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Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer cation Cs2He+n

Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer cation... PCCP View Article Online PAPER View Journal | View Issue Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer Cite this: Phys. Chem. Chem. Phys., cation Cs He † 2019, 21,25362 2 n a a a Lorenz Kranabetter, Nina K. Bersenkowitsch, Paul Martini, ab a a a Michael Gatchell, Martin Kuhn, Felix Laimer, Arne Schiller, a a a Martin K. Beyer, Milan Oncak * and Paul Scheier * ˇ´ We investigate the photodissociation of helium-solvated cesium dimer cations using action spectroscopy and quantum chemical calculations. The spectrum of Cs He shows three distinct absorption bands into both bound and dissociative states. Upon solvation with further helium atoms, considerable shifts of the 1 + absorption bands are observed, exceeding 0.1 eV (850 cm ) already for Cs He , along with significant 2 10 Received 29th August 2019, broadening. The shifts are highly sensitive to the character of the excited state. Our calculations show Accepted 30th September 2019 that helium atoms adsorb on the ends of Cs . The shifts are particularly pronounced if the excited state orbitals extend to the area occupied by the helium atoms. In this case, Pauli repulsion leads to a DOI: 10.1039/c9cp04790e deformation of the excited state orbitals, resulting in the observed blue shift of the transition. Since the rsc.li/pccp position of the weakly bound helium atoms is ill defined, Pauli repulsion also explains the broadening. absorption lines for the first 32 He atoms attached could be Introduction 31–33 used to extrapolate to the absorption line of the bare ion. In Vibrationally resolved electronic spectroscopy in combination combination with theory, the experimental results of the matrix with theoretical calculations is a powerful technique to determine shift as well as the width of the absorption lines provide details of the geometrical arrangement and electronic structure unprecedented insight into the details of the solvation of ions of molecules or clusters. The quality of vibrationally resolved with helium, including phase transitions and isomeric effects. spectra depends critically on the temperature, as recently demon- Alkali metal atoms reside on dimples at the surface of + 1 21,34–40 strated for the assignment of the structure of Au . This has helium droplets, as shown in numerous experimental 41–46 triggered the development of several cryogenic techniques, i.e., and theoretical studies. Alkali metal clusters, however, are 2,3 4,5 6,7 46–48 molecular beams, matrix isolation, helium droplets, cryo- submerged into the cluster above a critical cluster size. The 8–20 21–25 genic traps and cryogenic storage rings. The low density groups of Ernst and Stienkemeier investigated photoionization + 34,39 of ionic targets requires special techniques, such as cavity ring of Cs atoms and the submersion of Cs into He droplets. 26 27,28 + down or action spectroscopy. Tagging of ions with a weakly Chen et al. managed to solvate Cs ions with several million bound messenger turns out to be particularly suitable to measure helium atoms via pickup of Cs formed upon thermionic 10,29 49 absorption lines of ions, albeit leading to a matrix shift. The emission into large He droplets. Dopant ions ejected from binding energy of helium to ions is lower than for any other atom charged helium droplets are sometimes complexed with a few or molecule, leading to a minimum matrix shift. helium atoms when recorded via mass spectrometry. The first Whereas only very few He atoms can be attached to singly- solvation layer is particularly strongly bound, and its closure charged ions in cold traps, ions can be solvated with almost any is typically reflected as a clear intensity drop of the ion series + + 48,50–52 number of He atoms when formed in doped helium nanodro- MHe (with M the alkali metal ion) in mass spectra. plets. In the case of C , a remarkable linear red-shift of the Path integral or basin-hopping methods reproduce the experi- 44,45 ments very well. It was also shown previously that helium a 53 Institut fu ¨r Ionenphysik und Angewandte Physik, Universita¨t Innsbruck, adsorption might lead to appreciable spectral shifts. Technikerstr. 25, A-6020 Innsbruck, Austria. E-mail: Milan.Oncak@uibk.ac.at, Spectroscopic properties of the Cs ion were investigated Paul.Scheier@uibk.ac.at 54,55 56–58 previously in experimental and theoretical studies within Department of Physics, Stockholm University, 106 91 Stockholm, Sweden the series of alkali metal dimers, assigning the lowest electro- † Electronic supplementary information (ESI) available: Relative stabilities of nically excited states. In the present study, we explore photo- calculated isomers, method benchmarks, spin–orbit couplings, complete experi- mental spectra, Cartesian coordinates. See DOI: 10.1039/c9cp04790e dissociation of Cs He via action spectroscopy to assess helium 2 n 25362 | Phys. Chem. Chem. Phys., 2019, 21,25362--25368 This journal is © the Owner Societies 2019 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper PCCP solvation effects in a seemingly simple diatomic system. Single- floppy excited state in Cs He , even stricter criteria for solving the reference and multi-reference ab initio calculations including EOM-CCSD equations were used, with convergence criteria of 10 , 11 10 spin–orbit coupling reproduce the experimental results and 10 and 10 for energy, wavefunction and CCSD and ground- provide an assignment of the electronic transitions. A pronounced state Z-vector iterations, respectively. In all