Molecular Spectrocopy,1e

2.8.3 Spectrum of Helium Atom (Two-Electron System)

After one-electron atoms, the spectrum of helium is the simplest to study. Helium atom contains two valence electrons (1s²) and total binding energy of He-atom is 78.98 eV. The energy levels and the observed emission spectrum of He-atom are shown in Figure 2.20. The energy levels and spectrum results due to excitation of one of the two 1s electrons while the state of the second 1s electron is held constant. The lines in the emission spectrum fall into two distinct sets of hydrogen like series, which are quite independent of each other. The earlier workers thought that these two distinct and independent sets of series belong to two different types of helium christened by them ortho- and parahelium. ...