COLLISIONS OF FLUOROCARBON IONS AT SOLID-SURFACES - ELECTRONIC EXCITATION, SURFACE-INDUCED DISSOCIATION AND CHEMICAL SPUTTERING

Collisions of C3F6+. at self-assembled hydrocarbon, deuterated hydrocarbon and fluorocarbon surfaces yield fragment ions which are characteristic of both electronic excitation and vibrational excitation. Direct electronic excitation is indicated by loss of F., which has been shown previously to be diagnostic of this type of excitation process. Electronic excitation is favored by low-energy collisions at the hydrocarbon surface. Even the change to the corresponding deuterated surface produces a large effect in favor of the normal vibrational excitation process. This change in mechanism with the nature of the target shows up as a dramatic isotope effect in the surface-induced dissociation (SID) mass spectra. The control over the excitation process exhibited by the effective mass of the target is probably exerted through its effect on the relative velocity of the collision partners. The fluorinated surface is more effective than the others in conversion of translational into internal energy and in minimizing ion loss through neutralization and other processes which compete with SID. The fluorinated surfaces yield spectra that are largely free from chemical sputtering, a process which occurs even at ultra-high vacuum for stainless-steel surfaces which are not rigorously cleaned. The internal energy deposition associated with chemical sputtering increases with increasing collision energy. Several of the fluorocarbon fragment ions generated from perfluoropropylene have also been examined at self-assembled monolayer surfaces and they are well behaved in their SID and chemical sputtering reactions. This in contrast to the low efficiency of SID and high sputtering efficiency observed in previous studies at uncharacterized multi-layer hydrocarbon-covered surfaces.