15min:

YANGSOO KIM, ANATOLY V. KOMISSAROV, GREGORY E. HALL AND TREVOR J. SEARS, Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000.

We have studied rotational energy transfer by optical-optical double resonance methods in order to understand the close connection between rotational energy transfer and the mixed-state mechanism of intersystem crossing. Singlet CH₂ is produced by 308 nm photolysis of CH₂CO. After thermalization of the rotational and translational distribution, a bleaching tunable dye laser pulse depletes the population of a selected rotational level of singlet CH₂ while the population of the same (in saturation recovery mode) or a different (in saturation transfer mode) singlet CH₂ rotational level is monitored with FM absorption spectroscopy on a different vibrational band of the b-a system. The bleaching laser is blocked and unblocked to record a reference and a bleached transient waveform. The difference of the two signals as a fraction of the reference signal is the saturation recovery signal, which is found to follow a single exponential decay to a final value closed to half of the initial thermal Boltzmann fraction of the population in the bleached level. The rate coefficients for saturation recovery by collision with CH₂CO and He are 1.6 × 10^-8 and 2.1 × 10^-9 cm³ molec^-1 s^-1, respectively. The saturation transfer kinetics is multi-exponential and depends on which pair of rotational states is saturated and probed.