The vacuum UV photoabsorption spectroscopy of the cis-1 , 2-dichloroethylene ( 1 , 2-ClHC = CHCl ) in the 5-20 eV range . An experimental and theoretical investigation

The photoabsorption spectrum of cis-1,2-C2H2Cl2 has been examined in detail in the vacuum UV range between 5 eV and 20 eV photon energy by using synchrotron radiation. Quantum chemical calculations are proposed and applied to the electronic transitions and to the vibrational structures belonging to these transitions. The broad band observed at 6.568 eV includes the X̃A1→1B1, 3B2 and 2B1 transitions. The two latter excitations correspond to the valence 2b1(π)→π and to the Rydberg 2b1(π)→3s transitions. The former excitation is described by a more complex 2b1(π)→nCl +σCH/Rpσ transition where valence and Rydberg characters are strongly mixed. For these transitions short vibrational progressions are observed, analyzed and tentatively assigned. The abundant structure observed between 7.0 eV and 10.0 eV has been analyzed in terms of vibronic transitions to one ns(δ̄ = 0.960), two np(δ̄ = 0.525 and 0.337), and two nd-type (δ̄ = 0.080 and 0.002) Rydberg series, all converging to the X̃B1 ionic ground state. The vibrational structure analysis of the Rydberg states leads to the following average wave numbers: ω2 ≈ 1420 cm-1 (C=C stretching), ω3 ≈ 1190 cm-1 (symmetric C-H bending), ω4 ≈ 800 cm-1 (symmetric C-Cl stretching) and ω5 ≈ 190 cm-1 (symmetric C-Cl bending). These numbers are compared to previously reported values. Many other transitions are observed between 10 eV and 20 eV and are assigned to transitions to Rydberg states converging to the successive excited states of cis-1,2-C2H2Cl2. For several of these Rydberg states, a vibrational structure is also observed and interpreted. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5066368


I. INTRODUCTION
The chlorinated derivatives of ethylene are important reactants in several industrial applications.They are also used to replace long-living ozone-depleting agents. 1 As a consequence, these compounds became widely spread but were found to be abundant atmospheric air pollutants. 2 Their toxicity and potential carcinogenicity have been investigated. 3Several groups studied the release of the highly ozone-depleting Cl atom and HCl molecule by UV photons [4][5][6] or atmospheric OH attack 7,8 in the three isomers of dichloroethylene.Gas-phase ion chemistry of these compounds has also been reported. 9These aspects were almost only investigated in the two 1,2-C 2 H 2 Cl 2 isomers.However, the studies on the photoabsorption spectrum of both compounds remain scarce and mainly restricted to the 160-135 nm (or 7.75-9.18eV) wavelength region.
The first vacuum UV photoabsorption spectrum (PAS) of the two 1,2-C 2 H 2 Cl 2 isomers has been reported by Mahncke and Noyes 10 in 1935.They recorded the spectra between the visible region and 75 nm (16.53 eV).They roughly divided them into three parts: (i) a broad continuum starting at 240 nm (5.17 eV) with a maximum at 190 nm (6.52 eV), (ii) a discrete absorption spectrum extending from 157 nm (7.90 eV) to 135 nm (9.18 eV) and (iii) a continuous absorption below 128 nm (9.69 eV) in the cis-isomer and a series of diffuse bands in the trans-isomer.A Rydberg series classification and a vibrational analysis were attempted.Ionization energies were obtained by Rydberg series extrapolation.
Walsh 11 recorded the vacuum UV PAS of all chloroethylenes, except the 1,1-C 2 H 2 Cl 2 species.Beside the broad continuum peaking at 185-195 nm (6.70-6.36eV), vibrational progressions of different Rydberg series were observed and analyzed in all spectra.
Walsh and Warsop 12 reported a detailed analysis of the vacuum UV PAS of the cis-1,2-C 2 H 2 Cl 2 .They mention the continuum peaking at 190 nm (6.52 eV) but their attention was focused on the analysis and discussion of the discrete spectrum measured between 167 and 143 nm (7.42-8.67 eV).
In a paper on the photochemical HCl-loss from chloroethylenes, Berry 4 measured the vacuum UV absorption spectrum of all chlorinated derivatives of ethylene in the 260-140 nm range (4.77-8.86eV) and analyzed in more detail the broad band located between 240 and 180 nm (5.17-6.88eV).The discrete part of the spectrum has not been analyzed.
Closely related to photoabsorption spectroscopy, the low electron energy-loss spectroscopy of all chloro-substituted ethylenes C 2 H x Cl 4-x has been reported by Koerting et al. 13 This work was essentially focused on the singlet-triplet valence transitions.Excitations to singlet valence and Rydberg states below 7.0 eV were also observed.
On the quantum chemical level, Arulmoziraja et al. 14 presented an extended high-level theoretical study on the electronic transitions in the cis-and trans-dichloroethylenes, and in tetrachloroethylene using the SAC-CI theory.The main purpose was to obtain the electronic spectra and to assign excitations to valence and Rydberg states of both singlet and triplet multiplicity.More recently, Khvostenko 15 reported DFT calculations at the B3LYP/6-311-G(d,p) level of the transitions to excited valence singlet and triplet states of chloroethylene molecules.
We recently reported on the vacuum UV PAS study of 1,1-C 2 H 2 Cl 2 16 observed and analyzed between 5 eV and 20 eV photon energy.In the frame of our work on this series of molecular systems, the aim of the present paper is to propose a detailed analysis of the vacuum UV photoabsorption spectrum of the cis-1,2-C 2 H 2 Cl 2 isomer (i) at medium resolution but for the first time in the 10-20 eV photon energy range and (ii) at higher resolution between 5.0 and 13.0 eV, with vibrational structure analysis.Quantum chemical calculations will be used to support the assignments.In a near future a similar work on the trans-1,2-C 2 H 2 Cl 2 isomer will be reported. 17

