Knowledge Bank

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 $Q1(J1)+Q1(J2)+Q1(J3)$ corresponding to the triple collisions of hydrogen at 77,201 and 298 K in the spectral region $12,300-12,700cm-1$ around the center of the collision-induced infrared 3-0 band of $H2$ 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 $cm-2amagat-3$ 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.