A quantum-cascade laser operating at a wavelength of 8.1 μm 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 v4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 106 (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 × 10-4. The content of 13CH4 and CH3D species in a CH4 sample was determined.
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics