The photoelectron spectroscopy of the dichloroethylenes: the geminal isomer 1,1-Cl2C2H2. An experimental and quantum chemical study

This work analyzes the threshold photoelectron spectrum (TPES) and selected constant ion state (CIS) spectra of the 1,1-C2H2Cl2 isomer measured using synchrotron radiation. The TPES is compared to the HeI-, NeI- and ArII-photoelectron spectra (PES). In the HeI-PES nine photoelectron bands have been observed at vertical ionization energies of 9.992 eV, 11.652 eV, 12.157 eV, 12.536 eV, 13.633 eV, 14.195 eV, at about 15.9 eV and 16.2 eV and at 18.496 eV successively. For most of these bands the adiabatic ionization energy could be determined and a detailed vibrational analysis of the first four bands is presented. All these bands exhibit an extended vibrational structure. In particular, the ground electronic state and the third excited state display an extended vibrational structure upon excitation by the ArII-resonance line. The assignments of the electronic bands and of the vibrational structure are based on quantum chemical calculations. These allowed us to assign the nine electronic states in terms of ionization and double excitation in a molecular orbital scheme. The good correlation between predicted vibrational wavenumbers and the experimental values allowed us to assign all observed vibrational structures. The CIS spectrum of the eight first electronic states of 1 , 1 ‐ C 2 H 2 C l 2 + have also been recorded. Vibrationally resolved CIS spectra of the molecular ion ground state have been obtained and are discussed.


Introduction
We recently reported a detailed investigation of the vacuum UV photoabsorption spectrum (PAS) and the photoelectron spectroscopy (PES) of the disubstituted halogenated ethylenes, i.e. 1,1-C 2 H 2 F 2 , 1,1-C 2 H 2 FCl and cis-and trans-1,2-C 2 H 2 FCl [1]. Quantum chemical calculations were performed to support the interpretation and assignments of the successive electronic bands with their associated vibrational structure.
In contrast to these compounds, spectroscopic investigations devoted to the three isomers of dichloroethylene have received much more attention in the literature. This is likely linked to the broader field of chemical and industrial applications as well as to the environmental impact of these molecular species.
The HeI-PES of the geminal 1,1-C 2 H 2 Cl 2 isomer has been reported several times in the early 70s [2][3][4]. The only HeII (30.4 nm) spectrum of this molecule has been published by Von Niessen et al [5] who interpreted the spectrum and assigned the numerous bands based on quantum chemical calculations.
More recently, Parkes et al [6] published the threshold photoelectron spectrum (TPES) of the three isomers of C 2 H 2 Cl 2 by using dispersed synchrotron radiation. These authors used the threshold photoelectron-photoion coincidence technique to investigate the dissociative photoionization dynamics of these molecules. Quantum chemical calculations at different levels were applied to assign the photoelectron bands.
At about the same time Zhou et al [7] measured the infrared spectra of the dichloroethylene molecular ions trapped in a solid Argon matrix at 4 K. The + -1,1 C H Cl 2 2 2 and trans-1,2-C 2 H 2 Cl 2 + IR spectra have been reported.
These authors also assigned the spectral patterns based on quantum chemical wavenumber calculations.
In an ongoing work on 1,1-C 2 H 2 Cl 2 , its vacuum UV PAS [8] and photoelectron spectrum have been remeasured. These data are complemented by the measurements of the NeI-and ArII-PES. The TPES at improved resolution and constant ion state (CIS) spectra have been recorded using synchrotron radiation. Ab initio quantum chemical calculations have been performed to assign the electronic bands and identify the individual vibrational wavenumbers of the cation in their ground electronic state as well as in several excited states. The present paper focuses on the photoelectron, TPES and CIS spectroscopy.

