Measurement of activation energies for field evaporation of tungsten ions as a function of electric field

Activation energies for the field evaporation of tungsten ions have been determined as a function of electric field in the range from 4.70 to 5.90 V/Å. Field-evaporation rates covering 2-3 orders of magnitude were measured as a function of temperature at seven different field strengths with the use of calibrated imaging atom-probe mass spectra, and the activation energies and frequency prefactors were obtained from standard Arrhenius plots. The activation energies were found to decrease from 0.89±0.05 to 0.12±0.03 eV as the electric field was increased from 4.70 to 5.90 V/Å. These experimental results strongly disagree with calculations based on the image-force model of field evaporation, and a correction to the model involving field-induced charge transfer is suggested. As expected, the activation energies for field evaporation of ${\mathrm{W}}^{3+}$ and ${\mathrm{W}}^{4+}$ were found to be the same and give support for recent post-ionization models. Contrary to expectations, the frequency prefactor, which had been assumed to be constant, was found to be field dependent. This report also includes new measurements of the temperature dependence of the W evaporation field and the field dependence of the W evaporation rate. Measurements of the field-evaporation activation energy in background pressures above 5 × ${10}^{-10\mathrm{}}$ Torr indicate that very low levels of contamination can reduce the activation energy by more than a factor of 4.