Linear dichroism in the NEXAFS spectroscopy of n-alkane thin films

Linear dichroism in Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy has been used to determine molecular orientation at surfaces and in microscopic domains. However, the molecular orientation of n-alkanes cannot be derived unambiguously from their NEXAFS spectra due to the inadequate understanding of the character of the relevant spectroscopic features in the NEXAFS spectra of n-alkanes (i.e. C 1s → σ* C-H, C 1s → σ*C-C transitions).;Linear dichroism for the NEXAFS resonances of n-alkanes has also been studied by ab initio calculations. These calculations were carried out on an isolated n-alkane molecule and a cluster of n-alkane molecules. The calculations on an isolated n-alkane molecule are used to study linear dichroism for the NEXAFS resonances above the C 1s ionization potential. The cluster calculations account for the matrix effects, particularly for the NEXAFS resonances below the C 1s ionization potential. These calculations help us to understand the character and linear dichroism in the NEXAFS spectra of n-alkanes.;The film morphology and molecular orientation of n-alkane thin films have also been investigated by the NEXAFS spectroscopy and microscopy. Thin films of hexacontane (n-C60H122) and hexatriacontane (n-C36H74) were epitaxially grown onto cleaved NaCl (001) surfaces by physical vapor deposition. It is found that the film morphology and molecular orientation of n-alkane thin films depend on the alkane chain length and deposition parameters, such as substrate temperature. These observations have been rationalized by consideration of the kinetics and thermodynamics of nucleation and film growth.;We have studied linear dichroism in the NEXAFS spectra of hexacontane (HC, n-C60H122) thin films by using angular dependent NEXAFS spectroscopy. The HC thin films were epitaxially grown onto the cleaved NaCl (001) surfaces by physical vapor deposition. NEXAFS spectra of these HC thin films were acquired at different angles using STXM microscopy. A detailed analysis of the angular dependence of the NEXAFS spectra of the HC thin film helps us to understand the relationship between the form of linear dichroism and the molecular orientation in n-alkane molecules. This linear dichroism in the NEXAFS spectroscopy of n-alkanes is relevant for quantitative measurements of molecular orientation, such as for the microanalysis of crystalline organic materials.