Transmission and Trapping of Low-Energy (1-10 eV) Electrons in Crystalline Ice Films

We studied the interactions between low-energy (1-10 eV) electrons and a crystalline ice film in a low fluence condition, where incident electrons interacted mostly with the pristine ice lattice. The electron beams were irradiated onto an ice film sample of large thickness (>100 monolayers) at 95 K. The kinetic energy and flux of incident electrons were maintained constant by applying an offset bias potential to the sample to compensate the charging voltage of an ice film developed by electron trapping. A Kelvin work-function probe was used to measure the charging voltage and estimate the electron trapping efficiency. The measurements for ice films with different thicknesses showed that low energy electrons transmitted through the ice films very efficiently. When electrons were trapped in the ice films, the trapping occurred preferentially at the surface of ice films. The electron trapping cross-section for the ice surface was estimated to be <=(2.6 +/- 0.3) x 10(-23) m(2) at the incident electron energy of 1-10 eV. The electron trapping cross-section for the ice interior was below similar to 10-25 m(2). In comparison, adsorbed SO2 and CFCl3 molecules on the ice surface were able to trap incident electrons much more efficiently than the surface water molecules, with corresponding cross sections of (1.1 +/- 0.4) x 10(-21) m(2) and (2.8 +/- 0.3) x 10(-21) m(2), respectively.