The level crossing method and recent achievements of high-resolution laser spectroscopy

The level crossing method is compared with current methods of laser spectroscopy. The compared methods supplement one another in the solution of certain problems of atomic physics and are based on quite different physical phenomena. The level crossing method is one of the simplest and most effective methods for investigating the structures of atomic levels. It makes it possible to measure the separations of very close atomic states that could not be resolved by classical spectroscopic methods. This is due to the fact that the crossing signal is determined by the characteristics of the emitting state and is not masked by the Doppler broadening that usually limits the spectroscopic resolution of close lines. For the same reasons, however, the method does not permit one to investigate phenomena associated with the line shape. Special laser-spectroscopy procedures have recently been developed (two-photon excitation and saturated absorption spectroscopy) in which the resolving power is determined, as in the crossing method, by the natural line width, and not by the Doppler width. This has made it possible in certain cases to achieve high resolution of spectral components and to investigate phenomena taking place “under cover” of the Doppler broadened line.