Knowledge Bank

Semiconductor diode laser (SDL) spectrometers, such as the Laser Analytics Model LS-3, are capable of measuring high resolution ($0.0001cm-1$) zero, first and second derivative spectra. We wish to use such an instrument to measure the temperature and pressure dependence of the line profiles of rotational-vibrational lines of linear molecules such as CO and $CO2$. Since all SDL spectrometers are currently single-beam instruments, direct measurement of the transmittance or absorbance of lines is not possible. However useful line parameters can be obtained from the second derivative spectrum provided that the slope of the background spectrum is known. To minimize errors due to the nonflat background, two parameters can be derived from the second derivative of the single-bear spectrum from which a good estimate of the line profile can be obtained. The first of these is the half-width of the line measured at the inflection points (the ""inflection half-width""), and the second is the ratio of the maximum negative excursion of the second derivative curve, $R1$, to the value at the line center, $R2$. In this paper, the relationship between these two parameters and the Doppler, collision and Voigt half-widths, the peak absorbance and the slope of the background will be discussed It will be shown that a knowledge of the peak absorbance and the Doppler half-width, combined with measurements of either the inflection half-width of $R1$/$R2$ is sufficient to determine the collision half-width, and hence the line shape, even in the presence of substantial background slope. We discuss several approaches and the regimes in which they are most effective.