Vector correlations from doppler–broadened lineshapes. State-selected dissociation of methyl nitrite

Abstract

The theoretical framework needed to analyse sub-Doppler lineshapes obtained by 2 + 1 LIF spectroscopy is outlined and applied to data for the state-selected photodissociation of cis-methyl nitrite (CH₃ONO). Excitation into the second overtone of the terminal NO stretch (ν′= 2) produces NO fragments with zero, one or two vibrational quanta and a highly correlated set of vector properties. These vector properties can be quantified in terms of five bipolar moments, which include the rotational alignment, the translational anisotropy parameter, the second Legendre moment of the (v, j) correlation and tow moments which characterise the three-vector (µ,ν,j) correlation [β²₀(22) and β²₀(24)]. In principle, these bipolar moments can be used to separate the dissociation dynamics into the influence of a dissociation lifetime and a dissociation geometry. The experimental evidence for the photolysis of methyl nitrite implies an essentially planar dissociation geometry leading to a strong perpendicular (ν, j) correlation [β⁰₀(22)=–0.34 ± 0.05]. The slight deviation from complete planarity is probably caused by torsional forces. The dissociation lifetime derived from the value of the translational anisotropy parameter appears to be a function of the fragment vibrational level. A simple model which explains many of the observations is presented and discussed in the light of preliminary classical trajectory calculations.