Experimental setups
The two experimental setups used in this work have been described in detail in [9,10]. The most relevant information is reproduced here.
The fixed-wavelength photoelectron spectrometer used in this work is part of a PEPICO experiment [9] equipped with an effusive gas inlet. The VUV light produced by a discharge in He (58.43 nm or 21.22 eV), Ne (73.58-74.37 nm or 16.850-16.671 eV) or ArI (104.82-106.67 nm or 11.828-11.623 eV)-ArII (91.97 nm or 13.480 eV), is introduced in the ion chamber through a capillary of 0.5 mm diameter. A photoelectric light detector (a gold diode) continuously monitors the light source. Photoelectrons are extracted in the opposite direction to the ions and transmitted through a retarding potential analyzer with a differential output [11]. The photoelectron energy resolution, obtained under coincidence conditions, is 30 meV as measured on Ar + /e − coincidence data.
The TPES and CIS spectroscopy experimental setup uses the synchrotron radiation available from the BESSY facility (Berlin, Germany). It was dispersed by a 3m-NIM monochromator equipped with a platinumgrating of 2400 lines mm -1 with maximum transmission in the 10-40 eV (124-31 nm) range. An adequate signal-to-noise ratio is reached by adjusting the entrance and exit slits between 100 and 200 μm.
The light beam is focused into an ion chamber, in the focussing plane of a tandem electron spectrometer consisting of two 180°electrostatic deflectors [10]. Under constant pass energy (E 0 ) conditions, the full width at half-maximum energy resolution (ΔE) remains constant and is given by ΔE/E 0 =w/104, where w is the slit width (in millimeters). In the present work, w=0.5 or 1.0 mm while E 0 is chosen between 1.0 and 10.0 V to obtain a sufficient signal for each type of experiment.
The TPES operating mode corresponds to 'constant photoelectron energy' spectroscopy. The electron accelerating voltage V acc is kept as close as possible to E 0 , transmitting only TPE. A global photoelectron energy resolution of 5 meV is achieved.
In a CIS spectroscopy experiment, the relative partial ionization cross sections of well-defined electronic/ vibrational states of the molecular ion are measured as a function of the incident photon energy hν. hν and the electron kinetic energy E e kin have to be scanned in parallel to keep the difference n = - where IE E,ν corresponds to the ionization energy of the selected electronic/vibrational ionic state.
Both the TPES and CIS spectra have to be corrected for (i) the decreasing synchrotron storage ring beam current and (ii) the monochromator transmission function measured by the photoelectron signal of a gold diode, positioned in the ion chamber at the opposite of the monochromator exit slit. In addition, the CIS spectrum has to be corrected for the electron energy analyzer transmission function. For this purpose, the CIS curve of Xe + ( 2 P 1/2 ) is recorded and assimilated to the transmission function. This curve is structureless indeed over a broad energy range except for weak autoionization resonances assigned to the Rydberg Xe( 1 S 0 )→Xe( 2 S 1/2 )6p transition [12] at about 7.5 eV above threshold. Through the normalization process, they bring about an undesired but well-identified contamination of the CIS data in a range where the dichloroethylene ionization cross section is usually flat. No attempt has been made to remove these extremely weak structures. In practice the CIS spectra are just analyzed with an awareness of this contamination.

Data handling and error estimation
To facilitate the characterization of weak sharp peaks superimposed on an intense continuum, a continuum subtraction procedure developed by Carbonneau et al [13,14] has been applied. This method has proved successful in other spectral analyses [1,[14][15][16][17]. In practice, we first simulate the underlying continuum by a severe smoothing of the experimental spectrum. Upon subtraction of this continuum from the original data, we obtain a spectral curve called Δ-plot.
The fixed-wavelength PES spectra have been deconvoluted with an iterative procedure [18], improving the effective resolution to about 15 meV as measured on the HeI-PES of 1,1-C 2 H 2 Cl 2 .
In the fixed-wavelength PES, the photoelectron energy scale has been calibrated by recording the PES of a rare gas X-1,1-C 2 H 2 Cl 2 mixture (X=Ar, Kr and Xe), giving ±2 meV accuracy for the energy scale.
The Ar absorption spectrum between the 2 P 3/2 and the 2 P 1/2 ionization thresholds has been used to calibrate the 3m-NIM monochromator, with an accuracy better than 2 meV.
Energy steps of about 10 meV have been chosen for recording the TPES between 9 and 24 eV photon energy. The uncertainty on the energy position of a feature is estimated to be 6 meV. Between 10 and 20 eV, the TPES is recorded with an energy increment of 4 meV leading to an uncertainty for the energy positions of the order of 3 meV. These estimations are confirmed by the reproducibility of energy positions in different spectra over several measurement sessions.

Experimental results
3.1. HeI-, NeI-and ArII/ArI-PES The deconvoluted HeI-PES of 1,1-C 2 H 2 Cl 2 as measured between 9.5 and 21 eV is shown in figure 1(a). A recording of the 9.5-13.0 eV range is shown in figure 1(b) with 4 meV increments. Electronic states and vibrational assignments discussed below are included.
Seven well-defined bands and presumably nine electronic states are observed between 9.5 and 21 eV. Their adiabatic and vertical ionization energies are listed in table 1 together with previously reported results obtained with HeI and HeII. The NeI-PES of 1,1-C 2 H 2 Cl 2 is shown in figure 2(a) between 9.5 and 16.8 eV. The HeI-PES is reproduced in the same energy range for comparison. Figure 2(b) displays the 9.5-13.0 eV energy range on an expanded scale. In both parts of the figure, several structures are marked by an asterisk to indicate the probable contribution of the NeI-16.67 eV resonance line. On comparing with the HeI-PES, differences are obvious and will be discussed in section 5. The results on the ionization energies are listed in table 1.
The ArI/ArII-PES of 1,1-C 2 H 2 Cl 2 is represented in figure 3. Asterisks mark the probable contributions of the ArI-11.828 eV line. In the same figure the HeI-PES is reproduced, normalized to the intensity of the first vibrational structure in the first band. This comparison clearly shows dramatic differences over the whole ionization energy range.

