LDPI TOF MS signals for a UV-irradiated H₂¹⁶O (upper panel) and H₂¹⁸O (lower panel) ice at 20 K for increasing photon fluence. The lowest trace in each plot shows the signal without UV irradiation. Besides H₂O and its fragmentation pattern, these spectra reveal the presence of the photolysis products, H₂, O₂ and H₂O₂. In addition, protonated cluster ions (H₂O)_nH⁺ are seen to form in the desorption process (see text for details).