calculations, only 17 1 + blue-shift of more than 0.1 eV (850 cm ) for absorption into the electrons in Cs were treated explicitly, with the rest described 2 + + 1 S state for Cs He with n = 10 provides clear evidence that the within an effective core pseudopotential (ECP). u 2 n He atoms preferentially occupy positions along the axis of the Purely dissociative as well as bound excited states are cesium dimer cation. encountered in Cs . The absorption spectra of the dissociative states were modeled using the linear reflection principle (LRP) within 66–68 the harmonic approximation and by the standard reflection principle projecting the vibrational wavefunction (calculated for the Methods Cs potential on a grid) onto the excited state potential energy Cs He ions are formed upon electron irradiation (85 eV, 260 mA) surface. The spectra of the bound states were calculated using the m n of Cs doped He nanodroplets (average size about 10 atoms, Franck–Condon approximation within the harmonic regime as 2.7 MPa, 9.95 K expansion conditions). Penning ionization via implemented in the Gaussian software or directly by calculating electronically excited He* will be the dominant ionization channel the overlap of the ground state vibrational wavefunction in the 7,59–61 for the heliophobic cesium atoms. Ionized cesium atoms and electronic ground state with both ground and excited vibrational clusters have a substantially stronger interaction with the He wavefunctions in the electronically excited state. matrix than their corresponding neutral precursors, thus leading Gaussian 16 was used for TDDFT and (EOM-)CCSD calculations, to their submersion into the droplet. Low-mass ions ejected from Molpro for MRCI and spin–orbit calculations. the large multiply charged droplet are deflected by 90 degrees relative to the neutral beam via electro-static lenses. The beam is Results and discussion guided into the extraction region of a high-resolution time of flight mass spectrometer, where it is merged with a laser beam from a Let us start with the photodissociation spectrum of Cs He pulsed tuneable light source (EKSPLA NT 242, 210–2600 nm). Every (Fig. 1a). Upon electronic excitation, the He atom is lost, and tenth extraction pulse of the mass spectrometer operated at 10 kHz is irradiated with the laser (repetition rate 1 kHz), thereby enabling simultaneous measurement of mass spectra with and without laser light. Upon photon absorption, all adsorbed He atoms are typically lost from Cs He , and an optical photodissociation spectrum is m n derived from the depletion signal. A detailed description of the experiment can be found in the ESI† (Fig. S9) and elsewhere. To obtain a quantum chemical description of the optical spectra, ground state structures of Cs He ions were first 2 n modelled at the coupled cluster singles doubles (CCSD) level with the def2QZVP basis set on Cs and def2TZVP on He. The position of the weakly bound He atoms, residing in shallow potential wells, is very sensitive to the quality of the basis set used on Cs. With the triple-zeta quality basis set def2TZVP used + + on Cs, a bent structure is predicted for Cs He ; a linear Cs He 2 2 structure is obtained with the def2QZVP basis set on Cs and def2TZVP on He, and this does not change when the def2QZVPPD basis set is used on both Cs and He. To validate the theory as much as possible, excited states were modelled on various theory levels: time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional; equation of motion – CCSD (EOM- CCSD); multi-reference configuration interaction (MRCI) with an active space of one electron in 5 or 17 orbitals, MRCI(1,5) and MRCI(1,17), respectively; def2QZVPPD basis set was used on all atoms for excited state calculations. Spin–orbit coupling was Fig. 1 (a) Photodissociation spectra of Cs He ions, n =1,2,3,6,8,10, 12 2 n calculated using the state-interacting method as implemented obtained as a differential spectrum with and without laser irradiation of 63,64 65 clusters and scaled with laser power (see ESI† for details). A fit of the peak in the Molpro program along with the ECP46MDF basis set. at 2.8 eV by two Gaussian functions is shown for n = 1. Band centers for The D symmetry group was used for calculations; irreducible 2h Cs He are marked by vertical lines to guide the eye. Electronic state representations (IRs) in the D symmetry group were recon- Nh assignment is based on calculations (Table 1). (b) Multiple Gaussian fit to 2 + structed from the respective IRs in D D . ‘‘Very Tight’’ optimi- 2h 2h the vibrational progression of the transition into the 1 P state in Cs He u 2 zation criteria were used for optimization; for optimization in the and determination of the vibrational spacing. This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21,25362--25368 | 25363 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online PCCP Paper Table 1 Experimental and theoretical properties of excited states in Cs 2 (see Table S3 in the ESI†), the S states are only slightly affected, and Cs He : energy position E (in eV), width w of the Gaussian peak fit with shifts of up to 5 meV. However, the doubly-degenerate P states (= 2s), experimental splitting DE , spin–orbit splitting DE (all in meV). split SO 1 split into two states, separated by B10–35 meV (80–280 cm ). Excitation energies were calculated at the EOM-CCSD/def2QZVPPD level Comparison of the calculations with experiment, summarized of theory, spectral width was modelled within reflection principle approxi- in Table 1, allows us to assign the three main bands observed in mation, spin–orbit coupling at the MRCI(1,17)/ECP46MDF level 2 + 2 2 the experiment as 1 S ,1 