A laser is used in this experiment to excite a transition from the lowest vibrational level of the ground electronic state of the molecule to a vibrational level of quantum number v' in the excited state. Therefore the frequency of the transition is:
The wavelength of the laser light is 514.5 nm. The frequency of this light,
, and the energy
of the transitions it can induce, 
E, may be calculated from:
Spontaneous emission of light occurs from the excited level. The frequency of this fluorescence light is:
and thus, frequency difference between the laser and the fluorescence is:
In practice, many final vibrational states of the ground state may be reached by the fluorescence and a good picture of the vibrational energy level structure of the ground state may be obtained. G"(0) has a special name: it is the zero-point energy of the molecule. (G'(0) is the vibrational zero point energy in the upper electronic state).