Isotope effects and Born-Oppenheimer breakdown in excited singlet states of the lithium dimer

Observation of infrared electronic transitions involving the 1 (1)Delta(g) state of Li-7(2) has instigated an investigation of Born-Oppenheimer breakdown in four singlet electronic states correlating with (2s+2s), (2s+2p), and (2p+2p) lithium atoms. The 1 (1)Delta(g) state, which correlates at long range with (2p+2p) atoms, has been observed in emission from the (5p) (1)Pi(u) Rydberg state and in 1 (1)Delta(g)-B (1)Pi(u) bands, in both instances following optical-optical double-resonance excitation. The latter transition was observed previously for the lighter isotopomer, Li-6(2) [C. Linton, F. Martin, P. Crozet, A. J. Ross, and R. Bacis, J. Mol. Spectrosc. 158, 445 (1993)]. By analyzing multiple-isotopomer data for several electronic systems simultaneously, we have determined the electronic isotope shifts and the leading vibrational and/or rotational Born-Oppenheimer breakdown terms for the X (1)Sigma(g)(+), A (1)Sigma(u)(+), B (1)Pi(u), and 1 (1)Delta(g) states of the lithium dimer. This paper also reports Fourier transform measurements of the B-X absorption spectra of Li-6(2) and Li-7(2), which were required to better define the bottom portion of the B (1)Pi(u) state potential. (C) 2004 American Institute of Physics.