Imaging the Photodissociation Dynamics of Nitrous Acid (HONO): The Role of Torsion

We present the findings of the first imaging study of trans-HONO and cis-HONO photodissociation through the 202 band and 201 band of the A1A″–X1A′ transition. The NO photofragment was probed by (1 + 1) resonance-enhanced multiphoton ionization and studied using the direct-current slice imaging technique with our finite slice reconstruction method. The NO state-specific translational energy distributions show some rotational structure corresponding to the internal state distribution in the OH cofragment. All images showed a perpendicular angular distribution with a recoil anisotropy parameter from ca. −0.6 to −0.8. In both bands, cis-HONO showed greater anisotropy than trans-HONO. Deviation from the limiting value expected for a pure perpendicular dissociation is ascribed to deviation of the transition moment from normal to the heavy atom plane owing to OH torsion, rather than lifetime effects assumed in the large body of previous work.