Quantum theory of photodetachment spectra of transition states

Photodetachment of ions of the type XHY where X and Y are halogens produces the unstable neutral species XHY at a geometry that is often close to the transition state for the X + HY → XH + Y reaction. The photodetachment spectra generally show a series of bands that can be interpreted as spectra of this transition state. This article discusses recent theoretical work that has attempted to interpret these spectra, with an emphasis on accurate three-dimensional quantum scattering calculations of the photodetachment spectra of ClHCl^- and IHI^-. The quantum scattering calculations use a recently developed hyperspherical coordinate based coupled channel method to calculate scattering wave functions and photodetachment Franck-Condon factors. Semiempirical potential energy surfaces have been used, and this provides a significant source of uncertainty in the theoretical analysis. In spite of this, the theoretical and experimental spectra are in reasonable agreement, with the same number of major peaks at about the same energies for both ClHCl^- and IHI^-. These peaks can arise from either transition-state resonances or direct scattering reactive thresholds. The lowest energy peak in each spectrum is mainly controlled by thresholds, and as a result is quite broad, with structure arising from variation in the threshold energy with rotational state. Higher energy peaks usually have some resonance contributions, and for IHI^- and IDI^-, certain peaks are almost completely determined by resonances. For IHI^-, the theoretical results predict that each major peak is composed of several narrow peaks corresponding to either individual transition-state resonances or individual rotational-state reaction thresholds depending on which major peak is considered. Very recent high-resolution measurements have observed what appears to be both of these possibilities.