Application of a tunable spectrometer based on difference frequency generation in AgGaS₂ for line broadening experiments on NO in the 5 μm region

The application of nonlinear crystals which enable the difference frequency generation (DFG) of two input lasers is a promising tool for light sources in the mid-infrared spectral region beside the well known lead salt diode laser and sideband CO- and CO₂-lasers. We present a high resolution spectrometer based on the DFG in the nonlinear crystal AgGaS₂ applying an AlGaAs index-guided single mode diode laser and a ring dye laser as pump sources. With an input power of only 5 mW (diode laser) and 150 mW (dye laser) by applying non-critical type I 90° phase matching, it was possible to achieve an infrared output power of 15 nW. Applying diode lasers in the wavelength region between 770 nm and 785 nm and a dye laser with DCM the wavenumber range from 1920 cm^-1 to 2020 cm^-1 could be covered. The typical phase-matching bandwidth was about 1 cm^-1. Thus it was possible to measure several NO Λ -doublets. For the doublet ²Π_3/2R(25.5) the line broadening for collisions with NO, N₂, He, Ne, Ar, and Kr was investigated. The values for the self- and air broadening of this doublet, not studied previously, are smaller than those measured for lines with smaller quantum number. This corresponds to the known tendency that the broadening coefficient decreases with increasing quantum number.