Population Trapping Effect for System With Double Autoionizing Levels

Takashi Nakajima, P. Lambropoulos

Xin Chen, John A. Yeazell

Three distinct phenomena are found to affect the autoionization of a quasicontinuum. For certain initial superpositions of the states within the quasicontinuum, the autoionization may be inhibited by quantum interference and the population may be trapped. For those initial superpositions that do decay, a self-trapping effect can be observed which causes the superposition to evolve into a trapped state. The self-trapping is due to a redistribution of the population within the quasicontinuum via Raman-like transitions driven by the channel interaction with the continuum acting as the intermediate state. The physical system of a two-electron atom is studied. There the autoionizing continuum is created by an isolated core excitation that embeds a range of highly excited Rydberg states into the continuum. Another mechanism appears, in this system, that affects the autoionization. When the channel interaction is strong compared to the strength of the core excitation a stabilizing effect occurs which slows the autoionization. Analytical models examine these effects in a system with degenerate quasicontinua. Perfect population trapping, self-trapping, and stabilization are all clearly displayed by this system. Numerical models of nondegenerate quasicontinua show that the stabilization process is virtually unchanged by the lifting of the degeneracy. These models also show that, in general, the trapping effects largely disappear. However, a synchronization of the free evolution of the quasicontinuum states with Rabi oscillation of the core excitation returns them to significant roles in the decay process. A wide variety of nondecaying states can be created. Competition between these mechanisms is explored and the impact of these phenomena on experiments is considered.

P. Thornton Greenland

Emmanuel Paspalakis, M. Protopapas, P. L. Knight

We investigate the effects of smooth laser pulses in both an autoionization system and in the laser-induced continuum structure scheme. We are particularly interested in the phenomenon of population trapping in these systems and discuss the effects of time-dependent pulses on this trapping. We present analytical solutions to both systems which hold for any pulse shape, provided the adiabatic approximation holds. We also pay special attention to the effects of large pulse energies on the trapped population, which we find to be detrimental and propose a possible solution to this problem by using chirped (time-dependent) frequencies.

Guanhua Yao, Zhizhan Xu, CHEN RONG-QING

In the last decade, laser-induced autoionization has been an active subject of Strong laser-atom interaction. The main interest in the subject arises from many interesting high-field effects in the problem, notably the stabilization of the autoionizing state (AIS) and population trapping, which leads to possible applications in lasers without inversion.1 However, many decay mechanisms may destroy this interference effect to various extents. Many authors have addressed this problem, but few 2,3 have investigated the effects of high-order processes in strong-field autoionization.

Heider N. Ereifej