Maarten Vos studied experimental physics at Groningen University.  His PhD in Groningen (1987) was focussed on ion implantation and ion channeling.  Afterwards Vos did a post-doc in the group of John Weaver at the University of Minnesota (electron spectroscopy of interfaces) followed by a post-doc in the group of Ian Mitchell at University of Western Ontario, London,  Canada (ion beam studies of surfaces).

In 1992 Vos took up a position  with the Electronic structure of Matter center at the Flinders University of South Australia (headed by Erich Weigold and Ian McCarthy) exploring the possibilities of electron momentum spectroscopy (EMS) of thin films.  Since 1998 this project was continued at the Australian National University, but in recent years his interest has shifted somewhat to different flavours  of high-energy electron spectroscopy, which can be done in a  unique way using the (somewhat modified)  EMS spectrometer.

The role of recoil in elastic electron scattering

If an electron is deflected from a nucleus, it will necessarily transfer momentum to this nucleus.  The change in momentum of the nucleus will change necessarily its kinetic energy.  This energy change is mirrored in a change of the kinetic energy of the scattered electron.

If the scattering nucleus is part of a molecule or solid, then this scattering process is non-trivial and has attracted little attention, as these effects were considered too small to measure, due to the low mass (1800 times less than the mass of a proton)  of an electron.  At high energies (keV and more) and high energy resolution (better than 0.5 eV) these effects are, however, clearly visible.

The energy transfer from the electron decreases with the mass of the scattering atom.  Thus we can identify the scattering atom from the energy loss of the peaks in the spectrum. Details of the shape depend on the chemical bonding [*].  Here the theory is not completely developed, and requires additional experimental input.  

These recoil processes are not restricted to electron scattering but also play a role in high-energy photoemission and neutron scattering.

In the project the student will focus either on scattering from a gas-phase target or from surfaces.  The gas-phase work focuses on the quantum physics description of the process, the surface studies aim at establishing these scattering experiments as a surface analytical technique.  In all cases there is an emphasis on understanding the results based on quantum physics, as well as improving the all-important energy resolution of the experiment using state-of-the-art  techniques.

* M. Vos, J. Chem. Phys. 132 074306 (2010).

Kikuchi lines studied with an electrostatic analyser

Kikuchi lines are usually studied using a phosphor screen and are used in e.g. electron backscatter diffraction (EBSD) to determine the orientation of crystallites.  Here one integrates over all the energies of electrons appearing from a surface.

We use our electrostatic analyser to study these diffraction effects, and we can determine the energy of the diffracted electrons with sub-eV accuracy.  Using the recoil effect we can separate the Kikuchi lines of electrons scattered from atoms of different mass.  The huge differences between profiles of different elements makes a new way of looking at atomic configurations at surfaces possible[*]. This project aims at further developing these techniques, and see if they can be used to monitor e.g. epitaxy during thin film growth.

[*] A. Winkelmann and M. Vos, Phys. Rev. Lett 106 085503 (2011).