ENERGY  LEVELS AND  TRANSITIONS  IN  GAS  LASERS

     The transitions that bring about light emissions in a spectrum tube also give rise to laser action. In a helium–neon (He Ne) laser, for example, the active lasing species is the neon atom, which has a red transition between two upper-energy levels at 632.8 nm, the red light emitted by a common He Ne laser (Figure 1.1). A neon atom at ground state has an electron configuration of 1s² 2s² 2p⁶. It is an inert gas in that all outer orbitals are filled, so it is not reactive (it does not react to form compounds). If neon is excited sufficiently, it can achieve a level of 1s² 2s² 2p⁵ 5s¹ (the excitation mechanism). From that level it can fall to the 1s² 2s² 2p⁵ 3p¹ level and in doing so emit a photon of red light at 632.8 nm. This is the transition used for the red He Ne laser.

    As one might imagine, the two levels involved are not singular levels but rather a group of closely spaced energy levels. The upper level (2p⁵ 5s¹) is actually four hyperfine levels, and the lower level (2p⁵ 3p¹), 10 separate levels! Unlike sodium or hydrogen, there is more than one electron in the outer shell; in the case of neon, a total of eight electrons exist in the outer (n = 2) shells. These electrons interact to produce many possible energy levels. As a result, there are numerous transitions at which the helium–neon laser can operate. Some transitions are favored

Figure 1.1. Energy levels for neon transitions.

over others, and not all will produce laser light. Although the red transition at 632.8 nm is the strongest of the visible transitions (and the most commonly available), a green transition also exists at 543.5 nm, allowing a green He Ne laser. In all, there are 10 possible wavelengths between red and green at least four are commercially available as off the shelf He Ne lasers (the red and green mentioned above plus a yellow and orange transition). These energy levels are shown in Figure 1.1 as well as the corresponding emission wavelengths.