Enrichment of binary van der Waals clusters surviving a surface collision

We present experimental results on the collision of large binary noble gas clusters with a hot graphite surface at an incident velocity of 430 ms(-1). The mixed ArnXm clusters (X=Kr, Xe) with average sizes n+m between 1000 an d 13 000 are obtained from pure argon clusters by the pickup technique. The surface scattering dynamics belongs to the thermal evaporation regime and large surviving binary fragments are detected at grazing angles. As expecte d from simple binding energy considerations, in all cases the surviving clu sters are richer in the dopant species X. This enrichment has been measured as a function of incidence angle, incident cluster size, and dopant molar fraction x. For the lowest values of x (about 0.7%), the measured enrichmen t is the same for Kr and Xe, and decreases slowly with increasing size and incidence angle. On the other hand, when x is raised up to 15%, this enrich ment decreases substantially for Xe, and much less for Kr. This unexpected behavior clearly shows that collision induced evaporation cannot be viewed as a mere distillation of an homogeneous binary mixture. The possible role of the incoming cluster structure is discussed in connection with the prepa ration of the binary clusters by pickup. (C) 1999 American Institute of Phy sics. [S0021-9606(99)01805-X].