Theoretical studies of angle-resolved ion yield spectra of core-to-valence transitions of acetylene

Abstract

Recent experimental results on angle-resolved photoion-yield spectroscopy (ARPIS) spectra near the core-to-valence excitation in acetylene show significant anisotropies in the spectral profile measured at 0 degrees and 90 degrees regarding to the polarization direction of x-ray photons. In the present work, a theoretical model is proposed to simulate the fine structure and anisotropy in ARPIS. This employs two-dimensional potential energy surfaces of the ground and core-excited states, as well as transition dipole moments, including symmetric and antisymmetric bending modes to account for Duschinsky effect. The ARPIS is simulated by evaluation of the ion flux, which is found as a projection of the excited state wave packet on a particular direction in the molecular frame. Numerical simulations explain qualitatively the angular dependence of the experimental spectra of the 1s-->1pi(g) ( *) and 1s-->3sigma(u) ( *) transitions. The effects of the lifetime of the core-excited state, the direction of the ion flux, and the transition dipole moment are discussed.