THE EFFECT OF PRESSURE AND SLIT WIDTH ON SHAPES OF INFRARED BANDS

Abstract

The effect of gas pressure and slit width on the band shapes of infrared vibration-rotation bands has been studied. There are two limiting cases. {\bf High Pressure} At pressures sufficiently great to essentially smooth out the fine structure of bands of equally spaced lines, the spectrometer tracing assumes an especially simple form:\frac{{T}(\nu)}{{T}_{o}(\nu)}=\exp\left\{-\frac{\alpha(\nu){p}\ell}{\Delta\nu}\right\} where $\Delta\nu$ is the spacing between individual rotation lines. Quantities of interest such as the frequency separation of the P and R maxima and the intensity ratio of the P and R maxima are unaffected in this limit, if the function exp ($-\alpha p\ell/\Delta\nu$) is essentially linear over the slit function. {\bf Low Pressure} With a band of equally spaced lines at pressures sufficiently low so that the lines do not overlap, the quantity $1 - {T}(\nu)/{T}_{o}(\nu)$ is proportional to the pressure of absorbing gas and inversely proportional to $\Delta\nu$, the line spacing, providing the slit width is sufficiently large to smooth out the fine structure. The frequency separation of the P and R maxima is left unchanged in this situation, while the intensity ratio of the P and R maxima is a function of the line spacing times a function of $\alpha(\nu^{R}_{max}$) and $\alpha(\nu^{P}_{max}$), which reduces to ($\alpha^{P}_{max}/\alpha^{R}_{max})^{1/2}$ for Lorentzian line shapes. Measurements on CO, HCl, and HBr have been made and the results will be discussed.