Use of Amount-of-Substance Terminology and Equations in Field Desorption Theory
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ORIGINAL PAPER
Use of Amount‑of‑Substance Terminology and Equations in Field Desorption Theory Richard G. Forbes1
© The Author(s) 2020
Abstract This note proposes that the theories of field evaporation and field desorption, as used in atom-probe microscopy and related atomic-level contexts, should be consistently formulated in terms of a set of “seven-dimensional (7-D)” formulae and equations that involve the physical quantity “amount of substance”, but make use of an atomic-level constant effectively equal to “one atom” (or, more generally, “one entity”). It is argued that the term “count” should be introduced as an alternative name (more suited to atomic-level contexts) for the quantity “amount of substance”. For field evaporation/desorption theories, relevant definitions and formulae are proposed, and compared with the “six-dimensional” system (based on the dimensionless quantity “number of atoms/entities”) sometimes used in the literature. Advantages of using a 7-D system are noted. It is argued that there is also an increasing need for a comprehensive system of official nomenclature for atomic-level constants and units, for all three of the extensive quantities “mass”, “electric charge” and “amount of substance”. It is also argued that, in the longer term, considerations of the kind being proposed here for field evaporation/desorption theories might usefully be applied more generally in atomic-level rate theory. Keywords Field evaporation theory · Amount of substance · Counting units · Atomic-level constants
1 Introduction In the 1970s, the International Standards Organization and related organizations recommended a system of quantities and equations for use in science and engineering. Since 2009, this has been called the International System of Quantities (ISQ). The existence of this formal name is helpful in making a distinction between (a) the modern system of quantities and equations and (b) the formal system of units (the SI system) based on it. Obviously, in the ISQ there are seven base-quantities and (correspondingly) seven ISQ “dimensions” [1]. In the discussion below, the ISQ dimensions of amount of substance, of length, and of time, are denoted by N, L and T, respectively, in accordance with the usual convention [1]. In the ISQ, there are three core physical quantities that quantify “how much of a basic physical property is present”, * Richard G. Forbes [email protected] 1
Advanced Technology Institute and Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, Surrey, UK
namely mass (m), electric charge (q) and amount of substance (n). In the materials science technique called atom-probe tomography (APT) [2, 3], atoms are removed from a pointed specimen surface by a process called field evaporation (FEV), and are then subject to analysis using a positionsensitive time-of-flight mass spectrometer. A very similar technique (“pulsed field desorption mass spectrometry”) has been used by Kruse [4, 5], in the context of catalysis, to analyse molec
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