Multiscale Dynamics of Cluster Fragmentation

The fragmentation of rare-gas clusters is theoretically investigated over time scales ranging from ionization and electronic excitation ($\sim \mathrm{fs}$) up to experimentally relevant times ($\sim \mathrm{ms}$). For this purpose a combination of methods are used, including nonadiabatic molecular dynamics, classical dynamics on the ground electronic state surface, and a kinetic description for the final evaporative cascade. The present multiscale protocol shows that, although the clusters are strongly out of equilibrium upon excitation, the long-time properties appear as statistical already after 1 ps.

Figure 1

Average size of the main fragment, for the dissociation of ${\mathrm{Ar}}_{20}$ and ${\mathrm{Ar}}_{30}$ upon electron impact. The solid black lines result from combining nonadiabatic molecular dynamics, followed by adiabatic dynamics on the ground state surface up to 10 ps, followed by the kMC/PST statistical approach. The solid red lines result from applying the statistical treatment at the onset of internal conversion, bypassing the intermediate ground state dynamics. The dashed lines are obtained by approximating the vibrational state densities as harmonic in the PST calculations. The vertical blue line marks the switching between the MD and kMC/PST calculations at 10 ps.

Figure 2

Error on final size distribution at the waiting time 1 ms, resulting from applying the full PST statistical treatment after some fixed duration of the adiabatic MD simulation in the range between internal conversion and 100 ps, for the fragmentation of ${\mathrm{Ar}}_{20}$ upon electron impact. The distribution at 100 ps is taken as the reference, and the error at 1 ms resulting from the harmonic approximation at this waiting time is highlighted. The inset shows the cluster size distributions $P_n(t_0,t_w)$ for different values of $t_0$ and $t_w$, denoted as $t_0/t_w$.

Figure 3

(a) Distributions of translational kinetic energy carried by $\mathrm{Ar}_9{}^{+}$ (solid line) after 1 ms, and by all clusters (red bars) after 10 ps, for the fragmentation of ${\mathrm{Ar}}_{20}$ by electron impact. The blue dashed line highlights the exponential, Boltzmann-like behavior. (b) Distributions of angular momentum carried by $\mathrm{Ar}_9{}^{+}$ (solid line) after 1 ms, and by all clusters (red bars) after 10 ps. (c) Time evolution of the average vibrational (solid black line), translational (solid red line), and rotational (dashed line) temperatures of the main cluster. The vertical blue line marks the switching between the MD and kMC/PST calculations at 10 ps.