The impact of vibrationally excited levels of the transition state on CO (v=0,J ) distributions resulting from dissociation of H2CO

Abstract

The dissociation of formaldehyde into molecular products is known to proceed through vibrationally excited, quantum ergodic levels of the ground electronic state (H2CO*) and then a transition state (H2CO‡) according to the following mechanism: H2CO(S0)+hν→H2CO(S1)→H2CO*(S0) →H2CO‡→H2+CO. This paper reports the distribution of rotational energy in the CO (v=0) fragment following excitation to single rotational levels of the S1 2141 band of formaldehyde. The maxima of the rotational distributions resulting from photolysis on different rotational lines are observed to vary between J=38 and 49. Variations in the widths of the distributions are also observed. These variations show no systematic trends in the quantum indices or radiative lifetime of the parent molecule. Distributions calculated using an infinite-order-sudden approximation-based algorithm show fluctuations similar to those observed experimentally when each of the vibrational levels of the transition state through which the molecule can decay is randomly weighted. Thus it is understood that the ergodic character of each H2CO* S0 eigenstate uniquely projects onto the vibrational levels of the transition state, and since different levels of the transition state give rise to differing product distributions, decay through different S0 levels produces differing rotational distributions.