Study of Electronic Correlation in Noble Gas Atoms by means of Two Electron Photoemission Spectroscopies

The study of electronic correlations (EC) is a very general problem that has been long debated in different fields of physics of matter. The behaviour of an atom or a molecule is dominated by Coulomb forces nucleus-electron and electron-electron, whose nature is well known. Anyway the mathematical complexity connected with the description of the quantum behaviour of a many-body system under the action of the mutual Coulomb-forces makes the exact calculation of the atomic properties of an atom rather cumbersome also in the simplest systems. Thus different approximations have been introduced to overcome these difficulties.
In a Mean Field theory each electron is thought in a stationary field, which is the average of all its interactions with the nucleus and the other electrons. These theories have been used as valid tools to calculate the energy levels of atoms and molecules, and the main features of interaction of matter with radiation. The perturbation generated by modifying the atomic structure, as it occurs when an electron is removed from one shell, can be accounted by introducing an average screening potential. Anyway, the minor interactions which are not considered within this picture can cause not negligible effects, which are fundamental in the description of some physical processes, such as double photoionisation process that we shall treat in this work. All the effects that are not accounted in a mean field theory are usually defined as Electron Correlations.
In order to measure effects due to EC, high efficiency and/or high resolution conditions are often required. These conditions have been seldom achieved in the past years. The recent experimental developments permitted to overcome part of these difficulties, and the study of EC is actually a very active field of research. In particular the third generation Synchrotron sources gave availability of high brilliance-, high polarised- light, tunable with high resolution over a wide energy range (namely form infra-red up to hard X-ray). The improvement of electron/ion spectroscopy techniques as well as the electronic standard for signal processing and data acquisition, increased the overall resolution and efficiency performances of the experimental apparatus.
In this work we shall discuss two processes which are particularly suited for the study of EC: the direct double photoionisation (DPI) of He and the Auger cascade decay of the Ne 1s-13p resonantly excited state. Both of them can be thought as different aspects of the same physical process, namely the double photoemission, i.e. the emission of two electrons following the absorption of one photon.
When the photon absorption is treated within the dipole approximation, the double photoionisation process is entirely due to EC, so DPI measurements are supposed to be particularly useful to understand EC. The measurement of EC in an Auger cascade turns out to be a powerful tool to increase the number of physical quantities that can be determined in one experiment. The simplest closed-shell systems where the direct DPI and the Auger cascade can occur are Ne and He respectively. We measured the He DPI at three different photon energies, namely 20, 40 and 80 eV above the DPI threshold. Auger cascade in Ne has been observed along 5 different decay paths. The investigation of the cascade processes has been complemented by a series of spectroscopic measurements of the Auger decay of the Ne 1s-1 mp (m=3,4,5) excited states. These measurements provide original angle- and energy-resolved spectroscopic information of the Ne+ satellite states at moderate resolution, and they will be presented here.
The kinematics of double photoemission is fully determined by measuring correlated in time the momenta, i.e. kinetic energy and directions, of two out of three final products of the reaction. This implies to measure in coincidence two electrons, or one electron and the ion, selecting their kinetic energy and direction. It is well known that the coincidence spectroscopy has the disadvantage of the reduced detection efficiency respect to the measurements not-correlated in time. This is particularly dramatic when the cross section of the process under exam is low, as it is the case in the systems we studied.
All the experiments were performed at the GasPhase Photoemission beamline at Elettra storage ring (Trieste-Italy). The multicoincidence end station there operating holds 10 hemispherical electrostatic analysers for angle- and energy-resolved electron spectroscopy, which can be positioned at different angles respect to the photon beam. The original solution of a multi-detector array permits to cover a larger solid angle, without degrading the angle-resolution. The electronic data acquisition system permits the simultaneous detection of up to 21 pairs of electrons in coincidence.