Proton energy degradation, the fate of the energy lost by protons in slowing down, is analyzed from a microscopic point of view for protons of <tex-math>$0.5-10^{4}$</tex-math> keV in water vapor. The analysis is based on the assignment of cross sections for the excitation of the H2 O molecule by proton and neutral hydrogen impact. Given these cross sections and the efficiencies for electron degradation in water vapor, we calculate the efficiency for population of ionization continua, dissociative continua, and bound excited states of H2 O after complete degradation of the proton. Each efficiency includes the contribution from direct proton excitation, excitation by neutral hydrogen produced in charge exchange reactions, and excitation by secondary electrons. We also calculate the average energy lost per ion pair and obtain a value of approximately 25 eV at high energies.

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