Some comments on the simultaneous desorption of 3He and 4He

Drachsel and co-workers have recently reported that, when a balanced isotopic mixture of 3He and 4He is used as the imaging gas, the simultaneous emission of a 3He+ ion and a 4He+ ion is observed in a small fraction of cases. This note comments on the explanatory models they put forward, and suggests some further possibilities. These are based on the idea that the creation of an ion in the critical surface temporarily changes the potential structure for the field-adsorbed atom lying beneath it, and hence could significantly affect the dynamics of the motion of this field-adsorbed atom if it is already in a vibrationally excited state. One possibility is the temporary formation of a HeHe+ dimer. It is unlikely that such an effect could by itself explain the observed results, but a combination of such an effect with the collision model proposed by Drachsel and coworkers might lead to a satisfactory explanation of the experimental observations.