Presentation Speech by Professor Ingvar Lindgren of the Royal Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

This year’s Nobel prize in physics is shared between three scientists – Nicolaas Bloembergen and Arthur Schawlow, both from the United States, and Kai Siegbahn from Sweden – for their contributions to the development of two important spectroscopic methods – laser spectroscopy and electron spectroscopy.

Both of these methods are based upon early discoveries of Albert Einstein. One of the major problems for physicists of the last century was to explain, with the “classical” concepts, the so-called photoelectric effect, i.e. the emission of electrons from a metal surface irradiated with light of short wavelength. In 1905 Einstein explained this phenomenon in a simple and elegant way, using the quantum hypothesis introduced by Max Planck five years earlier. According to this model, light is a wave motion, but it is quantized, i.e. it is emitted in small pieces – light quanta or photons – which in some respects behave like particles. This discovery was the first foundation stone to be laid in the building of the “new” physics – the quantum physics – which was to develop rapidly during the first decades of this century.

The photoelectric effect is the basis of the spectroscopy which Kai Siegbahn has developed together with his collaborators in Uppsala. When a photon of high energy – e.g. from an X-ray tube – hits an atom, it can penetrate deeply into the atom and expel an electron. By analyzing the electrons expelled in this way, it is possible to extract valuable information about the interior of the atom. Early experiments of this kind where performed in the second decade of this century, but the method was not sufficiently developed to probe the atomic structure until the 1950’s. At that time Kai Siegbahn had for a number of years developed more and more sophisticated instruments for analyzing electrons emitted at the decay of certain radioactive nuclei – so called beta decays. When he and his collaborators applied this technique to analyze the electrons emitted in the photoelectric process, the new era of electron spectroscopy was born. With this spectroscopy it became possible to determine the binding energy of atomic electrons with higher accuracy than was previously possible. This was of great importance for testing new atomic models and computation schemes, which were being developed at the same time, partly due to the simultaneous rapid development of computers. It was furthermore found that the electronic binding energy was to some extent dependent upon the chemical environment of the atom, and this led to a new method for chemical analysis – ESCA – which stands for “Electron Spectroscopy for Chemical Analysis”. Nowadays, this method is being applied at hundreds of laboratories all over the world, particularly in the investigation of surface reactions, such as corrosion and catalytic reactions, i.e. reactions where a substance may initiate or stimulate a chemical reaction, seemingly without taking part in it. Such reactions are of vital importance for the process industry, and the spectroscopy developed by Siegbahn and his collaborators can be of great help in our efforts to understand processes of this kind.

The second form of spectroscopy which is awarded with this year’s Nobel prize – the laser spectroscopy – is based upon another early discovery of Einstein. It had been known for a long time that atoms and molecules could absorb light as well as spontaneously emit light of certain wave lengths. In 1917 Einstein found that light could also stimulate atoms or molecules to emit light of the same kind. This is the basic process in the laser. Photons emitted in such a stimulated process have not only the same wave length but they also oscillate in phase with each other. We call light of this kind coherent.

Coherent light could be compared with a marching military troop – where the soldiers correspond to the photons – while non-coherent light in this model could be compared with people on a busy shopping street on a Saturday morning. Those who have done their military service know that marching soldiers should keep in step. However, there is one occasion, namely when the troop crosses a small bridge, when it is necessary to break step, otherwise the strong, coherent vibrations of the troop could break the bridge. The situation is similar for the coherent light. Due to the fact that the photons oscillate in phase, such light will have a much stronger effect than incoherent light on the irradiated material, and this gives laser light its very special character.

Coherent radiation was first produced in the microwave region, using an instrument we call maser (MASER=Microwave Amplification by means of Stimulated Emission of Radiation). The idea of the maser was conceived in the middle 1950’s by the American Charles Townes and by Basov and Prokhorov from the Soviet Union, who shared the Nobel prize in Physics in 1964. Townes and Arthur Schawlow extended the idea of the maser to the optical region – i.e. for visible light – and this led to the construction of the laser two years later (LASER= Light Amplification by means of Stimulated Emission of Radiation). At Stanford University Schawlow has led a research group, which has developed a number of advanced methods, where the laser is used to study the properties of atoms and molecules with extreme accuracy. This has stimulated the development of new theoretical models and improved appreciably our knowledge of these building blocks of matter.

Nicolaas Bloembergen has contributed to the development of laser spectroscopy in a different way. Laser light is sometimes so intense that, when it is shone on to matter, the response of the system could not be described by existing theories. Bloembergen and his collaborators have formulated a more general theory to describe these effects and founded a new field of science we now call non-linear optics. Several laser spectroscopy methods are based upon this phenomenon, particularly such methods where two or more beams of laser light are mixed in order to produce laser light of a different wave length. Such methods can be applied in many fields, for instance, for studying combustion processes. Furthermore, it has been possible in this way to generate laser light of shorter as well as longer wave lengths, which has extended the field of application for laser spectroscopy quite appreciably.

Professor Bloembergen, Professor Schawlow, Professor Siegbahn: you have all contributed significantly to the development of two spectroscopic methods, namely the laser spectroscopy and the electron spectroscopy. These methods have made it possible to investigate the interior of atoms, molecules and solids in greater detail than was previously possible. Therefore, your work has had a profound effect on our present knowledge of the constitution of matter.

On behalf of the Royal Swedish Academy of Sciences I wish to extend to you the heartiest congratulations and I now invite you to receive this year’s Nobel prize in Physics from the hands of His Majesty the King!