spectrum of a dopantmolecule, coronene, embedded in He nanodroplets.Within the experimental error, the spectrum closely re-sembles that of solid coronene, indicating that predomi-nantly corone clusters are photoionized. Penning ioniza-tion electron spectra can be measured with more thana factor 100 higher count rates owing to the large reso-nant absorption cross section of He droplets and the ef-ficient channeling of the optical excitation to the dopantmolecule. However, those electron spectra are nearlystructureless, indicating strong perturbation of the Pen-ning process by the He droplet.Although coronene is a favorable case due to its largephotoionization cross section in a suitable range of pho-ton energies, this work proves the principle that ultra-violet photoelectron spectroscopy (UPS) and likely x-ray photoelectron spectroscopy (XPS) of dopants in Hedroplets is possible. This opens the way to more system-atic structural investigations of molecular complexes andnano-clusters formed in the unique ultracold environmentof superfluid He nanodroplets. However, the low targetdensity and the lack of enhancement of dopant ionizationby the He droplet requires sensitive detection techniquesand long acquisition times.
ACKNOWLEDGEMENT
M.M. and L.B.L. acknowledge financial support byDeutsche Forschungsgemeinschaft (DFG, German Re-search Foundation, projects MU 2347/10-1 and BE6788/1-1) and by the Carlsberg Foundation. SRK thanksDST and MHRD, Govt of India, through the IMPRINTprogrammes, and the Max Planck Society. M.M. andS.R.K. gratefully acknowledge funding from the SPARCProgramme, MHRD, India. The research leading tothis result has been supported by the project CALIPSO-plus under grant agreement 730872 from the EU Frame-work Programme for Research and Innovation HORI-ZON 2020. [1] M. F´arn´ık and J. P. Toennies, J. Chem. Phys. ,014307 (2005).[2] A. Mauracher, O. Echt, A. Ellis, S. Yang, D. Bohme,J. Postler, A. Kaiser, S. Denifl, and P. Scheier, PhysicsReports , 1 (2018).[3] T. K. Henning and S. A. Krasnokutski, Nature Astron-omy , 568 (2019).[4] L. F. Gomez, E. Loginov, and A. F. Vilesov, Phys. Rev.Lett. , 155302 (2012).[5] A. Boatwright, C. Feng, D. Spence, E. Latimer, C. Binns,A. M. Ellis, and S. 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