Knowledge Bank

The Ar-HCN dimer has been $described1$ as ``a highly nonrigid molecule, with several spectroscopic anomalies attributed to strong angular--radial coupling in the interaction $potential.1-4$ In earlier work only $K=0$ transitions were observed and the dimer has been viewed as pseudolinear. However, a previously calculated potential $surface2$ exhibits a local maximum for the linear structure and a minimum for structures bent by $\sim 150$, with smaller angles favored at larger Ar to HCN separations. We now report the observation of four very weak $K\ne 0$ transitions which suggest that the Ar-HCN ground state structure is indeed bent, with (B-C) -- 16 MHz. The line centers are given below, in MHz. Those for $K=0$ are from earlier $work.1,2$ [FIGURE] Analysis of the hyperfine structure of the $K\ne 0$ transitions, produced by quadrupole interactions of the $14N$, gives coupling constants which are quite different from those determined for the $K=0$ transitions. Further analysis is in progress and will be $discussed.1$ K. R. Leopold, G. T. Fraser, F. J. Lin, D. D. Nelson, JR., and W. Klemperer, J. Chem. Phys. 81, 4922 (1984). $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}T$. D. Klots, C. E. Dykstra, and H. S. Gutowsky, J. Chem. Phys. 90, 30 (1989). $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}G$. T. Fraser and A. S. Pine, J. Chem. Phys. 91, 3319 (1989), R. E. Burngarner and G. A. Blake, Chem. Phys. Lett. 161, 308 (1989). $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}D$. C. Clary, C. E. Dateo, and T. Stoecklin, J. Chem. Phys. 93, 7666 (1990).