Knowledge Bank

A multispectrum nonlinear least-squares fitting $techniquea$ has been used to determine Lorentz broadening and pressure- induced shift coefficient for a large number of transitions in the $\nu 3$ fundamental band of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3D$ in the region from 1150 to $1430cm-1$. We analyzed a total of 14 high-resolution $(0.006cm-1)$ room temperature absorption spectra recorded with the 1-m Fourier transform spectrometer (FTS) at the McMath-Pierce facility of the National Solar Observatory at Kitt Peak. The data set included 10 spectra of 98% pure $CH3D$ and 4 spectra of $CH3D$ in $N2$. Our multispectrum analysis technique allowed us to simultaneously analyze both self-broadened and $N2$-broadened spectra. The measurements in the $\nu 3$ band included transitions with rotational quantum number as high as $J\prime \prime =16$ and $K\prime \prime =16$. We determined $N2$-broadening and shift coefficients for about 300 transitions and self-broadening and shift coefficients for over 400 transitions. The broadening coefficients (both self and $N2$) range between 0.02 and $0.10cm-1atm-1$ 2 at 296K. Both the self-shift and the $N2$-shift coefficients vary between about -0.012 and $+0.009cm-1$ $atm-1$. At least 95% of the measured shift coefficients are negative, and the small number of positive shift coefficients often involve transitions with $J\prime \prime =K\prime \prime$. The $J^{\prime\prime} = K^{\prime\prime} $ transitions in the sub-band show the smallest broadening coefficients. The present results will be compared to previous measurements in this parallel band in the nearby perpendicular $\nu 5$ and $\nu 6$ bands.