Threshold-PES
The TPES spectrum of 1,1-C 2 H 2 Cl 2 has been recorded between 9.5 and 30 eV with a pass energy E 0 =5 V. Owing to the low transmission of the grating above 24 eV, an unfavorable signal/noise ratio is observed in this range of the spectrum. A well resolved spectrum between 9.5 and 20 eV, recorded at a pass energy E 0 =1 V and with 5 meV energy increments, is shown in figure 4. For comparison, the same figure also displays the HeI-PES normalized to the intensity of the adiabatic transition in the first PES band. Table 1 lists the vertical ionization energies inferred from the TPES together with those from the HeI-, NeI-and ArII-PES.
Large differences between the HeI-PES and the TPES are observed. Compared to the intensity of theX 2 B 1 -band at 9.826 eV, the relative intensities of the PES-bands corresponding to the ionic excited states are considerably enhanced by the resonant photoionization process governing the TPES. The very long vibrational progression in theX B 2 1 band displays several intensity maxima at 9.99, 10.72, 11.06, 11.25 and 11.35 eV. As already pointed out earlier in the case of 1,1-C 2 H 2 F 2 [1] and 1,1-C 2 H 2 FCl [1], the HeI-PES intensity is more or less evenly distributed among the ground and the excited states of the ion, whereas the excited states dominate the TPES between 11.5 and 21 eV. The difference in the shape of the bands at about 14 eV and at 18.5 eV is also noteworthy. In the former, several maxima are observed between 13.6 and 15.2 eV. For the latter band, the signal Table 1. Adiabatic and vertical ionization energies (eV) measured by HeI-, NeI-ArII-PES and by TPES using synchrotron radiation in this work and in previous reports [2][3][4][5][6]. increases very sharply at threshold (18.496 eV) in the HeI-PES and is narrow, whereas it rises up smoothly from 18.05 eV in the TPES and is much broader.

The CIS spectra
The CIS spectra have been recorded for the first eight electronic states of + -1, 1 C H Cl . 2 2 2 These are displayed in figure 5(a) together with the TPES. To facilitate the interpretation and discussion, the Δ-plot of the vacuum UV PAS of 1,1-C 2 H 2 Cl 2 has been reproduced in figure 5(b) [8] between 9.8 and 20 eV.
Owing to the wide extent of the vibrational population of the ground electronic state of observed in the TPES, a sequence of vibrationally resolved CIS spectra has been recorded between 9.83 and 20.0 eV. The result is shown in figure 6. The corresponding TPES spectrum is included in this figure. In this case too, marked differences of the spectral shape are noteworthy.
As mentioned in section 2.1, CIS spectra extending over large energy ranges are contaminated by Xe autoionization resonances at about 7.5 eV above threshold, due to the normalization procedure.
The molecular orbital (MO) configuration of 1,1-C 2 H 2 Cl 2 in the C 2v symmetry group is described by where 1a 1 is the first outer-valence shell orbital. The wavenumbers characterizing the twelve vibrational normal modes were determined at the M06-2X, B3LYP and TDDF/M06-2X levels.

Results of the calculations
The result of the geometry optimization of the neutralX 1 A 1 ground state, of the cationic ground state,X 2 B 1, and of several excited states are presented in table S1 which is available online at stacks.iop.org/JPCO/1/ 055030/mmedia (supplementary data), according to the atomic numbering shown in the same table, at three or four different calculation levels.
The vibrational normal modes for the different electronic states of the cation figure S1 (see supplementary data). The corresponding wavenumbers are listed in table 2. The present theoretical results can only be compared to those obtained by Zhou et al [7] and Takeshita [27]. This latter author reported on the ab initio calculated photoelectron spectrum of the three dichloroethylene isomers [27].
The vertical and adiabatic ionization energies calculated at several levels are presented in table 3. The contour maps of the involved MO, calculated at the RHF level for the neutral molecule optimized at the CCSF(FC) level, are represented in figure S2 (supplementary data).

