Figure 3

Simulations of incident alpha radiation on thin water layers. Left: a demonstration of the inhomogeneous reaction system reaching equilibrium. The system models incident \(\alpha\) radiation with a dose rate of \(10^{-2}\) Gy ns\(^{-1}\) uniformly in a thin film of water of thickness \(L = 0.5\) nm, with the dynamics tracked at \(x = 0.5\)nm (water gas boundary) (see Supplementary Fig. S1 online for sample simulations at varying time steps). Right: results from 160 simulations of incident alpha radiation, with various dose rates, in a thin film of water of different thicknesses ranging from typical dose rates up by over 12 orders of magnitude to illustrate the wide parameter space available. The plotted quantity is the concentration of \(\hbox {H}_2\) at the water-gas boundary at equilibrium, as a function of the thickness of water layer for various dose rates. The black crosses show the data points gathered from the simulations, and the gradient triangle in the bottom right verifies that the gradients of the plots show a quadratic dependence between yield of \(\hbox {H}_2\) at equilibrium and water thickness \(L \in\) [0.25 nm, 5 nm]. G-values⁵³ were converted to reaction rate coefficients for the zeroth order reactions used to simulate the constant input of chemical species as a result of irradiation in the physical stage (see Supplementary Table S1 in Supplementary Data online).