STRUCTURE AND VIBRATIONAL PREDISSOCIATION OF THE $HcI_{2}$ VAN DER WALLS' COMPLEX.

Abstract

The $HeI_{2}$ van der Waals’ complex was prepared from a dilute mixture of iodine in helium at a pressure of 100 atm by supersonic expansion through a nozzle into a vacuum. Laser-induced fluorescence excitation spectra were recorded for the $\tilde{B} \leftarrow \tilde{X}$ transition of $HeI_{2}$ as well as corresponding spectra for the $He_{2}I_{2}$ and $I_{2}$ species also found in the expanded gas. These species are observed to have extremely low internal temperatures as a result of the supersonic expansion: $T_{rot} \approx 0.4K, T_{vib} \approx 50K$. Evidence was found for vibrational predissociation of the $HeI_{2}$ complex in both the $\tilde{X}$ and $\tilde{B}$ electronic states. The vibrational predissociation rate was found to depend weakly upon the degree of excitation of the I-I stretching mode, $\nu_{1}.$ For $\nu_{1} = 1$ in the $\tilde{X}$ state the predissociation rate was found to be greater then $5 \times 10^{6} sec^{-1}$ in the B state vibrational predissociation rate is $\sim 5 \times 10^{10} sec^{-1}$ for $v_{1}=27,$ decreasing to $< 5\times 10^{9} sec^{-1}$ for $\nu_{1} \leq 7$. A study of the dispersed fluorescence from the $I_{2}$ fragment produced in the predissociation indicated that the $\Delta\nu = -1$ channel accounts for over 98% of all predissociation. Our progress in the interpretation of the detailed rotational structure of these transitions and in the measurement of the relative importance of the possible values for $\Delta J$ in the vibrational predissociation will be discussed.