Knowledge Bank

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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The maximum peak is located at 6055.17 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$/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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$/atm, and a little higher value of CO$2$, - 0.013 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$/atm, were found.