Discussion
5.1. HeI-, NeI-and ArII/ArI-PES of 1,1-C 2 H 2 Cl 2 (figures 1-3) The PES of 1,1-C 2 H 2 Cl 2 strongly varies with the energy of the exciting light source. As shown in figures 1-3 the extent of vibrational excitation involved with the three light sources is quite different. The relative intensities between the successive bands and within each band are altered. Owing to the numerous structures observed, each band as produced by the three excitation sources will be analyzed separately.  table 4). Using the NeI-and ArII-resonance lines the cationic ground state is observed up to 11.076±0.006 eV (levels in brackets in table 4) and 11.383±0.004 eV (levels in square brackets in table 4) respectively.
The adiabatic ionization energy averaged over the results of five HeI-measurements and two NeI-and ArIIspectra is equal to IE ad .002 eV, where the indicated error is the largest deviation from the average between all the measurements. A weak signal is observed at 9.746±0.010 eV. It represents 8.6% of the intensity of the adiabatic transition and is assigned to a hot band involving ω 4 =603 cm −1 (0.075 eV) of the neutral 1,1-C 2 H 2 Cl 2 molecule [28], for which a contribution of 5.5% is predicted at room temperature, based on the Boltzmann distribution law.
As shown in table 1 this value of IE ad is in good agreement with most previously reported results [2][3][4][5], ranging from 9.74 to 9.83 eV. The published value of 10.00 eV [2][3][4][5] corresponds to the IE vert =9.992±0.001 eV as measured in the present work. The calculated values listed in table 3 are in very good agreement with those reported in earlier and in the present work (see table 1).
The energy positions of the structures observed in the first PES band are listed in table 4. Considering only the results obtained by the HeI resonance line the transition energies and assignments are listed in columns 1 and 3. The strongest vibrational transitions correspond to an average spacing of 0.163±0.006 eV (1320±50 cm −1 ). It is assigned to the C=C stretching vibrational normal mode involving ν 3 of a 1 symmetry (see figure S1). This assignment is consistent with the characteristics of the ionized 2b 1 MO (with π character) (see figure S2). This value is in good agreement with previous determinations, i.e. 1290 cm −1 [4], 1320 cm −1 [3] or 0.16 eV (1290 cm −1 ) [2]. Surprisingly, this vibration has not been observed by infrared spectroscopy of + -1, 1 C H Cl 2 2 2 trapped in a solid Argon matrix [7]. In previous quantum chemical calculations, the harmonic wavenumber of 1366.1 cm −1 and the anharmonic value of 1340.6 cm −1 were predicted [7]. In the present work, the calculated wavenumbers that can be related to a C=C stretching are equal to ω 2 =1499 (1477) cm −1 and ω 3 =1364 (1330) cm −1 at the M06-2X (B3LYP) level (see table 2). Takeshita [27] calculated ω 2 =1592 cm −1 and ω 3 =1435 cm −1 . The second predicted value (ω 3 ) is obviously in better agreement with the experimental results.
However, integrating the HeI-PES data in the list of results obtained with NeI and ArII, a lowering of the spacing is observed and ascribed to anharmonicity. The data analysis leads to ω=1347±8 cm −1 (0.167±0.001 eV) and ωx=3.9±1.1 cm −1 (0.48±0.14 meV). This wavenumber is in good agreement with the predicted anharmonic value [7].
The further analysis of the first PES band reveals a more complex structure with at least two other vibrational spacings. The strongest of these transitions is characterized by ω=653±24 cm −1 (0.081±0.003 eV) in very good agreement with 650 cm −1 reported by Lake and Thompson [3] by HeI-PES and 642.8 cm −1 measured by infrared spectroscopy [7]. Other HeI-PES determinations were reported i.e., 560 cm −1 [4] and 730 cm −1 [2].  This wavenumber is close to the predicted harmonic value of 681.8 cm −1 or anharmonic value of 672.4 cm −1 [7] and 682 cm −1 [27] for ν 4 (symmetric C-Cl stretching) of a 1 symmetry (see figure S1). In the neutral molecule this vibration is characterized by ω 4 =603 cm −1 [28]. The present work provides 624 (595) cm −1 for the neutral and 672 (642) cm −1 for the cation as calculated at the M06-2X (B3LYP) level (see table 2). Finally, a third and very weak vibrational excitation, combined with the excitation of ν 3 and ν 4 , is observed with ω=470±20 cm −1 (0.058±0.003 eV). Surprisingly, it has mainly been observed so far only in the HeI-PES. Its intensity is rapidly decreasing as the excitation of ν 3 increases . It could not be observed in the fairly well resolved ArII-PES. The doublet structure of the excitation source likely prevents its observation in the NeI-PES. A first explanation would be the large difference in the population mechanism involved in the HeI-PES on the one hand, where direct ionization prevails, and in NeI-and ArII-PES on the other hand, where autoionization is expected to be involved. The latter two excitation lines are resonant with broad and fairly strong photoabsorption bands [8] of 1,1-C 2 H 2 Cl 2 , displayed in figure 5(b). This wavenumber fits reasonably well with our calculated ν 8 wavenumber of 486 (475) cm −1 (see table 2), corresponding to an out-of-plane deformation of b 2 symmetry. This symmetry forbidden excitation is expected to be of very low intensity (see figure S1). A similar observation and assignment was proposed in the HeI-PES of 1,1-C 2 H 2 F 2 [30].
Combined with the excitation of the 1347 cm −1 normal mode, a fourth vibrational transition has to be invoked to assign three structures in the HeI-PES, i.e. at 10.179, 10.347 eV and at 10.488 eV. Extending our observations to the NeI-and ArII-PES, the combination of 1347 cm −1 with this vibration dominates these spectra. Figure 3 illustrates how far the nν 3 progression extends in combination with this vibrational motion. In the ArII-PES an additional sequence of structures is generated extending from 10.8 to 11.4 eV with a maximum at 11.06 eV. The associated spacing, averaged over all observed levels up to 11.383 eV, is ω=274±16 cm −1 (0.034±0.002 eV). This wavenumber could correspond to the vibrational Cl-C-Cl scissor motion ν 5 (see figure S1) of a 1 symmetry which is predicted at 331.2 cm −1 in [7], at 346 cm −1 in [27] and at 327 (322) cm −1 in the present work. The corresponding transition has not been observed by infrared spectroscopy [7]. In the neutral molecule, this normal mode has a wavenumber of 299 cm −1 [28].

