Robin Shakeshaft

Evidence for highly correlated electron dynamics in two-photon double ionization of helium

Experiments using new sources of XUV pulses now tackle the difficult problem of few-photon direct double ionization of atoms. Despite its apparent simplicity, the fundamental process of two-photon direct double ionization of helium is far from being understood. Here, we use a time-dependent approach to study the process. Our results for the electron angular and energy distributions demonstrate that the dominant mechanism for double-electron escape involves a highly correlated electron motion. Angular correlations strongly favour back-to-back electron emission along the polarization axis, while dynamical screening leads to an equipartition of the electron energy for a broad range of field frequencies. These features are reflected in the recoil-ion-momentum distributions that are presently accessible to experiments.

Absolute cross sections for double photoionization of helium at energies from 0 to 80 eV above threshold

We have performed, using an ab initio basis-set method, calculations of the absolute cross section for double photoionization of helium for energies from 2 to 80 eV above threshold, and we have extended the results down to 0 eV through spline interpolation. Our results are consistent with the Wannier threshold law, and agree well with the near-threshold measurements. At higher energies (20 eV or so above threshold) significant discrepancies with experiment are evident. We find a simple relation between the integrated cross section for double photoionization and the singly differential cross section at the midpoint of the energy distribution.

Theory of multiphoton ionization of atoms by intense laser fields

We review some basic elements of the theory of multiphoton ionization of atoms by intense fields. Our emphasis is on physical interpretation, but we also give considerable attention to mathematical details.

Multiphoton ionization of an atom; the choice of gauge

We argue that the velocity gauge is the natural one in which to formulate the theory of multiphoton ionization of atoms by intense fields. If the length gauge is used, careful attention must be paid to the time evolution of the eigenvector which characterizes the state in which the photoelectron emerges when the atom is ionized.

A note on the Sturmian expansion of the Coulomb Green's function

It is shown that any physical matrix element of the Coulomb Greens function can be written in a form such that its Sturmian expansion is convergent both below and above the continuum threshold.

Sturmian basis functions in the coupled state impact parameter method for H++H scattering

When Sturmian basis functions are used in the standard coupled state impact parameter method for proton-hydrogen atom scattering, the correct boundary conditions cannot, in general, be satisfied. This means that for most choices of the initial and final states the approximate transition amplitude, as usually defined, does not exist. This difficulty is resolved and an approximate transition amplitude is defined which is a variational estimate of the exact transition amplitude for arbitrary initial and final states. In the Sturmian representation the exchange matrix elements can be calculated more directly by solving a set of coupled differential equations which is somewhat different from the set of equations given by Cheshire (1967). A procedure that does not require storage and interpolation of the matrix elements in order to exploit their time-symmetry is also described.

A note on the exchange integrals in the impact-parameter treatment of heavy-particle collisions

The one-electron three-dimensional exchange integrals which arise in the impact-parameter treatment of heavy-particle collisions, are reduced to one-dimensional integrals whose ranges are finite, and whose integrands contain only simple exponential functions.

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.

Possibility of observing the second Born contribution to electron capture at high impact velocities

The second Born contribution to electron capture has not yet been unambiguously detected even though it almost surely dominates over the first Born term at sufficiently high energies. Three methods are suggested whereby it might be possible to detect the second Born contribution to the cross section for forward electron capture from a hydrogen atom or molecule or from a helium atom by a lightly charged (bare) ion incident with an energy in the range 10-80 MeV per nucleon. The first method involves measuring the cross section for 1s to 3d capture to check that this cross section exhibits a 1/v11 velocity dependence. The second method involves measuring the angular distribution for scattering angles ranging from zero to a few minutes; at a small but non-zero angle there should be a sharp peak in the angular distribution. For the target a He atom or H2 molecule, a third possibility is to detect a peak in the energy spectrum of electrons ejected from the target in a direction perpendicular to the beam direction, in coincidence with capture.

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.