Possible observation of the isotope effect during field evaporation

The field evaporation of tungsten at high temperatures ($T\sim$ 2000 K) has been studied using a magnetic mass spectrometer equipped with a field ion source. Only low-charge ions (W$^{+2}$ and W$^{+}$) have been observed in the course of evaporation for all tungsten isotopes. For singly charged ions only, the number of ions of the heaviest isotope, $^{186}$W$^{+}$, was about one order of magnitude lower than that corresponding to the standard isotope ratio for natural tungsten. An explanation of this anomalous phenomenon is proposed.