Dependence on accelerating voltage of crystal structural changes in water ice thin film under electron beam irradiation

The dependence on accelerating voltage of crystal structural changes in water ice thin film under electron beam irradiation was investigated by cryogenic transmission electron microscopy (cryo-TEM) at 95 K and accelerating voltages of 25, 50, 75, 100, or 125 kV. Ice thin film was prepared by depositing residual moisture in the TEM column onto both sides of a carbon membrane at 95 K. The major phase of the deposited film at 95 K was identified as ice Ic by transmission electron diffractometry. We found that the mass loss rate of the ice thin film decreased sharply as the accelerating voltage was increased. From this result, we conclude the mass loss mechanism was the ionization of water by inelastic scattering of incident electrons. Moreover, the phase transition from ice Ic to ice Ih was observed at accelerating voltages of 75 kV or greater. At 50 kV or lower, however, the phase transition was hardly observed by TEM. Because the phase transition can also be attributed to inelastic scattering of incident electrons, the results suggest that whether mass loss or a phase transition occurs depends primarily on the accelerating voltage.