Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Abstract

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X1Σ+ → (1π−14σ)A1Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π−1 npπ)-valence (3σ−14σ) mixing in X1Σ+ → 1Σ+ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X1Σ+ → 1Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X1Σ+ → 1Σ+ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X1Σ → (1π−14σ)A1Π, → (1π−13pπ)B1Σ+, → (1π−13sσ)C1Π, and → (3σ−14σ)D1Σ+ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π−1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F2, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.