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

Abstract

This work analyzes the threshold photoelectron spectrum (TPES) and selected constant ion state (CIS) spectra of the 1,1-C 2 H 2 Cl 2 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 H Cl 2 2 2 have also been recorded. Vibrationally resolved CIS spectra of the molecular ion ground state have been obtained and are discussed.

The vacuum UV photoabsorption spectrum of the geminal dichloroethylene (1,1-C2H2Cl2) in the 5–20 eV range. A vibrational analysis of the valence and Rydberg states

“…Internuclear distances in Ǻ and angles in degree. We recently measured the HeI-photoelectron spectrum (PES) and the threshold photoelectron spectrum (TPES) of 1,1-C 2 H 2 Cl 2 and the results will be reported in a forthcoming paper [17]. The first adiabatic ionization energy is located at IE ad (1,1-C 2 H 2 Cl 2 + ,X 2 B 1 )=9.826±0.002 eV and the corresponding vertical value is IE vert (1,1-C 2 H 2 Cl 2 + ,X 2 B 1 )=9.993±0.004 eV, in very good agreement with available data [18][19][20][21].…”

“…At higher energies eight bands were observed by HeI-PES. For most of them the adiabatic ionization energy (IE ad ) could be measured at 11.520, 12.157, 12.497, 13.521, 15.539 and 18.496 eV [17]. The PES-band shape corresponding to the first excited state has a symmetric bell-shaped envelope and its maximum is measured at IE vert =11.647 eV.…”

“…The fifth and sixth PES-bands at 13.521 and 15.539 eV involve two overlapping electronic states [17]. For the two components vertical ionization energies (IE vert ) were measured at 13.633 and 14.195 eV for the former and at about 15.82 and 16.16 eV for the latter [17].…”

“…No uncertainty estimation is provided by Walsh et al [6]. For the assignments reported in the present work the adiabatic ionization energy value IE ad (1,1-C 2 H 2 Cl 2 + ,X 2 B 1 )=9.826±0.002 eV [17] has been used whereas the value determined by Walsh et al [6] was 9.85 eV. Table 1 displays the excitation energies corresponding to the π→ns Rydberg series converging to the first ionization energy limit.…”

“…This approach has been shown efficient to interpret the vacuum UV spectra of the methyl halides [28], of C 2 H 3 F [1], and 1,1-C 2 H 2 F 2 [3]. The convergence limit of most of the Rydberg states involved between 6.7 and 9.9 eV is IE ad (1,1-C 2 H 2 Cl 2 + ,X 2 B 1 )=9.826±0.002 eV [17]. The cationicX 2 B 1 vibrational structure is dominated by the excitation of two vibrational normal modes characterized by the wavenumbers ω 3 + =1322±48 cm −1 (0.163±0.006 eV) and ω 4 + =653±24 cm −1 (0.081±0.003 eV) [17] (notation ω i for the neutrals and ω i + for the ions).…”