The Rigid Rotor

• Two particles whose separation is fixed (the "rigid" part) are allowed to rotate (the "rotor" part) about their common center of mass.
• The angular momentum of the system is quantized. This also quantizes the energy of rotation.
• The orientation of the angular momentum is also quantized.
• The wavefunctions are the spherical harmonics.
• For diatomic molecules, pure rotational spectra (in the microwave to far infrared region of the spectrum) show a series of evenly spaced transitions. The separation is 2B, where B is the rotational constant for the molecule, which depends upon its masses and its internuclear separation.