Abstract
The near-edge X-ray absorption fine structure (NEXAFS) spectrum of naphthalene was analysed theoratically using the final state approximation rule. ROHF/SCF calculations were carried out for a localized core-hole cationic state under the Bagus-Schaffer scheme for each channel of inequivalent excitations. CI calculations were performed allowing single and double excitations from carbon 1s electrons to the first ten virtual orbitals. The energies and oscillator strengths were calculated. The final spectrum was generated by convolution of Gaussian peaks with 0.5 eV as FWHM of the statistically-averaged calculated transitions. The theoretical result shows remarkable agreement with the experimental high-resolution NEXAFS data measured at BESSY II. Work is in progress to get better control of the variational wave function collapse problem at CI level through the use of symmetry breaking core-hole excited states (MEG-CI). This methodology might offer a clear and transparent way to take into account the relaxational and correlation energy, the virtual orbital specificity and the required restricted configuration interaction needed to the NEXAFS prediction problem.
Highlights
Poly-p-phenylene and polyacene compounds have been chosen as prototype molecular probes for several conceptual patterns in the interpretation of the carbon K-edge X-ray absorption spectrum.[1]
For several molecular systems the near-edge X-ray absorption fine structure (NEXAFS) spectrum is a typical fingerprint of the molecular system, a problem which poses a great deal of controversial questions such as: the nature of the observed peaks, the lack of dependency of chain length increase, the diminished oscillator strength for the HOMO-LUMO transition, the nature of the final state and whether it possesses localized or delocalized character, and the role of the relaxational processes
Most of the methods currently employed for the prediction of NEXAFS spectra deal with this property in limited ways by: a) allowing relaxation of the core-hole orbital; b) artificially imposing a core hole over a specific site; or c) calculating the virtual orbitals with the previous restrictions
Summary
Poly-p-phenylene and polyacene compounds have been chosen as prototype molecular probes for several conceptual patterns in the interpretation of the carbon K-edge X-ray absorption spectrum.[1]. A second alternative interpretation to the complex features of near-edge excitations has been proposed on empirical grounds and is known as the initial state proposition.[1] For large molecular systems, built from heterogeneous block units the whole spectrum might be convoluted from the spectra of the basic block units This model suggests an additive pattern among these block units but their results do not take into account the molecular coupling among the block units. Intense work has been carried out in recent years by Hollauer and coworkers to understand the gains of stepping further and employing fully-symmetrical wavefunctions built from multistructural generalized configuration interaction calculations (MEG-CI)[11] to describe inner-shell excitations This procedure was successfully used to study the He + molecular ion,[12] and nitrogen and carbon dioxide inner-shell excitations.[13] In all cases it was possible to show that reliable oscillator strengths and excitation energies can be obtained by this procedure. In this model all states are allowed to interact and couple and detailed interpretation of the spectra should emerge
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