Introduction

The concept of free radicals has long roots in chemistry; according to Herzberg [1.1] it already existed in the 19th century. An identification of a chemically stable free radical triphenylmethyl in the early 20th century gave impetus to the study of free radicals. However, it remained a quite difficult task for chemists to characterize free radicals, because most of them appeared only for a short period of time. A number of simple free radicals, mostly diatomic, were later identified in flames and electrical discharges, by recording and analyzing their emission spectra, e.g., CH, OH, CN, and C₂. The advent of new quantum mechanics provided spectroscopy with a sound basis, and brought about remarkable progress in spectroscopic studies of diatomic free radicals in the late 1920s and early 1930s, as compiled by Herzberg in [1.2]. After World War II, Herzberg and his students started high-resolution studies of transient molecules; a new technique of flash photolysis was introduced to generate such species, and emission and absorption spectra were recorded by large grating spectrometers [1.3].