P , and 2 P , reproducing the u u u 2 + 2 2 State 1 S 1 P 2 P u u u excitation energy to within 0.1 eV. The fourth allowed transition + 2 + Experiment, Cs He E 1.41 1.55; 1.59 2.82; 2.86 into 2 S at B3.1 eV (see Table S2, ESI†) is not observed in the w 83 — 39; 30 experiment due to insufficient laser power in this region. For DE —39 42 split 2 + 2 the dissociative states 1 S and 2 P , calculations within the u u Theory, Cs E 1.43 1.51 2.85 reflection principle approximation also predict a realistic width w 64 — 26 of the observed bands, see Table 1. DE —28 13 SO As mentioned above, the 1 P state is bound and, therefore, Theory, Cs He E 1.44 1.51 2.84 the measured photodissociation spectrum reflects its vibrational DE —28 14 2 SO structure. The 1 P state is calculated to have a very similar 2 + equilibrium distance as the 1 S ground state (Fig. 2). The excited state potential well is less steep, reflected in the calculated the spectrum is recorded following the depletion of the ion harmonic vibrational frequencies of 30 and 20 cm for the signal. There are four clearly observed absorption bands at 2 + 2 + 1 S and 1 P state, respectively, for Cs (at the (EOM-)CCSD/ about 1.4, 1.55, 1.60 and 2.8 eV (Table 1). The band at 2.8 eV is g u 2 def2QZVPPD level). The respective values for Cs He are 31 and further split, with spacing of about 40 meV (300 cm ). The 2 1 + 19 cm , with a stronger Cs –He interaction in the excited states, band at 1.60 eV has pronounced vibrational structure, with a 2 2 + shortening the Cs–He distance from 4.4 Å in the 1 S state to spacing of 22 cm (Fig. 1b); the broad bands at 1.4 and 2.8 eV g 2 + 4.0 Å in 1 P . The harmonic Cs He vibration is located at are, on the other hand, structureless. u 2 1 2 1 30 cm . The calculated frequency in the 1 P state of 19 cm For a more quantitative interpretation and reliable assignment u fits well to the measured spacing of 22 cm . Note that only the of these features, we performed EOM-CCSD and MRCI calculations higher 3/2u(1 P ) state shows clear vibrational resolution shown and analyzed the contribution of spin–orbit coupling effects. Fig. 2 u + + shows the potential energy curves of Cs .The Cs ion has D in Fig. 1b. The 1/2u(1 P ) state interacts with the repulsive 2 2 Nh 2 + symmetry and, when disregarding spin–orbit coupling, its ground 1/2u(1 S ) state, and the potential energy surface will be deformed 2 + due to the avoided crossing of the states. From a computational electronic state is 1 S . From this state, the only symmetry allowed perspective, the equilibrium distance in the 1 P state depends transitions are perpendicular and parallel transitions into P and 2 + 2 + 2 heavily on the choice of the basis set (Fig. S4, ESI†). As a direct S states, respectively. Within 3 eV, two S and two P states u u u consequence, our attempts to reproduce the experimental spectrum can be found. Among these states, only the 1 P state is bound, shown in Fig. 1b using Franck–Condon simulations failed to grasp the others are dissociative. When accounting for spin–orbit effects correctly the relative intensity of the contributing peaks (see the ESI† for details). Finally, the calculated weak spin–orbit splitting of the P states results into two states 1/2u and 3/2u with a different O value following Hund’s rule 3. The splitting is well reproduced 2 2 for 1 P (see also inset in Fig. 2). For 2 P , the calculated spin– u u orbit splitting is significantly smaller than the experimentally observed splitting. These observations can be rationalized if one takes into account the additional splitting induced by the asymmetric distribution of the He atom relative to the singly occupied molecular orbital (SOMO) in the excited state. Fig. 3 illustrates how the energy of the several electronic states in + + Cs He depends on the position of the He atom in Cs He . For 2 2 this calculation, the Cs–Cs–He angle is fixed, and the Cs–Cs and Cs–He distances are optimized in the electronic ground 2 + state, 1 S . From 1801 to 1501, the ground state energy hardly changes, i.e. the He atom moves more or less freely in this + 2 + Fig. 2 Potential energy curves in Cs calculated at the EOM-CCSD/ range of Cs–Cs–He angles. The repulsive 1 S state reacts more 2 u def2QZVPPD level. Excited states to which electronic transitions are sensitively to the position of the He atom, and this effect allowed from the ground state are shown with full lines, forbidden ones contributes to the broadening of the absorption band, which with dashed lines. In the inset, the vicinity of the 1 P minimum is shown, is 19 meV larger than expected from the linear reflection along with states including spin–orbit coupling (dotted lines, calculated at 2 2 principle approximation, Table 1. For the 1 P and 2 P states, the MRCI(1,17)/ECP46MDF level of theory). Only selected states are shown. u u See the ESI† for excluded electronic states and method benchmarking. the degeneracy of the two contributing p orbitals (see Fig. 4) is 25364 | Phys. Chem. Chem. Phys., 2019, 21,25362--25368 This journal is © the Owner Societies 2019 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper PCCP Let us now concentrate on the shift of the spectra induced by solvation, i.e. the He matrix shift. As shown in Fig. 1a and Table 2, pronounced shifts to higher energies are recorded for 2 + 2 the 1 S and 2 P states, reaching values of more than 0.1 eV u u + 2 + already for Cs He . Note also that the shift in the 1 S state 2 10 u exceeds the He binding energy. In the n = 1–10 range, the 2 + energy of the 1 S state shifts linearly with solvation by 12 meV 1 2 (100 cm ) per helium atom. The 2 P band at B2.8 eV seems to shift slightly