Collision-induced spectra and current astronomical research

Collision-induced spectra are the spectra of complexes of two or more atoms or molecules in a 'fly-by' collisional encounter. Collision-induced absorption (CIA) has been observed in many dense gases and gas mixtures, in most cases at infrared frequencies in the form of quasi continua, and also in liquids and solids. CIA spectra of several binary complexes have been computed using modern quantum chemical methods, combined with molecular scattering theory, which couples the collisional complex to the radiation field as usual in other spectroscopic work. Binary collisional systems, such as H₂ interacting with another H₂ molecule, or with a helium or hydrogen atom, are first candidates for such computational work, owing to their small number of electrons and the astrophysical interest in such systems. The computed CIA spectra are found to be in close agreement with existing laboratory measurements of such spectra. Laboratory measurements exist at a limited selection of temperatures around 300 K and lower, but theory currently also provides CIA data for temperatures up to 9000 K and for higher frequencies (well into the visible), on a dense grid of temperatures and frequencies. For such calculations, detailed potential energy surfaces (PES) of the supermolecular complexes, along with the induced dipole surfaces (IDS), are needed so that the rotovibrational matrix elements of PES and IDS may be computed for the molecules involved, which may be highly rotovibrationally excited. Modern astronomical research needs opacity tables for analyses of the atmospheres of 'cool' objects, such as cool white dwarfs, solar and extrasolar planets and their big moons, cool main sequence stars, and 'first' stars, which are briefly described in a concluding section. (author)