Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X 1 Σ g + → 1 Σ u + bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π g → n pπ u Rydberg and 3σ g →3σ u intravalence excitations. A depleted X→V σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F 2 below the 3σ g −1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X 1 Σ g + → 1 Π u excitations the origins of which are attributed to configuration mixing between 1π g → n pσ u and 1π u → n sσ g Rydberg series.