Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate

Photoinitiated unimolecular decomposition rate constants of rotationally excited NO2 molecules have been measured near dissociation threshold (D0) by employing a double resonance technique. Rotational selectivity has been achieved by using narrow-linewidth ~0.015 cm! infrared excitation to prepare specific rotational levels (N851,3,...,15, Ka850) of the ~1,0,1! vibrational level. The picosecond-resolution pump–probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appearance of the NO product. Data have been obtained for two progressions of average excess energies, ^E&2D0 : ~i! 10 cm 1E101 rot and ~ii! 75 cm1E101 rot , where ^E& denotes an average over the pump laser linewidth and E101 rot is the rotational energy of the ~1,0,1! X̃ A1 intermediate vibrational level. The measured rate constants do not display any noticeable dependence on N8, which is a reflection of significant rovibronic interaction. Spin– rotation interaction, which has been implicated as the main source of rovibronic coupling for small values of N8, is not likely to yield such a result. A model is proposed to describe the influence of rotation on the dissociation rate. The experimental data are consistent with a Coriolis coupling mechanism causing transitions to occur between Ka levels. © 1999 American Institute of Physics. @S0021-9606~99!01744-4#