II. EXPERIMENTAL
The experimental setup used in this work at the BESSY I and II synchrotron radiation facilities (Berlin, Germany) has been described previously. 18Briefly, two monochromators have been used.For medium resolution spectra (resolving power of 1200 at hν = 10 eV), a modified 1.5 m-NIM 225 McPherson monochromator equipped with a 1200 lines/mm gold-coated laminar Zeiss grating was used.High resolution measurements (resolving power of about 15000 at 10 eV) have been reached with a 3m-NIM monochromator equipped with an Al/MgF 2 spherical grating of 600 lines/mm.The synchrotron radiation intensity and the pressure inside the cell were monitored, ensuring reliable absorption data.The sample pressure was kept in the 30-40 µbar to prevent saturation.The cis-1,2-C 2 H 2 Cl 2 sample purchased from Aldrich (99% stated purity) was used without further purification.
As described and discussed in detail in previous works, a continuum subtraction procedure has been applied to better identify and characterize weak and/or diffuse structures superimposed on an intense continuum. 19,20The resulting spectrum is denoted as ∆-plot.This data handling procedure has been validated by Marmet 21 and Carbonneau. 22e wavelength scales calibration is based on the Ar absorption spectrum between the 2 P 3/2 and the 2 P 1/2 ionic states, with an accuracy better than 2 meV.Between 6 eV and 20 eV photon energy, the photoabsorption spectrum has been recorded with energy increments of 15 meV, so that the energy positions have an uncertainty of about 8 meV.The 5-15 eV range has also been investigated with energy increments of 2 meV.Several parts of the spectrum, e.g., 7.0-10.0eV and 11.6-12.2eV, have also been recorded with 500 µeV energy steps.The energy positions are here known within ±2 meV.Reproducibility of energy positions has been checked with spectra recorded over several years.(iii) the region above 9.2 eV and up to 13 eV consists of a series of weak, fairly narrow structures superimposed on a strong continuum; (iv) the spectral range above 13 eV is displayed in Fig. 2a and consists of several successive broad bands up to 20 eV; the corresponding ∆-plot is displayed in Fig. 2b.The positions of the Rydberg transitions and the convergence limits 23 are identified.

III. EXPERIMENTAL RESULTS
Tables I and II list the energies of the vibrationless transitions to Rydberg states converging respectively to the ground  a The vertical ionization energy is used as convergence limit for these series.
and to excited ionic states.[12] IV.AB INITIO CALCULATIONS: METHODS AND RESULTS

A. Computational tools
Quantum chemical calculations were performed with the Gaussian 09 program package. 25The aug-cc-pVDZ basis set containing polarization as well as diffuse functions 26 was used throughout.In specific cases, calculations with the basic cc-pVDZ set, which does not include diffuse functions, were also performed.
The wavenumbers of the twelve vibrational normal modes were computed at the DFT(M06-2X) and TDDFT(M06-2X) levels.

