State-selected photodissociation dynamics: Complete characterization of the OH fragment ejected by the HONO Ã state

Abstract

T r a n s-HONO is optically prepared in specific −N=O stretching vibrational levels (2n, n=1,2,3) of the à state at 369, 355, and 342 nm. The ejected OH fragment is completely characterized by Doppler and polarization laser excitation spectroscopy. In this manner the OH translational energy, angular distribution, rotational alignment, and internal state distribution (vibration, rotation, spin-orbit and Λ-doubling components) are probed through the OH A–X system. The OH fragment is found to be translationally hot (∼0.5 eV) with a nearly sin2 θ angular distribution about the electric vector of the photolysis laser. The corresponding line shapes are Doppler split. However, the line shapes and widths do not noticeably depend on either fragment rotation or parent vibration. The internal motion of the OH fragment is vibrationally and rotationally cold; the spin-orbit components and the Λ doublets are not in equilibrium. The OH fragment is aligned and its π lobe lies preferentially in the plane of rotation. With increasing rotational excitation, these effects become more pronounced. This information allows us to construct a detailed photodissociation mechanism. The fragmentation is prompt and the trajectories of the recoiling fragments lie close to the initial HONO plane. The impulse associated with the central O–N bond fission contributes predominantly to OH translation while the rotation appears to arise from the zero-point motion of the parent in-plane bending and torsional vibrations. The OH energy content is found to be quite insensitive to the parent ν2 vibration, suggesting that the à state surface is rather ‘‘flat’’ along the −N=O stretch compared to the steep fragmentation coordinate.