A NEW METHOD FOR THE DIRECT MEASUREMENT OF SPECTRAL LINE WIDTHS AND STRENGTHS.
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Date
1967
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Ohio State University
Abstract
It is well known that spectral line strengths and widths may be determined from observed lines by the use of correction curves obtained by numerically passing a spectrometer slit function over a spectral line of known shape (e.g., a Cauchy function for a Lorentz line$^{1}$ ). One such curve is a plot of $\frac{\gamma_{0}(4a)}{\gamma_{0}(2a)}$ vs $\frac{2a}{\gamma_{r}}$ where is the half width at half height of the spectrometer slit function, $\gamma_{r}$ is the true line half width at half height and $\gamma_{0}(4a)$ is the half width at half height of the absorption coefficient measured directly from a spectrometer having a spectral resolution of 4a. Once $\frac{\gamma_{0}(4a)}{\gamma_{0}(2a)}$ has been determined from two measurements on a given line, the ratio $\frac{2a}{\gamma_{r}}$ is thus determined, and $\gamma_{r}$ and the line strength S may be determined from other correction curves. A new procedure has been developed which makes use of the fact that the $\frac{\gamma_{0}(4a)}{\gamma_{0}(2a)}$ vs $\frac{2a}{\gamma_{r}}$ curve is quite insensitive to the location of the line of 100\% transmittance as long as the measurement extends into the wings of the line a distance at which instrumental distortion of the line is small. This is the case, typically, for measurements carried out a distance of 6 or 7 half widths from the line center for a ratio $\frac{2a}{\gamma_{r}}\sim 1$. Families of correction curves have been constructed, each member of which corresponds to the distance into the wings a measurement has been taken. The line widths of the first overtone band of the hydrogen fluoride molecule have been measured at pressures of 1/4, 1/2, and 1 atmosphere using this procedure. The accuracy of this correction method has also led to a slight revision of the line strengths reported $previously^{2}$, and to a slightly different value of the dipole moment expansion coefficient M$_{2}$.
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This work was supported by the Advanced Research Projects Agency, under Contract DAHC15 67 C 0062, as a part of Project DEFENDER. $^{1}$ H.J. Kostkowski and A. M. Bass, J. Opt. Soc. Amer. 46, 1060, (1956). $^{2}$ R. E. Meredith, Bull. Amer. Phys. Soc. January, 1966
Author Institution: Infrared Physics Laboratory, Willow Run Laboratories The University of Michigan
Author Institution: Infrared Physics Laboratory, Willow Run Laboratories The University of Michigan