Speaker
Description
Recent treatments of field-ion (FI) imaging theory (e.g., [1]) differ from older treatments (e.g., [2]). Modern theories consider the availability of in-specimen final electron states. Particularly with alloy specimens, this is long overdue and welcome. However, aspects of modern theories seem incompatible with older theories firmly backed by experiment, or incompatible with scientific thinking outside atom-probe.
This presentation points out the separate limitations of both old and modern imaging theories, attempts to combine their separate strong points into a qualitative-level integrated theory, and explores possible future developments.
Topics covered (as time permits) are:
– Using the "7-dimensional" (or "amount-based") imaging equation r_FI=CQ_e. The "amount of operating-gas ionized per unit volume per unit time" (r_FI) at some point R in space is given by the product of the "effective" gas concentration (C) at R and the electron transfer rate-constant (Q_e) at R. Q_e is given by the product of the electron transition rate-constant (P_e) (rate-constant in free space) at R and a factor representing availability of specimen "final states".
– Gas atom history, "best image field", the "Assumption of corresponding potential structures", and the roles of C and Q_e in image formation.
– Charged-surfaces theory (charge and dipole charge distributions, electrical surface, critical surface).
– The choice between tunnelling-integral-based and overlap-integral-based theories of rate-constants, and evidence that prefers tunnelling integrals.
– The desirability of validating imaging theories using He-on-W imaging of the (111) plane near 80 K; Ruska blurring.
– Field adsorption and its possible effects on C and Q_e.
– The possible role of charge-transfer between surface atoms of different species.
– What theoretically needs doing next? Better field ionization tunnelling theory, certainly, but what else ?
[1] S. Bhatt et al., Phys. Rev. B 107 (2023) 235413.
[2] R.G. Forbes, Appl. Surf. Sci. 94/95 (1996) 1–16.
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