The ruby laser three-level lasers

The ruby laser, invented in 1960, was the first device to put the ideas just described into practice. Rubies are crystals of alumina (aluminium oxide, corundum, AI2O3), containing about 0.5 % chromium ions, Cr +, in place of aluminium ions, Al . Ruby is a dilute solid solution, and the Cr ions form substitutional defects. The laser action involves only the Cr ions and is due to the transition of electrons from the ground state to higher energy levels among the 3d orbitals. 

In the case of ruby there are two important spin-allowed transitions  

Lying at a lower energy than E2 is an energy level El- This energy level is due to an electron configuration in which two electrons are spin-paired, (tit ) A transition from Eq. 2 or E3 to Ei is not allowed under the total electron spin rule. None of these transitions would normally be involved in transitions that produce colour. However, in ruby, excited Cr ions in states E2 or 3 can lose energy to the crystal stmcture and drop down to level Ei. This process operates under different conditions from the optical transitions and is independent of spin. The energy is taken up in lattice vibrations and the ruby crystal warms up. This is called a radiationless or phonon-assisted transition. Typical rates of the transitions are  

It is energetically costly to obtain a population inversion in a three-level laser because one must 

5 The neodymium (NiF ) solid-state laser four-level lasers 

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