VIBRATION-ROTATION TUNNELING SPECTROSCOPY OF THE VANDERWAALS BOND - A NEW LOOK AT INTERMOLECULAR FORCES

Measurements of the low-frequency van der Waals vibrations in weakly bound complexes by high-resolution laser spectroscopy provide a means to probe intermolecular forces at unprecedented levels of detail and precision. Several new methods are presently being used to record vibration-rotation-tunneling (VRT) transitions associated with the motions of the weak bonds in van der Waals clusters. The most direct measurements are those probing only the van der Waals modes themselves, which occur at far-infrared wavelengths. This article presents a review of the information on both intermolecular forces and intramolecular dynamics that has been obtained from far-infrared VRT spectra of 18 complexes during the past several years. Some rotationally resolved measurements of van der Waals modes observed in combination with electronic or vibrational excitation are also discussed.