SPONTANEOUS DESORPTION

Spontaneous desorption (SD) denotes a process in which atoms and molecules are transferred as singly charged ions from the solid phase into the gas phase. For this purpose the solid sample does not need to be bombarded by particles (keV or MeV ions, electrons) from an external source. An electric field between the sample and an acceleration grid with a field strength at the sample surface below the threshold value for field desorption is the only prerequisite to initiate the SD process. The SD mechanism is the subject of the present paper. A number of experimental results obtained recently are summarized. It follows that SD is a two-step process. Primary ions are field desorbed in a first step from the edges of the grid where an enhanced electric field exists. These ions are mainly molecular ions and stem from adsorbates at the grid. They are accelerated in the electric field between the grid and sample to keV energies and sputter, in a second step, secondary ions from the sample with a sputter yield that exceeds the yield of atomic ions considerably.