TheÃ 2 B 2 -PES band (figures 1(b), 2(b) and 3)
The second HeI-PES band extending from 11.5 to 12.1 eV consists of a smooth and regular structure. In the NeI-PES, the relative intensity of several structures of the same band is strongly modified. Apart from ArI-PES contributions, the intensity distribution appears considerably enhanced in the ArII-PES as shown by the comparison with its HeI-PES (see figure 3). The average positions of the structures observed in these spectra are listed in table 5.
The transition observed at 11.47±0.02 eV has very likely to be ascribed, at least partially, to a hot band transition. The wavenumber separation of 347±60 cm −1 (0.043±0.007 eV) is close to 299 cm −1 measured for ν 5 in the neutral molecule [28]. In the NeI-PES the intensity ratio between the two transitions measured at   11.464 and 11.517 eV is about 17%, close to the Boltzmann law prediction of 23%. In the ArII-PES the intensity observed at 11.477 eV is contaminated by the ionization to theX B 2 2 ground state with the ArI resonance line at 11.830 eV. Table 5 reports the assignments of all the structures observed in this band. It shows that the whole band could be accounted for by one progression involving a single vibrational mode characterized by an average separation of 0.035±0.005 eV (282±40 cm −1 ). Lake and Thompson [3] report a wavenumber of 270 cm −1 , perfectly compatible with the present determination. This progression is observed up to v=16. It could be assigned to ν 5 which has been calculated at 287 (261) cm −1 at M06-2X (B3LYP) level in this work. Takeshita [27] predicted 320 cm −1 for the Cl-C-Cl scissoring mode. From the shape of the ionized 4b 2 MO (see figure S2) and the results of the optimized geometry (see table S1), the Cl-C-Cl scissor mode should be an important component of the excited mode confirming the assignment of the progression to ν 5 (see figure S1).

TheB 2 A 2 -andC 2 A 1 -PES bands (figures 1(b), 2(b) and 3)
The third and fourth bands of the HeI-PES are well separated. Both contain well-resolved regular structures. Compared to the HeI-PES, the NeI-and ArII-spectra show noticeable modifications of these bands (see figures 2(b) and 3). Furthermore, in the NeI-PES these two bands are contaminated by the contribution of the NeI-16.670 eV resonance line (marked by asterisks in figure 2(b)). In contrast, the ArII-PES of these two bands cannot be perturbed by ArI-lines at 11.62-11.83 eV.
HeI-ionization to the third band provides unambiguously IE ad =IE vert =12.152±0.005 eV. This should correspond to theB 2 A 2 state of + -1,1 C H Cl . 2 2 2 Two lower-lying structures are observed at 12.122±0.004 eV and 12.079±0.004 eV. Their relative intensity with respect to the adiabatic transition is 20% and 9% respectively whereas the predicted value of the Boltzmann population for one and two quanta of ω≈300 cm −1 (0.037 eV) vibration is 24% and 6% respectively. This suggests assigning these two transitions to hot bands from v=1 and v=2 of the ν 5 vibration of the neutral 1,1-C 2 H 2 Cl 2 ground state (see table 2 table 3).
In the HeI-PE spectrum the intensity of the features observed in the third band steeply decreases upon increasing energy. The average energy positions and their assignment are listed in table 6. The progression Table 5. Transition energies (eV), assignments and average wavenumbers (cm −1 ) inferred from the structures observed in the HeI-, NeI-, ArII-/ArI-PES and in the TPES of theÃ 2 B 2 band of + -1,1 C H Cl . 2  (v x = 0 at 9.826 eV or 1 at 9.993 eV) state of + -1, 1 C H Cl 2 2 2 as ionized by the 11.830 eV (95417.6 cm −1 ) or 11.620 (93723.8 cm −1 ) lines of ArI-shown by asterisks in figure 3. b Contributions of the NeI-16.670 eV (134455.7 cm −1 ) line in the NeI-PES shown by asterisks in figure 2. c Uncertainty estimated to be ±0.003 eV.
involves a single vibrational normal mode with ω=258±16 cm −1 (0.032±0.002 eV). This value has to be compared with the only measurement reported by Lake and Thompson [3] of 270 cm −1 . Furthermore, it has to be stressed that in the NeI-and ArII-PES (i) IE ad =IE vert no longer holds and IE vert =12.181±0.005 eV and 12.174±0.003 eV respectively, and (ii) a second intensity maximum is observed at 12.347 eV or 12.363 eV, respectively. Such changes of the intensity profile have already been observed earlier when autoionization is involved [18,31] (see also section 5.1.1). The wavenumber of 258 cm −1 is close to that measured for ν 5 in the neutral molecule, i.e. 299 cm −1 [28]. The shape of the ionized 1a 2 MO (see figure S2), the results of the geometry optimization, and the M06-2X (B3LYP) calculated wavenumber value of 281 (279) cm −1 validate the assignment of ν 5 to the progression (Cl-C-Cl scissoring in figure S1). Previously Takeshita's quantum chemical calculation [27] led to ω 5 =308 cm −1 .
HeI-PES leads to IE ad =IE vert =12.488±0.007 eV for the fourth band. The structure lying at 12.458±0.005 eV is ascribed to a hot band transition from v=1 of ν 5 of the neutral molecule. This band is Table 6. Transition energies (eV), assignments and average wavenumbers (cm −1 ) as deduced from the structures observed in the HeI-, NeI-, ArII-/ArI-PES and in the TPES of theB 2 A 2 andC 2 A 1 bands of + -1,1 C H Cl . 2 2 2 Conversion factor 1 eV=8065.545 cm −1 [29]. Weak to very weak structures are mentioned in italics. assigned to theC 2 A 1 state of + -1, 1 C H Cl . 2 2 2 The intensity of the vibrational structure of this band decreases regularly up to 12.656 eV for HeI-, 12.734 eV for NeI-and 13.432 eV for ArII-PES. As for the third band, the IE vert is blue shifted in the latter two spectra: at 12.536±0.006 eV in NeI-and ArII-PES. The IE ad in the HeI-PES can be compared with previous determinations at 12.50 eV [2], 12.51 eV [3], 12.48 eV [4] and 12.5 eV [5] by HeII-PES. Our calculated M06-2X values for IE ad and IE vert are 12.44 and 12.52 eV respectively (see table 3).

