Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90–140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of $2kBTtrans=0.39±0.04 eV$ $(2300±200 K)$ and 0.02 eV $(120±20 K),$ respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, $(H2O)6+1,$ has been theoretically calculated, where $(H2O)6+1$ is a $(H2O)6$ cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190–230 nm in the region between 80 and 85 km altitude.