The singlet electronic excited states of the F2 molecule

Abstract

By using multireference single excitation configuration interaction calculations and multireference single and double excitation CI calculations, we consider the Σu+1, Πg1, and Πu1 excited states of the F2 molecule which lie between 4.3 and 14.1 eV above the ground state. The basis set is composed of 13s, 10p, 7d, and 2 f contracted Gaussian-type functions, and covers molecular orbitals spanned by 4s, 4p, and 3d Rydberg orbitals. Of the Σu+1 states, G 1Σu+ is sometimes disregarded, presumably because it is not directly observed by optical measurements, but is inferred from perturbations in the visible and ultraviolet spectra. We find that G 1Σu+ arises from the shallow local minimum in the lowest Σu+1 potential curve, which also has a stable minimum corresponding to the state designated C 1Σu+. The experimental excitation energies (T0 values) for G 1Σu+ are 12.81–12.87 eV according to electron energy loss spectroscopy, and our theoretical value is 13.06 eV. Agreement between the experiment and the calculation is quite close. The state has a mixed ionic-Rydberg character with an interesting Rydberg portion. The experimental and calculated T0 values for C 1Σu+ are, respectively, 11.57 and 11.59 eV, suggesting that the present calculation for the state is reliable. Ambiguity found in experimental assignments of the vibrational levels for C 1Σu+ is settled here. The Πg1 and Πu1 states are also discussed.