Triply differential photoelectron studies of non‐Franck–Condon behavior in the photoionization of acetylene

Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19-40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C2H2+ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4π-collection geometry of the velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

High-resolution photoelectron spectroscopy with angular selectivity - a tool to probe valence-Rydberg states and couplings in HCl(+)

Due to strong electron correlation effects and electron coupling with nuclear motion, the molecular inner-valence photoionization is still a challenge in electron spectroscopy, resulting in several interesting phenomena such as drastic changes of angular dependencies, spin-orbit induced predissociation, and complex interplay between adiabatic and nonadiabatic transitions. We investigated the excited electronic states of HCl(+) in the binding energy range 27.5-30.5 eV using synchrotron radiation based high-resolution inner-valence photoelectron spectroscopy with angular resolution and interpreted the observations with the help of ab initio calculations. Overlapping electronic states in this region were disentangled through the analysis of photoelectron emission anisotropies. For instance, a puzzling transition, which does not seem to obey either an adiabatic or a nonadiabatic picture, has been identified at ∼28.6 eV binding energy. By this study, we show that ultrahigh-resolution photoelectron spectroscopy with angular selectivity represents a powerful tool to probe the highly excited ionic molecular electronic states and their intricate couplings.