A quantum-cascade laser operating at a wavelength of was used for high-sensitivity absorption spectroscopy of methane . The laser frequency was continuously scanned with current over more than , and absorption spectra of the branch were recorded. The measured laser linewidth was 50 MHz. A concentration of 15.6 parts in in 50 Torr of air was measured in a 43-cm path length with accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be . The content of and species in a sample was determined.
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