[UPSC CSE 2007] Molecular Physics (Q261, 20M)

(i) State and explain Franck-Condon principle. (5) (ii) In three separate pairs of electronic states (one ground and another upper state) of diatomic molecules, the average internuclear distances are -

1. equal;
2. slightly greater;
3. appreciably greater. In accordance with Franck-Condon principle, obtain most favorable transitions from the ground state vibrational levels in each of the above situations. Illustrate the transitions by drawing suitable energy-level diagrams. (15)