Mass spectrometric analysis of low temperature field evaporation

Abstract

A first-order focusing mass spectrometer was built using a field ion microscope tip assembly as an ion source. The instrument is capable of detecting field evaporated ions under the normal, low temperature, operating conditions of a field ion microscope. At liquid hydrogen temperature field evaporation of Be, Fe, Cu, and Zn in a background vacuum of approximately 10 −7 Torr yields Be ++, Fe ++, Cu +, Cu ++, Zn + and Zn ++ respectively. At liquid nitrogen temperature nickel field evaporates as Ni ++. The charges of the detected ions of these metals are essentially those predicted from the image force theory of field evaporation. Beryllium exhibits a change in its evaporated ion species from Be ++ to Be + as the temperature is raised from 21 °K to about 300 °K. This result is wholly unexpected and requires extension of the present theory. The behavior of iron, nickel, copper, and zinc as a function of temperature is complicated by the appearance of molecular oxygen at about 150 °K and the subsequent formation of ionic oxides of these metals. In the presence of hydrogen nearly all of the metals exhibited hydride ions, and their identification was confirmed by producing the corresponding deuteride ions. Beryllium produces BeH + predominantly; some traces of BeH ++ are evident at 21 °K. The field reaction product of hydrogen and copper is CuH 2 +. FeH 2 ++ and NiH ++ are apparently the predominant ion species of iron and nickel respectively. No hydrides of zinc could be detected unambiguously.