Vibrational Autodetachment and Valence-to-Nonvalence Population Transfer in the Nitromethane Anion.

Vibrational Feshbach resonances (VFRs) constitute fundamental doorway states governing electron attachment and vibrational autodetachment in molecular anions. However, their real-time dynamics has remained largely unexplored for valence-bound radical anions because the exceptionally small electron affinities of these species make direct time-resolved measurements experimentally challenging. Here we investigate the nitromethane anion (CH3NO2-), a prototypical valence-bound radical anion with an electron affinity of only ∼0.17 eV, using picosecond mid-infrared pump-probe photoelectron spectroscopy. Selective excitation of the symmetric CH3 stretching mode reveals a vibrational autodetachment lifetime of approximately 10 ps, determined directly using a depletion-based time-resolved photoelectron detection scheme. This time scale is orders of magnitude longer than expected for prompt vibrational electron emission, demonstrating that autodetachment is limited by intramolecular vibrational redistribution (IVR), which redistributes the initially localized vibrational energy before the system gains access to the electron-detachment continuum. Picosecond IR photoelectron spectra further reveal transient population transfer from the valence-bound anion to a dipole-bound state (DBS), providing direct evidence for reverse internal conversion induced by mode-selective vibrational excitation. These findings establish that VFRs in valence-bound radical anions are not merely precursors to electron emission but constitute dynamical gateways that partition excess-electron relaxation between autodetachment and reverse internal conversion. This mechanistic picture provides a unified framework for understanding electron-driven chemistry in weakly bound molecular anions.