Spectral Region for Laser Isotope Measurements of Gases

NASA's Jet Propulsion Laboratory, Pasadena, California

A mid-infrared wave-number range has been identified as being useful for laser measurements of CO and CO₂gases that contain various combinations of isotopes of C and O. The isotopic species are ¹²C¹⁶O (common CO), ¹³C¹⁶O, ¹²C¹⁸O, ¹⁶O¹²C¹⁸O,¹²C¹⁶O₂ (common CO₂), and ¹³C¹⁶O₂.

More specifically, the wave-number range from about 2,036.2 to 2,038.4 cm^-1(wavelength range from about 4.9111 to about 4.9058 µm) contains two absorption spectral lines for ¹²C¹⁶O₂, a group of lines for ¹³C¹⁶O₂, and one line for each of the other isotopic species (see figure). The range is narrow enough that it can be spanned by a typical tunable laser.

The Transmission Spectrum of a mixture of isotopic species of CO and CO₂contains distinct absorption lines for each of the species in narrow wave-number range centered at about 2,037 cm-1. The ¹³C¹⁶O₂Q-branch lines would provide an independent determination of temperature. The spectral simulation shown is typical for Mars atmospheric conditions.

From measurements of the absorption of laser light in this wavelength range, one can determine simultaneously the temperature of a gas mixture and the concentration of each of the six isotopic species. Potential applications for instruments based on this measurement principle include Earth and planetary gas spectrometers; analysis of exhaled, isotope-tagged CO and CO₂ for medical diagnosis; analysis of isotope-tagged CO and CO₂ for research on the operation of automotive engines and the resulting pollution; and other applications in industries that involve the sensing and/or measurement of gases.

This work was done by Christopher Webster of Caltech forNASA's Jet Propulsion Laboratory.

NPO-21163