Electronically forbidden (5σu→kσu) photoionization of CS2: Mode-specific electronic-vibrational coupling

Abstract

Vibrationally resolved photoelectron spectroscopy of the CS(2) (+)(B (2)Sigma(u) (+)) state is used to show how nontotally symmetric vibrations "activate" a forbidden electronic transition in the photoionization continuum, specifically, a 5sigma(u)-->ksigma(u) shape resonance, that would be inaccessible in the absence of a symmetry breaking vibration. This electronic channel is forbidden owing to inversion symmetry selection rules, but it can be accessed when a nonsymmetric vibration is excited, such as bending or antisymmetric stretching. Photoelectron spectra are acquired for photon energies 17</=hnu</=72 eV, and it is observed that the forbidden vibrational transitions are selectively enhanced in the region of a symmetry-forbidden continuum shape resonance centered at hnu approximately 42 eV. Schwinger variational calculations are performed to analyze the data, and the theoretical analysis demonstrates that the observed forbidden transitions are due to photoelectron-mediated vibronic coupling, rather than interchannel Herzberg-Teller mixing. We observe and explain the counterintuitive result that some vibrational branching ratios vary strongly with energy in the region of the resonance, even though the resonance position and width are not appreciably influenced by geometry changes that correspond to the affected vibrations. In addition, we find that another resonant channel, 5sigma(u)-->kpi(g), influences the symmetric stretch branching ratio. All of the observed effects can be understood within the framework of the Chase adiabatic approximation, i.e., the Born-Oppenheimer approximation applied to photoionization.

Vibrationally resolved photoionization dynamics of CF4 in the DA12 state

“…As described in previous studies, [18][19][20][21][22][23] vibrational branching ratios are obtained by measuring the relative intensities of the various vibrational excitations of interest relative to the ground vibrational level. The energy dependencies for the vibrational branching ratios are shown in Fig.…”

“…Our recent studies on linear triatomic systems demonstrate the utility of vibrationally resolved photoelectron studies to extract a detailed picture of the photoionization dynamics. [18][19][20][21][22][23] Thus, a study of the vibrationally resolved behavior of CF 4 photoionization might be useful in identifying fundamental aspects of the photoelectron scattering dynamics, particularly because shape resonances are well known to couple photoelectron and vibrational motion. [24][25][26] In the current study, we focus on the CF 4 + ͑D 2 A 1 ͒ state for two reasons.…”

“…11 In the current study, we cover a relatively wide range of incident photon energies, 26.5 eVഛ h ഛ 50 eV. Like the strategy applied to our recent studies on linear triatomics, [18][19][20][21][22][23] we determine the branching ratios of all of the available vibrational modes to explore the photoionization dynamics. In the case of CF 4 , there are four vibrational degrees of freedom: 1 ͑breathing vibration, i.e., symmetric C-F stretch͒, 2 ͑bend͒, 3 ͑asymmetric stretch͒, and 4 ͑another bend͒.…”

“…So, it is important to ascertain the mechanism responsible for the single quantum excitations of the nonsymmetric vibrational modes. In our previous work on symmetric triatomic systems, [18][19][20][21][22][23] analogous forbidden vibrational excitations have been characterized extensively. While such transitions have been observed previously, they had typically been ascribed to non-Born-Oppenheimer effects, such as Jahn-Teller, Renner-Teller, or Herzberg-Teller coupling.…”

“…This photoelectron-mediated intrachannel mechanism is contrasted with the interchannel vibronic coupling mechanism elsewhere, 18,20,21,31 but the key point is that the energy dependence of the vibrational branching ratios can elucidate the cause of the excitation of nonsymmetric vibrations. It is of interest to probe the excitation of nonsymmetric modes for a more complex polyatomic system than those previously studied, [18][19][20][21][22][23]31 and this is a motivation for the current study.…”