He is a physical chemist, specializing in spectroscopy of small molecules in the gas phase. He performed the first microwave-optical and optical-optical double resonance experiments on small molecules, and invented the Stimulated Emission Pumping (SEP, or "PUMP and DUMP") spectroscopic method. He is also particularly known for studies of the molecules acetylene (CH) and calcium fluoride (CaF) in the gas phase.
His active research group at MIT includes about eight graduate students and postdocs working on experimental, theoretical and computational physical chemistry of small molecules in the gas phase.