MEASUREMENT OF CH$_3$D ABSORPTION CROSS SECTIONS, PRESSURE BROADENING, AND SHFT COEFFICIENTS IN THE 1.65 $\mu$m SPECTRAL REGION BY USING CONTINUOUS AVE CAVITY RING-DOWN SPECTROSCOPY
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Date
2011
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Publisher
Ohio State University
Abstract
Quantitative spectroscopy of CH$_3$D in the near-IR is of importance for an ongoing project to build an instrument to measure the H/D isotopic ratio of methane gas. Continuous-wave cavity ring-down spectroscopy (CRDS) has been used to examine the absorption cross sections, the pressure-broadening and pressure-shift coefficients at around 1652 nm. The absorption cross sections of CH$_3$D were quantified in the wavenumber region between 6046 and 6060 cm$^{-1}$. The maximum peak is located at 6055.17 cm$^{-1}$, which gives ($8.58 \pm 0.37) \times 10^{-21}$\,cm$^2$/molecule at the total pressure of $\sim 8.2$\,Torr of the N$_2$ buffer gas. By using the small step size of the laser wavenumber scan, we measured the pressure-broadening effects, and the pressure-shift effects, on CH$_4$ and CH$_3$D absorption lines. The N$_2$, O$_2$ and CO$_2$ pressure broadening coefficients of CH$_3$D are 0.058, 0.054 and 0.049 cm$^{-1}$/atm, respectively, at the wavenumber we employed. Under the experimental conditions we used, N$_2$ and O$_2$ have very similar pressure broadening effects, and their effects on CH$_3$D is very similar to those of CH$_4$. At the wavenumber we employed, the same values of N$_2$ and O$_2$ pressure-shift coefficient , - 0.012 cm$^{-1}$/atm, and a little higher value of CO$_2$, - 0.013 cm$^{-1}$/atm, were found.
Description
Author Institution: Department of Chemistry and School of Medicine, University of Virginia, Charlottesville VA, 22904-4319; Department of Physics, College of William and Mary, Box 8795, Williamsburg, VA 23187-8795