a)   For a  heteronuclear diatomic molecule, draw an energy level diagram showing  the          four lowest rotational energy levels. Indicate the energy  of each level in terms of B.          Indicate all possible light  absorption transitions.
 
 b)   Draw a spectrum (absorbance vs. frequency) corresponding to  the transitions you          have shown in part a). Indicate which  transitions in the energy level diagram            correspond to which  peaks in the spectrum.