Energy redistribution in cluster-surface collision: I₂^- (CO₂)_n onto silicon surface

Fragmentation of I₂^- (CO₂)_n (n= 1-30) by its collision on a silicon surface was investigated by measuring the fragment anions and their translational energy parallel to the surface (surface-parallel translational energy) in a tandem time-of-flight mass spectrometer equipped with a collision chamber evacuated down to ∼ 10^-8 Pa. At the collision energy (per 1₂^-) of 50 eV and the incident angle of 26° with respect to the surface normal, the distributions of the surface-parallel translational energies of the fragment anions from a given parent cluster anion were found to obey the one-dimensional Maxwell-Boltzmann distribution with the same translational temperature, T_s∥ The results show that the cluster anion and its neighboring surface atoms reach quasiequilibrium before the fragment anions leave the surface. A general increasing trend of T_s∥ (6000-12 000 K) with n is interpreted as an increasing extent of cluster-impact heating with n, while the reduction of T_s∥ in the 13≤n≤∼ 19 range is attributable to efficient transmission of the I^- and I₂^- translational energies to the CO₂ solvent cage. The effective volume and pressure of I₂^-(CO₂)_n colliding on the surface were estimated; at n = 10, the volume and the pressure were 100 nm³ and 10 MPa, respectively.