Martin Dorr

High-Intensity Laser-Atom Physics

Publisher Summary This chapter illustrates that the development of lasers capable of delivering short pulses of very intense radiation, over a wide frequency range, has led to the discovery of new, nonperturbative multiphoton processes in laser interactions with atomic systems. In this article, we first give a survey of the main properties of multiphoton processes such as the multiphoton ionization of atoms, the emission by atoms of high-order harmonics of the exciting laser light, and laser-assisted electron-atom collisions. It then review the theory of these processes, giving particular attention to ab-initio nonperturbative methods such as the Sturmian-Floquet approach, the R-matrix-Floquet theory, and the numerical integration of the time-dependent Schrodinger equation. The chapter discusses relativistic effects that occur at ultra-high intensities. The chapter concludes by considering possible future developments of high-intensity laser-atom physics.

Atomic hydrogen in a superintense high-frequency field : testing the dipole approximation

We have solved the fully 3-dimensional time-dependent Schrodinger and Pauli equations for an electron bound in a Coulomb potential, interacting with a superintense electromagnetic (laser) field of high frequency. Adiabatic stabilization is observed at high intensities. Corrections to the dipole approximation modify the ionization probability only slightly, up to the maximum intensity considered (2.5 1019 W cm-2).

R-matrix-Floquet theory of multiphoton processes. II. Solution of the asymptotic equations in the velocity gauge

For pt.I see ibid., vol.24, no.4, p.751-90 (1991). A unified R-matrix-Floquet theory of multiphoton ionization and laser-assisted electron-atom collisions for a general atom has been proposed recently by Burke et al. (1990, 1991). The present paper describes two new computational methods for solving the asymptotic equations which occur in the external R-matrix region in the velocity gauge. The first method extends the Light and Walker (1976) propagator approach to include the first derivative coupling term which arises in the velocity gauge. The second method extends the Burke and Schey (1962) asymptotic expansion to include the first derivative coupling terms and the long-range multipole potential terms. These new methods, combined with the solution in the internal R-matrix region, enable the complete solution of the R-matrix-Floquet equations to be obtained. By introducing a small r expansion of the one-electron system in the velocity gauge, numerical results are presented for multiphoton ionization of atomic hydrogen and compared with results obtained using other approaches.

R-matrix-Floquet theory of multiphoton processes. III: Multiphoton ionization of atomic hydrogen

Using the recently proposed R-matrix-Floquet theory we have analysed the multiphoton ionization of atomic hydrogen from the ground state and the metastable 2s state in an intense, linearly polarized and monochromatic laser field. Results are presented for total ionization rates, branching ratios into photon absorption channels (including ATI, above threshold ionization) and angular distributions, for several frequencies and a range of laser intensities. We also discuss the limits of lowest order perturbation theory and we study high-frequency stabilization at high intensities.

R-matrix Floquet theory of multiphoton processes. V. Multiphoton detachment of the negative hydrogen ion

Using the recently proposed R-matrix Floquet theory we have performed ab initio calculations for single- and multiphoton detachment of the negative hydrogen ion, H-. Our method is non-perturbative in the field strength and is a multielectron theory allowing for full correlation between the two electrons. We present results depending on frequency up to nonperturbative field intensities for H- in a single-frequency linearly polarized laser. We investigate a wide range of frequencies and intensities from the multiphoton detachment regime to the high-frequency regime. Our results include total detachment rates, shifts of the attachment energy in the field and partial rates into the various excess photon absorption and core excitation partial channels. We compare our results with other theoretical calculations and with experimental results.

The energy spectrum of photoelectrons produced by multiphoton ionization of atomic hydrogen

The authors present a comparison of theoretical and experimental data for the first three above-threshold ionization channels of the photoelectron kinetic energy spectrum of atomic hydrogen when the atom is irradiated by a short pulse of linearly polarized light of wavelength 608 nm and peak intensity 6*1013 W cm-2 or 1.2*1014 W cm-2.

Atomic hydrogen irradiated by a strong laser field: Sturmian basis calculations of rates for high-order multiphoton ionization, Raman scattering, and harmonic generation

We briefly review some technical aspects of our Sturmian basis calculations of rates for high-order multiphoton processes occurring in atomic hydrogen initially in its ground state. The processes under study include ionization, Raman scattering, and harmonic generation. We present results that illustrate the efficacy of the method and that also highlight some of the interesting physics of multiphoton processes. In particular, we show nonperturbative rates for harmonic generation by 1064-nm light and for ionization by light of wavelengths 532, 608, 616, and 1064 nm, with laser intensities in the range 1012–1014 W/cm2. We discuss in some detail the role of intermediate resonances in ionization processes. Some of these resonances strongly enhance the rates; others do not. The influential resonances can be characterized by the orbital angular quantum number of the corresponding intermediate states. At long wavelengths and moderate intensities, perturbation theory grossly overestimates the ionization rates. Consequently, at long wavelengths the peak intensity that atoms can experience in short pulses before undergoing ionization is far higher than would be anticipated on the basis of perturbation theory.

Evolution of a system undergoing multiphoton ionization

We describe an efficient method for solving the time-dependent Schroedinger equation for an atomic system subject to an oscillating radiation field. The method is based on solving the Schroedinger equation in momentum space, which obviates the need to impose artificial boundaries, and facilitates the extraction of the rapidly varying part of the wavefunction. We present results of a test application to a one-dimensional system with atomic potential −exp(−|x|/a0).

R-matrix Floquet theory of multiphoton processes. IV. Laser-assisted electron-proton scattering

For pt.III see ibid., vol.26, p.L275 (1993) We present the first application of the R-matrix-Floquet theory to the study of a collisional process in a laser field, namely laser-assisted electron-proton scattering. We have calculated total and angular differential cross sections for elastic scattering and processes involving a net absorption or emission of a number of photons in a linearly polarized laser field. The field induces resonances due to the temporary capture of the projectile electron into atomic hydrogen bound states, accompanied by the exchange of photons with the field. We also find structures corresponding to different sublevels due to the breaking of the spherical symmetry of the atomic system by the laser field. We analyse in detail these features and their interference patterns for various field parameters and collision geometries.

Theory of Multiphoton Ionization of Atoms

Laser pulses having intensities of the order of or exceeding the atomic unit of intensity I a = 3.5 × 1016 W cm−2, corresponding to the atomic unit of electric field strength e a = 5.1 × 109 V cm−1, can be readily produced in laboratories today. Such fields are strong enough to compete with the Coulomb forces in controlling the electron dynamics in atomic systems. As a result, atoms in intense laser fields exhibit a variety of phenomena, which are collectively referred to as multiphoton processes. In this contribution we shall review the theory of one important process: the multiphoton ionization of atoms. We begin by giving a brief overview of some of the basic features of the multiphoton ionization. Experimental results will be used to illustrate these features. Next, we discuss some general issues concerning the study of the interaction of atoms with intense laser fields. In particular, we introduce the general form of the time-dependent Schrodinger equation to be solved and outline perturbation theory for laser-atom interactions. Two non-perturbative approaches for solving the time-dependent Schrodinger equation, namely the use of Floquet theory and the direct numerical integration of the time-dependent Schrodinger equation, will then be considered. Subsequently, we outline approximation schemes for the low and high frequency regimes. Finally, we make some remarks concerning relativistic effects in intense laser-atom interactions. The reader is referred to the book edited by Gavrila [1], the articles by Burnett et al. [2], Joachain [3], DiMauro and Agostini [4], Protopapas et al. [5] and Joachain et al. [6] as well as the other contributions to this volume for more detailed discussions of multiphoton processes.