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Laser Physics Letters

Ionization of a model quantum system by a single mode electromagnetic cavity field

Olga V. Tikhonova, A. M. Popov

Published Dec 21, 2005• 8 citations
Publisher

Abstract

The ionization of a model atom in a strong single-mode non-classical electromagnetic field is studied beyond the perturbation approach using the model of ``one discrete level + continuum'' for the atom. Explicit analytical expressions for the time-dependent probability of atomic ionization are obtained for different initial states of radiative field: Fock state, coherent wave packet, squeezed vacuum state. The results are compared to the ionization decay of discrete atomic level in a classical light field, and are analyzed for different mean number of radiative photons. The time-dependent distribution of the quantized field over photon number states is examined.

DOI
10.1002/lapl.200510085

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The ionization of a model atom in a strong single-mode non-classical electromagnetic field is studied beyond the perturbation approach using the model of ``one discrete level + continuum'' for the atom. Explicit analytical expressions for the time-dependent probability of atomic ionization are obtained for different initial states of radiative field: Fock state, coherent wave packet, squeezed vacuum state. The results are compared to the ionization decay of discrete atomic level in a classical light field, and are analyzed for different mean number of radiative photons. The time-dependent distribution of the quantized field over photon number states is examined.
The ionization of atoms in a strong nonclassical unimodal electromagnetic field was considered. It was shown that the probability of ionization could substantially change depending on the quantum state of the field even at a constant mean number of quanta in the radiation mode. The difference of ionization rates was especially large for multiphoton ionization processes. It was, in particular, shown that a nonclassical field could be much more effective from the point of view of the ionization of atoms than a classical field of the same intensity. The characteristics of the decay of a bound atomic system state in a strong nonclassical field were studied without invoking perturbation theory.
We consider ionization of an atom by the electromagnetic field in a Fock state $|N\ensuremath{\rangle}$: An eigenvector of the operator ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{N}}_{\mathbit{k},\ensuremath{\lambda}}={\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}}_{\mathbit{k},\ensuremath{\lambda}}^{\ifmmode\dagger\else\textdagger\fi{}}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}}_{\mathbit{k},\ensuremath{\lambda}}$ of the number of photons in the mode $\mathbit{k},\ensuremath{\lambda}$. We treat the electromagnetic field in a quantized way, which allows us to obtain the photon picture of the strong field ionization. We obtain, in particular, probability distributions for the number of absorbed photons in the process of strong field ionization for atomic systems governed by short-range and Coulomb potentials.
The dynamics of an atom in a strong single mode non-classical electromagnetic cavity field is studied. The reconstruction of the energy spectrum of an atom and the effect of ionization suppression in the presence of a non-classical field are discussed. It is found that a mechanism of ionization suppression similar to the Kramers–Henneberger one is also realized in strong non-classical fields. For squeezed vacuum states the threshold of the stabilization is approximately three times lower than for a classical (coherent) state of the same intensity.
Double ionization is studied by direct numerical integration of the time-dependent Schrödinger equation for a model two-electron system in the field of an electromagnetic wave in the case when the photon energy exceeds the energy required for the removal of both electrons of the atom. The probabilities of single-electron and double ionization are obtained as functions of the radiation intensity. The energy spectra and double pulse distributions of photoelectrons are analyzed. It is shown that single-photon ionization plays a significant role only in the limiting cases of weak and strong fields. The obtained results are used in an analysis of the contributions from different channels to the double-ionization process (in particular, for clarifying the role of the electron-electron interaction). The results of numerical calculations are compared with the analytic model of the phenomenon.
The ionization of an atom by a nonclassical electromagnetic field has been studied by a method developed to obtain results for an arbitrary initial state of the field, including squeezed vacuum with a large average number of photons. Characteristics of the ionization of atomic systems by nonclassical squeezed light have been demonstrated for the first time. In contrast to the case of a coherent field, a significantly smoother decrease in the contributions of different above-threshold channels to ionization has been found. Features of photo-electron spectra in continuum have been revealed for the first time as a significant overlapping of peaks from different ionization channels.
We study the dynamics of an N-level atom coupled in a lossless cavity to a single-mode near-resonant quantized field. The atomic levels are coupled by the multiphoton transitions and the coupling constants between the field and the atomic levels are supposed to be intensity dependent. We find the exact solution for the state vector describing the dynamics of the atom-plus-field system. As an illustration we use the model for studying (i) the time evolution of the atomic occupation probability with the initially coherent field and (ii) the light squeezing, when the cavity field is initially in the vacuum state and the atom is prepared in the atomic ‘coherent state’ (a superposition of atomic states).