State-to-state rotational energy transfer and reaction with ketene of highly vibrationally excited b̃ 1B1 CH2 by time-resolved Fourier transform emission spectroscopy

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Abstract
Dispersed fluorescence spectra from the CH2 b̃ 1B1→ã 1A1 band were recorded with time-resolution by Fourier transform emission spectroscopy after pulsed excitation of a single rotational level of the b̃ 1B1 (0,160,0) state. Fluorescence observed from the initially excited level and from levels populated by rotational energy changing collisions with the bath gas (ketene) was used to deduce the state-to-state rate constants for rotational energy transfer and the state-resolved rate constants for total collisional removal of b̃ 1B1 CH2. The observed propensity rules for rotational energy transfer—ΔJ=±2, ΔKa=0, and ΔKc=±2—are consistent with a quadrupole–dipole interaction between b̃ 1B1 (0,160,0) CH2 and ketene. The existence of a quadrupole in the intermolecular interaction suggests that the structure of CH2 in the b̃ 1B1 (0,160,0) state, averaged over the time of a collision, must be linear. The state-to-state rotational energy transfer rate constants range from approximately equal to the hard sphere gas kinetic rate to four times the gas kinetic rate, with the largest rate constants between rotational levels with the smallest energy gaps. Examination of fluorescence spectra recorded with polarization analysis shows that rotationally elastic (ΔJ=0)M changing collisions are negligible. State-resolved rate constants for reactive collisions between b̃ 1B1 CH2 and ketene were obtained by subtracting the rotational energy transfer contribution from the total rate constants for collisional removal of b̃ 1B1 CH2 (obtained from a Stern–Volmer analysis). These rate constants vary from one to five times the hard sphere gas kinetic rate, and increase with rotational energy for the levels studied. Their magnitudes show that CH2 is about two times as reactive in its b̃ 1B1 state than its ã 1A1 state.