B. Results of the calculations
The results of the geometry optimizations in the C 2v , C 2 and C S symmetry point groups at different computational levels are presented in Table S1 (see supplementary material).These results are compared and discussed below.
The calculated vertical transition energies to several neutral excited states are listed in Table III  The vibrational wavenumbers associated with the twelve normal modes represented in Fig. 3 have been calculated for the first four excited states of the neutral molecule at the These optimizations of the 3 1 B 2 excited state lead to an equilibrium geometry corresponding to a twisted situation of C 2 symmetry (all real wavenumbers, see Table V, column 5).In this geometry, the π→π * state belongs to the 1 B representation.The C=C bond is moderately lengthened compared to the ground state but the bond angles are substantially modified (see Table S1).A saddle point is obtained in C s (one imaginary wavenumber, see Table V, column 6) and a third order critical point in C 2v (stationary point with three imaginary wavenumbers, see Table IV, column 4 and Table V, column 4).Compared to the C 2 minimum, the stationary points in C s and C 2v are destabilized by 1.13 eV and 1.24 eV, respectively.They are characterized by a very large C=C bond lengthening (nearly 0.2 Å) and large bond angle modifications.Furthermore, at the optimized twisted C 2 geometry of the 3 1 B 2 / 1 B state, the X1 A state is higher in energy.Internal conversion from the 1 B state to the X1 A state is expected to take place upon relaxation.Anticipating the publication of the calculation results on trans-1,2-C 2 H 2 Cl 2 , 17 it can be emphasized that the 3 1 B 2 optimized geometry is also that obtained from the optimization of the first excited state in the transisomer.It can therefore, be predicted that the isomerization is obviously photo-induced.Geometry optimizations in other symmetry point groups (C s , C 2 and C 1 ) for the 1 1 B 1 and 2 1 B 1 states have been unsuccessful owing to the large variation of the electronic energies upon geometry change, leading to numerous crossings.
We recently measured the HeI-PES and threshold photoelectron spectra (TPES) of cis-1,2-C 2 H 2 Cl 2, which will be the subject of a forthcoming paper. 23The lowest adiabatic ionization energy leading to cis-1,2-C 2 H 2 Cl 2 + ( X2 B 1 ) is equal to (9.666±0.006)eV.This value is in very good agreement with earlier [32][33][34] and with the most recent 35,36 determinations.The vertical value is measured at 9.843 eV. 23ght bands were observed by HeI-PES at higher energies and were characterized by their adiabatic ionization energies at 11.426 eV, 11.965 eV, 12.375 eV, 13.592 eV, 14.083 eV, 15.531 eV and 16.638 eV and at about 18.40 eV. 23The corresponding vertical values are 11.690 eV, 12.028 eV, 12.460 eV, TABLE V. Vibrational wavenumbers (cm -1 ) calculated for the twelve normal modes (VNM) (see Fig. 3) of the neutral ground and first excited states in the C 2v , C 2 and C S symmetry point groups at the DFT(M06-2X) level.For the neutral ground state comparison is made with experimental data. 31Imaginary wavenumbers are bold printed.The characteristic low energy broad band observed in the vacuum UV PAS of the ethylene compounds displays a doublet maximum at 6.568 eV-6.620eV in cis-1,2-C 2 H 2 Cl 2 (Fig. 4).Compared to our observations in the chlorinated derivatives investigated up to now, 16,37,38 this band has a similar intensity and extends between 5. has been applied.The red curve in Fig. 5a represents the continuum which is subtracted from the original signal.The result of this subtraction operation is displayed in Fig. 5b which clearly shows two different parts: (i) between 5.0 eV and 6.0 eV a very weak series of peaks with spacing suggesting a vibrational progression and (ii) between 6.0 eV and 7.0 eV a more intense series of alternating broad and narrow peaks.

State
The very weak signal between 5.0 eV and 6.0 eV may be interpreted as part of the transitions to a longer vibrational progression belonging to a singlet valence excited state.A π→σ * state is a reasonable candidate.By quantum chemical calculations, Arulmozhiraja et al. 14 calculated the π→σ * (1 1 B 1 ) transition energy at 6.45 eV in cis-1,2-C 2 H 2 Cl 2 whereas Khvostenko 15 predicted this transition at 6.52 eV.The present calculations indicate a more complex situation where the final 1 1 B 1 state has a strongly mixed Rydberg (R'p'σ) and valence [n Cl +σ * CH ] character (see Table III).The predicted oscillator strength is very weak.At both DFT and TDDFT (M06-2X) levels the optimized geometry of the 1 1 B 1 state in the C 2v point group corresponds, however, to a transition state whose vibrational wavenumbers have been calculated (see Table IV).As discussed in Section IV B, optimization attempts in lower symmetry groups were unsuccessful.The identified peaks of the very weak signal are shown by dashed lines in Fig. 5b.An average separation of 0.12±0.01eV (960±80 cm -1 ) could be determined.This compares favorably with the values of 933 cm -1 (TDDFT) or 944 cm -1 (DFT) predicted for the in-plane C-H bending mode excitation ν 3 (see Fig. 3) and is compatible with the large change in the optimized H-CC angle (see Table S1).The 6.0-7.0 eV energy range consists of a complex series of fairly narrow peaks entangled in a sequence of broader structures.As far as the diffuse features starting at 6.16 eV are concerned, a regularly distributed intensity is observed up to 7.0 eV at intervals of about 0.15±0.02eV (1200±160 cm -1 ).The maximum intensity for this progression is observed at 6.62 eV.The most probable transition at this energy is likely the π→π * (3 1 B 2 ) transition whose vertical energy is predicted at 6.55 eV at the TDDFT level (see Table III).As mentioned in Section IV B, this state has to cross the ground state, leading to an internal relaxation which may be responsible for the diffuseness of the detected structures.Arulmozhiraja et al. 14 calculated this excitation at 6.93 eV.The vibrational structure with ω≈1200±160 cm -1 , superimposed on the strong continuum may likely be assigned to the ν 2 (C=C stretching) or ν 3 (C-H bending) vibrations (see Fig. 3) calculated at 1269 cm -1 and 1112 cm -1 respectively (see Table IV).The large change in the C=C bond length of the optimized geometry of the 3 1 B 2 state (Table S1) is expected to induce an excitation of the C=C stretching vibration.
Fig. 4 compares the relative energy position of these two transitions in chlorinated compounds: C 2 H 4 is used as a reference.Fig. 4a shows that the presence of a second Cl-atom induces a shift to lower energy of the σ * and π * states.For the latter a systematic energy lowering from 6.880 eV (C 2 H 3 Cl) 37 to 6.620 eV (cis-1,2-C 2 H 2 Cl 2 ) and 6.482 eV (1,1-C 2 H 2 Cl 2 ) 16 is observed.The substitution in cis-position induces a shift to higher energy with respect to the 1,1-isomer.Fig. 4b clearly shows the expected shift to lower energy produced by the substitution of F by a Cl-atom in the cis-isomers: from 7.11 eV 38 to 6.62 eV.
The short sequence of narrower peaks observed between 6.4 eV and 7.0 eV and assigned to the π(2b 1 )→3s Rydberg transition will be discussed in the next section V B 1.
Above the photon energy of 7.0 eV, several valence transitions are predicted by quantum chemical calculations, i.e. at 7.32 eV, 8.58 eV and 9.15 eV successively (see Table III).In agreement with these predictions, a short but fairly well defined progression consisting of three broad weak peaks starts at 7.322 eV (see Fig. 1) and is followed by structures at 7.477 eV and at 7.613 eV (see Fig. 6a).These structures are superimposed on a broad background with its maximum at 7.6 eV (see red curve in Fig. 1).We suggest to assign this part of the spectrum to the n Cl +σ CH →π * transition predicted at 7.32 eV and leading to the 5 1 B 1 state (see Table III).The average interval of 0.146±0.009eV (1180±70 cm -1 ) could be assigned to the excitation of the C-H bending vibration (see Fig. 3 and Table IV).This assignment is also supported by the predicted geometry changes in the optimized conformation (see Table S1).Mahncke and Noyes 10 mentioned non-assigned "diffuse bands" at 60562 cm -1 (7.509 eV) and 61169 cm -1 (7.583 eV) which are probably related to the present observations.
Below and close to the first vertical ionization limit of cis-1,2-C 2 H 2 Cl 2 at 9.843 eV, 23 two additional broad structure are observed, with maxima at 8.5 eV and 9.8 eV (see red curve in Fig. 1).They may correlate with the transition energies predicted at the TDDFT level for n pz,Cl +π CCl →π * at 8.58 eV (14 1 A 1 ) and n Cl +σ CC +σ CH →π * at 9.15 eV (20 1 B 2 ) respectively (see Table III).For comparison, Arulmozhiraja et al. 14 calculated seven valence as well as Rydberg transitions taking place between 8.04 eV and 8.43 eV: the lowest n→π * excitation is at 8.04 eV whereas two n→σ * transitions were predicted at 8.24 eV and 8.38 eV.It is thus likely that both valence and Rydberg states are implied in this region of the absorption spectrum and the fine structure superimposed on the broad background might be interpreted as resulting from transitions to Rydberg states as discussed in the next section.

