Rydberg transitions in X-ray absorption spectroscopy of alkanes: The importance of matrix effects

Precise ab initio electronic structure calculations on isolated alkanes (pr opane, butane) and on propane embedded in a matrix of several propane molec ules were used in order to characterize the near-edge x-ray absorption fine structure (NEXAFS) resonances of condensed saturated hydrocarbons. The res ults demonstrate that upon condensation the dominating NEXAFS spectral feat ures, i.e., the Rydberg resonances located between 287.4 and 288.1 eV, have a significant blue shift from the corresponding transition energies of an isolated alkane molecule. Furthermore, the theoretical results confirm the excitonic character of the final state. Additional calculations carried out for different conformations (trans/gauche) of isolated butane demonstrate that the transition energies of the corresponding NEXAFS resonances do not differ significantly. Previously observed changes in the electronic structu re of disordered thin organic films of calciumarachidate are thus attribute d to changes in the intermolecular spacing of the molecules rather than to changes in the electronic structure of the different, trans and gauche, con formations of the alkane chain. (C) 1999 American Institute of Physics. [S0 021-9606(99)30838-2].