Photodissociation of state-selected hydrogen iodide molecules following excitation at vacuum ultraviolet wavelengths: a tuneable source of high velocity H atoms.

The vacuum ultraviolet absorption spectrum of gas phase hydrogen iodide molecules displays numerous sharp features attributable to excitations to (often perturbed) predissociating levels of Rydberg states belonging to series converging to the lowest ionisation limit(s). The present work explores and discusses the fate of photoexcited HI molecules formed when exciting on more than 140 such parent resonances in the wavelength range of 126-141 nm, via measurements of the branching into dissociation channels yielding an H atom together with ground (I) and spin-orbit excited (I*) products, and the recoil anisotropies of these respective products. It also serves to highlight the very high, and very tightly defined, velocities (and kinetic energies) of the H atom products, which can attain v ∼ 36 km s-1 (E ∼ 647 ± 3 kJ mol-1 (6.70 ± 0.03 eV)), with a measured velocity spread Δv/v ∼ 0.25%, which can be expected to find application in future dynamical studies of atom-molecule collisions and atom-surface scattering experiments.