Infrared laser stark spectroscopy of the pre-reactive Cl···HCl complex formed in superfluid 4He droplets

Chlorine atoms, generated through the thermal decomposition of Cl$_{2}$, are solvated in superfluid helium nanodroplets and clustered with HCl molecules. The H–Cl stretching modes of these clusters are probed via infrared laser spectroscopy. A broad band centered at $\approx$2880.9 $\wn$ is assigned to the binary Cl$\cdot$$\cdot$$\cdot$HCl complex. The band center is red shifted by only 7.4 $\wn$ from the “free” HCl stretch (\nub{1}) of (HCl)$_{2}$ and, as such, is consistent with an assignment to a similarly “free” HCl stretch. Also, the breadth of the band ($\approx$2 $\wn$ FWHM) is consistent with assignment to a mostly \textit{b}-type component of the H–Cl stretch; the band is lifetime broadened to a similar extent as the predominantly \textit{b}-type \nub{1} stretch of (HCl)$_{2}$, due to fast rotational relaxation facilitated by the helium droplet environment. Despite the lack of rotational structure, which would verify our assignment, the spectrum is consistent with stabilization of a weakly-bound complex having an L-shaped geometry. Computations reveal that the projection of the transition dipole moment onto the \textit{a}-axis results in a dramatic decrease ($\approx$700 times) in the intensity of the \textit{a}-type band relative to the \textit{b}-type band intensity; indeed, the signal-to-noise ratio in our experiment precluded observation of an \textit{a}-type band for this complex. No bands were observed that could derive from a strongly H-bonded Cl$\cdot$$\cdot$$\cdot$HCl complex. Additionally, we located two bands at 2764.0 and 2798.5 $\wn$ that are consistent with the pick-up of two HCl molecules and are therefore assigned to vibrations of the Cl$\cdot$$\cdot$$\cdot$(HCl)$_{2}$ complex