EXPERIMENTAL DETERMINATION OF GROUND STATE BINDING ENERGIES IN HETERO- AND HOMONU-CLEAR RARE GAS-DIHALOGEN COMPLEXES

A simple Arrhenius model based on the assumptions of a thermodynamic equilibrium between the T-shaped and linear isomers of the $He\\cdots I^{35}Cl(X,\\nu^{\\prime\\prime} = 0)$ rare gas-heteronuclear dihalogen complex and of a Boltzmann rotational distribution for each isomer is used to find the relative binding energies of the isomers. The model was tested by comparing the intensities of the T-shaped and linear bands observed in laser-induced fluorescence spectra recorded in the ICI B-X, 3-0 spectral region in varying temperature regions in the expansion. The results reveal that the T-shaped isomer is $3.2(1.0) cm^{-1}$ higher in energy than the linear isomer. Using two-laser, pump-probe and high-resolution action spectroscopy the linear binding energy was precisely determined to be $21.97(12) cm^{-1}$, thereby indicating a T-shaped $He\\cdots I^{35}Cl(X,\\nu^{\\prime\\prime})$ binding energy of $18.8(1.0) cm^{-1}$. The approach was then extended to the $He\\cdots ^{79}Br_{2}(X,\\nu^{\\prime\\prime} = 0)$ rare gas-homonuclear dihalogen complexes. The intensities of the features associated with transitions of the T-shaped and linear isomers observed in the ro-vibronic spectra recorded in the $Br_{2} B-X, 11-0$ region are used to monitor changes in the relative populations of the isomers. The T-shaped $He\\cdots ^{79}Br_{2}$ isomer is found to be only $0.7(2) cm^{-1}$ higher in energy than the linear isomer. Action spectra reveal that the ground state linear isomer is bound by $17.6(4) cm^{-1}$, and thus a binding energy of $16.9(4) cm^{-1}$ for the T-shaped isomer is determined. The experimental energies agree remarkably well with those obtained using a two-dimensional $He + Br_{2}(X,\\nu^{\\prime\\prime} = 0) ab initio$ potential energy surface,17.4 and $16.6 cm^{-1}$ for the linear and T-shaped isomers, respectively.