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.
