Angular anisotropy parameters and recoil-ion momentum distribution in two-photon double ionization of helium

We present convergent-close-coupling (CCC) calculations of the angular anisotropy parameters β2,β4 and the recoil ion momentum distribution dσ∕dp in two-photon double ionization (TPDI) of helium. In a stark contrast to single-photon double ionization (SPDI), where the β2 parameter varies widely changing the angular distribution from isotropic to nearly dipole for slow and fast photoelectrons, respectively, the β parameters for TPDI show very little change. The angular distribution of the recoil ion is fairly isotropic in TPDI as opposed to a strong alignment with the polarization of light in SPDI

A study of cross sections for excitation of pseudostates

Using the electron-hydrogen scattering Temkin-Poet model we investigate the behavior of the cross sections for excitation of all of the states used in the convergent close-coupling (CCC) formalism. In the triplet channel, it is found that the cross section for exciting the positive-energy states is approximately zero near-threshold and remains so until a further energy, equal to the energy of the state, is added to the system. This is consistent with the step-function hypothesis [Bray, Phys. Rev. Lett. {\bf 78} 4721 (1997)] and inconsistent with the expectations of Bencze and Chandler [Phys. Rev. A {\bf 59} 3129 (1999)]. Furthermore, we compare the results of the CCC-calculated triplet and singlet single differential cross sections with the recent benchmark results of Baertschy et al. [Phys. Rev. A (to be published)], and find consistent agreement.Comment: Four pages, 5 figure

Different escape modes in two-photon double ionization of helium

The quadrupole channel of two-photon double ionization of He exhibits two distinctly different modes of correlated motion of the photoelectron pair. The mode associated with the center-of-mass motion favors a large total momentum which is maximazed at a parallel emission. However, the mode associated with the relative motion favors an antiparallel emission. This difference is manifested in a profoundly different width of the angular correlation functions corresponding to the center-of-mass and relative motion modes.Comment: 4 pages, 3 figure

Calculation of the free-free transitions in the electron-hydrogen scattering S-wave model

The S-wave model of electron-hydrogen scattering is evaluated using the convergent close-coupling method with an emphasis on scattering from excited states including an initial state from the target continuum. Convergence is found for discrete excitations and the elastic free-free transition. The latter is particularly interesting given the corresponding potential matrix elements are divergent

Atomic photoionization: When does it actually begin?

We analyze a time delay of one- and two-electron photoemission from an atom after absorption of an attosecond XUV pulse. We establish this delay by solving the time dependent Schrödinger equation and by subsequent tracing of the field-free evolution of

Angular correlation in the two-electron continuum

Following absorption of a single photon, angles of simultaneous emission of two electrons from a He(n 1S) atom become more correlated with increasing n. We find that the strength of this correlation is due to the two-electron continuum of the electron-impact ionization of the He+(ns) ion. The strength is determined by the width of the momentum profile of the ionic ns state but not the strength of the electron correlation in the He initial state. This can explain the increasing (over He) angular correlation strength found in double photoionization of targets such as Be, Ne, and H2

Pseudostate description of diatomic-molecule scattering from a hard-wall potential

A collinear scattering of a structured particle from a hard wall is studied with consideration of vibrational transitions initiated by the collision. It is shown that this problem can be solved analytically in the framework of the source-function method. With the use of the continuum discretization technique we are able to take into account both discrete and continuum states. No approximations of the interatomic potential is required. We illustrate our approach for the case of a hydrogen molecule bound by the realistic Morse potential

Convergent Close-Coupling Approach to Electron-Atom Collisions

It was with great pleasure and honour to accept the invitation to make a presentation at the symposium celebrating the life-long work of Aaron Temkin and Richard Drachman. The work of Aaron Temkin was particularly influential on our own during the development of the CCC method for electron-atom collisions. There are a number of key problems that need to be dealt with when developing a general computational approach to such collisions. Traditionally, the electron energy range was subdivided into the low, intermediate, and high energies. At the low energies only a finite number of channels are open and variational or close-coupling techniques could be used to obtain accurate results. At high energies an infinite number of discrete channels and the target continuum are open, but perturbative techniques are able to yield accurate results. However, at the intermediate energies perturbative techniques fail and computational approaches need to be found for treating the infinite number of open channels. In addition, there are also problems associated with the identical nature of electrons and the difficulty of implementing the boundary conditions for ionization processes. The beauty of the Temkin-Poet model of electron-hydrogen scattering is that it simplifies the full computational problem by neglecting any non-zero orbital angular momenta in the partial-wave expansion, without loosing the complexity associated with the above-mentioned problems. The unique nature of the problem allowed for accurate solution leading to benchmark results which could then be used to test the much more general approaches to electron-atom collision problems. The immense value of the Temkin-Poet model is readily summarised by the fact that the initial papers of Temkin and Poet have been collectively cited around 250 times to date and are still being cited in present times. Many of the citations came from our own work during the course of the development of the CCC method, which we now describe

Convergent calculations of double ionization of helium: from (γ\gamma,2e) to (e,3e) processes

The first absolute (e,3e) measurements, by Lahmam-Bennani et al [Phys. Rev. A {\bf 59}, 3548 (1999)], have been recently approximately reproduced by Berakdar [Phys. Rev. Lett. {\bf 85}, 4036 (2000)] and supported by Jones and Madison [Phys. Rev. Lett. {\bf 91}, 07321 (2003)], but with widely differing conclusions. The former indirectly implied that the Born-CCC-based calculations of Kheifets et al [J. Phys. B {\bf 32}, 5047 (1999) were invalid due to the reliance on the 1st Born approximation. The latter argued that the 1st Born approximation was valid, but the wrong initial state was used. We investigate these claims and find that the original calculations of Kheifets et al are reproduced whether the 2nd Born approximation is incorporated or if we use a ground state similar to that of Jones and Madison, but appropriately corrected as done by Le Sech and co-workers [J. Phys. B {\bf 23}, L739 (1990)].Comment: 4 pages, 3 figure

Spin-resolved electron-impact ionization of lithium

Electron-impact ionization of lithium is studied using the convergent close-coupling (CCC) method at 25.4 and 54.4 eV. Particular attention is paid to the spin-dependence of the ionization cross sections. Convergence is found to be more rapid for the spin asymmetries, which are in good agreement with experiment, than for the underlying cross sections. Comparison with the recent measured and DS3C-calculated data of Streun et al (1999) is most intriguing. Excellent agreement is found with the measured and calculated spin asymmetries, yet the discrepancy between the CCC and DS3C cross sections is very large