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 H-2() ion bombardment. The distributions show that depending on the physic al and chemical nature of the substrate, the neutral molecules have st rikingly different desorption profiles, For C6H6/Ag{111}, at low cover age the C6H6 molecules eject with energies in the range 0.25-1 eV whil e at high coverage most of the molecules desorb with thermal kinetic e nergies (similar to 0.04 eV). At intermediate coverage two peaks are p resent in the time-of-flight distribution indicating that two differen t mechanisms contribute to the desorption process. For self-assembled monolayers of phenylethanethiol on Au, while a minor ejection is obser ved at higher kinetic energy (similar to 1eV) most of the molecules de sorb with thermal kinetic energies (similar to 0.03 eV). Pyrenebutyric acid molecules ejected from monolayer and multilayer samples have kin etic energies close to 0.2 eV, One ejection mechanism is observed in t his case. For tryptophan, most molecules eject with kinetic energies c lose to 0.1 eV, In addition, a feature unique to this case is the cont inuous emission of molecules from the surface that extends beyond 200 mu s after ion impact. For all the multilayer samples investigated, a molecular collision cascade in the solid leads to ejection of molecule s with kinetic energies in the range 0.1-0.3 eV, (C) 1998 John Wiley & Sons, Ltd.