B. The Rydberg transitions
In Fig. 1 the PAS of cis-1,2-C 2 H 2 Cl 2 measured in the photon energy range of 7.0 eV to about 13.0 eV displays numerous structures of variable intensity.Particularly from 7.4 eV up to about 9.0 eV, it exhibits a series of weak to very weak sharp features besides one stronger and broad transition.The original spectrum and the corresponding ∆-plot are displayed on an expanded energy scale in Fig. 6(a-d) for the 7.0-13.0eV range.Between 7.0 and 10.0 eV, the spectrum has been recorded with 500 µeV energy increments.
As no prior information on the coupling among Rydberg states is available for this molecule, we used the Rydberg formula (1) as a zero-order assumption for the assignment of the spectral transitions: In (1), R=13.6057 eV 24 is the Rydberg constant, δ is the quantum defect, and IE represents the ionization energy corresponding to the ionic state to which the Rydberg series converges.Any significant Rydberg-Rydberg coupling is then expected to affect the quality of the fits of the Rydberg formula to the experimental data.Furthermore, in the absence of coupling between Rydberg series, the quantum defect δ has typical values which are characteristic of the angular momentum of the Rydberg orbital.When couplings are involved, these values may be perturbed.Such perturbations already allowed us to identify and quantitatively analyze transition between Hund's coupling cases. 39The quantum defect values are therefore important parameters to be determined.
The observed intensity distributions of the spectral features show Franck-Condon-like behaviors and do not follow the n -3 law, so that vibrational excitation of the successive Rydberg states dominates the spectrum.If Rydberg-Rydberg couplings may be neglected, it is sensible to assume that very similar potential energy surfaces characterize the Rydberg states and the ionic state to which they converge, so that similar vibrational structures are expected for a Rydberg state and the corresponding ionic state. 38The quality of the fits indicates a posteriori whether this assumption holds.This procedure has already been successful to disentangle the vibrational structure in the vacuum UV spectra of C 2 H 3 F, 40   and 1,1-C 2 H 2 FCl. 42In the present case, to make the comparison easier, the ∆-plot of the appropriate PAS energy range will be compared to the HeI-PES of the cis-1,2-C 2 H 2 Cl 2 + in its ground or excited states as measured in our laboratory. 23Vibrationless Rydberg transitions between 6.4 eV and 10 eV (see Fig. 1) The vibrationless Rydberg transitions observed for cis-1,2-C 2 H 2 Cl 2 and converging to the first ionization limit lie between 6.4 eV and 9.7 eV and are labeled in Fig. 1.Table I lists the energies and wavenumbers of these transitions with their associated effective quantum numbers and compares the results of the present work with previous investigations by Mahncke and Noyes, 10 Walsh 11 and Walsh and Warsop. 12The assignments are based on an adiabatic ionization energy for the ionic ground state equal to 9.666±0.006eV, 23 in good agreement with the value provided by Walsh and Warsop (IE = 9.652 eV). 12As mentioned in section II, the estimated uncertainty on the transition energies is 2 meV (16 cm -1 ).[12] The observed π(2b 1 )→ns Rydberg series converging to the first ionization energy limit are listed in Table I.The average quantum defect over the n=3 to 8 range is equal to δ= 0.96±0.01.By quantum chemical calculation at the TDDFT level, the lowest π→R's'(2 1 B 1 ) vertical transition energy is equal to 6.35 eV (see Table III), in good agreement with the present experimental value at 6.398 eV.
Transitions from the π(2b 1 ) to np Rydberg orbitals lead to two well-identified series (Table I), as was also observed in previous studies on 1,1-C 2 H 2 Cl 2 , 16 1,1-C 2 H 2 F 2 , 41 1,1-C 2 H 2 FCl, 42 and CH 3 X (X=Cl, Br and I). 39The two series are observed up to n=8.We assign the series starting at 7.414 eV to the π(2b 1 )→npσ transitions whereas the series starting at 7.740 eV is assigned to π(2b 1 )→npπ.These assignments are based on the good agreement with the theoretical predictions of the present work listed in Table III: π→Rpσ (8 1 B 1 ) and π→Rpπ (11 1 A 1 ) transitions were calculated at 7.44 eV and 7.90 eV successively.Both transitions have noticeable oscillator strength.The splitting of 0.326 eV between the two 3pλ -type states is very close to the value of 0.356 eV observed in 1,1-C 2 H 2 Cl 2 . 16n 1,1-C 2 H 2 F 2 , however, the splitting is substantially smaller (0.183 eV), 41 while it is again of the same order of magnitude in 1,1-C 2 H 2 FCl (0.404 eV). 38,42erage quantum defects δ=0.53±0.01 and 0.34±0.02are determined for the npσ-and the npπ-type Rydberg series respectively.The small dispersion of the quantum defects along the series is compatible with our assumed neglect of the Rydberg-Rydberg couplings.The difference between the quantum defects for the npσ-and the npπ series can be interpreted as follows.The presence of more σ orbitals than π orbitals in the ionic core leads to a stronger core-Rydberg interaction for σ-type Rydberg orbitals, so that a larger quantum defect should be observed for npσ compared to npπ Rydberg orbitals.In the C 2v point group, the npσ and npπ orbitals should rigorously be denoted by npa 2 and npb 1 .However, assuming the molecular ion field to be nearly cylindrical (diatomic-like), the σ, π, . . .nomenclature is usually used.
A first π(2b 1 )→ndλ series starts at 8.089 eV and its members are characterized by an average quantum defect δ=0.08±0.03.The second series starting at 8.170 eV is characterized by an average quantum defect δ= 0.002±0.008.These δ values suggest ndσ and ndπ Rydberg states respectively.In the vacuum UV spectrum of 1,1-C 2 H 2 F 2 41 and of 1,1-C 2 H 2 FCl, 38,42 a ndσ series is observed with δ=0.13±0.03 41and 0.125±0.025 41respectively.For the corresponding ndπ series δ=0.04±0.03 40and δ=-0.11±0.02 41were respectively determined.Surprisingly, the splitting between 3dσ and 3dπ is found to be equal to 0.081 eV in cis-1,2-C 2 H 2 Cl 2 , although it is much greater, 0.214 eV, in 1,1-C 2 H 2 FCl 42 and 0.109 eV in 1,1-C 2 H 2 F 2 . 41nally, a fairly strong broad and complex structured band is observed at about 8.5 eV, as already mentioned in Section V A, where the broad signal is interpreted as resulting from a valence-valence n pz,Cl +π CCl →π * transition.Superimposed fine structure is observed, however, whose energy positions are listed in Table VI.In the densitometer trace of the PAS published by Walsh and Warsop 12 a strong broad and roughly structured band is observed at 68 310 cm -1 (8.469 eV) showing several shoulders between 67 294 cm -1 (8.343 eV) and 68697 cm -1 (8.517 eV) but these structures were not discussed or assigned by the authors.The structures reported in Table VI, with an adiabatic excitation energy of 8.446 eV, do not fit into the possible Rydberg series converging to the ground ionic state and which have been detailed above.Based on our quantum chemical TDDFT computations (Table III), which predict the vertical (4b 2 :n Cl +σ CH )→3s transition at 8.24 eV, we rather suggest to assign the structures starting at 8.446 eV to transitions to the vibrational levels of the lowest Rydberg state (3s) converging to the Ã2 B 2 ionic state, whose vertical ionization energy is equal to 11.690 eV. 23Based on the information from the photoelectron spectrum, 23 we infer n * =2.05±0.01 or δ=0.95±0.01,which is perfectly compatible with a 3s Rydberg state.
