  In the present article, we introduce a model to investigate the energy
spectrum of a relativistic rotor by considering the Klein-Gordon Hamiltonian.
Rotational spectral lines are a signature of homonuclear and heteronuclear
systems and play a key role in understanding diatomic molecules. We show that
the energy-correction term arising due to unequal masses influences the line
separation. Determining the rotational constant enables one to calculate the
moment of inertia and bond length of the molecule.
