Ion-induced electrostatic charging of ice at 15–160 K

We studied electrostatic charging of ice films induced by the impact of 1−200-keV Ar+ ions and their subsequent discharging postirradiation. We derived the positive surface electrostatic potential from the kinetic energy of sputtered molecular ions and with a Kelvin probe. Measurements were performed as a function of film thickness, temperature, and ion energy. Charging requires that the projectile ions are stopped in the ice and that the ice temperature is below 160 K. The decay of the electrostatic charge after irradiation is determined by two time constants, corresponding to the detrapping of trapped charges in shallow and deep traps within the ice. Amorphous solid water films are found to charge to a higher electrostatic potential than crystalline ice films. The surface potential of crystalline ice increases and decreases during cooling and warming, respectively, without hysteresis. We present a model to describe the charging and discharging processes.