Simulation of field-induced molecular dissociation in atom-probe tomography: Identification of a neutral emission channel

We investigate the dynamics of dicationic metal-oxide molecules under large electric-field conditions, on the basis of ab initio calculations coupled to molecular dynamics. Applied to the case of ZnO^2+ in the field of atom probe tomography (APT), our simulation reveals the dissociation into three distinct exit channels. The proportions of these channels depend critically on the field strength and on the initial molecular orientation with respect to the field. For typical field strength used in APT experiments, an efficient dissociation channel leads to emission of neutral oxygen atoms, which escape detection. The calculated composition biases and their dependence on the field strength show remarkable consistency with recent APT experiments on ZnO crystals. Our work shows that bond breaking in strong static fields may lead to significant neutral atom production, and therefore to severe elemental composition biases in measurements.