Using parallel beam illumination, electron beam-induced structural transformation has been studied as radiation damages for decades. Recently, however, more attention may need to be paid to beam-induced chemical transformation which happens under illumination by a finely focused beam. Such a small probe is often used in analytical electron microscopy, particularly the core-electron energy loss spectroscopy (EELS). Since the signal for the spectroscopy is very weak long radiation is required and makes the induced chemical reactions happen. Therefore, without attentions to the reactions, the acquired data could lead to misinterpretation of specimen properties. In the present study, using a single crystal calcium fluoride (CaF2), we demonstrated by time-resolved EELS that radiation with a nanometer electron probe could induce chemical reactions between the specimen and the typical residual gases in a microscope column.

The thin films of CaF2 were fabricated by vaporizing flakes of single crystal CaF2 with an electron beam and by dissolving the vapors onto a (001) surface of sodium chloride (NaCl) heated at 300°C.