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Double photoionization is a fundamental process in which one absorbed photon creates simultaneously two photoelectrons. Unless the electromagnetic field is prohibitively strong this is only possible through many-electron correlation in the system of interest. Many-electron correlation is a generic term which describes deviation of the properties of the system from what one may expect if all the electrons are considered independent. For instance, in atoms independent electrons populate well defined energy levels. However, the amount of energy required to remove an electron from the atom is different from its one-electron energy since it involves disturbance of the other electrons as well. In solids, because of larger electron densities, the many-electron correlation is much stronger. It is responsible for such fundamental and important effects as ferromagnetism and superconductivity.
The simplest target to study double photoionization is a system of two interacting electrons, i.e. the helium atom. The target many electron correlation in helium is treated by employing very accurate ground state wave functions such as multiconfiguration Hartree-Fock or explicitly correlated Hylleraas ones. The Coulomb interaction of the two photoelectrons in the continuum is described by the convergent close-coupling method in which one of the electrons is represented by a complete set of the target pseudostates with positive energies. With this two key ingredients the total double photoionization cross-section has been calculated very accurately in a large range of photon energies. The theory is now developed to calculate the angular correlation of the two photoelectrons. In addition, the theory may be expanded to describe the helium double ionization by electron impact. The more complex atoms will be considered as well as the double photoionization from condensed matter.
This project will involve both analytical work and numerical
computations. It will help the student to build up analytical skills
and to get first-hand experience with high-performance computers.
References:
A. S. Kheifets and I. Bray.
Calculation of circular dichroism in helium double photoionization.
Phys. Rev. Lett., 81(21):4588-4591, 1998.
Electron momentum spectroscopy (EMS) is a novel technique in condensed matter physics. It relies on the electron impact ionization of one of the target electrons by a fast projectile. Then two outgoing electrons, scattered and ejected, are detected in time coincidence with fully determined kinematics. By using the laws of energy and momentum conservation the quantum state of the initially bound target electron can be fully restored.
Theoretically the energy-momentum distribution of electrons in solids can be obtained from solution of the Schroedinger equation with a model Hamiltonian which accounts for the correlation and exchange in the system of interacting electrons. A convenient choice of the Hamiltonian is provided by the density functional theory in its localized form. The energy-momentum distribution of electrons in a wide range of amorphous and crystalline solids have been calculated using this approach and were found to be in a good agreement with the experimental data.
Now experiment is about to be taken into a new dimension. The measurement will soon be possible to resolve the spin of the electrons in the solid target. Spin-dependent effects are very important in solids and responsible for strong collective effects such as ferromagnetism. Some theoretical models of high-temperature superconductivity also rely on electron spin. However, spin aspect of the electron-momentum spectroscopy have not been investigated so far. This is proposed for the present project.
The project will involve both analytical work and numerical
computations. It will help the student to build up analytical skills
and to get first-hand experience with high-performance computers.
References:
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