The low temperature oxidation of lithium thin films on HOPG by O₂ and H₂O

Lithiated graphite and lithium thin films have been used in fusion devices. In this environment, lithiated graphite will undergo oxidation by background gases. In order to gain insight into this oxidation process, thin (< 15 monolayer (ML)) lithium films on highly ordered pyrolytic graphite (HOPG) were exposed to O_2(g) and H₂O_(g) in an ultra-high vacuum chamber. High resolution electron energy loss spectroscopy (HREELS) was used to identify the surface species formed during O_2(g) and H₂O_(g) exposure. Auger electron spectroscopy (AES) was used to obtain the relative oxidation rates during O_2(g) and H₂O_(g) exposure. AES showed that as the lithium film thickness decreased from 15 to 5 to 1 ML, the oxidation rate decreased for both O_2(g) and H₂O_(g). HREELS showed that a 15 ML lithium film was fully oxidized after 9.7 L (L) of O_2(g) exposure and Li₂O was formed. HREELS also showed that during initial exposure (< 0.5 L) H₂O_(g), lithium hydride and lithium hydroxide were formed on the surface of a 15 ML lithium film. After 0.5 L of H₂O_(g) exposure, the H₂O_(g) began to physisorb, and after 15 L of H₂O_(g) exposure, the 15 ML lithium film was not fully oxidized.