A model for plasma formation in the plume of desorbed material in UV l
aser MALDI (matrix assisted laser desorption/ionisation) is proposed b
ased on thermal ionisation of the photoexcited matrix vapour. Signific
ant ion yields are obtained at modest plume temperatures and the predi
cted ion yield-fluence scaling is in considerably better agreement wit
h experiments than results obtained using a photoionisation model.