Two-channel opto-acoustic diode laser spectrometer and fine structure of methane absorption spectra in 6070–6180 cm−1 region

We describe the hardware and software of the high-sensitive two-channel opto-acoustic spectrometer with a near infrared diode laser. A semiconductor TEC-100 laser with outer resonator generates a continuous single-frequency radiation in the range of 6040–6300 cm−1 with spectral resolution better that 10 MHz. The newly designed model of photo-acoustic cells in the form of a ring type resonator was used in the spectrometer, and the system allows the measurement of a weak absorption coefficient equal to 1.4 × 10−7 cm−1 Hz−1/2 with a laser radiation power of 0.003 W. The methane absorption spectra within a range of 6080–6180 cm−1 were measured with a spectral resolution of 10 MHz and the signal to noise ratio more than 103. Six hundred absorption lines were recorded, which is twice as many as in HITRAN-2004. The accurate measurements of the half-width and shift of methane unresolved triplet R3 of 2ν3 band permit us to determine values of the broadening and shift coefficients for CH4–air, CH4–N2, and CH4–SF6 mixtures.