PES
The ArII-PE spectrum displays a well resolved vibrational progression up to 13.432 eV as shown in figure 3 and table 6. The intensity nearly vanishes above 12.919 eV and rises up again to 13.432 eV. From 13.1 to 13.5 eV, the intensity of the vibrational structure is affected by the strong zero-energy (ZEKE) electron signal.
The value found for ω 4 is in the range predicted by our quantum chemical calculations (see table 2), that is, between 520 and 620 cm −1 . Furthermore, the ν 4 vibrational excitation is combined with the excitation of the ν 5 mode with ω 5 =260±30 cm −1 (0.032±0.004 eV). The latter assignment is based on the satisfactory agreement with our theoretical predictions, in the 250-300 cm −1 range. These assignments are compatible with the characteristics of the 5a 1 MO depicted in figure S2 and with the normal modes described in figure S1.

TheD 2 B 2 -Ẽ 2 B 1 andF 2 A 1 -G 2 B 2 -PES bands (figure 1)
The fifth and sixth PES-bands are broad and spread over 1.0-1.5 eV. They both involve two overlapping electronic states likely consisting of a dense vibrational structure. This structure is difficult to disentangle owing to overlapping progressions.
A closer examination of the fifth PE-band allows us to measure the IE ad =13.521±0.004 eV for the fifth electronic state and assign it to theD 2 B 2 state of + -1, 1 C H Cl . 2 2 2 An IE vert =13.633 eV is also measured and could be compared to the literature values 13.84 [2], ∼13.7 eV [3] and 13.8 eV by HeII-PES [5]. The calculated TD-DFT IE values are respectively 13.30 eV and 13.82 eV, which are compatible with the experimental data (see table 3 and figure S2).
In the same way, but less reliable, IE ad =14.05 or 14.09 eV and IE vert =14.19 or 14.24 eV could be proposed for the fifth electronic state. These IE's are assigned to theẼ 2 B 1 state of + -1, 1 C H Cl . Only the few first structures could be analyzed up to 18.8 eV. The intensity is steeply decreasing and a fairly short likely anharmonic progression is observed with ω=621±8 cm −1 (0.077±0.001 eV). This progression is combined with a low energy vibration characterized by ω=194±24 cm −1 (0.024±0.006 eV). Only Lake and Thompson reported an analysis of the vibrational structure and mentioned a wavenumber of 1290 cm −1 . This value is about twice the value proposed in the present work.
Our quantum chemical calculations (table S1) suggest that the ionization from the 3a 1 MO would lead to the vibrational excitation of the C-H and C=C stretching modes and also of the H-C-H and Cl-C-Cl bending modes (see figures S1 and S2). Comparing the experimental results with the predicted wavenumbers listed in table 2, the most satisfactory assignment is ω 4 =621±8 cm −1 and ω 5 =194±24 cm −1 .

Threshold PES of 1,1-C 2 H 2 Cl 2 (figures 4 and 7)
The ionization and excitation mode are markedly different in the TPES and in the fixed wavelength PES. To record a TPES, the molecular system is resonantly excited by continuously scanning the vacuum UV light energy. The usefulness of this technique is that besides the successive ionic states, also the neutral states interacting with the ionization continuum through resonant autoionization and producing low-energy electrons, are involved. The Franck-Condon zones associated with these intermediate transitions may strongly differ from those involved in the direct ionization process usually governing HeI-PES, thus giving rise to a quite different PES.
The TPES observed in this work is reproduced in figure 4 between 9.5 and 20 eV and is compared to the HeI-PES normalized to the adiabatic transition for the ionic ground state. Major differences between the two spectra are obvious. The most visible aspect is the large increase in the relative intensity of almost all TPES bands. The successive adiabatic and/or vertical ionization energies are listed in table 1. To enhance the vibrational structure of the successive TPES bands, the subtraction method has been applied (see section 2.2). The resulting Δ-plot is shown on an expanded ionization energy scale in figures 7(a)-(d).
The energy position of the features observed in theX 2 B 2 -TPES band is listed in table 4. For most of the structures the agreement with the fixed-wavelength PES data is satisfactory. It has to be pointed out that the intensity at 10.990 eV is noticeably higher than in the HeI-PES (see also the NeI-PES in figure 2). However, in the 9.830-10.480 eV energy range, the TPES shows (and confirms) the excitation of the ν 5 vibrational mode at 9.865, 9.940, 10.110 and 10.385 eV successively. These features are not observed in the low-energy region of the first HeI-PES band, contrary to the more favorable situation in the better resolved TPES. The average energy separation is 0.037±0.004 eV in the TPES whereas this value is 0.034±0.002 eV as deduced from the HeI-PES (see column 4 in table 4).
As shown in figures 4 and 7(a) the ground ionic state is populated well above 10.480 eV. The signal/noise ratio in the Δ-plot is quite low. However, several maxima are observed and listed in table 4 and are compared with observations in NeI-and ArII-PES. Also in these cases the agreement is satisfactory.
The Δ-plot corresponding to theÃ 2 B 2 , theB 2 A 2 and theC 2 A 1 TPES bands is displayed in figure 7(b). They clearly reveal the associated vibrational structure which is listed in tables 5 and 6.
For the broad and abundantly structuredÃ 2 B 2 TPES band the shape is very close to the fixed wavelength PES. Only its relative intensity is considerably enhanced in the TPES. The average separation energy of 0.035±0.004 eV for the single vibrational progression involved in this ionic state is in very good agreement with the value of 0.035±0.005 eV deduced from the fixed wavelength PES. Similarly to the large enhancement of the intensity of theÃ 2 B 2 TPES band, theB 2 A 2 and theC 2 A 1 TPES bands are the strongest in the TPES. Here also, the features observed in these bands confirm those detected by HeI-and ArII-PES. The energy positions are listed in table 6. Owing to the fairly low signal/noise ratio in the Δ-plot only approximate energy values could be measured for the features observed between 13.5 and 15 eV (see figure 7(c)). Additional abundant unresolved autoionization structures seem to bring about a significant contribution. For theẼ 2 B 2 band only an intensity maximum could be estimated at about 14.25 eV. It seems that, in this energy region, autoionization strongly perturbs the regular progression which was observed by HeI-PES. The energy range of 13.5-17.3 eV corresponds to several broad absorption bands of 1,1-C 2 H 2 Cl 2 , with strong to weak intensities [8] (see figure 5(b)).
The Δ-plot of the energy range covered by the three last TPES bands is represented in figure 7(d). Also in these cases only fairly broad but well resolved features are observed. Between 16.1 and 17.6 eV several features are observed at 15.82, 16.21, 16.56, 16.9, 17.2, 17.4 and 17.6 eV. This fairly surprising regular pattern might be related to the oscillation of the Franck-Condon factors governing the autoionizing transitions from the neutral excited states to the finalF 2 A 1 andG 2 B 2 ionic states. The vibrational structure is likely hidden in the noise on the successive maxima. Similar effects have been observed and investigated in NeI-PES of diatomic molecules [31].
The last ionicH 2 A 1 state is observed at IE=18.506±0.003 eV which can be compared with the adiabatic IE=18.496±0.003 eV measured by HeI-PES (see section 5.1.5 and table 1). As already pointed out earlier and clearly observed in figure 4, the base of the peak at 18.5 eV is strongly broadened towards lower energy, i.e. to about 17.9 eV. In the Δ-plot of this energy range (see As drawn in figure 7(d), these structures could be assigned to two major vibrations, i.e. hcω≈0.36 eV (2900 cm −1 ) and hcω≈0.17 eV (1370 cm −1 ). A closer examination of the major peak at 18.5 eV reveals several shoulders with intervals of 0.025-0.035 eV (200-280 cm −1 ). These can be compared with the value of 0.024±0.006 eV inferred from the HeI-PES. The only vibrational analysis of this band in the He-I PES is that of Lake and Thompson [3], who reported a wavenumber of 1290 cm −1 . Their interpretation differs however from that of the present work (see section 5.1.5). Comparing the TPES results with the quantum chemical predictions, the ν 1 (ω 1 =2900 cm −1 ), ν 3 (ω 3 =1370 cm −1 ) and likely ν 5 (ω 5 =200-280 cm −1 ) are excited. The first two vibrations have not been observed in the present HeI-PES. Again, resonant autoionization is suspected to strongly perturb the branching among the photoionization and, as a consequence, the vibrational band profile.

