Abstract
The nonlinear susceptibility of a gas of molecules under level crossing in a single-mode laser field is calculated by an iteration procedure. The higher-order solutions up to the fourth order are explicitly given for arbitrary number of nearly degenerate levels. Contributions of the second-, third-, and fourth-order terms are discussed with particular emphasis on the third-order term. The third-order solution reveals the level-crossing effect in saturated absorption with the half-width almost twice as broad as the Lamb clip when the decay rates of the upper and the lower states are different to some extent. Finally the zero-field level-crossing experiment of methane at 3.39 µm by Luntz et al. is reexamined to result in the pressure broadening of its half-width to be 7.4 MHz/Torr.