Recently, experimental methods for 3D-atom probe microscopy for a wide class of oxides and semiconductor materials have been developed by Hono et al using laser assisted field evaporation. Though this method is really epoch-making, the mechanism of laser assisted field evaporation has not been clarified so far and the best operation condition is still unexplored. Therefore we studied theory on the mechanism of laser assisted field evaporation from MgO crystal, as a prototype of oxides. As a result, we found the mechanism is explained by electronic excitation model, which is remarkably different from the usual field induced evaporation from metal tips.

An important fact we clarified is that a certain amount of holes at the evaporation region exert a driving force of the evaporation by reducing the potential barrier of the ion desorption. The holes are generated by a strong positive electric field on the tip, as well as by the laser irradiation.

Using MRMP2 ab initio calculations, we obtained the potential energy surfaces of the ground states and photo-excited states of somewhat larger MgO clusters leading to the photo-induced desorption of corner ions. We found when two holes are captured in the MgO clusters, the activation barrier height of the excited diabatic potential energy surface is remarkably reduced, indicating such an excited potential energy surface provides an effective evaporation pathway. Using ab initio embedded cluster method with 6-311G*/B3LYP for much larger MgO cluster models including more than 1000ions, we calculated the effect of positive charging on the sublimation energy of the corner ions and found only two holes captured by the cluster remarkably reduce the sublimation energy. We also obtained analytic expressions of the potential energy surfaces from phenomenological analyses, from which evaporation rates are estimated as the function of the applied voltage. The holes generated and accumulated near the evaporation region play an important role for triggering the ion desorption. We also discuss how such surface hole accumulation can be detected by the FIM images.

Based on theoretical analyses as above, we establish a systematic scenario leading to the laser assisted field evaporation from oxides and semiconductors. Moreover, theoretical clue will be provided towards finding a condition for the best resolution of the 3D-atom prove method using the laser assisted field evaporation.