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Frontiers in Optics

Fully Microscopic Studies of Strong-Field Atom Ionization

K. Schuh, J. Hader, Jerome V. Moloney, Stephan W. Koch

Published Jan 1, 2014• 0 citations
Publisher

Abstract

This talk summarizes the results of our microscopic approach to study short-pulse ionization in a many atom system. The interaction of a highly off-resonant light pulse with an atomic gas is modeled fully microscopically. The theory systematically includes the light-atom interaction, as well as the electron-electron and electron-ion interaction effects on the distributions and the optically induced coherent polarization. The resulting equations are solved numerically for an atomic model system excited by a strong light pulse.

DOI
10.1364/ls.2014.lw1i.3

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This talk summarizes the results of our microscopic approach to study short-pulse ionization in a many atom system. The interaction of a highly off-resonant light pulse with an atomic gas is modeled fully microscopically. The theory systematically includes the light-atom interaction, as well as the electron-electron and electron-ion interaction effects on the distributions and the optically induced coherent polarization. The resulting equations are solved numerically for an atomic model system excited by a strong light pulse.
The interaction of a short, few-cycle light pulse and an atom which is prepared initially in a superposition of two stationary states is shown to exhibit strong signatures of atomic coherence. For a given waveform of the laser pulse, appropriate quantum mechanical relative phase between the constituents of the initial superposition can increase the ionization probability by a factor of three. A similarly strong effect can be observed when the waveform of the ionizing pulse is changed. These results allow for intuitive explanations, which are in agreement with the numerical integration of the time dependent Schr ¨odinger equation.
Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.
Until recently, the nonlinear interaction between light and matter has been restricted to only low photon energies produced by optical lasers. However, about a decade ago, the rise of free-electron laser facilities revolutionized the field of nonlinear light-matter interaction by delivering intense high-energy light pulses. Today, such lasers are used for research in materials science, chemical technology, biophysical science, solid state physics as well as fundamental research. It is the new experimental possibilities provided by free-electron lasers that motivated the work presented in this thesis. Two-photon ionization process is one of the simplest nonlinear interactions in which absorption of two photons by an atom (or a molecule) leads to promoting one of its bound electrons to continuum. This work presents studies of two-photon ionization of neutral atoms. After a brief historical introduction to the topic of nonlinear light-matter interaction, the density matrix describing the state of an atom and a photoelectron following two-photon ionization is derived. In each successive chapter, part of this density matrix is used to obtain characteristic quantities such as total two-photon ionization cross section, photoelectron angular distributions, ion polarization or degree of polarization of fluorescence photon produced by subsequent decay of the photoion. Physical properties of these quantities are studied and intriguing phenomena, such as elliptical dichroism, polarization transfer as well as relativistic and screening effects are investigated.
Quantum light-specifically light in noncoherent quantum states-can now be generated with intensities high enough to induce nonlinear responses in atoms, marking the emergence of a new "quantum era" in strong-field atomic physics. In this Letter, we explore the influence of quantum states of light on one of the key processes in this field: atomic double ionization. We develop a theoretical framework to model the interaction between a two-electron atom and light in arbitrary quantum states, such as phase-squeezed coherent states or bright squeezed vacuum states. Our results reveal that the quantum state of light significantly impacts the atomic double ionization process, leading to substantial changes in ionization probability and correlated electron momentum distribution.
In the present thesis, the interaction between atoms in superposition states and attosecond pulses is studied from the microscopic to the macroscopic regime. Using a semi-classical framework, three different models have been explored in order to investigate attosecond light--matter interaction processes. The first model is a microscopic, single-atom model. Through using this model, the absorption and the radiation of an external attosecond pulse was studied for several atomic superposition states and pulse energies. The second model was composed by two coupled atoms, where the interaction was described by either dipole-dipole or radiation interaction. The model was validated by simulating the Förster resonant phenomena. In addition, this model was used to determine the behaviour of the interactions with respect to separation distance and the initial states of the atoms. It was found that dipole-dipole interaction is dominant in the near field, whilst radiation interaction is dominant in the far field. Additionally, it was found that superpositions with high principal quantum numbers increase the strength of the dipole-dipole interaction but not the radiation interaction. The simulated results have been validated, in the short-time regime, using perturbation theory. The last model investigated in this thesis is a macroscopic many-atom model. It was implemented in order to simulate the propagation of an attosecond pulse through a large sample of atoms. Utilising this model, collective effects of many-atom systems were studied. It was found that a macroscopic sample emits collimated radiation and that dissipation of pulse energy in a macroscopic sample retains the behaviour of radiation produced by a single atom but that it is deformed by the propagation through the sample. The models explored in this thesis can be used in the study of light-matter interaction on the attosecond timescale with inter-atomic interactions and collective effects. The work presented here can be built on further to investigate research areas such as the dipole-blockade effect in cold atom physics, ionisation due to inter-atomic interaction or collective coherent effects.
A simple, "exactly solvable" atomic model will be presented that helps to discuss some of the interesting features of multiphoton ionization of atoms. The electromagnetic field is treated non-perturbatively, it can be strong. The atomic electrons are assumed to be non-interacting (Hartree model). They are bound in a harmonic oscillator potential (one- or three-dimensional). At the ionization energy, the potential is cut off and the free electron has no Coulomb attraction. The Schrodinger equation has exact analytic solutions valid in the inner or outer regions. If the field is turned on adiabatically, the exact solution can be found by matching these solutions at the boundary; both of their value and the normal derivative at all times. Thus the machine calculation reduces to repeated application of fast Fourier transforms and to the solution of linear equations (unfortunately a lot of them).