Fragmentation lifetimes and the internal energy of sputtered clusters

We have compared the distribution of internal energies and fragmentation rate constants determined experimentally for sputtered Fen+ cluster ions with theoretical Molecular Dynamics computer simulations. It is found that the experimental data and the simulation are complementary with respect to the fragmentation time scale involved. While the experiment is sensitive to fragmentation times of nanoseconds and above, the simulation can provide information about the time interval ranging from femtoseconds to about one nanosecond. From the experimental data, it is found that the distribution of fragmentation rates as a function of time after the emission of the clusters follows a power law rather than an exponential decay, thus indicating a broad distribution of fragmentation rate constants. From the simulation, we conclude that this dependence continues down into the sub-nanosecond time range with, however, increasing exponent as the times get shorter. Around fragmentation times at or below 10-12 s, the rate distribution levels off due to a maximum possible rate constant of the order of the vibrational frequency of the cluster. The fragmentation rate constants are connected with the internal energy distribution of the sputtered clusters by means of statistical RRK theory. While the average internal energy determined by experiment and simulation agree quite well, significant differences are found in the width of the respective distributions, the origin of which is attributed to the different times scales explored by both techniques. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)