Knowledge Bank

About 30 new transient molecules including free radicals, molecular ions and unstable molecules have been studied at the Institute for Molecular Science in 1993 to 1999 by using a high-sensitivity submillimeter-wave spectroscopy. The spectrometer is a combination of quiet klystrons and multipliers, a Woods-type free space dc-discharge cell and a liquid helium-cooled InSb detector, covering the frequency region up to 620 GHz. The minimum detectable absorption coefficient of the spectrometer is $7.5\times 10-10cm-1$. This high sensitivity has enabled us to study various kinds of transient molecules including promising interstellar molecular ions, diatomic molecules in the electronically excited states, simple metal hydrides and oxides, fundamental triatomic hydrides, transition-metal hydoixes, a linear tetra-atomic radical showing Renner-Teller effect of two bending modes, a fluorinated methyl radical showing hyperfine structure altenation in inversion levels, and completely new phosphorus-bearing molecules. The spectroscopic method and several interesting features of the transient molecules studied will be presented.