Hydrogen analysis in APT: methods to control adsorption and dissociation of H₂.

Experimental factors that influence adsorption of hydrogen from the residual gas on a nickel-rich alloy during atom probe tomography are investigated. The rate of adsorption has a maximum value at field strengths between 24 and 26 V/nm. It is found that by selecting sufficiently high laser energies, or alternatively high DC fields, it is possible to significantly reduce adsorbed quantities. Some of the physical mechanisms for hydrogen supply to the analyzed area of the tip are discussed, and it is concluded that the dominating supply mechanism is most likely direct adsorption from the gas phase. Low hydrogen adsorption at high fields is attributed to autoionization, and a decline at low fields is explained by reduced field adsorption.