On the formation of molecular ions from liquids and solids

The ionization and desorption of thermally labile molecules from surfaces is accomplished by a number of ionization methods based on different primary excitations such as electrical charging and heating, absorption of radiation and collision processes. The possible role of temperature in the formation of molecular ions from liquids or solids is discussed. It is shown that a purely thermal desorption mechanism exists for some complex cations and anions of salts. However, for most organic molecules a purely thermally induced emission of molecular ions is rather improbable even at high rates of sample evaporation. Other mechanisms are considered to be important for the production of molecular ions under various conditions such as ionization of desorbing molecules in the gas phase for example via alkali ion attachment and surface ionization of (salt) molecules and clusters, not in thermal equilibrium with the heated surface. (orig.)