EXAMINATION OF THE STRUCTURE AND MELTING BEHAVIOR OF THIN-FILM N-ALKANES USING ULTRA-SOFT POLARIZED NEAR-EDGE X-RAY-ABSORPTION SPECTROSCOPY

The use of ultra-soft near-edge X-ray absorption tine structure (NEXAF S) spectroscopy as a tool for determining the molecular orientations, and changes in the orientations and conformations, of thin organic fil ms is presented. Polarisation-dependent NEXAFS measurements made at th e carbon K-edge reveal that vapour-grown n-alkanes, deposited onto Si( 111), exhibit structures with a high degree of long-range order. The m olecular tilt angles, with respect to the substrate surface, for n-alk anes CnH2n+2 in the carbon chain-length range n = 22-60 are found to b e consistent with their bulk crystallographic structures. Temperature dependent measurements carried out using simultaneous electron yield a nd fluorescence yield techniques reveal evidence of surface freezing i n the longer even n-alkanes (n greater than or equal to 46). Such a pr ocess is not evident in the shorter alkanes. The potential causes of t his chain-length dependence are discussed.