Study of adsorption on radiation-damaged CaF2(111) surfaces

Thin CaF2(111) films (60-130 Angstrom) have been grown in ultra-high vacuum by vapor deposition on Si(111)-(7 X 7) surfaces. Damage by low-energy (50 eV) electrons and the subsequent adsorption of small molecules (O-2, CO2, N-2, C2H6, and C2H4) have been examined using photoemission and electron energy loss spectroscopies. The damaged surface is highly reactive with O-2 and CO2. The initial rapid uptake of O saturates at exposures of less than or equal to 10 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 banding in the surface-sensitive Ca 3p photoemission spectrum. The O Is 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 less than or equal to 10(3) L of N-2, or less than or equal to 300 L of C2H6. For C2H4, deposition of C is shown by the appearance of the C Is in photoemission and of the 6.6 eV energy loss peak characteristic of graphitic C. O Published by Elsevier Science B.V.