Ejection of neutral molecules from ion-bombarded organic surfaces

Time-of-flight distributions of neutral molecules ejected from various organic surfaces have been measured subsequent to 8 keV Ar⁺ and H₂⁺ ion bombardment. The distributions show that depending on the physical and chemical nature of the substrate, the neutral molecules have strikingly different desorption profiles. For C₆H₆/Ag{111}, at low coverage the C₆H₆ molecules eject with energies in the range 0.25-1 eV while at high coverage most of the molecules desorb with thermal kinetic energies (~0.04 eV). At intermediate coverage two peaks are present in the time-of-flight distribution indicating that two different mechanisms contribute to the desorption process. For self-assembled monolayers of phenylethanethiol on Au, while a minor ejection is observed at higher kinetic energy (~1 eV) most of the molecules desorb with thermal kinetic energies (~0.03 eV). Pyrenebutyric acid molecules ejected from monolayer and multilayer samples have kinetic energies close to 0.2 eV. One ejection mechanism is observed in this case. For tryptophan, most molecules eject with kinetic energies close to 0.1 eV. In addition, a feature unique to this case is the continuous emission of molecules from the surface that extends beyond 200 μs after ion impact. For all the multilayer samples investigated, a molecular collision cascade in the solid leads to ejection of molecules with kinetic energies in the range 0.1-0.3 eV.