Understanding the essentials of energy transfer

The reluctance of nuclei to undergo a change in their momenta has so far been discussed in terms of the Franck-Condon principle. In its simplest form it says that a photon-induced electronic excitation is not accompanied by a change in the kinetic energy of the nuclei. But an inelastic collision is necessarily characterized by such a change, that we called the gap. We have seen ample experimental evidence that the transition is more likely when the gap is small. How small is small is the subject of this section. 

The opposite extreme is that of a very loose BC oscillator spring. Here the atom C is so very weakly connected to B that it initially does not respond to a change in the velocity of atom B. hr the limit of a vanishingly small oscillator 

We next need to decide what is the scale against which we regard the oscillator as stiff or loose. 

In physical terms, the limit of a loose BC oscillator spring is the limit where the duration, of flie A—B collision is short compared to the time required by atom 

C to know that atom B has changed its momentum. B communicates with C via their mutual force. So the time required for C to respond is the vibrational period, c, of the oscillator. The limit of a loose spring is therefore when 

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