2. Vibrational analysis (Fig. 3, Fig. 5 and Fig. 6(a-c)) After having identified the vibrationless Rydberg transitions, the vibrational structure of the identified electronic states has to be disentangled.To this purpose we will rely on the assumption stated above, i.e. the vibrational structure of the Rydberg states should be close to that of the cationic states to which they converge. 39This information will be inferred from the HeI-PES. 23A comparison between the PAS and the HeI-PES of the ground state of the cation is illustrated in Fig. 5, Figs.6a and 6b.Most of the Rydberg states observed between 6.4 eV and 9.9 eV converge to IE ad ( X2 B 1 ) = (9.666±0.006)eV of cis-1,2-C 2 H 2 Cl 2 . 23The electronic ground state of the cation mainly shows three vibrational modes: 23,32 ω 2 + =1460±30 cm -1 (181±4 meV), ω 3 + =1160±20 cm -1 (144±3 meV) and ω 4 + =860±60 cm -1 (107±8 meV).These values correspond to the C=C stretching, C-H in-plane bending and C-Cl stretching vibrations respectively.
The application of the same procedure is displayed in Fig. 6(a-c).Table S2(a-c) reports the transition energies and the proposed corresponding assignments.The 7.4-8.1 eV photon energy range shows weak broad bands alternating with stronger sharp peaks.The broad weak features have already been assigned earlier in this work (see section V A).The sharper features are assigned to the 2b 1 →3pσ and 2b 1 →3pπ Rydberg transitions with adiabatic excitation energies at 7.414 eV and 7.740 eV respectively (see section V B 1 and Table I).Both states show an extended vibrational structure as analyzed in Table S1a and illustrated in Fig. 6a.Energy positions in square brackets indicate that at least two different assignments are possible and that these should be considered with caution.Intensity fluctuations may result from the overlap of several contributions.At least two vibrational modes are excited in both states, i.e. ω=1420±20 cm -1 (176±2 meV) and ω =795±15 cm -1 (98±2 meV).In addition, two weaker modes are detected with wavenumbers of 1220±20 cm -1 (151±3 meV) and of 190±30 cm -1 (24±4 meV).
Their assignment relies on the comparison with the ground state cation.The first two wavenumbers should correspond to ω 2 + (C=C stretch)= 1430±30 cm -1 and to ω 4 + (C-Cl symmetric stretch)= 860±60 cm -1 respectively. 23,32The latter value is significantly higher in the cation than in the Rydberg state.However, this result agrees with the quantum chemical calculations performed on the two systems 23 and in the present work: for the Rydberg excitation only the C=C stretching is significantly modified whereas the C-Cl stretching remains only slightly affected in most of the excited neutral states (e.g.see Table IV).
The wavenumber equal to 1220±20 cm -1 can be correlated with the value of 1160±20 cm -1 inferred from the HeI-PES for the ground state cation.This vibration corresponds to ν 3 (C-H bending) predicted at 1215 cm -1 by ab initio calculations. 23The wavenumber of 190±30 cm -1 is not detected in the HeI-PES.An assignment to ν 5 (C-Cl bending) is proposed based on quantum chemical predictions (193 cm -1 ). 23However, as mentioned in Table S2a, ω 3 is close to ω 4 +2ω 5 which introduce ambiguities in the assignments.12 shows a very good agreement and the present work brings a strong argument supporting the assignments.These authors also mentioned a wave number of 1224 cm -1 but rejected it because ". ..no frequency of this magnitude is observed in any other electronic transition of cis-dichloro ethylene or in the transitions of the other chloroethylenes".As mentioned above, this wavenumber is very probably involved in both the two 3pσ and 3pπ Rydberg states and very likely in combinations in the 3pσ Rydberg state.Fig. 6b shows the PAS in the 8.0-9.1 eV range on an expanded energy scale.This part of the spectrum is much more crowded and the energy positions of the features are listed in Table S2b.The procedure described earlier in this section (HeI-PES inserted in Fig. 6b) leads to the assignments presented in the same table .At least seven Rydberg transitions are observed.Six of them converge to the ground state of the cation and correspond to 2b 1 →3dσ, 3dπ, 4s, 4dσ, 4dπ and 5s successively.The strong broad peak around 8.5 eV has already been interpreted as a superposition of a valence-valence (n pz,Cl +π CCl →π * ) and a valence-Rydberg (4b 2 →3s) transition.(see section V B 1).
Fig. 6c shows the 9.0-10.0eV energy range on an expanded scale.A large number of weak to very weak structures as well as broad intense peaks at about 9.6 eV and 9.8 eV are observed.The ∆-plot shown in the lower panel of Fig. 6c enhances these structures.Their energy positions and the proposed assignments in terms of transitions to vibrationally excited Rydberg states converging to the ionic ground state are listed in Table S2c.The majority of the observed transitions are interpreted as leading to ndσ (n=5-8) Rydberg states.As expected, the same vibrational normal modes are involved as for the previous series.The average wavenumbers are ω 2 =1411±8 cm -1 (175±1 meV), ω 3 =1170±20 cm -1 (145±3 meV), ω 4 =807±20 cm -1 (100±2 meV) and ω 5 =190±20 cm -1 (24±2 meV).The present energy range being almost outside the prospected region of previous photoabsorption works, [10][11][12] no comparison can be made.Only Mahncke and Noyes 10 mentioned a few discrete bands which have been introduced in Table S2c.Most of them fit the present data and possible assignments are proposed.
We could not observe any significant breakdown of the Rydberg formula or of the assumption of similar vibrational structures for the Rydberg states and the corresponding cationic state.This suggests that Rydberg-Rydberg or Rydberg-continuum interactions are too small to affect the energy positions at the resolution and accuracy reached in our spectra.
Beside the long sequence of weak structures, the present region also shows a strong complex structured doublet peak spread between 9.6 eV and 9.9 eV (see Fig. 1 and Fig. 6c, upper panel).The steep absorbance increase is likely linked to the opening of the 2b 1 -1 ionization continuum at 9.666 eV.Rydberg states converging to higher lying ionization limits need to be considered.[34] Based on the vibrational structure of the B2 A 1 photoelectron band, 23 the unassigned features observed in the 9.6-9.9eV range could be interpreted (see Table VII and upper panel of Fig. 6c).The effective quantum number n * =2.426±0.005,averaged over eight vibronic transitions, suggests an assignment to a 5a 1 →3p Rydberg transition.

