Extreme processes in clusters impacting on a solid surface

A large high-energy cluster impacting on a solid surface (the number of particles in a cluster $N\ge 10^2-10^6$, the collision energy per particle $E_{\mathrm{col}}\ge 10-10^3$ eV) forms for a short time ($\le 50-500$ fs) a medium characterized by extremely high temperature ($\ge 10^4-10^5$ K), density (up to 4 to 5 times the solid state value), and pressure ($\ge 1-10$ Mbar). As this takes place, the cluster heating rate reaches the value $\ge 10^{15}-10^{16}$ К s$^{-1}$. In these extreme conditions, physical and chemical processes that are impossible in thermal equilibrium can occur both in the cluster itself and the collision zone. In this paper, extreme processes induced in clusters as a result of their strong excitation at collisions with a solid surface are reviewed, including ionization, light and charged-particle emission, fragmentation, breaking and making of chemical bonds, microshock wave generation, nuclear fusion, and surface bombardment. Conditions for these processes to proceed are examined and models to describe them discussed. It is shown that the characteristics of the processes depend significantly on the velocity, size, and composition of the cluster, as well as the material and temperature of the surface. Cluster excitation by an impact with a surface and that by a superhigh-power ultrashort laser pulse are compared and practical applications of the above processes are discussed.