Using Millimeter-Wave Spectroscopy to Probe the Dynamics of Weakly Bound Complexes

The lowest vibrationally excited states of ArHCN and HeHCN, as well as the ground state of HeHCN, were studied using electric resonance optothermal spectroscopy. Transitions in ArHCN were measured directly from the ground state to the Sigma_1 and Pi_1 states, each having a band origin near 6 cm^{-1}. Rotational constants, dipole moments, and quadrupole coupling constants were determined for each state, as well as Sigma - Pi coupling parameters characterizing the Coriolis interaction and transition dipole moment. Analysis of the hyperfine and Stark effect data indicates a broad angular distribution for each state, centered near the T-shaped geometry. Extreme angular-radial coupling was confirmed by the rotational constants, which indicate that in the bending states, the average separation between the argon and the HCN center of mass contracts by roughly 0.5 A compared to the linear ground state. In HeHCN, Sigma and Pi designations for the bending states are probably not useful, as the angular anisotropy is apparently not strong enough to couple the HCN rotational motion to the intermolecular axis. Hyperfine resolved lines, correlating in the free internal rotor limit to the HCN subunit j = 1 >=ts 0 transition, were recorded near 100 GHz. Low frequency transitions, involving end -over-end rotation (l) of the complex, have been observed at 16 GHz and 31 GHz using mm-wave/microwave double resonance. Tentative assignment of J and F quantum numbers, aided by an ab initio (F.-M. Tao, J. Chem. Phys. 98, 2481 (1993)) calculation of the potential, has permitted determination of the ground state quadrupole coupling constant. The corresponding angular expectation value indicates that a nearly free internal rotor description of HeHCN is most appropriate. A ground state l = 1 to 0 assignment for the 16 GHz transition provides an effective rotational constant, whose magnitude reveals large amplitude zero -point stretching motion. The spectrometer construction and performance are described. The design follows that of Fraser (G. T. Fraser, R. D. Suenram, and L. H. Coudert, J. Chem. Phys. 90, 6077 (1989)), and exploits the strong rotational cooling of the supersonic jet source to eliminate the need for state selection by molecular beam deflection.