We have recently determined the induced dipole surface (IDS) and potential energy surface (PES) of collisional H2-He complexes. We have used these surfaces to compute the binary collision-induced absorption spectra of H2 molecules interacting with He atoms and of D2 molecules interacting with He atoms. Here we extend these calculations to the case of T2 molecules interacting with He atoms. Whereas the electronic structure of X2-He is virtually the same for all hydrogen isotopes X = H, D, or T, the collisional dynamics and molecular scattering wave functions are different for the different collisional pairs. We have calculated spectra up to a temperature of 9000？K and frequencies up to 20,000？cm？1. Here we compare the calculated collision-induced absorption spectra for the different hydrogen isotopes. While we have observed reasonable agreement between our calculations and laboratory measurements for the collisional H2-He and D2-He complexes, there are no laboratory measurements for T2-He collisional complexes, and one must rely on the fundamental theory, supported by the agreement between theory and experiment for the other isotopes. It is an interesting fact that even the so-called infrared-inactive gases, such as hydrogen and its homonuclear isotopes, absorb infrared radiation, if sufficiently high gas densities are encountered [1–4]. This absorption can be traced back to transient electric dipole moments that are induced during collisions of two or more molecules by the same mechanisms that result in the intermolecular forces, that is, by exchange, dispersion forces, and multipolar induction. Modulation of these induced dipole moments due to vibration, rotation, and relative translational motion leads to collision-induced absorption (CIA) of radiation from applied electromagnetic fields. Collision-induced absorption is omnipresent in dense media—it has been observed in dense gases, liquids, and solids [1, 5, 6]. Laboratory measurements of CIA are performed at a very limited range of selected temperatures and frequencies. In contrast to that, the fundamental theory [1] makes it possible to compute collision-induced absorption spectra reliably, over a wide range of temperatures and frequencies, thereby providing numerical values for the absorption intensities even where laboratory measurements do not exist. To compute the binary collision-induced absorption spectra from the fundamental theory, the induced dipole surfaces (IDSs) and potential energy surfaces (PESs) of the binary van der Waals complexes under consideration have to be known. These
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