Excited state displacement gives rise to the line shape
The nuclear factor gives rise to line shapes. The reason for this is that the intensity of absorption lines is distributed among the various excited state vibrational levels due to the shift in the potential energy profile between the grond and excited state. The bond order of a molecule is affected because of promotion of an electron. Usually it is decreased since the electron is promoted from a bonding molecular orbital to an anti-bond orbital. Thus, molecular bond lengths tend to increase in the excited state.
