STATE-TO-STATE PHOTOIONIZATION DYNAMICS PROBED BY ZEKE SPECTROSCOPY

Abstract

ZEKE spectroscopy, as first experimentally realized by using pulsed field extraction to detect electrons from a small energy interval within the ionization threshold region, provides an experimental resolution around $0.1cm^{-1}$ (three orders of magnitude better than conventional photoelectron spectroscopy) thus allowing for the spectroscopy of ions down to molecular eigenstates. The ZEKE method has now been applied by a large number of groups worldwide to a variety of molecular systems including the ``activated complex'' and REMPI and VUV excitation schemes are employed. Our group's interest has focused on i) rotationally resolved ZEKE spectra of $NO, NH_{3}, H_{2}S$, and $C_{6}H_{6}$ revealing the ionization dynamics and new symmetry correlations and selection rules, and ii) molecular clusters like phenol-solvent for which the very low frequency intermolecular modes in the cation and their dependence on the vibration selected in the intermediate $S_{1}$ state have been investigated. An example of the resolution of the ZEKE method is demonstrated by the ZEKE spectrum of benzene. The rotational structure of the $^{2}E_{1g}$ electronic ground state of the cation is fully resolved. For ionization from $J^{\prime} = 1$ and $K^{\prime} = 1$ in the intermediate $S_{1} 6^{1}$ state only two rotational progressions in $J^{\prime}$ with $K^{\prime} = 1$ and 5 are observed i.e. the ionization dynamics is described with only two matrix elements. This is the first fully rotationally resolved structure of the benzene cation, the textbook example of a molecule subject to the dynamic Jahn-Teller effect. The rotational structure of the Jahn-Teller states arising from linear and quadratic JT coupling has also been fully resolved for excitation of the $6^{1} (e_{2g}), 6^{2}$ and $16^{1}6^{1}$ vibrations in the cation. The ZEKE spectra of phenol-X (hetero)dimer hydrogen bonded clusters with X = -methanol, -ethanol, -propanol, -dimethylether and -phenol show rich structure (now known as the stegosaur - dimetrodon bands) which is assigned to intermolecular vibrations of the phenol-(hetero)dimer cation.