266-NM CH3I PHOTODISSOCIATION - CH3 SPECTRA AND POPULATION-DISTRIBUTIONS BY COHERENT RAMAN-SPECTROSCOPY

High resolution coherent anti-Stokes Raman spectroscopy has been used to study the symmetric CH stretching mode of methyl radical formed by UV laser photolysis of CH I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (similar to 3 to similar to 6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the v(2) out-of-plane bending coordinate [v(2)=1/v(2)=0, population ratio 0.27(10)]. This is in accord with recent theoretical calculations favoring slow, adiabatic CH3 relaxation from a pyramidal to planar structure as the C-I bond breaks. Extensive N,K rotational structure is resolved and the distributions obtained lend support to those deduced for nascent CH3 by Chandler and co-workers from modeling of unresolved resonance enhanced multiphoton ionization spectra. The results are consistent with conservation during dissociation of CH3 orthopara nuclear spin forms and of K spinning angular momentum about the symmetry axis. CH3 product tumbling motion is slightly greater (1-2 units of angular momentum) than predicted theoretically. Rapid collisional excitation of higher rotational levels is seen and the rich spectrum observed after similar to 50 collisions has been analyzed to give improved or new vibrational-rotational parameters for 1000 and 1100 states. The 1100 band origin is determined to be 2996.21(4) cm(-1), yielding -8.23(5) cm(-1) for the x(12) anharmonicity constant.