The observation of strong pseudo-Jahn–Teller activity in the benzene cation B̃ 2E2g state

Abstract

Articles you may be interested in K selection in one-photon mass-analyzed threshold ionization of C H 3 I and C D 3 I to the X ̃ E 3 ∕ 2 2 state cations J. Chem. Phys. 128, 044310 (2008); 10.1063/1.2823032 Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: Electronic and rovibronic structures of the cyclopentadienyl cationThe Jahn-Teller effect in the lower electronic states of benzene cation. II. Vibrational analysis and coupling constants of the B̃ 2 E 2g state Photoinduced Rydberg ionization ͑PIRI͒ spectroscopy has been applied to the problem of recording vibrationally resolved spectra of the dipole-forbidden B 2 E 2g ←X 2 E 1g transition of C 6 H 6 ϩ and C 6 D 6 ϩ . PIRI spectra of the B state have been recorded via a number of vibrational states of the ionic ground state. A combination of Herzberg-Teller and pseudo-Jahn-Teller vibronic coupling between the B state and a close lying C state were used to explain the complex B state vibrational structure. The two pseudo-Jahn-Teller vibrational modes, 16 and 17 , were both found to be active in the PIRI spectra. In addition, evidence for a strong pseudo-Jahn-Teller interaction within each of these modes was found. The perturbed vibronic bands resulting from this pseudo-Jahn-Teller activity were successfully modeled using a two-mode pseudo-Jahn-Teller vibronic coupling model. Using a combination of the spectral results and the pseudo-Jahn-Teller calculations, we were able to generate absolute vibronic symmetry assignments for many of the lower B state vibrational levels, from which vibrational assignments were made. The calculations also produced unperturbed vibrational frequencies for both pseudo-Jahn-Teller active modes along with values for their respective linear coupling constants.

The infrared spectrum of the benzene–Ar cation

“…It is interesting to note that all three components of the Fermi triplet have also been observed in ZEKE and MATI spectroscopy on C 6 H 6 at the same ͑within 2 cm Ϫ1 ) frequency positions. 8,11 Moreover, the almost completely mixed outer two components from this triplet indeed appear with the same intensity in these spectra. The two lowest frequency components of this triplet had been assigned to the two doubly degenerate components of the 6 16 combination band, 8,11 but it was already noted that these modes would then be significantly shifted away from their expected frequencies.…”

“…19 The strong IR active mode observed at 485 cm Ϫ1 in the spectrum of the C 6 D 6 -Ar complex is consequently safely assigned to this 11 mode, which is of electronic-vibrational E 1u symmetry in the C 6 D 6 cation. This mode has been observed before at 488 cm Ϫ1 as a weak peak in ZEKE and MATI spectra, 8,11 but has never been assigned. The corresponding mode in the C 6 H 6 -Ar complex is expected as a single strong peak around 650 cm Ϫ1 .…”

“…In particular, six ungerade modes of electronicvibrational A symmetry and ten ungerade modes of electronic-vibrational E symmetry result, which are all IR active from the vibrationless level in the ion. Of these, two A modes and two E modes are in the high-frequency C-H͑D͒ stretching region, whereas the E 1u component of the 16 mode, known from ZEKE and MATI experiments to be located at 306 cm Ϫ1 ͑264 cm Ϫ1 ) for the C 6 H 6 ͑C 6 D 6 ) cation, 8,11 is outside of our scan region at the low frequency side.…”

“…3,4 Its low fluorescence quantum yield has prohibited such gas-phase LIF studies on the benzene cation. Photoelectron spectroscopy 5 and threshold-electron ͑zero electron kinetic energy, ZEKE͒ [6][7][8] and threshold-ion detection ͑mass analyzed threshold ionization, MATI͒ schemes [9][10][11] have been successfully used, however, to measure and assign several vibrational modes in the benzene cations.…”

“…The IR spectrum of the C 6 H 6 -Ar cation is dominated by a Fermi resonance between the IR active 11 mode and two components of the combination mode of the lowest frequency modes 6 and 16 . A stringent upper limit of 316 cm Ϫ1 is found for the value of the dissociation limit D 0 of the neutral C 6 D 6 -Ar complex.…”