Population inversion, and laser action

Such a situation suggests the possibility of creating a population inversion and laser action between two such states, since any molecules in the repulsive ground state have an extremely short lifetime, typically a few picoseconds. A laser operating by this mechanism is a... 

Whereas population inversion can often be achieved relatively easily with a pulsed pumping source, it is considerably more difficult, and sometimes impossible, to continuously maintain population inversion while laser action prevails. Clearly, lasing means that excited atoms decay with the emission of stimulated radiation and thus the laser action itself will cause the lasing action to stop. Efficient pumping mechanisms are required. We will now study some important types of continuous fixed-frequency lasers which all work with gaseous laser media. 

Lasers are devices for producing coherent light by way of stimulated emission. (Laser is an acronym for light amplification by stimulated emission of radiation.) In order to impose stimulated emission upon the system, it is necessary to bypass the equilibrium state, characterized by the Boltzmann law (Section 9.6.2), and arrange for more atoms to be in the excited-state E than there are in the ground-state E0. This state of affairs is called a population inversion and it is a necessary precursor to laser action. In addition, it must be possible to overcome the limitation upon the relative rate of spontaneous emission to stimulated emission, given above. Ways in which this can be achieved are described below, using the ruby laser and the neodymium laser as examples. 

Exciplex lasers (also called excimer lasers) use reactive halogen atoms to form excited pseudo-molecules with noble-gas atoms. Molecules such as XeF are stable on ly in excited electronic states and quickly dissociate after transition to the ground state. This makes possible a large population inversion and produces laser action in the ultraviolet region. A simple prototype for such behavior is the Hcj excimer, which is an entry in Table 11.1. 

Population inversions have been observed in a number of chemical and photochemical reactions. In a few of these cases, laser action has been produced in a suitable cavity. In most cases of molecular laser emission, there is only partial inversion282 in which several vibration-rotation transitions are inverted even though the total population in the upper vibrational state does not exceed that in the lower. In this case there is laser action in P branch transitions only. 

---