Abstract
The vibrational distribution of CH3 radicals produced by the 248 nm photodissociation of CH3I has been investigated by infrared diode laser kinetic spectroscopy. The ν2 bands of CH3, v2=1←0 up to 4←3, were observed as functions of time for the sample pressure ranging from 9 to 100 mTorr. The observed Doppler line shape showed a clear correlation between the transition dipole moment and the recoil velocity (μ−v) in the photodissociation, and the observed linewidth indicated that CH3 fragments were predominantly generated through the I(2P1/2) adiabatic dissociation channel. The relative population in each vibrational level was determined to be 0.66:0.26:0.08:0.004 for v=0, 1, 2, and 3, in sharp contrast to the inverted vibrational distribution reported previously. The present result suggests that the geometry of the CH3 moiety in the precursor CH3I is relaxed gradually to its equilibrium planar structure before the cleavage of C–I bond takes place.
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