non-linearly, its structure is however not well resolved in the experiment for larger ions. The 1 P state at 1.6 eV, on the other hand, is shifted only byB20 meV (160 cm )to lower energies within n =1–12 (Table2). The shifts can be directly connected to the interplay between the position of He atoms around the Cs ion and the character of the respective excited state. Our calculations show that Cs is preferentially solvated by He atoms on its ends, along the Cs–Cs axis for the smallest ions (see Fig. 4). For Cs He and Cs He , the most stable configuration is linear, but deformation 2 2 from linearity down to a Cs–Cs–He angle of 1501 requires very little energy, see Fig. 3; in Cs He , conformations with six and five/seven 2 12 Fig. 3 Dependence of ground state and excited state energies in Cs He He atoms on each end are the most stable ones (Table S1, ESI†). on the Cs–Cs–He angle. Calculated at the EOM-CCSD/def2QZVPPD As expected, the binding energy of helium atoms is low, within level of theory with ground-state structures optimized at the CCSD/ 2.4–3.1 meV He for up to 12 helium atoms. In addition to the def2QZVP(Cs),def2TZVP(He) level with a constrained Cs–Cs–He angle. shifts, the presence of a large variety of almost isoenergetic Spin–orbit effects were neglected. isomers leads to significant broadening of the spectra upon solvation (see Fig. S1, ESI†). Due to Pauli repulsion, the presence of He atoms deforms the orbitals of both ground and excited electronic states that extend along the Cs–Cs axis, shifting them to higher energy. This is the case for the HOMO orbital as well as the orbitals 2 + 2 corresponding to excitations into the 1 S and 2 P states u u (Fig. 4). The diffuse orbitals in the excited state are deformed more than the HOMO orbital, shifting the excitation energy considerably to higher values (Fig. 1a). The p orbital corres- ponding to the excitation into the 1 P state at 1.6 eV repre- sents a different case. Here, the surrounding helium atoms do not approach the orbital and the shift to lower energies reflects the slight destabilization of the HOMO orbital. Our calculations 2 + can reproduce the shifts for excitations into 1 S and under- estimate those observed for 1 P (Table 2); for n = 12, the Fig. 4 Highest occupied molecular orbital (HOMO) and natural transition orbitals for Cs He ions calculated at the CAM-B3LYP/def2QZVPPD level 2 n of theory. A low iso value (0.005) was chosen so that orbital deformation Table 2 Experimental and calculated energy shift of excited states upon induced by helium is clearly visible. solvation (in meV) with respect to Cs He . See Fig. S12 for experimental 2 + 2 + fits. Electronic states are correlated to 1 S and 1 P states in Cs . Vertical u u 2 transitions were calculated at the EOM-CCSD/def2QZVPPD//CCSD/ lifted by the presence of the He atom, which leads to an def2QZVP(Cs), def2TZVP(He) level of theory. For the 1 P state, the additional splitting. For 1 P , this effect is negligible down to average calculated value of the two SOMO orientations is given. Isomers 1501 Cs–Cs–He angle, because the He atom does occupy the with equal distribution of He atoms to each Cs end are chosen for the calculation same space as the singly occupied p orbitals. In contrast, the SOMO of the 2 P state is much more diffuse, and in one of Experiment Theory the two orientations exhibits significant overlap with the position 2 + 2 2 + 2 n 1 S 1 P 1 S 1 P He u u u u of the He atom. The splitting of the two orientations of the SOMO in the 2 P state amounts to 31 meV at 1501, which constitutes211 3.4; 3.7 6 0.8 323 6.1; 6.6 15 2.1 the dominant contribution to the experimentally observed 659 13.3; 15.2 41 6.1 splitting. For larger clusters, the same effect is responsible for 10 98 9.2; 19.3 96 12.0 the significant broadening of the 2 P band. 12 124 18.5; 41.0 109 14.2 This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21,25362--25368 | 25365 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online PCCP Paper 6 J. P. Toennies and A. F. Vilesov, Angew. Chem., Int. Ed., 2004, sudden increase observed in the 1 P experimental shift might 43, 2622–2648, and references therein. be induced by the fitting procedure (see Fig. S12, ESI†)or 7 A. Mauracher, O. Echt, A. M. Ellis, S. Yang, D. K. Bohme, appearance of new isomers. This is most likely caused by the J. Postler, A. Kaiser, S. Denifl and P. Scheier, Phys. 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Backstrom, This work was supported by the Austrian Science Fund FWF, J. D. Alexander, S. Leontein, D. Hanstorp, H. Zettergren, project numbers W1259 (DK-ALM), P29174 and P31149, and the M. Kaminska, R. Nascimento, L. Liljeby, A. Kallberg, Swedish Research Council, contract number 2016-06625. The A. Simonsson, F. Hellberg, S. Mannervik, M. Larsson, W. D. tunable OPO system is part of the Innsbruck Laser Core Facility, Geppert, K. G. Rensfelt, A. Paal, M. Masuda, P. Hallden, financed by the Austrian Federal Ministry of Science, Research G. Andler, M. H. Stockett, T. Chen, G. Kallersjo, J. Weimer, and Economy. The computational results presented have been K. Hansen, H. Hartman and H. Cederquist, EPJ Web Conf., achieved using the HPC infrastructure LEO of the University of 2015, 84, 01004. Innsbruck. 22 M. Gatchell, H. T. Schmidt, R. D. Thomas,S.Rosen,P.Reinhed, P. Lofgren, L. Brannholm, M. Blom, M. Bjorkhage, E. Backstrom, J. D. Alexander, S. Leontein, D. Hanstorp, H. 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Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer cation Cs2He+n

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Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer cation Cs2He+n

