Complete population trapping in two-photon ionization

PAUL M. RADMORE, P. L. Knight

M. Kaivola, Nis Bjerre, Ove Poulsen +1 more

Zhifang Deng

We study a theoretical model of photoionization. An initially occupied ground electronic state is ionized to a flat continuum by two-photon excitation. There are two near-resonant intermediate excitation states. The line-narrowing confluence effect and population trapping are studied. We show that we cannot define an effective Hamiltonian to relate our model to the Fano model. In connection with recent studies of laser-induced autoionization (LIA), we show several differences between our model and LIA. For example, we show that the zero in the absorption spectrum and in the photoelectron spectrum has different features compared with that in the Fano model [ Phys. Rev.124, 1866 ( 1961)]. We also show that a laser-power-independent confluence condition and a complete population-trapping condition can be found in our model.

Kazuyuki KATOH, Atsuyuki Suzuki

The 4-photon 3-step ionization scheme was analyzed by using Bloch equations. The effect of laser bandwidth was considered in accordance with the decorrelation method. For monochromatic lasers, the coherent population trapping was observed and the effects of detunings and Rabi frequencies on the ionization characteristics were studied. For nonmonochromatic lasers, the coherent population trapping was not observed and the enhancement of ionization at large detunings was observed. In order to evaluate the selectivity, the ionization probability of atoms which have detuning of 5 GHz for each excitation step was examined. Two small ionization peaks were observed for nonmonochromatic lasers while only peak was observed for monochromatic lasers. However sufficiently high selectivity can be expected over detunings.

Nikolai L. Manakov, S. I. Marmo, S. A. Sviridov

Xi‐Jing Ning

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 235U atoms, population trapping takes place when the laser fields are weak and can be gotten rid of by an increase in laser intensity.