An experimental and theoretical view of energetic C₆₀ cluster bombardment onto molecular solids

Recent experimental measurements and calculations performed by molecular dynamics computer simulations indicate, for highly energetic C₆₀ primary ions bombarding molecular solids, the emission of intact molecules is unique. An energy- and angle-resolved neutral mass spectrometer coupled with laser photoionization techniques was used to measure the polar angle distribution of neutral benzo[a]pyrene molecules desorbed by 20-keV C ₆₀⁺ primary ions and observed to peak at off-normal angles integrated over all possible emission energies. Similarly, computer simulations of 20-keV C₆₀ projectiles bombarding a coarse-grained benzene system resulted in calculations of nearly identical polar angle distributions. Upon resolving the measured and calculated polar angle distributions, sputtered molecules with high kinetic energies are the primary contributors to the off-normal peak. Molecules with low kinetic energies were measured and calculated to desorb broadly peaked about the surface normal. The computer simulations suggest the fast deposition of energy from the C₆₀ impact promotes the molecular emission by fluid-flow and effusive-type motions. The signature of off-normal emission angles is unique for molecules because fragmentation processes remove molecules that would otherwise eject near normal to the surface. Experimental measurements from a Ni {001} single crystal bombarded by 20-keV C₆₀⁺ demonstrate the absence of this unique signature.