Rydberg transitions between 10 eV and 20.0 eV (see Figs. 1, 2, (6)d)
The PAS of cis-1,2-C 2 H 2 Cl 2 between 10 eV and 13 eV has been recorded with 2 meV increments and is reproduced in Fig. 6d.The absorbance steadily increases over the whole range.It shows a sequence of weak broad bands.Several of these features are clearly consisting of very weak substructures.They become more apparent in the ∆-plot (lower  6d) and show intervals of the order of 30 meV (240 cm -1 ).To try to disentangle this range of the PAS it would be reasonable to use the same hypotheses and assumptions applied earlier.Using the present data, term values and effective quantum numbers could be derived.
In this energy range three vertical ionization limits, lying close together, are involved, i.e. 11.690 eV (4b 2 -1 ), 12.028 eV (5a 1 -1 ), and 12.460 eV (1a 2 -1 ) 23,32-34 successively.They are inserted in Fig. 6d (upper panel).The sum of the associated continua contributes to the strong increase of the background observed in the PAS in this energy range.All three cationic states are characterized by an extended vibrational progression. 23 we assume that the maxima at 10.252 eV, 10.584 eV and 10.792 eV (Table II) correspond to transitions to Rydberg states converging to the 4b 2 -1 continuum at 11.69 eV, effective quantum numbers of 3.08, 3.51 and 3.89 are respectively obtained.This is compatible with 4b 2 → 4s, 4p and 4d assignments (Figure 6d).
The next four well identified bands lye at 10.880 eV, 11.120 eV, 11.275 eV and 11.536 eV (Table II).Based on their effective quantum numbers n * =3.44, 3.87, 4.25 and 5.25, they may be assigned to 5a 1 → 4p, 4d (or 5s), 5p and 6p transitions, respectively.The last two bands at 11.342 eV and 11.75 eV will be assigned to 1a 2 → 4p and 5p transitions for which effective quantum numbers n * =3.49 and 4.38 are obtained.The substructures detected in these bands could likely be assigned to vibrational excitations, e.g. of C-Cl bending for which a wave number of the order of 180 to 270 cm -1 has been predicted for several Rydberg and valence states (see Table IV) and cationic states as well. 23P Advances 9, 015305 (2019); doi: 10.1063/1.5066368 9, 015305-12 The width of all these bands has likely to be related to the short lifetimes of these Rydberg states.In a forthcoming paper 23 it will be shown that these Rydberg states mainly undergo resonant autoionization to the ionic ground state of cis-1,2-C 2 H 2 Cl 2 + ( χ2 B 1 ) giving rise to threshold photoelectrons.
A broad band spreads from 12.054 eV to 12.412 eV and consists of five narrower well resolved regularly spaced features with a regular intensity distribution.Assuming the D2 B 1 state of the cation at 13.592 eV as convergence limit 23 (see Table VII) an average effective quantum number n * =3.00±0.02 is obtained.Therefore, this band is tentatively ascribed to a 1b 1 →4s/3d Rydberg transition.The vibrational analysis leads to an averaged wavenumber ω= 722±30 cm -1 which could likely be assigned to ν 4 (C-Cl stretch/C-H bending) as predicted for other lower lying neutral excited states (see Table IV).
The energy range between 13 eV and 20 eV has been recorded with the 1.5m-NIM monochromator and with 15 meV energy increments.This high energy range only shows a few very broad bands superimposed on a slower increasing continuum as shown in Fig. 2a.The band maxima are listed in Table II.[34] In Fig. 2b the ∆-plot has been reproduced and slightly smoothed by FFT to increase the signal/noise ratio.At least six broad bands are characterized by a full width half-maximum (FWHM) ranging from 0.4 eV to 0.7 eV.A last very broad band extends from 16.8 eV to 19.5 eV.The energy position of the successive maxima is listed in Table II.A tentative classification based on the vertical ionization energies and the inferred effective quantum numbers is also provided in Table II.

