Excited-state reaction dynamics of the radical anions revealed by time-resolved photofragment depletion spectroscopy.

The excited-state reaction dynamics of radical anions are investigated using a newly developed technique: time-resolved photofragment depletion spectroscopy. This method leverages differences in photodetachment cross-sections among transient anionic species involved in the reaction pathway. It offers a distinct advantage for studying radical anions, which are typically challenging to probe using conventional spectroscopic techniques due to their low electron affinities. As a benchmark, the method is first applied to I₂⁻, whose excited-state behavior is well characterized. The technique is then extended to CH₃NO₂⁻ and (CH₃NO₂)₂⁻, enabling real-time probing of the excited-state dynamics of their nonvalence-bound states. Our findings reveal that ultrafast internal conversion from a nonvalence orbital to a valence orbital is followed either by prompt chemical bond dissociation or by comparatively slower cluster decomposition. These results highlight the dynamic role of the nonvalence orbital in driving chemical reactivity.