266 nm CH3I photodissociation: CH3 spectra and population distributions by coherent Raman spectroscopy

Abstract

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 CH3I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (∼3 to ∼6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the ν2 out-of-plane bending coordinate [v2=1/v2=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 ortho-para 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 ∼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 x12 anharmonicity constant.