Highly Excited Rotational and Vibrational Levels of the Lowest Triplet States of He 2: Level Positions and Fine Structure

Abstract

We have used fast neutral beam laser spectroscopy to investigate transitions in the near-infrared, from the metastable a 3Σ + u state to predissociating levels of the electronic state c 3Σ + g of He 2. A number of transitions were not readily identified, while others were easily identified using the tables of M. L. Ginter ( J. Chem. Phys. 42, 561-568, 1965). Predictions of the width and the position of lines were obtained using the potential for the c 3Σ + g state, and led to identification of most lines as transitions between a 3Σ + u and c 3Σ + g in highly rotationally excited states. In a number of transitions the fine structure could be resolved. For these lines, the fine structure of the lower state was measured by laser-radiofrequency double-resonance spectroscopy. This gave the finestructure splittings for the N values: 7, 9, and 11 in ν = 2 of a 3Σ + u and N = 25, 27, and 29 in ν = 0 of a 3Σ + u , where the latter were identified by their fine-structure splittings. We discuss the possible upper state of the transitions from these highly excited rotational levels, and argue that the upper state must have predominantly c 3Σ + g character, but must also include a significant admixture of the b 3Π g state due to rotational coupling. The measured fine structure is described by an effective fine-structure Hamiltonian including the spin-spin parameter, λ, and the spin-rotation parameter, γ. These parameters and their dependence on rotation, N, and internuclear distance, R, has been determined.