Dynamic Interplay between the Mode-Randomization and Autodetachment of the Dipole-Bound States of the Anion.

State-specific dynamics of the dipole-bound state (DBS) of the cryogenically cooled deprotonated 4,4'-biphenol anion have been investigated by picosecond time-resolved pump-probe spectroscopy. For DBS vibrational states below the electron-detachment threshold, the relaxation rate is slow to give a lifetime (τ) longer than ∼5 ns, and it is attributed to the nonvalence-to-valence orbital transformation. For the DBS resonances above the detachment threshold, however, the lifetime decreases with the activation of autodetachment, whereas the otherwise zeroth DBS modes seem to be randomized by intramolecular vibrational energy redistribution (IVR), as manifested in the biexponential transients. As the DBS internal energy increases further, the lifetime shows a monotonic decrease to give τ ∼ 50 ps at E ' vib ∼ 1700 cm -1 . This study demonstrates that IVR may play an important role in the autodetachment dynamics when the density of states rapidly increases with increasing vibrational energy, giving important implications for the electron-transfer dynamics taking place in large biological or astrochemical systems.