VI. CONCLUSIONS
The measurement of the VUV photoabsorption spectrum of cis-1,2-C 2 H 2 Cl 2 at higher resolution by using synchrotron radiation enabled us to extend the data above the 10.5 eV photon energy limit and, for the first time, up to 20 eV.In this energy range the absorbance drastically increases and numerous bands are identified.Interpretations and assignments have been proposed.
Quantum chemical calculations allowed us to describe and assign valence-valence (2b 1 (π)→σ * /π * ) as well as valence-Rydberg (2b 1 (π)→3s) transitions which are involved at the low energy end of the spectrum, i.e. between 5 eV and 7.4 eV.Below 6.0 eV the 1 1 B 1 state has a strongly mixed valence (n Cl +σ * CH )/Rydberg (Rpσ) character.Above 6.0 eV the π→π * (3 1 B 2 ) valence and the π→3s (2 1 B 1 ) Rydberg transitions are entangled.For the vibrational structure of both states an assignment is proposed.Above 7.0 eV several valence transitions are predicted by the calculations and compared to the observations.In the intermediate photon energy range, i.e. between 6.4 eV and 10.0 eV, a large number of narrow vibronic Rydberg transitions, i.e., 2b 1 →ns (n=3-8), two types of np (up to n=8) and two nd-type (up to n=9) series, have been identified and interpreted.The involved Rydberg states converge to the cis-1,2-C 2 H 2 Cl 2 + ( X2 B 1 ) ground ionic state.By reference to the first band of the cis-1,2-C 2 H 2 Cl 2 HeI-PES, 23 the vibrational structure has been assigned to four vibrational modes, including harmonics and combinations: ω 2 ≈ 1420 cm -1 (C=C stretching), ω 3 ≈ 1190 cm -1 (C-H symmetric bending), ω 4 ≈ 800 cm -1 (C-Cl symmetric stretching) and ω 5 ≈ 190 cm -1 (C-Cl symmetric bending).These assignments are compared to those proposed on a more empirical basis in previous reports. 11,12 8.5 eV, 9.6 eV and above 10.0 eV numerous Rydberg transitions are observed, all assigned to members of series converging to the successive excited states of cis-1,2-C 2 H 2 Cl 2 + .For a few of them vibrational progressions are observed.

