Julius E. Lilienfeld Prize Lecture (2021): Laboratory studies using lasers and molecular beams and their application to astronomy over the past 60 years

I was determined after my Ph.D., to develop experimental techniques that I could use to study the reactive intermediates that are important in chemical reactions. In 1964 I started at Goddard Space Flight Center of NASA in the Astrochemistry section assigned to develop a laboratory program to study the elementary chemical reactions in comets. At that time only a few atoms like O, H, Na, & K, and diatomic and triatomic radicals and ions, e.g., C₂, CN, OH, CO⁺, OH⁺, N₂⁺, C₃, NH₂, and CO₂⁺ had been observed by their emission spectra excited by resonance fluorescence of sunlight. In 1971, I built a tunable dye laser and used it to demonstrate that atoms and radicals observed in comets using ground and satellite-based telescopes can be formed by stable molecules that are likely to be in comets. Laboratory experiments using lasers, and molecular beams show that N, O, S, C, CN, C₂, C₃, CS, O₂, and S₂ can be produced when vacuum ultraviolet radiation from the sun is absorbed by simple molecules like CO, N₂, C₂N₂, O_2, CO₂, CS₂, C₂H_2, C₃H₄, and H₂O. Lasers have been combined with quadrupole and time-of-flight mass spectrometers to detect the fragments that do not radiate and in this case, imaging and other techniques have been used to determine the velocity and angular distribution of the non-radiating fragment. The experiments have provided new data about basic photochemical processes occurring in isolated molecules on excited potential surfaces such as state-to-state dynamics, transition probabilities, intersystem crossing, and internal conversion. This information has been used to expand our understanding of the chemical processes that can happen when these simple molecules are excited in the many different environments observed in astronomy.