Photodissociation dynamics of m- and p-cresol in the S1 state: Interplay between the mode-randomization and H atom tunneling reaction.

The H atom tunneling dissociation dynamics of the S₁ state of meta- or para-cresol has been investigated by using the picosecond time-resolved pump-probe spectroscopy in a state-specific manner. The S₁ state lifetime (mainly due to the H atom tunneling reaction) is found to be mode-dependent whereas it quickly converges and remains constant as the rapid intramolecular vibrational energy redistribution (IVR) starts to participate in the S₁ state relaxation with the increase of the S₁ internal energy (E_int). The IVR rate and its change with increasing E_int have been reflected in the parent ion transients taken by tuning the total energy (hν_pump + hν_probe) just above the adiabatic ionization threshold (so that the dissipation of the initial mode-character could be monitored as a function of the reaction time), indicating that the mode randomization rate into the S₁ isoenergetic manifolds exceeds the tunneling rate quite early in terms of E_int for m-cresol (≤∼1200 cm⁻¹) or p-cresol (≤∼800 cm⁻¹) compared to the case of phenol (≤∼1800 cm⁻¹). Though the H atom tunneling dynamics of phenol (S₁) seems to be little influenced by the methyl substitution on the either m- or p-position, the IVR rate has been found to be strongly accelerated due to the sharply-increasing (S₁) density of states with increasing E_int due to the pivotal role of the low-frequency CH₃ torsional mode. [ABSTRACT FROM AUTHOR]