5.
3. CIS spectra of 1,1-C 2 H 2 Cl 2 (figures 5 and 6) The individual CIS spectra related to the first eight electronic states have been recorded over a range of about 10 eV photon energy ( figure 5(a)). In the case of theD 2 B 2 ,Ẽ 2 B 1 ,F 2 A 1 andG 2 B 2 TPES bands the CIS spectra have been recorded for the energy corresponding to the maximum of the band.
The difference between the CIS spectral profiles below and above 14 eV photon energy is noteworthy. Below 14.0 eV, the CIS spectra show a complex shape and a strong cross-section variation. Above 14.0 eV, a mainly monotonic behavior is observed. This observation might be related to a large difference in the population mechanism of the ionic states below and above 14 eV.
Above 14 eV, we may infer from the shape of the CIS spectra that the population of theD -Ẽ B 2 2 2 B 1 andF -G A 1 2 2 B 2 states has to occur resonantly or nearly resonantly through the autoionization of highly excited (Rydberg) states of the neutral molecule. For clarity of this discussion, the Δ-plot of the vacuum UV PAS of 1,1-C 2 H 2 Cl 2 is reproduced in figure 5(b) [8]. From 14 eV upward, several Rydberg states were identified, e.g., at  [8]. Both converge to theC 2 A 1 ionic state. Several shoulders and maxima are observed, which correlate with PAS bands. A sudden decrease of the cross-section is observed at 16.1 eV in the CIS spectra ofX 2 B 1 ,Ã 2 B 2 ,B 2 A 2 andC 2 A 1 states. This may be correlated to the onset of theF 2 A 1 -G 2 B 2 channel at 16.117 eV. It may also be related to the resonance observed at 16.1 eV in the PAS. This resonance shows an asymmetric profile in the absorption spectrum [8], which might be interpreted as a Fano profile. The decrease of the state-resolved ionization cross sections (CIS spectra) at this energy is compatible with a partial population transfer from the continuum to the discrete state, a situation linked to the interference between the continuum and Rydberg wave functions and which is expected to give rise to Fano profiles. Figure 5(a) shows a typical CIS spectrum obtained for theX 2 B 1 state. Figure 6 displays a set of seven CIS spectra recorded for well defined (as indicated in the figure) vibrational levels of the ground state of the molecular ion. Also in this narrow energy range the CIS spectral profiles for different vibrational levels show marked differences with respect to the structure and intensity. These observations are summarized in table 7.
The CIS spectrum of IE ad at 9.830 eV shows a nearly constant cross section up to 12.5 eV. Above this energy it increases as do the CIS spectra recorded at 9.905, 9.986, 10.076 and 10.400 eV which all show the same autoionizing contributions. These are identified in the vacuum UV PAS [8] and shown in table 7. In the HeI-PES these vibrations are all populated by direct Franck-Condon transition from the neutral ground state of the molecule. In contrast, the CIS spectra at 10.746 and 11.055 eV are related to vibrational states populated only through autoionization processes in the NeI-and ArII-PES and in the TPES. At 1.0-1.5 eV above onset, these cross-sections decrease regularly to reach the baseline at about 16 eV. A bold line in table 7 separates the two types of vibrational states.

Conclusions
A detailed photoelectron spectroscopic investigation of 1,1-C 2 H 2 Cl 2 up to 22 eV has been performed including constant wavelength HeI-, NeI-, and ArII/ArI-PES, as well as the variable wavelength TPES and CIS spectra for all accessible ionic electronic states and for selected vibrational states of the ground ionic electronic state. Seven electronic bands have been identified in this photon energy range. These bands were assigned with reference to quantum chemical calculations. The first four bands show an extended vibrational structure. For each of these bands, vibrational wavenumbers are inferred and compared to quantum chemical predictions. The very good correlation between the theoretical predictions and the experimental values allowed us to assign the normal modes. These vibrational data are also compared to previous investigations. The structures observed in the CIS spectra of the successive electronic states of the molecular ion below 14 eV exhibit strong correlation with those observed in the vacuum UV PAS of 1,1-C 2 H 2 Cl 2 . Vibrationally resolved CIS spectra recorded for theX 2 B 1 state of + -1, 1 C H Cl 2 2 2 show a similar behavior only for those vibrations which are populated by direct Franck-Condon transitions from the neutral ground state of the molecule. The additional vibrational levels which appear in the NeI and ArII PES involve, however, autoionization transitions. Table 7. Maxima or band limits (eV) observed (x) or missing (o) in the CIS spectra of the specified vibrations of the ground stateX 2 B 2 of + -1,1 C H Cl . 2 2 2 The assignments refer to the corresponding transitions in the vacuum UV PAS of 1,1-C 2 H 2 Cl 2 [8].