Multiphoton Spectra and States of Halogens

Abstract

Although the diatomic halogens are endowed with a wide range of intravalence-shell electronic states, the number and variety of these states is far from obvious in one-photon absorption from the ground state where only a few regions of structured absorption are present at visible and UV wavelengths. Selection rules on angular momentum and parity are key players in enforcing this sparse structure, but Franck- Condon effects also exercise considerable influence by directing transitions into regions where the transition dipole is small o r into a continuum where much of the detail is lost: further, a simultaneous excitation of two o r more electrons is formally forbidden in single-photon transitions. Some of the obstacles to one-photon absorption are avoided in the sequential absorption of two o r three photons using the conveniently placed, low-lying excited sQtes in the visible region for the initial step, in effect broadening the perspective from the ground state to include that from the lower excited states. This article summarizes progress realized by classical and nonclassical (multiphoton) methods of spectroscopy which, over the last 10–12 years, has led to the identification of many new states of halogens and materially improved the definition of several known ones. The advances are timely in that the same period has witnessed a renewal of interest in halogen spectra, largely because of their use in rare-gas halide lasers but also because the homonuclear halogens and certain interhalogens have potential value as laser systems in their own right [1, 2].