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 (ã ¹A₁) 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. [ABSTRACT FROM AUTHOR]