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PCCP View Article Online PAPER View Journal | View Issue Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer Cite this: Phys. Chem. Chem. Phys., cation Cs He † 2019, 21,25362 2 n a a a Lorenz Kranabetter, Nina K. Bersenkowitsch, Paul Martini, ab a a a Michael Gatchell, Martin Kuhn, Felix Laimer, Arne Schiller, a a a Martin K. Beyer, Milan Oncak * and Paul Scheier * ˇ´ We investigate the photodissociation of helium-solvated cesium dimer cations using action spectroscopy and quantum chemical calculations. The spectrum of Cs He shows three distinct absorption bands into both bound and dissociative states. Upon solvation with further helium atoms, considerable shifts of the 1 + absorption bands are observed, exceeding 0.1 eV (850 cm ) already for Cs He , along with significant 2 10 Received 29th August 2019, broadening. The shifts are highly sensitive to the character of the excited state. Our calculations show Accepted 30th September 2019 that helium atoms adsorb on the ends of Cs . The shifts are particularly pronounced if the excited state orbitals extend to the area occupied by the helium atoms. In this case, Pauli repulsion leads to a DOI: 10.1039/c9cp04790e deformation

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PCCP View Article Online PAPER View Journal | View Issue Considerable matrix shift in the electronic transitions of helium-solvated cesium dimer Cite this: Phys. Chem. Chem. Phys., cation Cs He † 2019, 21,25362 2 n a a a Lorenz Kranabetter, Nina K. Bersenkowitsch, Paul Martini, ab a a a Michael Gatchell, Martin Kuhn, Felix Laimer, Arne Schiller, a a a Martin K. Beyer, Milan Oncak * and Paul Scheier * ˇ´ We investigate the photodissociation of helium-solvated cesium dimer cations using action spectroscopy and quantum chemical calculations. The spectrum of Cs He shows three distinct absorption bands into both bound and dissociative states. Upon solvation with further helium atoms, considerable shifts of the 1 + absorption bands are observed, exceeding 0.1 eV (850 cm ) already for Cs He , along with significant 2 10 Received 29th August 2019, broadening. The shifts are highly sensitive to the character of the excited state. Our calculations show Accepted 30th September 2019 that helium atoms adsorb on the ends of Cs . The shifts are particularly pronounced if the excited state orbitals extend to the area occupied by the helium atoms. In this case, Pauli repulsion leads to a DOI: 10.1039/c9cp04790e deformation of the excited state orbitals, resulting in the observed blue shift of the transition. Since the rsc.li/pccp position of the weakly bound helium atoms is ill defined, Pauli repulsion also explains the broadening. absorption lines for the first 32 He atoms attached could be Introduction 31–33 used to extrapolate to the absorption line of the bare ion. In Vibrationally resolved electronic spectroscopy in combination combination with theory, the experimental results of the matrix with theoretical calculations is a powerful technique to determine shift as well as the width of the absorption lines provide details of the geometrical arrangement and electronic structure unprecedented insight into the details of the solvation of ions of molecules or clusters. The quality of vibrationally resolved with helium, including phase transitions and isomeric effects. spectra depends critically on the temperature, as recently demon- Alkali metal atoms reside on dimples at the surface of + 1 21,34–40 strated for the assignment of the structure of Au . This has helium droplets, as shown in numerous experimental 41–46 triggered the development of several cryogenic techniques, i.e., and theoretical studies. Alkali metal clusters, however, are 2,3 4,5 6,7 46–48 molecular beams, matrix isolation, helium droplets, cryo- submerged into the cluster above a critical cluster size. The 8–20 21–25 genic traps and cryogenic storage rings. The low density groups of Ernst and Stienkemeier investigated photoionization + 34,39 of ionic targets requires special techniques, such as cavity ring of Cs atoms and the submersion of Cs into He droplets. 26 27,28 + down or action spectroscopy. Tagging of ions with a weakly Chen et al. managed to solvate Cs ions with several million bound messenger turns out to be particularly suitable to measure helium atoms via pickup of Cs formed upon thermionic 10,29 49 absorption lines of ions, albeit leading to a matrix shift. The emission into large He droplets. Dopant ions ejected from binding energy of helium to ions is lower than for any other atom charged helium droplets are sometimes complexed with a few or molecule, leading to a minimum matrix shift. helium atoms when recorded via mass spectrometry. The first Whereas only very few He atoms can be attached to singly- solvation layer is particularly strongly bound, and its closure charged ions in cold traps, ions can be solvated with almost any is typically reflected as a clear intensity drop of the ion series + + 48,50–52 number of He atoms when formed in doped helium nanodro- MHe (with M the alkali metal ion) in mass spectra. plets. In the case of C , a remarkable linear red-shift of the Path integral or basin-hopping methods reproduce the experi- 44,45 ments very well. It was also shown previously that helium a 53 Institut fu ¨r Ionenphysik und Angewandte Physik, Universita¨t Innsbruck, adsorption might lead to appreciable spectral shifts. Technikerstr. 