Study of adsorption on radiation-damaged CaF 111/ surfaces

Thin CaF2 111. films 60–130 A°. have been grown in ultra-high vacuum by vapor deposition on Si 111.- 7=7. surfaces. Damage by low-energy 50 eV.electrons and the subsequent adsorption of small molecules O2, CO2, N2, C2H6, and C2H4. have been examined using photoemission and electron energy loss spectroscopies. The damaged surface is highly reactive with O2and CO2. The initial rapid uptake of O saturates at exposures of F10 Langmuirs L., leading to the attenuation of a damage-induced defect peak at 1.8 eV in the energy loss spectrum and the appearance of a feature indicating Ca–O bonding in the surface-sensitive Ca 3p photoemission spectrum. The O 1s and valence-band photoemission data indicate two distinct states of chemisorbed O differing in valence charge density. No chemisorption on the damaged surface is seen for F103 L of N2or F300 L of C2H6. For C2H4, deposition of C is shown by the appearance of the C 1s in photoemission and of the 6.6 eV energy loss peak characteristic of graphitic C. q Published by Elsevier Science B.V.