Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) an
d higher concentrations were recorded by use of a 16-mW continuous-wave qua
ntum-casacde distributed-feedback laser at 8.5 mu m whose wavelength was co
ntinuously temperature tuned over 15 nm. A sensitivity (noise-equivalent ab
sorbance) of 3.4 x 10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitro
gen at standard temperature and pressure, which corresponds to a detection
limit of 0.25 ppbv. (C) 2000 Optical Society of America.