Photodissociation dynamics of methylnitrite (CH3O–NO) in the 300–400 nm range: An ab initio quantum mechanical study.

We report the results of a two-dimensional, quantal study of the photodissociation of CH3O–NO within the first continuum (S0→S1, 300–400 nm) taking into account only the O–N and the N=O separations. The S1 potential energy surface is taken from recent ab initio calculations. The calculated absorption spectrum consists of two band progressions of narrow resonance lines with widths of ∼0.3 and ∼5 meV, respectively. These resonances can be associated with excitation of the O–N bond (m=0,1) and excitation of the N=O chromophore (n*=0,1,2,...). The intensities of the m=1 band are negligibly small compared to those of the m=0 band. The decay mechanism in the two cases is different: The m=0 resonances decay primarily via vibrational predissociation, i.e., a nonadiabatic transition from n* to n*-1, and yield NO products with a preferential population of the (n*-1) level. The m=1 resonances decay mainly via tunneling through a potential barrier yielding preferentially NO products in state n*. Several of the theoretical results agree qualitatively (ratio of peak intensities) or even quantitatively (energy spacing between peaks) with the measurements. Most important, however, is the good agreement found for the vibrational NO distributions at several excitation wavelengths of the parent, which reveals that vibrational predissociation within the S1 state is the main decay mechanism. [ABSTRACT FROM AUTHOR]