Examination of the structure and melting behaviour of thin film n-alkanes using ultra-soft polarised near-edge X-ray absorption spectroscopy

The use of ultra-soft near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a tool for determining the molecular orientations, and changes in the orientations and conformations, of thin organic films is presented. Polarisation-dependent NEXAFS measurements made at the carbon K-edge reveal that vapour-grown n-alkanes, deposited onto Si(111), exhibit structures with a high degree of long-range order. The molecular tilt angles, with respect to the substrate surface, for n-alkanes C_nH_2n+2 in the carbon chain-length range n = 22-60 are found to be consistent with their bulk crystallographic structures. Temperature dependent measurements carried out using simultaneous electron yield and fluorescence yield techniques reveal evidence of surface freezing in the longer even n-alkanes (n = 46). Such a process is not evident in the shorter alkanes. The potential causes of this chain-length dependence are discussed.