This is due to contamination of the tip, either from the vacuum, or more often from diffusion of adsorbed surface species to the tip. Thus the real nature of a field emission tip during use, and indeed of an STM tip, is somewhat shrouded in mystery. Three types of example are given here. An image from Gomer s book is shown in diagram A18. In-situ deposition of individual metal atoms on the tip has been shown to caused jumps in the emitted current, as illustrated in diagram A19.
Adsorption is useful for an electron source if the adsorbate increases emission.
The stringent vacuum requirements of field emission can be reduced somewhat if one both increases the operating T, and also uses an adsorbed layer which reduces the work function. When a sample is bombarded with charged particles, the strongest region of the electron energy spectrum is due to secondary electrons. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence. There is a lot of information in this secondary electron background , but, unlike Auger and other electron spectroscopies, it is not directly chemical or surface specific in general. However, there are cases where the secondary electrons can be seen to convey more specific surface information.