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

Abstract

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 to sigma*C-H, C 1s to sigma*C-C transitions).We have studied the linear dichroism in the NEXAFS spectra of n-alkane thin films by using angular dependent NEXAFS spectroscopy to explore the molecular orientation of hexacontane (HC, n-C60H122). The HC thin films were epitaxially grown onto the cleaved NaCl (001) surfaces by physical vapor deposition. NEXAFS spectra of the HC thin film 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 to understand the relationship between the 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. The linear dichroism of the NEXAFS resonances for 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 the linear dichroism for the NEXAFS resonances above the C 1s IP. The cluster calculations account for matrix effects in the NEXAFS features of condensed n-alkanes. A comparison of calculations on an isolated molecule and on a cluster of molecules provides information on how the NEXAFS spectra change from gas phase to condensed phase and determines the linear dichroism of each NEXAFS feature below the C 1s IP.In the process of preparing n-alkane thin films for the study of linear dichroism, the morphology and molecular orientation of n-alkane thin films with different chain length (n-C36H74 and n-C60H122) have also been investigated by the NEXAFS spectroscopy and microscopy. These thin films were epitaxially grown onto cleaved NaCl (001) surfaces by physical vapor deposition. The results revealed that the morphology and molecular orientation of n-alkane thin films depend on the chain length and deposition parameters, such as substrate temperature. These observations have been rationalized by the thermodynamics of nucleation and the kinetics of growth.