Speaker
Description
This presentation provides a reminder of our existing background knowledge about how hydrogen behaves in the context of field ion emitters. This may or may not prove to be of detailed relevance to the analysis of hydrogen-related atom-probe data, but it would probably be useful to have it available as background information. The following behavioural effects will be noted.
(1) The likely dynamic behaviour of any hydrogen molecules desorbed from the inner surface of a metal vacuum chamber. This will be similar to the behaviour of the operating gas in a field ion microscope or gas field ion source, which has been summarised before [1].
(2) The propensity for hydrogen molecules to dissociate into hydrogen atoms on reaching a metal surface [2].
(3) The phenomenon of "hydrogen promoted field ion imaging". In my view, the physical origin of this effect is still not clearly established (see [3] for a 1974 discussion). The issues are what form the hydrogen takes and precisely where it is located on the emitter surface. The relevance to this workshop is the question: "What happens to an internal hydrogen entity, as field evaporation moves the emitter surface back towards it?
(4) The possibility that what is field evaporated is a complex between a hydrogen molecule or atom and an atom of the material substrate.
(5) The possibility that a field-evaporated complex may break-up in flight.
Attention will also be briefly drawn to three aspects of conventional atom-probe theory where small improvements could possibly/probably be made, as follows.
(6) Better understanding of the full physics behind the shape of a field-evaporated endform.
(7) Better understanding of the theory behind the experimental fact that radial distance in field electron, field ion and atom-probe images is proportional to "crystallographic angle" between the local surface normal and the emitter axis.
(8) Better understanding of the relationship between the parameter called "image compression factor" in atom-probe data analysis and the various different parameters called "angular magnification" in the context of charged-particle optics as understood outside the atom-probe community.
Attention is also drawn to a recent topical review [4] of field emitter electrostatics, including reference to highly efficient numerical simulation procedures. This review applies to both field electron and field ion emitters, although the main focus is field electron emission. There is probably a need to correlate this with recent developments in field ion emitter electrostatics, as used for example in [5].
[1] R.G. Forbes, Appl. Surface. Sci. 94/95, 1 (1996).
[2] Carry out a websearch on "hydrogen dissociation at metal surfaces" to discover many relevant papers.
[3] T. Sakurai, T.T. Tsong, and E.W. Müller, Phys. Rev. B 10, 4205 (1974).
[4] T.A. de Assis, F.F. Dall'Agnol and R.G. Forbes. J. Phys.: Condens. Matter 34, 493001 (2022).
[5] C. Hatzoglou et al., Microsc. Microanal. 19, 1124 (2023).
