OPTICAL OPTICAL DOUBLE RESONANCE SPECTROSCOPY OF $Li_{2}$: THE POTENTIAL BARRIER IN THE $B^{1}\Pi_{u}$ STATE:

Abstract

The $F^{1}\Sigma^{+}_{g}$ state of $^{6}Li_{2}, ^{7}Li_{2}$ and $^{6}Li ^{7}Li$ has been excited via $X{^{1}}\Sigma_{g}^{+} \rightarrow A{^{1}}\Sigma_{4}^{+}\rightarrow F^{1}\Sigma^{+}_{g}$ double resonance using the red lines (647.1 nm and 676.4 nm) of a krypton ion laser plus a ring dye laser. High resolution spectra of the resulting fluorescence have been obtained with a Bomem Fourier Transform Spectrometer in various spectral regions covering the range $7000 - 22000 cm^{-1}$. Transitions observed in the fluorescence spectra include $F^{1}\Sigma_{g}^{+} - B{^{1}}\Pi_{u}$ (infra-red), $F{^{1}}\Sigma^{+}_{g} - A{^{1}}\Sigma_{u}^{+}$ (visible), $B^{1}\Pi_{u} - X{^{1}}\Sigma_{g}^{+}$ (visible), $1^{1}\Delta_{g}- B{^{1}}\Pi_{u}$ (infra-red), $G^{1}\Pi_{g} -A{^{1}}\Sigma_{u}^{+}$ (visible), the latter two transitions resulting from collisionally induced energy transfer from the $F{^{1}}\Sigma_{g}^{+}$ state. Our analysis has concentrated on the long $F{^{1}}\Sigma^{+}_{g} - B^{1}\Pi_{u}$ progressions in the infra-red. These extend the $B^{1}\Pi_{u}$ state to higher vibrational levels than have previously been observed. In 3 progressions $(2 in ^{7}Li_{2}, 1 in ^{6} Li_{2})$ the last lines in the spectrum are very broad, indicating that these transitions are terminating on $B{^{1}}\Pi_{u}$ levels very close to the top of the potential barrier. A survey of all the results will be presented including our analysis of both the $F{^{1}}\sigma_{g}^{+}$ and $B{^{1}}\Pi_{u}$ states which will hopefully include an extended potential curve for the $B{^{1}}\Pi_{u}$ state and a good description of the potential hump. Preliminary calculations using each of the three observed broad transitions have given consistent results and indicate that the barrier height is in the $495 - 500 cm^{-1}$ range.