Pump-probe spectroscopy of Rg-Br$_2$ linear isomers

We have recorded and analyzed the \mbox{\textit{X}$\rightarrow$\textit{B}} spectra for three Rg--Br-Br linear isomers [Rg = He, Ne, Ar] using pump-probe spectroscopy. This work is an interesting test case for the transition from quantum to quasi-classical dynamics, and how the dynamics are interconnected with changes in the potential energy surface. Helium is not only much lighter than argon, but the He-Br$_2$ potential well is much shallower than that of Ar-Br$_2$. Excitation spectra to individual Rg-Br$_2$ (\textit{B}, $\nu$') intermolecular potentials were recorded by probing the Br$_2$ (\textit{B}, $\nu$') asymptotic limit of the potential while scanning the pump laser. The continuum spectra of the three species are very different, with the He-Br$_2$ spectrum peaking at threshold while the Ar-Br$_2$ spectrum is negligible at threshold and strongly blue shifted. The linear Ne-Br$_2$ bond energy was measured to be \mbox{71 $\pm$ 3 cm$^{-1}$} by the threshold energy for the onset of the continuum. Since excitation tends to move electron density to the $\sigma^*$ orbital of the Br-Br bond near the rare gas atom, the intramolecular stretching vibration (Br-Br) and the intermolecular stretching vibration (Rg-Br) are strongly coupled. The experiments will be compared to a two dimensional model using the best available potential energy functions.