A COMBINED ABSORPTION-WAVELENGTH MODULATION TECHNIQUE FOR DIODE-LASERSPECTROSCOPY OF VANDERWAALS COMPLEXES IN A PULSED PLANAR JET

A tunable diode laser spectrometer modified for the investigation of m olecular gases and van der Waals complexes in a supersonic jet is desc ribed. Simply constructed, reliable pulsed sources with 15 mu m X (7-4 0 mm) slits were used at a repetition rate of the order of 100 Hz for the cooling of gases and for the formation of complexes. A double modu lation technique based on a simultaneous modulation of the jet and a 1 0 kHz wavelength modulation of the diode laser enabled measurements of absorbances 1.3 X 10(-5) (3 X 10(-5) in relative absorption) at opera ting: conditions corresponding to 1 s output time constant of the regi stration system. While type multireflection optics enlarged the absorp tion path of the laser radiation through the jet by 16 times. New spec tra of the van der Waals complexes Ar-CH4 and Kr-CH4 in the 7 mu m reg ion are presented in order to show the characteristics of the spectrom eter. The final resolution in the recorded spectra was better than 40 MHz. The relational temperature of the complexes produced by the prese nt jet source was 7-10 K, which could be compared to the 13 K excitati on temperature measured for the two lowest rotational states of F symm etry of the methane monomer. (C) 1997 American Institute of Physics.