MICROWAVE STUDIES OF SMALL CLUSTERS IN THE Ar/HF SYSTEM$^{\ast}$

Abstract

Expansion of an Ar/HF gas mixture from a supersonic nozzle generates a progressio of small clusters $Ar_{n}. (HF){m}$ and $Ar{n}. (HF){m}$. Or these, the Ar -HF and $(HF){2}$ dimers were among the first weakly bonded complexes to be detected and characterized by rotational spectroscopy. More recently, we have reported preliminary observations of $Ar_{2}-HF$, a T-shaped $trimer,^{1}$ and of $Ar_{3}-H/DF$, a symmetric top $tetramer,^{2}$ with the Flygare-Fourier transform microwave spectrometer. These and subsequent results for the Ar/HF system will be presented. Over 20 b-dipole transitions have now been observed in the 4-18 GHz region for both $Ar_{2}-HF$ and $Ar_{2}-DF$, and their hyperfine structur analyzed. Fitting of the line centers for $Ar_{2}-DF$ with Kirchoff's NBS $program^{3}$ gives $A^{\prime\prime}. B^{\prime\prime}$. and $C^{\prime\prime}$ rotational constants of 3 506.745(1), 1 744.094(1) and 1 144.478(1) MHz and centrifugal distrition constants (assuming planarity of -38(1), -0.49(1), and -178(3) KHz for $\tau I_{bbee}$. $\tau_{ccaa'}$ and $\tau {aabb}$. The significance of these and similar results for $Ar{2}-HF$ will be discussed. Analysis of the hyperfine structure shows that the torsional oscillations of the H/DF are anisotorpic. For $Ar_{2}-HF$ the average in-plane amplitude 15 $32.8^{\circ}$ and the out-of-plane, $29.7^{\circ}$. A sizeable number (-50) of as yet unassigned lines have been found for the Ar/HF system. Most likely they are from other small clusters such as $Ar-(HF){2}, Ar{2}-(HF){2}$, and $Ar{4}-HF$. Further efforts are in progress to identify the clusters involved, including a search specifically for $Ar_{4}-HF$. Any new results, or lack of them, will be reported.