LDPI TOF MS spectra of H₂O+Ar ice at 20 K prior and during the UV irradiation, followed by their quantitative analysis. Panel A: LDPI TOF MS spectra of porous amorphous H₂¹⁶O ice (20 ML) coated with an argon layer (50 ML) and collected at 20 K prior to UV photolysis. For clarity of the figure, the plume profile of Ar⁺ is separated and vertically offset. Panel B: LDPI TOF MS spectra of porous amorphous H₂O + Ar ice after a UV irradiation with a fluence of 3.6 × 10¹⁸ photons cm⁻². In plots shown in panels A and B, labeling of ions is representative for each full plume; however, for clarity of the figure, we kept the labeling minimal. Panel C: integrated plume profiles of LDPI TOF MS for different UV photon fluence increments. Panel D: photodepletion of the H₂O column density as a function of UV fluence from repeated experiments with H₂¹⁶O and H₂¹⁸O. Error bars represent the uncertainty regarding the ice thickness of ±1.5 ML.