Ab-Initio Simulation Laboratory

Field evaporation of silicon as positive and negative ions in the field ion microscope and scanning tunneling microscope configurations is investigated with the charge-exchange model using atomic potentials from an empirical potential due to Tersoff [Phys. Rev. B 37, 6991 (1988)] and an environment dependent potential developed by Bolding and Andersen [Phys. Rev. B 41, 10568 (1990)]. For the geometry of the field ion microscope, Si+ should be the observable ion species. In the close-spaced electrode geometry of the scanning tunneling microscope, Si2- should be the favored ion species since it requires the lowest evaporation field.

Following our previous work, we carry out a more detailed study of dissociation dynamics of diatomic ions field-evaporated from a metallic tip of field-ion microscopy, We have found that the partial fractal behavior of the dissociation probability near threshold field strength and the multiple peaks in the time-of-flight spectrum can be attributed to different combinations of vibrational and rotational cycles before ions dissociate, But more clear revelation of the origin of the fractal behavior comes from investigating the uniform-field limit, where we show that two types of chaos exist, one is associated with the rotational saddle orbit and the other associated with the moving potential barrier of a DC field-distorted anharmonic oscillator, The homoclinic tangles associated with these saddles give rise to the sensitive dependence of dynamics on initial conditions.

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