Physical-state effect in the stopping cross section ofH2O ice and vapor for 0.3-2.0-MeVαparticles

The stopping cross sections of ${\mathrm{H}}_2$O ice and vapor for $\alpha$ particles of energy 0.3-2.0 MeV have been measured, respectively, by a thick-target elastic-scattering method and with a differentially pumped gas-cell system. The stopping cross section of ${\mathrm{H}}_2$O vapor (probable error ≃ 1.1%) was found to be (4-12)% higher than that of ${\mathrm{H}}_2$O ice (probable error ≃ 4%). This difference is less than that found previously for protons [(10-14)%] in the same velocity interval. It is proposed that the physical-state effect in the stopping cross section of ${\mathrm{H}}_2$O is caused by changes in the modes of electronic excitation in the molecule caused by the state of aggregation.