Photoinitiated unimolecular decomposition of NO2: Rotational dependence ofthe dissociation rate

Photoinitiated unimolecular decomposition rate constants of rotationally ex cited NO2 molecules have been measured near dissociation threshold (D-0) by employing a double resonance technique. Rotational selectivity has been ac hieved by using narrow-linewidth (0.015 cm(-1)) infrared excitation to prep are specific rotational levels (N'=1,3,...,15, K-a'=0) of the (1,0,1) vibra tional level. The picosecond-resolution pump-probe technique has then been used to photodissociate the molecules thus tagged and to monitor the appear ance of the NO product. Data have been obtained for two progressions of ave rage excess energies, < E >-D-0: (i) 10 cm(-1)+E-101(rot) and (ii) 75 cm(-1 )+E-101(rot), where < E > denotes an average over the pump laser linewidth and E-101(rot) is the rotational energy of the (1,0,1) (X) over tilde (2)A( 1) intermediate vibrational level. The measured rate constants do not displ ay any noticeable dependence on N', which is a reflection of significant ro vibronic interaction. Spin-rotation interaction, which has been implicated as the main source of rovibronic coupling for small values of N', is not li kely 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 K-a levels. (C) 1999 American Institute of Physics. [S0021-9606(99)01744-4].