THE HIGH VIBRATIONAL LEVELS OF THE $A^{1}{\Sigma_{u}}^{+}$ STATE OF $Li_{2}$

Abstract

In experiments designed initially to examine the potential barrier in the $B^{1}\Pi_{u}$ state of $Li_{2}$, two tunable dye lasers were used to excite the $F^{1}{\Sigma_{g}}^{+}$ state via the $X^{1}{\Sigma_{g}}^{+} \to A^{1}{\Sigma_{u}}^{+} \to F^{1}{\Sigma_{g}}^{+}$ double resonance. The resulting fluorescence was examined in the $8000-10000 cm^{-1}$ region at high resolution using a Fourier Transform spectrometer. Several spectra showed long vibrational progressions which were found only to occur when the excited $F^{1}{\Sigma_{g}}^{+}$ level was nearly in resonance with an $E^{1}{\Sigma_{g}}^{+}$ "shelf state" level of the same J. The lower levels of the vibrational progressions were identified as high vibrational levels (v > 30) of the $A^{1}{\Sigma_{u}}^{+}$ state. In further experiments, different vibrational levels of $F^{1}{\Sigma_{g}}^{+}$ and $E^{1}{\Sigma_{g}}^{+}$ were excited systematically at J corresponding to the crossing point of the 2 states and very long progessions in the $A^{1}{\Sigma_{u}}^{+}$ 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 $A^{1}{\Sigma_{u}}^{+}$ state will be discussed.