Diode laser spectroscopy of pressure broadening of 12CO2 and 13CO2 lines

Abstract

Lead-salt tunable diode lasers (TDLs) emit with a very narrow linewidth of ~0.0003 cm−1. This spectral resolution is much narrower than typical Doppler broadened linewidths in the mid-IR range. It is thus possible to detect individual rotational lines within the vibrational band and measure their intensity, which can be used to determine gas concentration and isotopic ratio. The 2300-cm−1 spectral region is especially interesting because the 12CO2 and 13CO2 bands overlap so that their rotational lines have approximately equal absorbance at the anticipated isotopic ratio (~90) of atmospheric CO2 on earth and Mars. To characterize pairs of spectral lines used for isotopic ratio measurements in our stable isotope laser spectrometer, we measured the effect of pressure variations on the spectral line shape of 12CO2 and 13CO2 rotational lines in the 4.3-μm band. Two lines we have studied are (most likely) the R(20) lines of 13CO2 (2298.503 cm−1) and the P(54) line of 12CO2 (2298.278 cm−1). These two lines are separated by only 0.225 cm−1 but are well resolved using a TDL. They have approximately equal absorbance at the normal isotopic concentrations of CO2 on earth and Mars. These lines begin to broaden with increasing pressure at ~2 Torr. The measured profiles compare favorably with theoretical calculations.