Franck-Condon principle

When electron transitions (excitation, de-excitation) occur due to electron beams or photons in a molecular system, the time required for the transition is extremely short compared to vibration between atoms. Therefore, the distance between atoms may be considered constant. While the general rotational cycle of molecules is 10^-11~ 10^-12 sec and the vibrational cycle is 10^-13 ~ 10^-14 sec, electron tran-sitions are on the order of 10^-16 sec. These electron transitions are also called perpendicular transitions.