Influence of intramolecular vibrational dynamics on state-to-state photodissociation: <i>T</i> <i>r</i> <i>a</i> <i>n</i> <i>s</i> DONO (<i>A</i>) versus HONO (<i>A</i>)

Abstract

The state-to state fragmentation of the à state of trans DONO and HONO are compared. All the optically accessible vibrations (00, 2n) of the excited state are photoselected with a polarized tunable laser, and the dissociation dynamics is probed through measurements on the ejected OD and OH photofragments. DONO’s N=O stretching (ν2) vibration leaves a significant fingerprint on the OD fragment’s rotational energy distribution, and rotational and orbital anisotropy. On the other hand, the effect of ν2 on OH photoejected by HONO is very small. These differences in the photochemistry of the isotopic cousins are attributed to the coupling in DONO between the –N=O coordinate and the DON bending motion ν3, leading to ‘‘V→V transfer.’’ The in-plane ν3 oscillation transforms predominantly into the high-energy wing of the OD fragment’s rotational distribution, so that increasing the number of ν2 quanta in the parent leads to higher product rotational temperature and anisotropy. The ν2 motion in HONO, by contrast, is not coupled to the ν3 coordinate and, consequently, increasing the ν2 content in this case does not result in profound changes in fragment properties.