ASYMMETRY OF THE INTERCOLLISIONAL DIP IN PRESSURE-INDUCED INFRARED $SPECTRA^{*}$

Author Institution: McDonnell Douglas Research LaboratoriesInfrared spectra in the fundamental band of molecular hydrogen, including Q and S branch $quadrupole^{1,2}$ and pressure-induced $dipole^{1}$ features, have been analyzed. The absorption dip in the pressure-induced dipole contribution to the Q branch is seen to have to definite anti-symmetric component. The dip has been previously $attributed^{3}$ to anticorrelation between the dipole moments induced in successive binary collisions; this anticorrelation produces a dip symmetric about line center. In the work the original theoretical $argument^{3}$ has been extended to include phase shifts in the molecular wave functions resulting the collisional interaction. The appropriate dipole moment autocorrelation function has been constructed, and the resulting absorbing dip ``line shape"", obtained from the Fourier transform of the autocorrelation function, has both a symmetric and antisymmetric component given by $W(\omega) = (\omega^{2} \tau _{c}^{2} - \omega\tau_{c} \sin\alpha) (1 + \omega^{2}\tau_{c}^{2})^{-1}$ for cos $\alpha = 1.$ In this equation $\omega$ is the frequency measured from line center, $\alpha$ is the single collision phase shift, and $\tau _{c}$ characterizes the exponential distribution of intervals between collisions. The data for pressure-induced Q(0) and Q(1) absorption dips are shown to be well fit by W$(\omega)$, and the value obtained for $\alpha$ ($\sim$ 0.1 rad) is consistent with a simple repulsive interaction collision model. $^{*}$This work is supported by the McDonnell Douglas Independent Research and Development Program. 1. S. L. Bragg, ``An Experimental Study of the Vibrational -Rotational Spectrum of Molecular Hydrogen"", thesis, Washington University (1981). 2. S. L. Bragg, J. W. Brault, and W. H. Smith, Ap. J. 263, 999 (1982). 3. J. Van Kranendonk, Can. J. Phys. 46, 1173 (1968)