The emission charge state dependence of alkali metal ions back-scattered in binary collisions has been evaluated. For this purpose, mass-resolved LEIS in an SIMS instrument was utilized during bombardment of Cs-133(+) or K-39(+) on metal surfaces (Ta, Au or Cu) under dynamic sputtering conditions. Back-scattered singly positively charged alkali ions were found to be by far predominant (i.e. with an at least a factor of 1000 larger flux than other ejected alkali ions). Back-scattered negative alkali ions (Cs-133(+), K-39(-)) as well as doubly positively charged alkali ions (Cs-133(++), K-39(++)) were also observed, while other emission charge states (such as triply positively charged alkali ions) could not be detected. (C) 2002 Published by Elsevier Science B.V.