STRUCTURAL TRANSFORMATION IN (CO2)(N) CLUSTERS, N-LESS-THAN-100

In this paper, the structure of CO2 clusters made of less than a hundr ed of molecules is studied by means of electron diffraction experiment s and molecular dynamics simulations. According to previous works, CO2 clusters are expected to undergo a transition in this size range, fro m an icosahedral structure to the bulk cubic structure. Electron diffr action patterns have been recorded from clusters produced in a free je t expansion of gaseous CO2. These patterns provide evidence of a struc tural change occurring in clusters made of a few tens of molecules. (C O2)(N) models, with 13<N<80, have been constructed in the cubic struct ure and annealed through a molecular dynamics calculation. Calculated diffraction functions show significant changes for a critical size of about 30 molecules per cluster. In the N=25 model, carbon atoms exhibi t a polyicosahedral arrangement, similar to the one already identified in argon cluster models. In the N=32 model, most of the molecules bui ld up a cubic lattice, meanwhile some of them adopt a pentagonal arran gement allowing a higher cluster stability. In addition to what has be en observed previously, the present study confirms that the transforma tion from an icosahedral to a crystalline structure is a general proce ss which occurs during the growth of van der Waals clusters made of sm all molecules. The relation between the critical size and the molecula r species is briefly discussed. (C) 1996 American Institute of Physics .