Measurements of D₂Yields from Amorphous D₂O Ice by Ultraviolet Irradiation at 12 K

We have carried out experiments on the formation of D 2 molecules from amorphous D 2 O ice (thickness: 40, 120 Å) by UV irradiation (λ: 126, 172 nm) 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 D 2 O 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 D 2 O molecules, respectively. The formation reaction of D 2 was found to be a single-photon process via the first excited state of D 2 O. The reconstruction of D 2 O at 12 K (reverse reaction) was not observed within our experimental timescale (5 hr). We estimated the cross sections for the D 2 O destruction and the D 2 formation reactions by the photon impact at 12 K for the first time.