THE POTENTIAL BARRIER OF THE B(1)PI(U) STATE OF LI2

A resonant two-photon excitation process, X1SIGMA(g)+ --> A1SIGMA(u)+ --> F1SIGMA(g)+, has been used to populate single ro-vibrational levels of the F1SIGMA(g)+ state of both Li-7(2) and Li-6(2) using various combinations of lasers. High resolution Fourier transform infrared spectra of the F1SIGMA(g)+ --> B1PI(u) fluorescence show many broadened lines in transitions to the last vibrational level of the B1PI(u) state. The B1PI(u) state has a potential barrier and the levels near the top of the barrier are broadened by tunneling through the barrier. Systematic investigation of the positions of the broadened lines in Li-7(2) and Li-6(2) as a function of vibration and rotation has led to a determination of the barrier height as 500 +/- 2 cm-1. A potential curve has been constructed for the B1PI(u) state in the region of the barrier by combining experimental and ab initio data. Level widths, which have been derived from this potential are found to be consistent with the measured linewidths. (C) 1994 Academic Press, Inc.