Velocity-map ion-imaging of the NO fragment from the UV-photodissociation of nitrosobenzene

The velocity and angular distribution of NO fragments produced by UV photodissociation of nitrosobenzene have been determined by velocity-map ion-imaging. Excitation of the S2-state by irradiation into the peak of the first UV absorption band at 290.5 nm leads to a completely isotropic velocity distribution with Gaussian shape. The average kinetic energy in both fragments correlates with the rotational energy of the NO fragment and increases from 6% of the excess energy for j = 6.5 to 11% for j = 29.5. A similar isotropic distribution albeit with larger average velocity is observed when the ionization laser at 226 nm is also used for photodissociation, corresponding to excitation into a higher electronic state Sn (n ≥ 3) of nitrosobenzene. It is concluded that photodissociation occurs on a timescale much slower than rotation of the parent molecule, and after redistribution of the excess energy into the vibrational degrees of freedom.