Knowledge Bank

Measurements of the hydrogen iodide fundamental vibration-rotation spectrum at $4.3\mu$ with a resolution of $0.1cm-1$ or better and a precision hydraulic wavelength $drive1$ have yielded values of line intensity of unusual accuracy. The line intensities have, in turn, been used as input data in the determination of individual line widths. Two meters of absorbing hydrogen iodide were pressurized to one atmosphere with nitrogen as a non-absorbing broadening additive with the result that the line widths were comparable with the slit width. Thereupon, point by point conversion of the transmission curves to plots of the absorption coefficient permitted direct determination of the line intensity $S=\int k(\nu )d\nu$. Finally, small corrections for instrumental smearing were introduced in the manner of the Kostkowski and $Bass2$ treatment as well as wing corrections to replace the portions of the Lorentz curve which could not be differentiated practically from the noise. Present unsmoothed results are as follows: [FIGURE] These indicate a value for the integrated band intensity of only $0.37cm-2atm-1$ at $300\circ K$ Thus, the coefficient of the linear term in the electric dipole moment expansion is very small and the effect of vibration-rotation interaction may be expected to be quite marked. As the table shows, the R-branch is in fact considerably enhanced at the expense of the P-branch. The value of the interaction parameter computed from relative intensities near the center of the band is $\Theta =-5.1$.