Example: Fragmentation of argon tetramer after sudden ionisation Fragmentation dynamics of the singly ionised rare-gas tetramers Ivan Janeček, Daniel Hrivňák, and René Kalus Department of Physics, University of Ostrava, Ostrava, Czech Republic Supported by the Grant Agency of the Czech Republic (grant. no. 203/04/2146) Theoretical study of ionic clusters of rare gases (tetramers, pentamers) by dynamic simulations. Interaction model is based on diatomics-in-molecules approach (DIM) [1-4] used previously for theoretical study of rare gas trimer cation [5]. The additional interactions such as spin-orbit coupling (SO) can be included in DIM model. [1] F. O. Ellison, J. Am. Chem. Soc. 85 (1963), 3540. [3] J. S. Cohen and B. Schneider, J. Chem. Phys. 64 (1974) 3230 [2] P. J. Kuntz & J. Valldorf, Z. Phys. D (1987), 8, 195. [4] M. Amarouche et al., J. Chem. Phys. 88 (1988) 1010 [5] I. Janeček, D. Hrivňák, R. Kalus and F. X. Gadèa, J. Chem Phys 125, (2006) 1 – in print Neutral cluster in the static equilibrium configuration is vibrationally excited to determined energy from the zero point energy (ZPE) to the dissociation limit energy (DL). After MC equilibration the heated cluster had a random configuration different from the initial one. In the next step the cluster was suddenly ionised by putting out electron from random orbital. This state is initial state for MD simulations. EXCITATION FRAGMENTATION IONISATION Monte Carlo Simulation Molecular Dynamics Simulation The fragmentation times, proportions of fragmentation channels and proportions of final fragments emitted are detected. Example: Fragmentation of argon tetramer after sudden ionisation Models: DIM + ZPE DIM + SO + ZPE DIM + SO + DL For all three models the neutral monomer has the greatest proportion – about half of total number of fragments. Evaporation of neutral Ar from a cluster cation is the most probable process. The channel 211 has the smallest rate for all discussed models. We can expect that this type of decay is serial process (4→31→211), which needs longer time. For models with SO interaction a fragmentation of tetramer cation is slower than in case of pure DIM model. This effect is consistent with observation in case of trimer cations. The probability of channel 31 increases for model DIM + SO in comparison with DIM model. This increase is preferentially accompanied by decrease of probability of channel 211. So, many created trimers have not necessary energy for their fragmentation. However, the transfer from the channel 211 to the channel 31, which is observed after switching on the spin-orbit coupling, only slightly affects the fragmentation proportion for monomers because a decrease of monomers produced by the channel 211 (three fragments) leads also to a decrease of total number of created fragments.