FIG. 1 .
FIG. 1. Vacuum UV photoabsorption spectrum of cis-1,2-C 2 H 2 Cl 2 between 5 eV and 13 eV photon energy.The continuous red curve corresponds to the strongly smoothed PAS curve by fast Fourier transform.Vertical bars locate valence and Rydberg transitions and shaded areas show their convergence limit.Dotted areas correspond to vertical ionization energies.

Fig. 1 FIG. 2 .
Fig. 1 displays the vacuum UV PAS of cis-1,2-C 2 H 2 Cl 2 between 5 eV and 13 eV photon energy, with 2 meV energy increments.The whole 5-20 eV range may be split in four distinct regions: (i) the 5.0-7.2eV region consists of a number of weak broad bands superimposed on a continuum; (ii) the 7.2-9.6eV range contains a large number of weak to very weak sharp features and bands superimposed on a relatively weak continuum with steeply increasing intensity starting at 9.2 eV;

FIG. 5 .
FIG. 5. (a) Vacuum UV photoabsorption spectrum of cis-1,2-C 2 H 2 Cl 2 on an expanded.photon energy scale between 5.0 eV and 7.5 eV.The continuous red curve labeled FFT-Sm corresponds to the strongly smoothed PAS curve by fast Fourier transform.(b) ∆-plot of the PAS.Vertical bars indicate the vibrational progressions.The red curve corresponds to the HeI-PES band of the X2 B 1 cationic state of cis-1,2-C 2 H 2 Cl 2 .

TABLE II .
Rydberg series converging to the ionic excited states observed in the vacuum UV photoabsorption spectrum of cis-1,2-C 2 H 2 Cl 2 .

TABLE III .
Vertical excitation energies (eV) leading to neutral excited states of cis-1,2-C 2 H 2 Cl 2 as obtained at the TDDFT level and description of the transitions involved.The corresponding calculated oscillator strengths are indicated in parentheses.The calculations were performed with the aug-cc-pVDZ basis set.The notation R stands for "Rydberg state".
Cl + σ CH →Rs + Rpσ n Cl + σ CC + σ CH →π * (0.0112) AIP Advances 9, 015305 (2019); doi: 10.1063/1.5066368 9, 015305-4 © Author(s) 2019 FIG. 3. Schematic representation of the twelve vibrational normal modes of the cis-1,2-C 2 H 2 Cl 2 molecule in the C 2v symmetry point group (and their irreducible representations) for which the associated wavenumbers have been calculated.TDDFT level in the C 2v symmetry group and are listed in Table IV.The presence of imaginary wave numbers indicates that these C 2v geometries do not correspond to equilibrium situations.Table V reports the vibrational wavenumbers of the neutral ground state in the C 2v group 31 and of the first 1 B 2 excited state (π→π * ) in the C 2v ,C 2 and C S groups, computed at the DFT (M06-2X) level.

TABLE IV .
31brational wavenumbers (cm -1 ) calculated for the twelve normal modes (VNM) (see Fig.3) of the first four excited states in the C 2v symmetry group at the TDDFT(M06-2X) level.For the neutral ground state comparison is made with experimental data.31Imaginarywavenumbers are bold printed.

Table
S2a also compares the present results to those reported by Walsh and Warsop 12 (see TableS2a, col.6).These authors proposed a classification of the bands and determined two wavenumbers, i.e. of about 1400 cm -1 and 800 cm -1 .The comparison of TableVIIawith Table I in Ref.