Franck Condon Principle..

Statement states that: "An electronic transition takes place so rapidly that a vibrating molecule doesn't change its internuclear distance appreciably during the transaction".

a) Upper electronic state has the same equilibrium internuclear distance as the lower. Franck condon principle suggest that the transition occur vertically on this diagram since internuclear distance doesn't change and so if we consider the molecule to be initially in the ground state both electronically and vibrationally then the most probable transition is that indicated by the vertical line. Based on these line of V"=0 progression will be (0,0) line.

b) Upper state inter nuclear distance is little greater than that in the lower state. There is a small chance that the atom being near the extremities of the vibrational motion. Hence there is some chance of the transition starting from V"=0 state and finishing in V'=1,2,........ state in this case (1,0) line is the most intense line.

c) The upper state distance is considerably larger so in this case only few lines are seen in the spectrum and there is a continuum of the point at which continuum begins corresponds to the dissociation of the molecule.