Generalisation of the Landau-Zener calculation to three levels

The Landau-Zener calculation, for avoided crossing of two energy levels, is generalised by putting in a third level, so that three levels approach one another. A three-level model of an atom, with two transitions driven by laser beams, can be transformed into the present model by using the rotating-wave approximation. The resulting transition probabilities have a simple analytic form, like those of the original two-level model. (author)