 Solution

The pressure is $10^{-8}\mathrm{mbar}=10^{-6}\mathrm{Pa}$. The molecular arrival flux from an ideal gas isBecause the residual-gas composition, temperature and sticking probability are not specified, the answer is an order-of-magnitude monolayer-equivalent exposure time, not a unique coverage prediction. Take $T=300\mathrm{K}$, a nitrogen-like molecular mass $m=28u=4.65\times 10^{-26}\mathrm{kg}$, and unit sticking probability. ThenUse the given $0.5\mathrm{nm}$ molecular-layer scale as an approximate intermolecular spacing, giving $N_s\sim (0.5\mathrm{nm}{)}^{-2}=4\times 10^{18}{\mathrm{m}}^{-2}$. ThusFor $s=1$, this is a few minutes; assuming instead $10^{19}{\mathrm{m}}^{-2}$ sites gives about $350\mathrm{s}$, the same rough scale. A clean-looking vacuum pressure does not imply a contamination-free surface. Desorption and a small sticking probability can greatly lengthen net accumulation, while residual water or hydrocarbons can dominate the chemistry. Full saturation in a site-blocking adsorption model is not exactly the same as one monolayer-equivalent incident fluence.