A five-level quantum system including two low-lying states and three excited states of the ladder type coupled with four laser fields is investigated as it relates to dressed-atom models, and a dressed state is found to be independent of the ionization decaying from the highest level. Under general conditions, half of the initial population of the two low-lying states is trapped in the dressed state, leading to a low probability of ionization. Based on the analytical results, two schemes are proposed for removing population trapping. For a five-level system with hyperfine structures, such as the system for ionizing <sup>235</sup>U atoms, population trapping takes place when the laser fields are weak and can be gotten rid of by an increase in laser intensity.
© 2003 Optical Society of America
Xi-Jing Ning, "Removing population trapping in a five-level system with fast decay," J. Opt. Soc. Am. B 20, 2363-2367 (2003)