Excitation of the symmetry forbidden bending mode in molecular photoionization

Abstract

We present results on the energy dependence of the vibrational branching ratio for the bending mode in CO 2 3 u Ϫ1 photoionization. Specifically, we determine the v ϩ ϭ(0,1,0)/v ϩ ϭ(0,0,0) intensity ratio by detecting dispersed fluorescence from the electronically excited photoions. The results exhibit large deviations over a very wide energy range, 18Ͻh exc Ͻ190 eV. Production of the v ϩ ϭ(0,1,0) level of the ion from the v 0 ϭ(0,0,0) ground state is forbidden by symmetry, and while observations of such features are well established in photoelectron spectroscopy, their appearance is normally ascribed to vibronic coupling in the ionic hole state. In this case, we find that such explanations fail to account for the energy dependence of the branching ratio. These deviations indicate that the continuum photoelectron participates in transferring oscillator strength to the nominally forbidden vibrational transition. A theoretical framework is developed for interpreting the experimental data, and Schwinger variational calculations are performed. These calculations demonstrate that the continuum electron is responsible for the observation of the excited bending mode as well as its energy dependence. This is an intrachannel effect that is best described as photoelectron-induced vibronic symmetry breaking. This appears to be a general phenomenon, and it may be useful in illuminating connections between bond angle and photoionization spectroscopies. The magnitude of these deviations display the utility of vibrationally resolved studies, and the extent over which these changes occur underscores the necessity of broad range studies to elucidate slowly varying characteristics in photoionization continua.

Observation of the Symmetry-Forbidden5σu→kσuCS2Transition: A Vibrationally Driven Photoionization Resona

“…Specifically, we measure the response of the resonance to changes in alternative stretching vibrations, and the results display a striking mode-specific sensitivity to the antisymmetric stretching motion, which is a symmetry-forbidden excitation within the Franck-Condon framework. In recent CO 2 (C 2 g ) photoelectron studies, it has been shown that symmetry-forbidden vibrations can be induced by resonant variations of dipole amplitudes with changes in molecular geometry [8,9]. However, the current result is different from those previous investigations (indeed, as well as from all previous studies) in two significant ways.…”

“…If one does not invoke the Franck-Condon approximation, then there is a possibility of single quantum excitations of nonsymmetric modes. In our CO 2 studies, we have found that resonantly amplified vibronic symmetry breaking arises naturally from the dependence of the photoionization scattering amplitudes on changes of the molecular geometry that occur during zero-point vibrational motions [8,9]. The same methods are well suited for calculating the effects observed here.…”

“…The theoretical methods used in the calculations are discussed elsewhere [8]. Briefly, the target is treated using the configuration interaction method, and the final state is a single channel scattering function with a correlated target obtained using the Schwinger variational method [8].…”

“…Briefly, the target is treated using the configuration interaction method, and the final state is a single channel scattering function with a correlated target obtained using the Schwinger variational method [8]. Vibrational motion is incorporated into the calculations using the Chase adiabatic approximation, so the variation of the dipole amplitudes with changes in molecular geometry are included explicitly [13].…”

Rotationally resolved photoionization: Influence of the 4σ→kσ shape resonance on CO+(B 2Σ+) rotational distributions

“…The experimental method has been described elsewhere 34,36,62 and only details pertinent to the present study are discussed here. A schematic of the apparatus is shown in Fig.…”