A multiphoton ionization study of the photodissociation dynamics of the S₂ state of CH₃ONO

Two-color (1 + 1) REMPI (resonantly enhanced multiphoton ionization) photoelectron spectroscopy is used to probe the NO photofragments produced by the UV photodissociation of methyl nitrite, i.e., CH₃ONO + hv→CH₃ONO*(S₂)→CH₃O·(X) + NO(X, ν, J). The photofragments are produced in their ground electronic states but with high rotational and translational energy. NO fragment angular distributions, rotational state distributions, and spatial alignment are determined by photoion and photoelectron detection. The initial state alignment is obtained by the CDAD (circularly dichroic angular distribution) technique for the first time. CDAD measurements for rotational levels with 35.5≤J≤46.5 result in alignment parameters at the classical high-J limit of script A sign₀⁽²⁾ = -0.4. This alignment is consistent with an "impulsive" dissociation mechanism in which photofragment recoil along the CH₃O-NO bond imparts substantial rotational angular momentum to the NO molecule resulting in a high-J state distribution and preferential rotation in the plane of dissociation. These measurements clearly establish the utility of the CDAD method for probing chemical processes in which spatial alignment plays an important role. Photoion angular distributions are used to probe correlations between the CH₃ONO transition dipole moment, NO fragment velocity, and angular momentum. These correlations reveal additional details of the photolysis mechanism.