State-resolved thermalization of singlet and mixed singlet-triplet states of CH2

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH2 (a (1)A(1)) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH2 is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH2 population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times. (c) 2006 American Institute of Physics.