Knowledge Bank

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 $NO,NH3,H2S$, and $C6H6$ 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 $S1$ 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}E1g$ electronic ground state of the cation is fully resolved. For ionization from $J\prime =1$ and $K\prime =1$ in the intermediate $S161$ 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 $61(e2g),62$ and $16161$ 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.