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Physical Review A

Laser-induced stabilization of autoionizing states

Heider N. Ereifej, J. G. Story

Published Nov 1, 1999• 5 citations
Publisher

Abstract

Stabilization of autoionizing states of barium by laser-induced, stimulated emission of light is demonstrated. Relative to purely flourescent stabilization, the data clearly show an enhancement of the stabilization process for laser pulses short compared to the flourescent lifetime of the autoionizing states. Shakeup spectra in which the principal quantum number of both electrons changes during the stimulated emission process are also clearly demonstrated.

DOI
10.1103/physreva.60.3947

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Stabilization of autoionizing states of barium by laser-induced, stimulated emission of light is demonstrated. Relative to purely flourescent stabilization, the data clearly show an enhancement of the stabilization process for laser pulses short compared to the flourescent lifetime of the autoionizing states. Shakeup spectra in which the principal quantum number of both electrons changes during the stimulated emission process are also clearly demonstrated.
Atoms in doubly excited states above the first ionization limit can decay via autoionization in which an electron is emitted leaving an ion, or by photoemission which leaves the atom in a singly excited state. In this paper, it is demonstrated that interaction between the atoms and a laser pulse that is short compared to the autoionization lifetime can lead to large enhancement of the photoemission process by stimulating the atoms to emit a photon. Since the resultant singly excited atoms do not autoionize, this process can be viewed as an enhancement of the stabilization of the doubly excited atoms against autoionization. A simple theoretical model is outlined that shows good agreement with the experimental results.
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.
The atoms at autoionization state can be excited to higher continuum state, as well as transit to bond states by spontaneous emission a photon in laser induced autoionization. The effect of these processes on the laser induced autoionization process has been investigated by means of using the reaction matrix. The effect of spontaneous emission process is small in general case, but its spectrum shows that the emission is contributed mainly from the spontaneous Raman scattering enhanced by the cutoioni-zation state, and the Dressed atom picture was shown clearly. The multiphoton process makes the photoelectron spectra and laser induced autoionization process very complicated. For example, the photoelectron spectra of one and two-photon process have multi-maxium structure.
We present the results of our study on coherent control of photoionization of atomic barium. Our study focused on the understanding of the controllability, especially due to the effect of the coherent interaction between the atomic system and the laser field. The first half of the study investigates the mechanisms of the control behind the previously observed laser phase-insensitive product state control. The controllability of this excitation scheme, two-color two-photon resonantly enhanced excitation, was analyzed from two aspects, the role of ac Stark shift introduced by the strong laser field and the multi-pathway quantum mechanical interferences. We have analyzed the excitation scheme from the analysis of the photoelectron angular distribution measured using the excitation scheme and the monitoring of the intermediate state population. Analysis of the data as well as the numerical simulation showed clear understanding of the role of two mechanisms in the product state control reported. We also investigated the control of the phase lag during the product state control. We conducted the control of the phase lag in the study of asymmetric photoelectron angular distribution, which arises from the concurrent even-odd parity outgoing electron wave excitation. The phase lag was controlled in full range, 2π, and the results were analyzed in terms of the role of autoionizing resonance structures as well as the nature of outgoing electron waves at different locations of the autoionizing resonances.
We discuss the possibility of narrowing the atomic autoionization states via their resonance mixing in a field. The results show that, in contrast to the interference process [Yu. I. Geller and A. K. Popov, Laser Stimulation of Nonlinear Resonances in Continuous Spectra (Nauka, Novosibirsk, 1981)] or to the mixing of two isolated states [A. I. Andryushin et al., Sov. Phys.---JETP 61, 678 (1985)], with mixing of multiplets one may expect substantial narrowing of autoionization states owing to their intersection with bound-electron states.
A two-step process using internal conversion and fluorescence to stabilize the 6P3211d6{P}_{\frac{3}{2}}11d6P23​​11d autoionizing state of barium has been observed. The internal conversion produces very highly excited autoionizing Rydberg states, which then emit fluorescence to produce singly excited, bound states. Inclusion of this process should bring calculations closer to agreement with recent measurements of dielectronic recombination.