$N_{2}-$BROADENING AND LINE SHIFTS IN THE $\nu_{3}$ BAND OF $CO_{2}$ AND THE $\nu_{2}$ BAND OF $H_{2}O$

Abstract

On several of our spectra recorded with the McMath Fourier transform spectrometer (0.01) $-cm^{-1}$ resolution) the $\nu_{3}$ region of $CO_{2}$ and/or the $\nu_{2}$ region of $H_{2}O$ were obtained incidentally when neighboring spectral regions were being studied. Often the spectral lines of these gases were used for wavelength calibration of the spectra. The lines of these bands are extremely intense and arise from residual gas in the evacuated interferometer tank and the nitrogenpurged atmoshperic path. We have analyzed a number of spectra in which overlapping pairs of lines appear from both of these sources. In many cases, a difference in the line center positions of the narrow and broad components is clearly visible. Measuring this position difference gives a direct determination of the pressure-induced line shift. For each pair of lines, the two positions and the $N_{2}$-broadened halfwidth of the broader component have been measured using a nonlinear least squares spectrum fitting algorithm. For the $\nu_{3}$ band of $CO_{2}$ the measurements were made in the P(40) to R(40) spectral range. The halfwidths range from 0.086 to $0.062 cm^{-1}$/atm and vary smoothly with rotational quantum number within the uncertainties of the measurement (about 2\%). P and R branch lines with the same $J""$ have similar halfwidths. Pressure-induced line shifts in the P branch are almost the same for all lines to the accuracy of the measurements (-$0.0028 cm^{-1}$/atm with uncertainties of $0.0002 cm^{-1}$/atm). In the R branch, however, there is a strong dependence upon rotational quantum number, varying from a shift close to zero at $J""$ = 2 to a shift of -$0.0032 cm^{-1}$/atm near J"" = 40. Over 100 spectral lines of $H_{2}O$ were measured. The halfwidths varied from about 0.04 to $0.09 cm^{-1}$/atm. Unlike our results for $CO_{2}$ or those obtained for $CH_{4}$ or $O_{3}$ in our previous studies, approximately equal numbers of positive and negative line shifts were found for the $H_{2}O$ lines measured. The magnitude of the shift is typically a few thousandths of a $cm^{-1}$ per atmosphere.