25, A-6020 Innsbruck, Austria. E-mail: Milan.Oncak@uibk.ac.at, Spectroscopic properties of the Cs ion were investigated Paul.Scheier@uibk.ac.at 54,55 56–58 previously in experimental and theoretical studies within Department of Physics, Stockholm University, 106 91 Stockholm, Sweden the series of alkali metal dimers, assigning the lowest electro- † Electronic supplementary information (ESI) available: Relative stabilities of nically excited states. In the present study, we explore photo- calculated isomers, method benchmarks, spin–orbit couplings, complete experi- mental spectra, Cartesian coordinates. See DOI: 10.1039/c9cp04790e dissociation of Cs He via action spectroscopy to assess helium 2 n 25362 | Phys. Chem. Chem. Phys., 2019, 21,25362--25368 This journal is © the Owner Societies 2019 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper PCCP solvation effects in a seemingly simple diatomic system. Single- floppy excited state in Cs He , even stricter criteria for solving the reference and multi-reference ab initio calculations including EOM-CCSD equations were used, with convergence criteria of 10 , 11 10 spin–orbit coupling reproduce the experimental results and 10 and 10 for energy, wavefunction and CCSD and ground- provide an assignment of the electronic transitions. A pronounced state Z-vector iterations, respectively. In all calculations, only 17 1 + blue-shift of more than 0.1 eV (850 cm ) for absorption into the electrons in Cs were treated explicitly, with the rest described 2 + + 1 S state for Cs He with n = 10 provides clear evidence that the within an effective core pseudopotential (ECP). u 2 n He atoms preferentially occupy positions along the axis of the Purely dissociative as well as bound excited states are cesium dimer cation. encountered in Cs . The absorption spectra of the dissociative states were modeled using the linear reflection principle (LRP) within 66–68 the harmonic approximation and by the standard reflection principle projecting the vibrational wavefunction (calculated for the Methods Cs potential on a grid) onto the excited state potential energy Cs He ions are formed upon electron irradiation (85 eV, 260 mA) surface. The spectra of the bound states were calculated using the m n of Cs doped He nanodroplets (average size about 10 atoms, Franck–Condon approximation within the harmonic regime as 2.7 MPa, 9.95 K expansion conditions). Penning ionization via implemented in the Gaussian software or directly by calculating electronically excited He* will be the dominant ionization channel the overlap of the ground state vibrational wavefunction in the 7,59–61 for the heliophobic cesium atoms. Ionized cesium atoms and electronic ground state with both ground and excited vibrational clusters have a substantially stronger interaction with the He wavefunctions in the electronically excited state. matrix than their corresponding neutral precursors, thus leading Gaussian 16 was used for TDDFT and (EOM-)CCSD calculations, to their submersion into the droplet. Low-mass ions ejected from Molpro for MRCI and spin–orbit calculations. the large multiply charged droplet are deflected by 90 degrees relative to the neutral beam via electro-static lenses. The beam is Results and discussion guided into the extraction region of a high-resolution time of flight mass spectrometer, where it is merged with a laser beam from a Let us start with the photodissociation spectrum of Cs He pulsed tuneable light source (EKSPLA NT 242, 210–2600 nm). Every (Fig. 1a). Upon electronic excitation, the He atom is lost, and tenth extraction pulse of the mass spectrometer operated at 10 kHz is irradiated with the laser (repetition rate 1 kHz), thereby enabling simultaneous measurement of mass spectra with and without laser light. Upon photon absorption, all adsorbed He atoms are typically lost from Cs He , and an optical photodissociation spectrum is m n derived from the depletion signal. A detailed description of the experiment can be found in the ESI† (Fig. S9) and elsewhere. To obtain a quantum chemical description of the optical spectra, ground state structures of Cs He ions were first 2 n modelled at the coupled cluster singles doubles (CCSD) level with the def2QZVP basis set on Cs and def2TZVP on He. The position of the weakly bound He atoms, residing in shallow potential wells, is very sensitive to the quality of the basis set used on Cs. With the triple-zeta quality basis set def2TZVP used + + on Cs, a bent structure is predicted for Cs He ; a linear Cs He 2 2 structure is obtained with the def2QZVP basis set on Cs and def2TZVP on He, and this does not change when the def2QZVPPD basis set is used on both Cs and He. To validate the theory as much as possible, excited states were modelled on various theory levels: time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional; equation of motion – CCSD (EOM- CCSD); multi-reference configuration interaction (MRCI) with an active space of one electron in 5 or 17 orbitals, MRCI(1,5) and MRCI(1,17), respectively; def2QZVPPD basis set was used on all atoms for excited state calculations. Spin–orbit coupling was Fig. 1 (a) Photodissociation spectra of Cs He ions, n =1,2,3,6,8,10, 12 2 n calculated using the state-interacting method as implemented obtained as a differential spectrum with and without laser irradiation of 63,64 65 clusters and scaled with laser power (see ESI† for details). A fit of the peak in the Molpro program along with the ECP46MDF basis set. at 2.8 eV by two Gaussian functions is shown for n = 1. Band centers for The D symmetry group was used for calculations; irreducible 2h Cs He are marked by vertical lines to guide the eye. Electronic state representations (IRs) in the D symmetry group were recon- Nh assignment is based on calculations (Table 1). (b) Multiple Gaussian fit to 2 + structed from the respective IRs in D D . ‘‘Very Tight’’ optimi- 2h 2h the