Brief History of Spectroscopy

Optical spectroscopy is the investigation of the interaction between light and matter. It is a vital tool for scientific research, as it allows an unparalleled depth of information to be quickly and easily obtained from a wide range of materials and systems in a completely non-destructive manner.

The technique has its origins in the 17th Century discovery by Isaac Newton that sunlight is composed of bands of coloured light which are bent differently when passed through a glass prism. Subsequent measurements of sunlight by Wollaston and Fraunhofer revealed dark lines in the solar spectrum, although it was not understood that these lines were due to absorption by elements at the surface of the sun until 1859. This understanding opened up the way to spectroscopic analysis of different materials.

Further breakthroughs came at the turn of the century with the development of the quantum mechanics. Following on from the work by Planck and Einstein in which electromagnetic radiation is described as consisting of individual particles with finite energy, Neils Bohr proposed the model of electrons orbiting the atomic nucleus with fixed energy. The previously observed spectral lines could then be explained as transitions between energy levels of electrons in the atom.

Most of the experiments leading to these advances in understanding used sunlight, emission from elements excited in Bunsen burner flames, and from the late 19th century, emission from electric arc lamps. That is to say, most experiments were on systems in a steady state, with equal rates of population and decay of excited states. This is despite the fact that difference between short lived fluorescence and long-lived phosphorescence had already been observed, first as a physical curiosity, and then experimentally by Bequerel in 1858 using a mechanical phosophoroscope. This ingenious design allowed measurements of phosphorescence lifetimes down to below 100 us.

While these developments showed optical spectroscopy to be a vital tool to characterise materials and probe the interior workings of the atom, the most significant advance in the field came with the development of the laser in 1960. The narrow linewidth, vastly improved beam collimation, coherence, and intensities made possible by the first lasers opened up new avenues for investigation. The development of lasers with tuneable wavelength and pulsed operation further extended the range of possible measurements. Dye lasers and Q-switched lasers were developed in the 1960s, just a few years after the first ruby laser was developed and mode-locked dye lasers with pulses below 100 fs in duration were reported as early as 1981. Solid-state modelocked lasers and amplifiers driving parametric oscillators and parametric amplifiers have become the most commonly used systems for generating ultrashort pulses with wide spectral tunability, as they are far simpler to maintain and operate than dye lasers.