Lasers inversion mechanisms

Classification of Lasers Inversion Mechanisms in Gas and Plasma Lasers and Lasers on Self-Limited Transitions... 

The medium infrared spectral region contains typically vibrational transitions of molecules and their rotational substructure. Therefore it is obvious, that one can use vibration rotation transitions in a laser medium itself, provided there is an inversion mechanism available. However, in the gas phase such transitions are fairly narrow and therefore will not be the ideal source for spectroscopy, where one would like to have a continuously tunable laser source in order to scan across a series of vibration-rotation transitions of the molecular gas to be investigated. Although we can make use of it for very special situations e.g.for the spectroscopy of paramagnetic molecules, where Zeeman-tuning of the molecular transition can be achieved, we must use other types of gain media for a tunable infrared laser. 

The carbon monoxide laser normally operates at roughly twice the frequency of the CO2-laser. However, it can oscillate on several hundred lines between 4.8 and 8.4 jim [1,23]. This is due to the fact that many different vibrational levels in the deep, enharmonic potential curve of CO can be the starting point for laser emission. Under CW-condition, v=3= 2 up to v=37= 36-bands can be obtained and the frequency shift between adjacent bands is due to the anharmonicity of the CO-potential. The inversion mechanism is not as straightforward as in the CO2-laser. Usually no complete inversion for adjacent vibrational levels can be obtained and therefore only P-branch transitions do occur. Nevertheless, the wavelength region is completely covered with the rotational distribution of the vibrational bands (Fig.1.13). 

Willett, C.S. (1974). Introduction to gas lasers population inversion mechanisms. Pergamon, Oxford. 

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... 

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