Photodissociation dynamics of methyl nitrate at 193 nm: energy disposal in methoxy and nitrogen dioxide products

Abstract

The photodissociation dynamics of methyl nitrate, CH₃ONO₂, has been investigated at 193 nm by examining the products from the primary dissociation channel, namely CH₃O and NO₂. The CH₃O ( ²E) photoproducts were probed by laser-induced fluorescence (LIF) on the à²A₁– ²E transition under both nascent and jet-cooled conditions. The 3¹₀ and 3¹₁ bands originating from the vibrationless and C–O stretch (ν₃) levels, respectively, were characterized to obtain the internal energy distribution of the CH₃O products. Only a small fraction of the CH₃O products (≤10%) were produced with one quantum of C–O stretch excitation as determined from the relative intensities of the bands in combination with transition probabilities derived from dispersed fluorescence measurements and/or calculated Franck–Condon factors. The CH₃O products also had minimal rotational excitation: those produced in the ground vibrational state had a rotational temperature of 238 ± 7 K, corresponding to less than 1% of the available energy. Products with C–O stretch excitation were found to have a higher rotational temperature, but still a small fraction of the total energy. Combining the CH₃O internal energy findings with previous photofragment translational energy measurements [X. Yang, P. Felder and J. R. Huber, J. Phys. Chem., 1993, 97, 10903] indicates that most of the available energy is deposited in the NO₂ fragment. This is verified through dispersed fluorescence measurements which show that the NO₂ fragment is produced electronically excited with internal energies extending to the NO + O dissociation limit. Ab initio calculations confirm that the dominant initial excitation is strongly localized on the NO₂ moiety. The calculations are also used to reveal the forces that give rise to internal excitation of the CH₃O fragment upon electronic excitation.