INVESTIGATION OF WEAKLY DIPOLAR VAN DER WAALS COMPLEXES WITH A SUPERSONIC BEAM FOURIER TRANSFORM MICROWAVE SPECTROMETER

摘要

The technique of molecular beam Fourier transform microwave spectroscopy is used in more than 10 laboratories worldwide for the detection of rotational spectra of small to medium sized (2 to ～ 40 atoms) molecules in the gas phase. It is a particularly well suited method for the detection of molecular systems with only small dipole moments (<0.01 Debye) and for the investigation of weakly bound complexes.In brief, the operating principle of the spectrometer is as follows (see Figure below): van der Waals complexes are generated in a pulsed supersonic expansion of a high pressure (1 - 10 atm) sample gas into an evacuated microwave cavity. The low rotational and translational temperatures of less than 1 K achieved in such expansion stabilize the weakly bound systems. The molecular ensemble is then interrogated with a short (a few μs) microwave excitation pulse and the resulting emission signal is down-converted to radio frequencies using a heterodyne receiving system.