Study of adsorption on radiation-damaged CaF 2(111) surfaces

Abstract

Thin CaF 2(111) films (60–130 Å) 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 (O 2, CO 2, N 2, C 2H 6, and C 2H 4) have been examined using photoemission and electron energy loss spectroscopies. The damaged surface is highly reactive with O 2 and CO 2. The initial rapid uptake of O saturates at exposures of ≤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 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 ≤10 3 L of N 2 or ≤300 L of C 2H 6. For C 2H 4, 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.