Diode Laser Spectroscopy of Van Der Waals Complex Molecule

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Detailed studies of weakly bounded molecules are important because the qualitative knowledge about the nature of long range intermolecular forces can be obtained from the spectroscopy of such complexes. The weakly bounded molecules have been studied extensively by Fourier transform microwave spectroscopy combined with pulsed nozzle technique. In addition to rotational spectroscopy in the microwave region, the high resolution infrared studies of the molecular complexes are of great interest because the dynamical properties of the weakly bounded bond such as vibrational predissociation and large amplitude motion can be known directly.In the present study, we developed mid-infrared diode laser spectroscopy for the detection of weakly bounded molecular complexes, such as van der Waals complexes and hydrogen bonded complexes. The weakly bounded complexes were generated in the supersonic free expansion jet using pulsed-nozzle technique, and the infrared absorption spectrum of the complexes w … More ere observed directly. Tunable infrared radiation from a diode laser source was collimated and passed into a high vacuum molecular beam chamber.The laser light traversed the molecular beam approximately perpendicularly to the beam about ten times by means of two near-parallel mirrors. The chamber was evaluated by a 6-inch diffusion pump backed up by a buster pump and a rotary pump. The absorption signal was processed by a lock-in amplifier and detected by a boxcar averager and then accumulated in a micro-computer.Using the present instrument, the fundamental infrared band of CO molecule was measured and the rotational temperature of CO molecule in the supersonic free expansion jet was found to be low as 3 K, which was low enough to produce van der Waals complexes.The infrared absorption spectrum of (N_2O)_2 dimmer in the 1270 cm^<-1> region, which corresponded to the nu_1 band (N-O stretching) of monomer, was then measured and the characteristic features of the molecular beam was examined in detail with use of the spectrum. New van der Waals complexes such as OCS-HCI and OCS-N_2O were measured in the frequency range of 2200 cm^<-1> and we are now making their assignments.The diode laser microwave double resonance spectrometer for the detection of the weakly bounded complexes was also developed. The double resonance method has advantages for the detection of the complexes, for example, the assignments of the infrared spectrum becomes definite if a double resonance signal of the molecular complexes is detected. We also made the preliminary test of the double resonance system. Less