Theoretical investigations of population trapping phenomena in atomic four-color, three-step photoionization scheme

Abstract

The atomic four-color, three-step selective photoionization process is critical in laser isotope separation technology. The population trapping phenomena and their influences are studied theoretically in monochromatic and non-monochromatic laser fields based on the density matrix theory. Time evolutions of the photoionization properties of the four-color, three-step process are given. The population trapping effects occur intensely in monochromatic excitation, while it gradually turns into weakness as the laser bandwidth increases. The effects of bandwidth, Rabi frequency, time delay, and frequency detuning on the population trapping effects are carried out in monochromatic and non-monochromatic laser fields. Influences of laser process parameters and atomic parameters are also discussed for effective selective photoionization. The ionization probability and selectivity factors, as evaluation indexes, are difficult to be improved synchronously by adjusting systematic parameters. Besides, the existence of metastable state may play a negative role when its population is low enough.