Infrared Laser Spectroscopy of the L-Shaped Cl-HCl Complex Formed in Superfluid (4)He Nanodroplets

Chlorine atoms, generated through the thermal decomposition of Cl2, 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 band centered at ∼2880.8 cm(-1) is assigned to the binary Cl-HCl complex on the basis of HCl pressure dependence and difference mass spectra. The band lies in the "free" HCl stretching region, implying that the complex is not hydrogen bound. Furthermore, the breadth of the band (∼2 cm(-1) fwhm) is consistent with an assignment to a predominantly b-type component of the H-Cl stretch, as the dominant b-type selection rules and A rotational constant allow for high energy rotational excitations that efficiently couple to droplet excitations, resulting in fast rotational deactivation. Despite the lack of rotational fine structure, which would verify the assignment, the observed band is consistent with the stabilization of a weakly bound complex having an approximately L-shaped geometry. Frequency computations for a rigid, L-shaped complex reveal that the transition dipole moment vector points almost entirely along the b inertial axis; indeed, the signal-to-noise ratio in our experiment precluded the observation of an a-type component of the HCl stretching band for the complex. No bands were observed that could be assigned to a linear H-bonded Cl-HCl complex. Additionally, we located bands that are consistent with the formation of Cl2-HCl, Cl2-(HCl)2, and Cl-(HCl)2. Two vibrations of the Cl-(HCl)2 complex were found, and harmonic frequencies and intensities computed for a cyclic structure are consistent with the observations.