Observation of the New Triple Transitions $Q_1\left(J_1\right)+Q_1\left(J_2\right)+Q_1\left(J_3\right)$ in Molecular Hydrogen in Its Second Overtone Region

We report the first observation of simultaneous vibrational transitions $Q_1\left(J_1\right)+Q_1\left(J_2\right)+Q_1\left(J_3\right)$ corresponding to the triple collisions of the type ${\mathrm{H}}_2$-${\mathrm{H}}_2$-${\mathrm{H}}_2$ at 77, 201, and 298 K in the spectral region 12 300-12 700 ${\mathrm{cm}}^{-1\mathrm{}}$. The observed integrated absorption coefficients $\int \alpha \left(\nu \right)d\nu$ show the density-cube dependence confirming the triple collision mechanism. The experimental ternary absorption coefficients agree within a factor of 2 or 3 with the values calculated from a theoretical model of the overlap-induced, irreducible, ternary dipole component of the three interacting hydrogen molecules, developed by Moraldi and Frommhold [preceding Letter, Phys. Rev. Lett., 74, 363 (1995)].