vibrational progression of the transition into the 1 P state in Cs He u 2 zation criteria were used for optimization; for optimization in the and determination of the vibrational spacing. This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21,25362--25368 | 25363 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online PCCP Paper Table 1 Experimental and theoretical properties of excited states in Cs 2 (see Table S3 in the ESI†), the S states are only slightly affected, and Cs He : energy position E (in eV), width w of the Gaussian peak fit with shifts of up to 5 meV. However, the doubly-degenerate P states (= 2s), experimental splitting DE , spin–orbit splitting DE (all in meV). split SO 1 split into two states, separated by B10–35 meV (80–280 cm ). Excitation energies were calculated at the EOM-CCSD/def2QZVPPD level Comparison of the calculations with experiment, summarized of theory, spectral width was modelled within reflection principle approxi- in Table 1, allows us to assign the three main bands observed in mation, spin–orbit coupling at the MRCI(1,17)/ECP46MDF level 2 + 2 2 the experiment as 1 S ,1 P , and 2 P , reproducing the u u u 2 + 2 2 State 1 S 1 P 2 P u u u excitation energy to within 0.1 eV. The fourth allowed transition + 2 + Experiment, Cs He E 1.41 1.55; 1.59 2.82; 2.86 into 2 S at B3.1 eV (see Table S2, ESI†) is not observed in the w 83 — 39; 30 experiment due to insufficient laser power in this region. For DE —39 42 split 2 + 2 the dissociative states 1 S and 2 P , calculations within the u u Theory, Cs E 1.43 1.51 2.85 reflection principle approximation also predict a realistic width w 64 — 26 of the observed bands, see Table 1. DE —28 13 SO As mentioned above, the 1 P state is bound and, therefore, Theory, Cs He E 1.44 1.51 2.84 the measured photodissociation spectrum reflects its vibrational DE —28 14 2 SO structure. The 1 P state is calculated to have a very similar 2 + equilibrium distance as the 1 S ground state (Fig. 2). The excited state potential well is less steep, reflected in the calculated the spectrum is recorded following the depletion of the ion harmonic vibrational frequencies of 30 and 20 cm for the signal. There are four clearly observed absorption bands at 2 + 2 + 1 S and 1 P state, respectively, for Cs (at the (EOM-)CCSD/ about 1.4, 1.55, 1.60 and 2.8 eV (Table 1). The band at 2.8 eV is g u 2 def2QZVPPD level). The respective values for Cs He are 31 and further split, with spacing of about 40 meV (300 cm ). The 2 1 + 19 cm , with a stronger Cs –He interaction in the excited states, band at 1.60 eV has pronounced vibrational structure, with a 2 2 + shortening the Cs–He distance from 4.4 Å in the 1 S state to spacing of 22 cm (Fig. 1b); the broad bands at 1.4 and 2.8 eV g 2 + 4.0 Å in 1 P . The harmonic Cs He vibration is located at are, on the other hand, structureless. u 2 1 2 1 30 cm . The calculated frequency in the 1 P state of 19 cm For a more quantitative interpretation and reliable assignment u fits well to the measured spacing of 22 cm . Note that only the of these features, we performed EOM-CCSD and MRCI calculations higher 3/2u(1 P ) state shows clear vibrational resolution shown and analyzed the contribution of spin–orbit coupling effects. Fig. 2 u + + shows the potential energy curves of Cs .The Cs ion has D in Fig. 1b. The 1/2u(1 P ) state interacts with the repulsive 2 2 Nh 2 + symmetry and, when disregarding spin–orbit coupling, its ground 1/2u(1 S ) state, and the potential energy surface will be deformed 2 + due to the avoided crossing of the states. From a computational electronic state is 1 S . From this state, the only symmetry allowed perspective, the equilibrium distance in the 1 P state depends transitions are perpendicular and parallel transitions into P and 2 + 2 + 2 heavily on the choice of the basis set (Fig. S4, ESI†). As a direct S states, respectively. Within 3 eV, two S and two P states u u u consequence, our attempts to reproduce the experimental spectrum can be found. Among these states, only the 1 P state is bound, shown in Fig. 1b using Franck–Condon simulations failed to grasp the others are dissociative. When accounting for spin–orbit effects correctly the relative intensity of the contributing peaks (see the ESI† for details). Finally, the calculated weak spin–orbit splitting of the P states results into two states 1/2u and 3/2u with a different O value following Hund’s rule 3. The splitting is well reproduced 2 2 for 1 P (see also inset in Fig. 2). For 2 P , the calculated spin– u u orbit splitting is significantly smaller than the experimentally observed splitting. These observations can be rationalized if one takes into account the additional splitting induced by the asymmetric distribution of the He atom relative to the singly occupied molecular orbital (SOMO) in the excited state. Fig. 3 illustrates how the energy of the several electronic states in + + Cs He depends on the position of the He atom in Cs He . For 2 2 this calculation, the Cs–Cs–He angle is fixed, and the Cs–Cs and Cs–He distances are optimized in the electronic ground 2 + state, 1 S . From 1801 to 1501, the ground state energy hardly changes, i.e. the He atom moves more or less freely in this + 2 + Fig. 2 Potential energy curves in Cs calculated at the EOM-CCSD/ range of Cs–Cs–He angles. The repulsive 1 S state reacts more 2 u def2QZVPPD level. Excited states to which electronic transitions are sensitively to the position of the He atom, and this effect allowed from the ground state are shown with full lines, forbidden ones contributes to the broadening of the absorption band, which with dashed lines. In the inset, the vicinity of the 1 P