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The structure of molecules (Part 1)


The absorption of light by electrons and the Franck-Condon Principle
Edward Teller Scientist
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Now, to get back to my work in Göttingen, there was one more phase of it that again I want to talk to you about for its own sake and because of its consequences, its later continuation. In Göttingen, I met Hertzberg, a man interested in electronic absorption of light by the electrons, in molecules, where the absorption of light was accompanied by the excitation - by giving energy to - the vibration of the molecule. And this process had been already explained in some general simple cases, by what is called the Franck-Condon Principle. Franck, the same James Franck about whom I already told you, and Condon, an excellent American physicist. And the Franck-Condon principle said, very simply, the excitation of vibrations in a molecule occurs like this; we start from a configuration of the atoms in a molecule, with electrons not excited. The excitation of the electron occurs in first approximation instantaneously. The molecule gets excited without the atoms having time to change their position. But in the excited state, the old equilibrium position is no longer an equil- equilibrium position, therefore in the excited electronic state, the molecule starts to vibrate. This was a well-known situation, known as the Franck-Condon Principle, on which Hertzberg started to work. He and I then discussed in detail what happens if the molecule had the same symmetry in the lower state and in the upper state. And the simple conclusion is that in that case, if the molecule was not vibrating in an asymmetric fashion, there is no reason why it should start to vibrate in the excited state. And in this way you can see very easily which are the vibrations which disturb the symmetry and which are thos- those which don't disturb the symmetry. We discussed that in all kinds of detail. But I later, in the United States, with some of my friends, two ladies, one, Mrs Nordheim, and one, Herta Schpona, the assistant of Franck, both of whom had come to the United States.

The late Hungarian-American physicist Edward Teller helped to develop the atomic bomb and provided the theoretical framework for the hydrogen bomb. During his long and sometimes controversial career he was a staunch advocate of nuclear power and also of a strong defence policy, calling for the development of advanced thermonuclear weapons.

Listeners: John H. Nuckolls

John H. Nuckolls was Director of the Lawrence Livermore National Laboratory from 1988 to 1994. He joined the Laboratory in 1955, 3 years after its establishment, with a masters degree in physics from Columbia. He rose to become the Laboratory's Associate Director for Physics before his appointment as Director in 1988.

Nuckolls, a laser fusion and nuclear weapons physicist, helped pioneer the use of computers to understand and simulate physics phenomena at extremes of temperature, density and short time scales. He is internationally recognised for his work in the development and control of nuclear explosions and as a pioneer in the development of laser fusion.

Duration: 4 minutes, 21 seconds

Date story recorded: June 1996

Date story went live: 24 January 2008