Van Vleck-Weisskopf line shape

When the populations of the two levels involved in a transition are determined by thermal equilibrium conditions, the Van Vleck Weisskopf line shape function may be... 

Pressure broadening can be interpreted as the effect of collisions disrupting the processes of emission and absorption in a molecule. A number of different theoretical line shapes have been derived to explain pressure broadening. The one most widely used is the Van Vleck-Weisskopf line shape. Linewidths due to pressure broadening are proportional to pressure, the proportionality constant depending on the particular molecules involved in the collisions. For CO broadened by CO2, the linewidth of the ground-state transition of CO is approximately... 

The most important source of line broadening in microwave studies of bulk gas samples is collisional or pressure broadening, the theory of which was first developed by Van Vleck and Weisskopf [77], They developed the line shape function... 

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