The different reduced mass of the molecules that contain isotope atoms may also have an effect on the optical spectra of the isotope molecules. The phenomenon is called the spectroscopic isotope effect, and it can be observed in both atomic and molecular spectra.

Light emission is the result of the change of the energy of a particle from a greater level (E0) to a lower level (E"). In light absorption, the reversed process takes place. The energy levels mean rotation, vibration, and electron energies. The change in the rotation and vibration energies produces the molecular spectra, whereas the change of the electron energies gives the atomic spectra. As seen in Section 3.1, all the rotation, vibration, and electron energies depend on the reduced mass of the molecule (Eqs. (3.2) and (3.6)) or the atom (Eq. (3.9)), so the spectra of the isotope molecules and atoms are different. For example, the reduced mass of the H35Cl is д = 0.9722 and that of the H37Cl molecule is д = 0.9737. As can be calculated by Eq. (3.8), the ratio of the vibration energies of the two molecules is 1.00076. This value is very close to 1, so the difference of the spectra can be observed only by very high resolution spectrometers.

The spectroscopic isotope effects can be observed in some atomic spectra too. However, the difference in the reduced masses of the isotope atoms is very small. As a result, only hydrogen—deuterium spectroscopic isotope effects can be detected easily. The wave number of the hydrogen isotopes can be calculated by Eq. (3.9).

The wave number of a Ha line is 15,233 cm-1 and that of a Da line is 15,237 cm-1. The difference is 4 cm-1, which can be observed by traditional spec­trometers. As seen in Eq. (3.9), the reduced masses determine the Rydberg constant (Ry), the ratio of which for deuterium and hydrogen is:

This value is about three times less than the ratio for the vibration energies of the HCl isotope molecules (1.00076). The natural isotope ratio of hydrogen to deu­terium was determined on the spectral line intensities of hydrogen in 1939.