Determination of the critical ionization distance and ionization zone during high-temperature field evaporation of molybdenum

Steady-state field evaporation of molybdenum at high emitter temperatures ($T\sim$ 2000 K) has been studied using a magnetic mass spectrometer equipped with a field ion source. Only low-charge ions (Mo$^{+2}$ and Mo$^+$) have been observed in the course of evaporation. The measured ion energies and evaporating field strengths $(F_{ev})$ were used to determine the critical ionization distances $(x_{cr})$ and ionization zones $(\Delta)$ for singly and doubly charged ions. The obtained $x_{cr}$ and $\Delta$ values show that the formation of ions takes place at a certain distance from the emitter surface.