Proposal of Photoionization and Photofield Tunneling of Electrons as (Indirect) Evaporation Mechanisms during Laser-assisted Field Evaporation of Insulators

Artikel i vetenskaplig tidskrift, 2025

Atom probe tomography is a materials analysis technique that can attain highly localized chemical information with high sensitivity for all elements. It is based on field evaporation of ions in a high electrostatic field. To achieve this, a needle shaped specimen with an apex radius smaller than 100 nm is exposed to high voltages thereby creating electrostatic fields in the range of tens of V/ nm. Thesample is held at cryogenic temperatures during the experiment. For conducting materials an additional voltage pulse is used to ionize surface atoms towards a position-sensitive detector bytime-of-flight massspectrometry.Theionspeciesisidentified by time-of-flight mass spectrometry and an algorithm allows for the reconstruction of the evaporated volume. [1,2] The use of short laser pulses instead of voltage pulses opened up the technique for non-conductive materials [3,4]. So far, the widely accepted mechanism has been thermal activation, i.e. heating the specimen and thereby inducing ion desorption. Not all observations can be explained by this. Several studies show a wide range of possible laser pulse energies (LPEs) in insulators with UV laser light [5-7]. Recently the authors reported on similar observations for chromia in the newest generation of APT instruments with deep UV laser light [8]. The low endof the LPEvalues seemtoo lowfor creating asignificant thermal pulse. Similarly, APTmeasurementswithextremeUV laser lightcould evaporate oxidesbutnot metalsand the heat inputwas considered tobetoo small for classical thermal activation.