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Physical Review A

Rydberg electrons in intense fluctuating laser fields

G. Alber, B. Eggers

Published Jul 1, 1997• 4 citations
Publisher

Abstract

The influence of laser fluctuations on the optical excitation of Rydberg and continuum states close to a photoionization threshold is investigated within the framework of the phase diffusion model. Theoretical methods are developed for solving the relevant master equation of the atomic density operator which is averaged over the laser fluctuations. In the long-time limit this laser-induced excitation process exhibits nonexponential decay. This nonexponential time dependence reflects the fluctuation-induced diffusion of the Rydberg electron toward the ionization threshold.

DOI
10.1103/physreva.56.820

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The influence of laser fluctuations on the optical excitation of Rydberg and continuum states close to a photoionization threshold is investigated within the framework of the phase diffusion model. Theoretical methods are developed for solving the relevant master equation of the atomic density operator which is averaged over the laser fluctuations. In the long-time limit this laser-induced excitation process exhibits nonexponential decay. This nonexponential time dependence reflects the fluctuation-induced diffusion of the Rydberg electron toward the ionization threshold.
We develop a theory describing laser excitation of a Rydberg series by an intense laser field. Our theory is based on the fact that the radiative coupling is restricted to a region around the atomic nucleus (the reaction zone) which is small in comparison with the extent of the excited Rydberg states. The physical processes inside the reaction zone are characterized by a few parameters which are slowly varying functions of energy across the Rydberg threshold. This finite range of the radiative coupling allows us to apply methods from quantum-defect theory and to treat the laser interaction with the Rydberg series and the adjoining electron continuum as a whole. Within this approach we derive analytical expressions for transition probabilities as a function of time in either a dressed-state representation which is appropriate as long as only a few Rydberg states are excited or a multiple-scattering expansion which is particularly suited for a description of the excitation process close to threshold. The physical picture emerging in this limit is one of a radial electronic wave packet which is generated within the reaction zone and moves in the Coulomb potential of the ionic core. Every time it returns to the inner turning point of its orbit, i.e., to the reaction zone, it is either deexcited back to the initial atomic state or is scattered by the ionic core.
Rate equations are developed which describe the dynamics of a laser-excited Rydberg electron under the influence of a fluctuating laser field in the limit of large laser bandwidths. These equations apply whenever all coherent effects have already been damped out. The range of validity of these rate equations is investigated in detail for the case of phase fluctuations. The resulting asymptotic power laws are investigated which characterize the long-time dynamics of the laser-excited Rydberg electron and it is shown to what extent these power laws depend on details of the laser spectrum.
The dynamics of a laser-excited Rydberg electron under the influence of a fluctuating laser field are investigated. Rate equations are developed which describe these dynamics in the limit of large laser bandwidths for arbitrary types of laser fluctuations. These equations apply whenever all coherent effects have already been damped out. The range of validity of these rate equations is investigated in detail for the case of phase fluctuations. The resulting asymptotic power laws are investigated which characterize the long time dynamics of the laser-excited Rydberg electron and it is shown to which extent these power laws depend on details of the laser spectrum.
Excitation and photoionization of Rydberg wave packets are described. Interference of bound-free transitions from many coherently populated Rydberg levels is shown to affect the dependence of the photoionization probability on the duration of interaction. Strong-field interference effects in such a system are described also. The most important of these is the field-induced interference stabilization of Rydberg atoms in a strong ionizing field. The intensity range in which this effect can exist is determined. The results derived are used for a qualitative model describing supercontinuum emission in the process of multiphoton ionization.
An effect of phase fluctuations of a driving laser field on dissipative and dipolar forces of two-level atom is considered. The phase fluctuations are treated by a phase diffusion model where phase fluctuations follow Wiener–Levy process. An exact master equation for the relevant density operator is obtained and solved in the steady state. Optical forces are calculated. The effect of phase fluctuations on angular momentum imparted by a Laguerre–Gaussian beam and an ideal Bessel beam to the atom is investigated.
Laser-induced autoionization, a form of forced autoionization, is discussed. Highly excited atomic Rydberg states generally have a small cross section for photoionization due to their small overlap with the nucleus. The cross section for photoionization can be strongly enhanced if the Rydberg series is perturbed by an atomic state with a relatively large cross section for photoionization. The photoionization dynamics of such a perturbed Rydberg system shows strong similarities to that of an autoionizing Rydberg series. For intense laser pulses, which strongly couple the perturber to the continuum, the photoionization rate is limited by the coupling between the Rydberg states and the perturber or, in other words, the autoionization rate. The model describing this ionization dynamics is applied to the barium 6snd(1,3{(}^{1,3}(1,3D2{\mathit{D}}_{2}D2​) state which is perturbed by the 5d7d(1{(}^{1}(1D2{\mathit{D}}_{2}D2​) state. The calculations show good agreement with experimental results of photoionization of barium Rydberg states with subpicosecond laser pulses of light of 620 nm in wavelength.