HIGHLY EXCITED ROTATIONAL AND VIBRATIONAL LEVELS OF THE LOWEST TRIPLET-STATES OF HE-2 - LEVEL POSITIONS AND FINE-STRUCTURE

We have used fast neutral beam laser spectroscopy to investigate trans itions in the near-infrared, from the metastable a (3) Sigma(u)(+) sta te to predissociating levels of the electronic state c (3) Sigma(g)(+) of He-2. A number of transitions were not readily identified, while o thers 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 (3) Sigma(g)(+) s tate, and led to identification of most lines as transitions between a (3) Sigma(u)(+) and c (3) Sigma(g)(+) in highly rotationally excited states. In a number of transitions the fine structure could be resolve d. 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 upsilon = 2 of a (3) Sigma(u)(+) and N = 25, 27, and 29 in upsilon = 0 of a (3) S igma(u)(+), where the latter were identified by their fine-structure s plittings. We discuss the possible upper state of the transitions from these highly excited rotational levels, and argue that the upper stat e must have predominantly c (3) Sigma(g)(+) character, but must also i nclude a significant admixture of the b (3) Pi(g) state due to rotatio nal coupling. The measured fine structure is described by an effective fine-structure Hamiltonian including the spin-spin parameter, lambda, and the spin-rotation parameter, gamma. These parameters and their de pendence on rotation, N, and internuclear distance, R, has been determ ined. (C) 1995 Academic Press, Inc.