Question

Explain what a selection rule is and give the selection rules for rotational and electronic spectroscopy

Explain why Raman scattering and infrared absorption give different spectra despite the fact that they both probe vibrational transitions Please give detailed answer, thanks

Answer 1

Selection rule states that which radioactive transition is feasible between the different energy levels. The selection rule for rotational transitions for a rigid diatomic molecule are governed by the selection rule   $\Delta J= \pm 1$ i.e. only those transitions are allowed in which rotational quantum number changes by unity .the + sign refers to absorption and the - sign to emission of radiation.

The selection rule for electronic spectra is , transition will always takes place from singlet to singlet state and form triplet to triplet state.But never form singlet to triplet or triplet to singlet . Secondly, electronic transition consist of rotational as well as vibration transition, thus electronic transition always takes place from V=0 vibrational level in ground state as the population of electrons is maximum in this state.

In contrast with infrared spectroscopy, Raman spectroscopy deals with scattering of light and not with its absorption. Thus although their probe is same ,but as they deals with differently so they give different spectra.