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arXiv:2104.07956 · Journal of Physics B Atomic Molecular and Optical Physics

Non-adiabatic ionization with tailored laser pulses

Sajad Azizi, Ulf Saalmann, Jan Michael Rost

Published May 26, 2021• 1 citation
arXivHTMLPDF

Abstract

Abstract Non-adiabatic photo-ionization is difficult to control as it relies on the derivatives of the envelope and not on phase-details of the short ionizing pulse. Here, we introduce a catalyzing state, whose presence render non-adiabatic ionization sensitive to phase-details of tailored pulses. Since a catalyzing state is in general easy to create, this opens a perspective for coherent control of ultra-fast ionization.

DOI
10.1088/1361-6455/ac057a

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Abstract Non-adiabatic photo-ionization is difficult to control as it relies on the derivatives of the envelope and not on phase-details of the short ionizing pulse. Here, we introduce a catalyzing state, whose presence render non-adiabatic ionization sensitive to phase-details of tailored pulses. Since a catalyzing state is in general easy to create, this opens a perspective for coherent control of ultra-fast ionization.
How fast can a laser pulse ionize an atom? We address this question by considering pulses that carry a fixed time-integrated energy per-area, and finding those that achieve the double requirement of maximizing the ionization that they induce, while having the shortest duration. We formulate this double-objective quantum optimal control problem by making use of the Pareto approach to multi-objective optimization, and the differential evolution genetic algorithm. The goal is to find out how a precise time-profiling of ultra-fast, large-bandwidth pulses may speed up the ionization process. We work on a simple one-dimensional model of hydrogen-like atoms (the Pöschl-Teller potential) that allows to tune the number of bound states that play a role in the ionization dynamics. We show how the detailed shape of the pulse accelerates the ionization, and how the presence or absence of bound states influences the velocity of the process.
We have demonstrated an interference between multiphoton ionization processes that results in a phase-sensitive enhancement or inhibition of the net ionization rate. We focus two laser beams, either of which can induce an atomic transition between the same two states, into a cell containing atomic mercury. One of the fields is at a wavelength of 185 nm and can linearly induce the 6s 1S0 → 6p 1P1 transition in mercury. The other field is at a wavelength precisely three times as long, and it can excite the 6p resonance by a three-photon process. The absorption of two additional visible photons ionizes the mercury. In the presence of both fields, the transition amplitudes interfere, and their relative phases become very important. Their phases can be controlled by controlling the phases of the optical fields inducing the transitions. Coherence between the two fields, vital for the interference, is ensured by generating one field from the other using a nonlinear third-harmonic process. We have also demonstrated the effect of spatial phase variations between the two fields on the local photo-ionization rate.
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The strong-field coherent control of the nonresonant ionization of nitrous oxide using shaped pulses is investigated. We study the dependence of periodic coherent oscillation of the total ionization yield on the variation of laser phase parameters. The physical mechanism of the strong-field coherent control is investigated experimentally and theoretically by the nonresonant spectral phase interferences in the frequency domain. We show that the intense shaped pulses with broadband and off-resonance can be used as a robust strong-field coherent control method.
Abstract In this and subsequent chapters, we consider are other laser methods for controlling atoms and molecules that, in contrast to all the methods discussed above in Chapters 2–8, are destructive. They result in the ionization of the atoms or molecules being controlled and the dissociation and fragmentation of molecules. All these methods are united by the capability of the particles to be ionized or dissociated in a photo selective manner, which makes it possible to detect and separate atoms or molecules of certain species. They are based on the fact that all the structural details of atoms and molecules manifest themselves in their spectra, for example, some details manifest in the spectra of atoms are the atomic weight (the number of nucleons), the number of protons (the nuclear charge and the type of element), the number of neutrons (the isotopic state), and even the excitation energy of the nucleus (the isomeric composition). This enables one to excite, and, in some cases, ionize atoms and molecules in a photo selective way and thus accomplish their photo selective excitation and separation. Figure 9.1 lists all types of photo selectivity. All of them have been successfully realized by the method of resonance multistep ionization of atoms that is considered in the present chapter. Our primary attention here is concentrated on the realization of laser control of the photo selectivity of ionization. A detailed description of this method can be found in the monographs by Letokhov (1987) and Hurst and Payne (1988).