Fragmentation lifetimes and the internal energy of sputtered clusters

We have compared the distribution of internal energies and fragmentation ra te constants determined experimentally for sputtered Fe-n(+) cluster ions w ith theoretical Molecular Dynamics computer simulations. It is found that t he experimental data and the simulation are complementary with respect to t he fragmentation time scale involved. While the experiment is sensitive to fragmentation times of nanoseconds and above, the simulation can provide in formation about the time interval ranging from femtoseconds to about one na nosecond. From the experimental data, it is found that the distribution of fragmentation rates as a function of time after the emission of the cluster s follows a power law rather than an exponential decay, thus indicating a b road distribution of fragmentation rate constants. From the simulation, we conclude that this dependence continues down into the sub-nanosecond time r ange with, however, increasing exponent as the times get shorter. Around fr agmentation times at or below 10(-12) s, the rate distribution levels off d ue to a maximum possible rate constant of the order of the vibrational freq uency of the cluster. The fragmentation rate constants are connected with t he internal energy distribution of the sputtered clusters by means of stati stical RRK theory. While the average internal energy determined by experime nt and simulation agree quite well, significant differences are found in th e width of the respective distributions, the origin of which is attributed to the different times scales explored by both techniques. (C) 1999 Elsevie r Science B.V. All rights reserved.