Measurements of D2 yields from amorphous D2O ice by ultraviolet irradiation at 12 K

We have carried out experiments on the formation of D-2 molecules from amorphous D2O ice (thickness: 40, 120 Angstrom) by UV irradiation (lambda: 126, 172 Ka) at 12 K. Our results show that most of the D-2 products remain in and on the amorphous ice at 12 K; directly photodesorbed D-2 was little observed at 12 K. The amount of D2O destruction and D-2 yields after a dose of 10(18) photons cm(-2) at 12 K were about 10%-20% and 1%-2% of the initial number of D2O molecules, respectively. The formation reaction of D-2 was found to be a single-photon process via the first excited state of D2O. The reconstruction of D2O at 12 K (reverse reaction) was not observed within our experimental timescale (5 hr). We estimated the cross sections for the D2O destruction and the D-2 formation reactions by the photon impact at 12 K for the first time.