Generalized oscillator strength for core excitations of nitrous oxide M. Barbatti a , A.B. Rocha b , C.E. Bielschowsky a, * a Dept. de Fisico-Quimica, Instituto de Qu  ımica, CT Bloco, Cidade Universitaria, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21949-900, Brazil b Instituto de F  ısica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ 21945-970, Brazil Received 28 October 2003; accepted 21 November 2003 Abstract Generalized oscillator strength (GOS) and optical oscillator strength are calculated within the First Born Approximation for several electronic transitions from the three 1s-core orbitals to the ﬁrst unoccupied molecular orbitals of the N 2 O molecule. The molecular wave functions were obtained by using the conﬁguration-interaction method, with a Hartree–Fock basis for the occupied molecular orbitals relaxed for each electronic state and improved virtual orbitals for the unoccupied orbitals. For the N1s-p  ex- citations, the GOS as a function of the transferred momentum K has just a slight dependence on the chemical environment of the two inequivalent nitrogen atoms. However, for the N1s-nsr excitations, the GOS proﬁles have completely distinct behaviors, de- pending on the nitrogen atom from which the excitation takes place. A discussion on the origin of this GOS proﬁle is made and an explanation is proposed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 33.70.Ca; 34.80.Gs Keywords: Nitrous oxide; Generalized oscillator strength; Inner-shell 1. Introduction K-edge spectroscopy provides a singular way to study structure of molecules. While visible and ultraviolet spectra strongly depend on the overall structure of the molecular system, the orbitals involved in the inner-shell excitation processes are very localized and strongly re- semble the orbitals of isolated atoms. This lead to rel- atively simple spectra, which have well deﬁned regions concerning to each diﬀerent element present in the sys- tem. If a molecule has two or more atoms of the same element in inequivalent positions, changes in the energy spectrum and other molecular properties are expected and, in principle, they may be mapped so as to establish a relation connecting the diﬀerent molecular environ- ment to the measured property. In the present work, we have calculated the general- ized oscillator strength (GOS) for the inner-shell elec- tronic transitions of the nitrous oxide. The main motivation in the choice of this particular molecule is to explore the inﬂuence of the chemical environment on the inner-shell properties of the two inequivalent nitrogen atoms. We show that completely distinct behaviors are found in the proﬁles of the generalized oscillator strength as a function of transferred momentum for states arising from excitations of the chemically in- equivalent atoms. The nitrous oxide is the subject of a broad interest in science and in application. In particular, its electronic excitation and ionization processes from the core elec- tron region have been extensively studied by Auger [1], photoabsorption [2,3], mass [4–6], and electron energy- loss [7–9] spectroscopy techniques, as well as by theo- retical methods [10–12]. The diﬀerence between the terminal and the central nitrogen atoms constitutes one of the most interesting structural characteristics of the N 2 O molecule, and they have been researched in the most of these works. Eﬀects of asymmetry and dissoci- ation dynamics have been studied by means of angle- resolved spectra produced by polarized radiation * Corresponding author. E-mail addresses: barbatti@if.ufrj.br (M. Barbatti), rocha@ if.ufrj.br (A.B. Rocha), biel@iq.ufrj.br (C.E. Bielschowsky). 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2003.11.043 Chemical Physics 299 (2004) 83–88 www.elsevier.com/locate/chemphys