Absorption measurements using frequency modulation heterodyne spectroscopy

Direct absorption. measurements using Frequency. Modulation. Spectroscopy (FMS) have been made. Simple calibration is possible and DC absorptions as small as 1.4x10-4 may be detected. By modulating the absorption photochemically, the sensitivity may be extended to a lower limit of1.5x10-7. The radicals HCO1 and ND4 have been studied using this method. © 1983 SPIE.