Free radicals are very reactive and short-lived, making them hard to study. Herzberg measured their ultraviolet–visible and infrared spectra and used the peaks to deduce electron arrangements (electronic structure) and bond lengths/angles (geometric structure). His data help chemists understand reaction mechanisms and processes such as ozone formation in Earth’s atmosphere. They also aid in identifying molecules found in outer space, laying the groundwork for astrochemistry.

Using high‐resolution molecular spectroscopy, Herzberg analyzed vibrational–rotational and electronic transitions to map potential-energy curves of many free radicals. By comparing experimental spectra with quantum-chemical calculations, he precisely determined electron configurations, spin multiplicities, symmetries, and derived molecular constants (rotational constants, centrifugal distortion constants, etc.). These results were compiled in the classic ‘Molecular Spectra and Molecular Structure’ series, still a standard reference in spectroscopy. He further applied the laboratory data to astronomical spectra, confirming the presence of interstellar molecules such as H2O+ and CH+ in regions like the Orion Nebula.

Herzberg generated radicals such as CH, OH, NH, and CN via multi-stage discharge cells and flame spectroscopy, recording electronic band systems with resolutions down to ~10^−3 cm^−1. Detailed analysis of λ-doubling, Λ-type resonances, and other fine structures, combined with ab-initio calculations, allowed quantitative assessment of orbital mixing and relativistic effects. He applied the Rydberg–Klein–Rees inversion to derive potential curves, calculating Franck–Condon factors and dissociation energies, and published numerous molecular constants as international reference data. These parameters have become indispensable for atmospheric chemistry, plasma diagnostics, and modelling of the cosmic infrared background.