The theory of difference frequency generation has been further developed. Derived from basic principles, the equations developed here accurately predict the conversion of power to the idler wave. Critical phasematching can now be modeled with ellipticity in both beams and the effects of double refraction and non-normal incidence upon the crystal treated. It is no longer necessary to focus the beams into the center of the crystal nor is it necessary to have an equal confocal parameter in the two drive beams. A mid-infrared spectrometer based on titanium:sapphire pumped difference frequency generation in silver thiogallate (AgGaS\sb2) has been constructed in order to conduct high resolution spectroscopy on free radicals. Detector limited sensitivity has been demonstrated with an estimated source bandwidth of 0.00003 cm\sp−1. The process used to calibrate scans has been improved and calibration accuracy of 0.001 cm\sp−1 is now routine. With the silver thiogallate spectrometer, the infrared spectrum of the monodeuterated propargyl radical (CH\sb2CCD) has been acquired. Approximately 250 lines have been positively identified and assigned to the ν\sb1 C-D stretch in the region between 2300 cm\sp−1 and 2400 cm\sp−1. The spectrum consists of a-type transitions (ΔK\sba = 0) with a fully resolved K subband structure, in contrast to the CH stretch of the normal radical. This resolved structure is due to a P\sba-type Coriolis interaction or a Fermi resonance in the molecule. These interactions may be responsible for the fact that we were unable to identify the odd K subbands, though positive identification of the K\sba = 0, 2, 4, and 6 subbands was made and used to determine rotational constants for the radical. A spectrometer based on difference frequency generation in gallium selenide (GaSe) was constructed. Precise phasematching characteristics were determined as well as the potential power produced in the process. The instrument is continuously tunable in the 8.8-15.0 μm wavelength region.