The UV-photodissociation of jet-cooled nitrosobenzene studied by fluorescence excitation spectroscopy of the NO fragment

Monomeric nitrosobenzene was cooled in a supersonic jet and photolyzed with a tunable laser by excitation into the first and second UV absorption bands corresponding to the second and higher excited singlet states (S-n, n greater than or equal to 2). The populations of the rotational and vibrational states of the NO fragment were probed by fluorescence excitation spectroscopy with a second laser. No alignment of the photofragment could be observed, and the Doppler profiles indicate an isotropic velocity distribution. It is concluded that the dissociation occurs after fast internal conversion to the potential energy surfaces of the electronic states S-0 or S-1 which correlate both with the electronic ground states of the products. A strong preference for the A" component of the Lambda -doublets is observed. If orbital symmetry is conserved, the plane of rotation of the NO fragment is perpendicular to the plane of the phenyl radical, i.e. dissociation occurs from a highly excited torsional state in a propeller-like motion.