A tunable diode laser spectrometer modified for the investigation of molecular gases and van der Waals complexes in a supersonic jet is described. Simply constructed, reliable pulsed sources with15 &mgr;m×(7–40 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 modulation technique based on a simultaneous modulation of the jet and a 10 kHz wavelength modulation of the diode laser enabled measurements of absorbances1.3×10−5(3×10−5in relative absorption) at operating conditions corresponding to 1 s output time constant of the registration system. White type multireflection optics enlarged the absorption path of the laser radiation through the jet by 16 times. New spectra of the van der Waals complexesAr–CH4andKr–CH4in the7 &mgr;mregion are presented in order to show the characteristics of the spectrometer. The final resolution in the recorded spectra was better than 40 MHz. The rotational temperature of the complexes produced by the present jet source was 7–10 K, which could be compared to the 13 K excitation temperature measured for the two lowest rotational states ofFsymmetry of the methane monomer. ©1997 American Institute of Physics.