Infrared spectra of photoinduced excited transient species measured by FTIR spectroscopy with a matrix-isolation technique are reviewed. Comparison of the transient spectrum with density functional theory calculation results makes it possible to identify the transient species and determine their geometrical structures . For example, the photoinduced transient species of dicyanobenzene and its derivatives are identified as the lowest electronically excited triplet states. On the other hand, those of hydroquinones are identified as the higher-energy rotational isomers. The relaxation process from the higher-energy species to the most stable isomers in a low-temperature matrix is ascribed to matrix-induced hydrogen-atom tunneling effect.
The submillimeter-wave and terahertz region, whose wavelength is from 1mm to 0.1mm, is a frontier not only in spectroscopy but also in astronomy, but it will rapidly develop in the near future. In this article, the astronomical application in this wavelength region is reviewd both from technical and scientific points of view. As for the detection technique, we particularly focus on the heterodyne detection.