title: Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser.
abstract: A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans.  The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.
authors.affiliation: Rice Quantum Institute, Rice University, Houston, Texas 77251-1892, USA. akoster@rice.edu
authors.forename: A A
authors.initials: AA
authors.lastname: Kosterev
authors.forename: R F
authors.initials: RF
authors.lastname: Curl
authors.forename: F K
authors.initials: FK
authors.lastname: Tittel
authors.forename: C
authors.initials: C
authors.lastname: Gmachl
authors.forename: F
authors.initials: F
authors.lastname: Capasso
authors.forename: D L
authors.initials: DL
authors.lastname: Sivco
authors.forename: J N
authors.initials: JN
authors.lastname: Baillargeon
authors.forename: A L
authors.initials: AL
authors.lastname: Hutchinson
authors.forename: A Y
authors.initials: AY
authors.lastname: Cho
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D002247:Carbon Isotopes / Q000032:analysis*; D003198:Computer Simulation; D007834:Lasers; D008697:Methane / Q000032:analysis*; D008956:Models, Chemical; D013055:Spectrophotometry, Infrared / Q000295:instrumentation
chemical_list: D002247:Carbon Isotopes; D008697:Methane