15min:
HIGH RESOLUTION INFRARED SPECTRA OF AR-WATER AND NE-WATER AT 6 µm.

X. LIU AND Y. XU, Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2.

Ar- and Ne-water are highly floppy van der Waals complexes where the water subunit experiences nearly free internal rotation. Their ro-vibrational energy levels are characterized by the internal rotor states of the water subunit within the complex and a pseudo-diatomic rotational energy Hamiltonian. Large amplitude motions of the complexes lead to strong perturbations, such as Coriolis coupling and angular-radial coupling among the internal rotor states and the van der Waals bending and stretching states. Mid-infrared spectra of Ar- and Ne-water were measured with a direct absorption spectrometer with an external cavity quantum cascade laser at 6 µm and a 366-pass astigmatic absorption cell. The scan-to-scan frequency instability of the laser was addressed with a ``on-the-fly'' calibration procedure.

The infrared spectrum of Ar-water has been studied by Weida and Nesbitt, in which the Sigma 110 and Pi 110 states have been identified. At least three new overlapping bands at 1630 cm-1 have been observed and two of them have been tentatively assigned to the n=1, Sigma 101 \gets Pi 110 and Sigma 110 \gets Sigma 101 bands. The n=1, Pi 101 \gets Sigma 101 band that was missing in the previous study was found at 1639 cm-1. Four new bands in the 1645-1665 cm-1 region have been observed and assigned to the Pi 212 \gets Pi 101, Sigma 212 \gets Sigma 101, Pi 212 \gets Sigma 101, and n=1, Sigma 111 \gets Sigma 000. A global fit of the microwave, far-infrared, near-infrared and mid-infrared data was performed with Pickett's SPFIT program to determine the spectroscopic constants of these levels. Infrared spectrum of Ne-water is analogous to that of Ar-water. The Ne-water PES is much shallower than that Ar-water. As a result, there are fewer number of internal rotor states supported by the surface. Indeed, only the Pi 110 \gets Sigma 101, Sigma 110 \gets Pi 101, n=1, Sigma 000 \gets Sigma 000, and Pi 111 \gets Sigma 000 bands were observed in our measurements. Severe spectral perturbations have been detected and analyzed. No infrared transitions of He-water were detected in the same frequency region. This is consistent with the previous theoretical calculation which predicted that the He-water potential supports only one bound state and no infrared transitions could be observed in this region because of the b- type selection rule.