Spectrum (a): LES and RIS mass spectra obtained from a neat H2O ASW film (20 BL thickness) grown on Ru(0001). Spectrum (b): MA (0.2 ML) was adsorbed onto the ASW film surface. Spectrum (c): MA (0.15 ML) was adsorbed onto an ASW film (20 BL thickness) after UV irradiation of the film for an exposure of ∼2 × 1016 photons cm−2. The appearance of the MAH+ peak in this spectrum indicates proton transfer from H3O+ in the ice to MA adsorbates. The sample temperature was maintained at ∼53 K during UV irradiation and spectral data acquisition.
RIS signals of water isotopomers recorded from a H2O–D2O mixture ice surface (a) before and (b) after UV photolysis of the sample. In spectrum (b), the sample was exposed to UV light for 30 min at 90 K for a total exposure of ∼3.8 × 1016 photons cm−2. RIS measurements were made below 80 K. A thermal H/D exchange reaction at the surface was prohibited at these temperatures. Mass spectra were recorded in the centroid mode.
MAH+ signal intensity (displayed using a normalized scale) measured as a function of delay time (t delay) from the termination of UV irradiation. The MAH+ intensity indicates the quantity of hydroniums present in the ice film, as detected by MA titration experiments. The experimental conditions for MA adsorption (0.2 ML) and UV photolysis (∼2 × 1016 photons cm−2) were identical to those in the procedure used for Fig. 1. The ice film thickness was ∼15 BL for ASW samples at ∼53, 80, and 125 K and ∼150 BL for the CI sample at 140 K (see text).
Variation in surface population of HDO as a function of UV exposure time at different ice film temperatures [54 K (■), 80 K (○), 90 K (▲), and 100 K (▽)]. HDO is formed by the H/D exchange reaction on the H2O/D2O mixture ice film. The non-zero population of HDO at t = 0 is due to the adsorption of residual HDO gas during sample preparation (see text). A separate ice film was prepared for the kinetic measurement at each temperature. The dotted lines are used as a visual guide.
Kinetic plots for the change in HDO surface coverage with UV irradiation time. The lines represent the least square fit of Eq. (3) to experimental data.
Apparent quantum yield for the H/D exchange reaction (), a corrected quantum yield (Φ H/D), and the average length of proton migration (N hop) at various temperatures.
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