The effect of grain size on fluorine gas bubble formation in calcium fluoride during electron-beam irradiation

Fluorine loss caused by a high-current-density electron beam in calcium fluoride films 100 nm thick has been studied as a function of grain size using parallel electron-energy-loss spectroscopy and monitoring the change in transmitted intensity. Three samples were studied. These were a single-crystal film grown by molecular beam epitaxy and two thermally evaporated polycrystalline films with different average grain sizes. We present evidence to show that the fluorine gas bubbles form and then burst in the sample with the smallest grain size, but that gradual fluorine loss occurs on the other samples. An explanation is offered which is based on the formation of molecular fluorine at grain boundaries followed by a slower diffusion rate of molecular fluorine compared with atomic fluorine interstitials.