Gerhard Herzberg, who died on 3 March, was one of the world's foremost molecular spectroscopists. The myriad wavelengths of light emitted or absorbed by molecules offer a vast lode of knowledge about the arrangement of atoms within molecules, the properties of the chemical bonds linking atoms, and the electronic interactions governing those bonds. Moreover, insight into how bonds can be made or broken in chemical reactions is crucial for understanding phenomena in physics and astrophysics, chemistry, materials science, biology and medicine. Developing the means to observe and decipher molecular spectral patterns has been a mighty task, in which Herzberg had a leading role for 60 years.

Herzberg was born on Christmas day in 1904 in Hamburg, Germany. When he was a young student, inspiring teachers encouraged his interest in science. However, after the director of the Hamburg Observatory dissuaded him from pursuing advanced study in astronomy, he entered the Technische Hochschule in Darmstadt, where he completed a doctorate in engineering physics in 1928. The period of his doctoral work coincided with the advent of quantum mechanics, the essential theoretical foundation for molecular spectroscopy. Herzberg liked to say “this fortunate accident made it possible for me to learn the subject while it was being developed”.

At Göttingen, then one of the prime centres for quantum mechanics, Herzberg worked both in the experimental institute headed by James Franck and the theoretical institute led by Max Born. During that time, Herzberg made his first measurements of electronic transitions in polyatomic molecules. He also did important theoretical work. In particular, in collaboration with Walter Heitler, Herzberg made use of a theorem, proven by Eugene Wigner a few months before, to show that intensity variations seen in rotational lines of the spectrum of the diatomic nitrogen molecule were in conflict with the prevailing view of nuclear structure. This was a striking result, not cleared up until three years later, when James Chadwick discovered the neutron. Herzberg also began work on molecular orbital theory as a systematic approach to describing the electronic configurations of molecules and their spectral transitions.

At the invitation of J. E. Lennard-Jones, who was also working on molecular orbital theory, Herzberg continued his studies of spectra at the University of Bristol, and diligently practised his English. He submitted his work on molecular orbitals as a habilitation thesis to Darmstadt and in 1930 was appointed Privatdozent there. This meant he did not receive a salary, but was able to earn a modest income by supervising laboratories and research students. He pursued research vigorously, and collaborated with Edward Teller in working out selection rules for the vibrational structure of electronic transitions of polyatomic molecules. When deuterium was discovered by Harold Urey in 1932, Herzberg recognized how this isotope of hydrogen might benefit studies of molecular structure. He demonstrated this by synthesizing several deuterium-labelled compounds and analysing the spectral shifts induced by replacing hydrogen with deuterium.

After Hitler came to power in 1933, anyone with a Jewish wife was not allowed to teach at a German university, and Herzberg looked for a position in another country. In 1935, the Herzbergs relocated to the University of Saskatchewan in Saskatoon, Canada. This haven came about because of the efforts of a young physical chemist, J. W. T. Spinks, who had worked with Herzberg on deuterated molecules. Here Herzberg began some of his most seminal work, including experiments in which he identified molecules in interstellar space by replicating their spectra in the laboratory.

In 1945 Herzberg went to the Yerkes Observatory of the University of Chicago, where he discovered the quadrupole spectrum of molecular hydrogen, destined to have a major role in identifying hydrogen both in interstellar space and in planetary and stellar atmospheres. In 1948 he returned to Canada to join the National Research Council laboratory (NRC) in Ottawa, in which he served as director of physics for 20 years. During his era, the NRC became the foremost laboratory for molecular spectroscopy in the world, by virtue of his own fruitful work, and the enterprising spirit and creative freedom he fostered in the research staff.

Herzberg was a mentor to legions of scientists by means of his definitive series of books on atomic and molecular spectroscopy, published in five volumes between 1936 and 1979. Extraordinary in scope, yet readable by the neophyte, his books have been translated into several languages. Researchers find them essential guides, with their comprehensive tables of molecular parameters and abundant illustrations of characteristic spectra and diagrams explaining theoretical concepts.

Herzberg's unpretentious, forthright manner, integrity and devotion to science were greatly admired by colleagues and students. His voice was a powerful baritone, enhanced by singing lessons with which he rewarded himself after completing his first book. Also relished were his ready humour and storytelling. A revealing tale stemmed from a visit to the Soviet Union in 1959. His books had been reprinted for Soviet scientists, and at his request the royalties were delivered: a shoebox full of roubles that could only be spent within the Soviet Union. Cheerfully, he applied a strategy akin to that he had used when leaving Nazi Germany over 25 years before. With the roubles he purchased several round-the-world aeroplane tickets, which he later exchanged for cash.

Herzberg received many honours, including the 1971 Nobel Prize in Chemistry. Particularly cited were his intrepid studies of free radicals, extremely reactive chemical species that are transient intermediates in many reaction processes. In 1975 the NRC Herzberg Institute of Astrophysics was created in his honour, and he continued active research until the age of 90. His legacy will long endure, and continue to grow, as methods he developed or elucidated enable new generations of scientists to read intriguing messages encoded in molecular spectra.