Isotopic probing of very weak intermolecular forces: Microwave and infrared spectra of CO-He isotopomers

Abstract

The pure rotational transition (J,j,l)=(101)←(000) of the weakly bound CO-He complex has been observed in the 17 GHz region for five isotopomers, namely C12O-16He4, C13O-16He4, C12O-18He4, C13O-18He4, and C13O-17He4, using a pulsed-jet cavity Fourier-transform microwave spectrometer. Hyperfine structure due to the quadrupolar O17 (I=5/2) nucleus has been observed and analyzed to yield the quadrupole coupling parameters. A new microwave-terahertz double resonance spectrometer has been used to carry out an experiment on C13O-16He4 in which a submillimeter-wave pump transition (J,j,l)=(110)←(000) and a microwave signal transition (211)←(110) were detected. Infrared spectra of C13O-16He4 and C12O-18He4, have been obtained in the 2100 cm−1 region of the C-O stretch using a tunable diode laser spectrometer and a long-path (200 m), low-temperature (46 K) equilibrium gas cell. The combined data are sufficient to construct essentially complete experimental energy level schemes for the bound states of the C12O-16He4, C13O-16He4, and C12O-18He4 forms of the complex. These energies are compared with calculated levels derived from two intermolecular potential models, V(3,3,3) and XC (fit). It is shown that the new spectra, in particular those of C13O-He16, discriminate between the two models and indicate that V(3,3,3) provides a better representation of the potential in the region of the attractive well probed by the bound states. A quasibound state of CO-He was observed for the first time, and its width due to predissociation was approximately determined, providing a sensitive measure of the depth of the potential.