Knowledge Bank

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$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ ). One such curve is a plot of $\gamma 0(4a)\gamma 0(2a)$ vs $2a\gamma r$ 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 $\gamma 0(4a)\gamma 0(2a)$ has been determined from two measurements on a given line, the ratio $2a\gamma r$ 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 $\gamma 0(4a)\gamma 0(2a)$ vs $2a\gamma r$ 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 $2a\gamma r\sim 1$. 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 $previously2$, and to a slightly different value of the dipole moment expansion coefficient M$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$.