Date: 2009. 4. 23, 5:00 PM Place: Mogam Hall, Bldg 500 - Abstract - Physical chemistry is a fairly mature field based on quantum mechanics and statistical thermodynamics and I think that fundamental works are more or less exhausted. It is natural, therefore, that our interest points toward less developed interdisciplinary fields. Those of us fascinated by rigorous studies of simple molecules are attracted to astronomy and others interested more in the rich application of chemical physics of complicated molecules point toward biology and applied engineering. Being the former, I have been concentrated in the study of H3+, and other molecular ions in plasmas in the laboratory and in space. Composed of three protons and two electrons, protonated hydrogen molecule, H3+, is the simplest polyatomic molecule. It was discovered by J. J. Thomson in the early days of mass spectrometry and theoretically shown to be an equilateral triangle by Coulson. In this talk, I would like to discuss how this fundamental molecular ion plays the central role in interstellar chemistry and thus in star formation, how its spectrum was discovered in the laboratory, how it has been discovered in interstellar space, how observations of H3+ clarify chemistry and physics of interstellar gas, and how the studies of H3+ and other molecular ions enrich chemistry at the most fundamental level.