minimum is shown, is 19 meV larger than expected from the linear reflection along with states including spin–orbit coupling (dotted lines, calculated at 2 2 principle approximation, Table 1. For the 1 P and 2 P states, the MRCI(1,17)/ECP46MDF level of theory). Only selected states are shown. u u See the ESI† for excluded electronic states and method benchmarking. the degeneracy of the two contributing p orbitals (see Fig. 4) is 25364 | Phys. Chem. Chem. Phys., 2019, 21,25362--25368 This journal is © the Owner Societies 2019 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Paper PCCP Let us now concentrate on the shift of the spectra induced by solvation, i.e. the He matrix shift. As shown in Fig. 1a and Table 2, pronounced shifts to higher energies are recorded for 2 + 2 the 1 S and 2 P states, reaching values of more than 0.1 eV u u + 2 + already for Cs He . Note also that the shift in the 1 S state 2 10 u exceeds the He binding energy. In the n = 1–10 range, the 2 + energy of the 1 S state shifts linearly with solvation by 12 meV 1 2 (100 cm ) per helium atom. The 2 P band at B2.8 eV seems to shift slightly non-linearly, its structure is however not well resolved in the experiment for larger ions. The 1 P state at 1.6 eV, on the other hand, is shifted only byB20 meV (160 cm )to lower energies within n =1–12 (Table2). The shifts can be directly connected to the interplay between the position of He atoms around the Cs ion and the character of the respective excited state. Our calculations show that Cs is preferentially solvated by He atoms on its ends, along the Cs–Cs axis for the smallest ions (see Fig. 4). For Cs He and Cs He , the most stable configuration is linear, but deformation 2 2 from linearity down to a Cs–Cs–He angle of 1501 requires very little energy, see Fig. 3; in Cs He , conformations with six and five/seven 2 12 Fig. 3 Dependence of ground state and excited state energies in Cs He He atoms on each end are the most stable ones (Table S1, ESI†). on the Cs–Cs–He angle. Calculated at the EOM-CCSD/def2QZVPPD As expected, the binding energy of helium atoms is low, within level of theory with ground-state structures optimized at the CCSD/ 2.4–3.1 meV He for up to 12 helium atoms. In addition to the def2QZVP(Cs),def2TZVP(He) level with a constrained Cs–Cs–He angle. shifts, the presence of a large variety of almost isoenergetic Spin–orbit effects were neglected. isomers leads to significant broadening of the spectra upon solvation (see Fig. S1, ESI†). Due to Pauli repulsion, the presence of He atoms deforms the orbitals of both ground and excited electronic states that extend along the Cs–Cs axis, shifting them to higher energy. This is the case for the HOMO orbital as well as the orbitals 2 + 2 corresponding to excitations into the 1 S and 2 P states u u (Fig. 4). The diffuse orbitals in the excited state are deformed more than the HOMO orbital, shifting the excitation energy considerably to higher values (Fig. 1a). The p orbital corres- ponding to the excitation into the 1 P state at 1.6 eV repre- sents a different case. Here, the surrounding helium atoms do not approach the orbital and the shift to lower energies reflects the slight destabilization of the HOMO orbital. Our calculations 2 + can reproduce the shifts for excitations into 1 S and under- estimate those observed for 1 P (Table 2); for n = 12, the Fig. 4 Highest occupied molecular orbital (HOMO) and natural transition orbitals for Cs He ions calculated at the CAM-B3LYP/def2QZVPPD level 2 n of theory. A low iso value (0.005) was chosen so that orbital deformation Table 2 Experimental and calculated energy shift of excited states upon induced by helium is clearly visible. solvation (in meV) with respect to Cs He . See Fig. S12 for experimental 2 + 2 + fits. Electronic states are correlated to 1 S and 1 P states in Cs . Vertical u u 2 transitions were calculated at the EOM-CCSD/def2QZVPPD//CCSD/ lifted by the presence of the He atom, which leads to an def2QZVP(Cs), def2TZVP(He) level of theory. For the 1 P state, the additional splitting. For 1 P , this effect is negligible down to average calculated value of the two SOMO orientations is given. Isomers 1501 Cs–Cs–He angle, because the He atom does occupy the with equal distribution of He atoms to each Cs end are chosen for the calculation same space as the singly occupied p orbitals. In contrast, the SOMO of the 2 P state is much more diffuse, and in one of Experiment Theory the two orientations exhibits significant overlap with the position 2 + 2 2 + 2 n 1 S 1 P 1 S 1 P He u u u u of the He atom. The splitting of the two orientations of the SOMO in the 2 P state amounts to 31 meV at 1501, which constitutes211 3.4; 3.7 6 0.8 323 6.1; 6.6 15 2.1 the dominant contribution to the experimentally observed 659 13.3; 15.2 41 6.1 splitting. For larger clusters, the same effect is responsible for 10 98 9.2; 19.3 96 12.0 the significant broadening of the 2 P band. 12 124 18.5; 41.0 109 14.2 This journal is © the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21,25362--25368 | 25365 Open Access Article. Published on 08 November 2019. Downloaded on 7/12/2021 4:42:48 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online PCCP Paper 6 J. P. Toennies and A. F. Vilesov, Angew. Chem., Int. Ed., 2004, sudden increase observed in the 1 P experimental shift might 43, 2622–2648, and references therein. be induced by the fitting procedure (see Fig. S12, ESI†)or 7 A. Mauracher, O. Echt, A. M. Ellis, S. Yang, D. K. Bohme, appearance of new isomers. This is most likely caused by the J. Postler, A. Kaiser, S. Denifl and P. Scheier, Phys. 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