We begin with gas lasers. In gas lasers, usually there is a mixture of gases. One of the gas types is excited by an electric discharge. The excitation energy of one kind of atom or molecule is transferred to a closely lying level of another kind of atom/molecule in a collision. Once the metastable level of the second molecule is populated, laser action becomes possible. The He - Ne laser is a typical example. The ground 1s2 state of He is excited to the 1s1 2s1 state, which transfers energy to Ne and the excited Ne atoms exhibit laser action at several wavelengths ranging from 633 nm to 3.4
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In the argon laser, the electric discharge produces several ions like Ar+ and Ar2+ in the excited states. These states are metastable states and exhibit laser action at wavelengths 514 nm (green) and 488 nm (blue). The krypton ion laser leases at 647 nm (red) and these Lasers are used in light shows as well as for generating radiation with high power.
While the above examples use electronic excitations the N2 - CO2 laser uses vibrational excitations of N2 molecules that are collisionally transferred to the excited vibrational levels of CO2 which then undergo laser transitions. Several vibrational levels of CO2 are involved and these lasers are in the IR frequency range.
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