Microwave spectroscopy of ternary and quaternary van der Waals clusters

The study of weakly bound complexes and clusters promises to provide a bridge between the properties of isolated molecules and those of dense phases. We describe briefly the method of pulsed nozzle Fourier transform microwave spectroscopy, which we use in our laboratory to study rotational and low-lying van der Waals vibrational spectra of weakly bound complexes and clusters. Systematic studies of three different classes of clusters are described: (i) mixed rare gas dimers, trimers, and tetramers, where the transition dipole moment is entirely provided by the weak van der Waals interactions; (ii) (rare gas)(1,2,3)-ammonia clusters where the internal motions of the ammonia subunit are affected by the rare gas atom solvation; (iii) (helium)(1,2,3)-molecule clusters, which provide a basis for the study of larger helium atoms containing clusters. We also discuss the effect of three-body non-additive contributions on spectroscopic parameters, such as dipole moments, rotational constants, and nuclear quadrupole coupling constants.