Knowledge Bank

In experiments designed initially to examine the potential barrier in the $B1\Pi u$ state of $Li2$, two tunable dye lasers were used to excite the $F1\Sigma g+$ state via the $X1\Sigma g+\to A1\Sigma u+\to F1\Sigma g+$ double resonance. The resulting fluorescence was examined in the $8000-10000cm-1$ region at high resolution using a Fourier Transform spectrometer. Several spectra showed long vibrational progressions which were found only to occur when the excited $F1\Sigma g+$ level was nearly in resonance with an $E1\Sigma g+$ "shelf state" level of the same J. The lower levels of the vibrational progressions were identified as high vibrational levels (v > 30) of the $A1\Sigma u+$ state. In further experiments, different vibrational levels of $F1\Sigma g+$ and $E1\Sigma g+$ were excited systematically at J corresponding to the crossing point of the 2 states and very long progessions in the $A1\Sigma u+$ state, up to v > 80, were observed almost to the dissociation limit. The results and the analysis of the spectra will be presented and the near dissociation behaviour of the $A1\Sigma u+$ state will be discussed.