FIRST OBSERVATION OF TRIPLE TRANSITIONS $Q_{1}(J_{1}) + Q_{1}(J_{2}) + Q_{1}(J_{3})$ IN $H_{2} - H_{2} - H_{2}$ COLLISIONS

Abstract

Since the discovery of collision-induced absorption (CIA) forty five years ago at Toronto the observed CIA spectra have been interpreted on the basis of double transitions in which two interacting molecules simultaneously absorb a single $photon.^{1}$ We now report the first observation of simultaneous vibrational transitions $Q_{1}(J_{1}) + Q_{1}(J_{2}) + Q_{1}(J_{3})$ corresponding to the triple collisions of hydrogen at 77,201 and 298 K in the spectral region $12,300 - 12,700 cm^{-1}$ around the center of the collision-induced infrared 3-0 band of $H_{2}$ for gas densities in the range 500 - 1000 amagat. The observed integrated absorption coefficients $\int \alpha (\nu)d\nu$ show the density-cube dependence confirming the triple collision mechanism. The ternary absorption coefficients in units of $cm^{-2} amagat^{-3}$ obtained from the analysis are $(8.8 \pm 0.1) \times 10^{-11}, (11.0 \pm 0.3) \times 10^{-11}$ and $(11.0 \pm 0.6) \times 10^{-11}$ at 77.201 and 298 K $respectively.^{2}$ These coefficients agree within a factor of 2 or 3 with the values calculated from a theoretical model of the overlap-induced, irreducible, ternary dipole (OITD) component of the three interacting hydrogen molecules, developed $recently.^{3}$ A sample analysis of a triple-collision absorption profile at 77 K will be also presented.