Ammonia Inversion Spectrum
This experiment is a beautiful realization of the effects of the quantum mechanical double well potential. The ammonia molecule is pyramidal shaped with the three hydrogen atoms forming the base and the nitrogen atom at the apex. The nitrogen atom sees a double-well potential with one well on either side of the plane defined by the H atoms. The wave function for the N atom can be either symmetric or anti-symmetric, the two states being split by a small energy difference. When a RF (microwave) field is applied at this energy difference, the resulting wavefunction (a combination of the symmetric and anti-symmetric functions) shows the N atom to be tunneling back and forth from one well to the other at the frequency of the applied field. The shape of the molecule and therefore that of the double well potential is a function of the rotational state of the molecule, with the result that the tunneling is observed at many different frequencies depending on the particular rotational state. Students measure a number of these frequencies and fit them to a model accounting for the rotational effects. Students also observe and measure hyperfine splitting due to the quadrupole moment of the N nucleus.
