Single Impacts of C60 on Solids: Emission of Electrons, Ions and Prospects for Surface Mapping

Abstract

We report on the co-emission of secondary ions and electrons resulting from 15 keV C60+ and 30 keV C602+ impacts on targets of Al, Si, Au, CsI, glycine, and guanine. The study has been performed by the combination of an electron emission microscope and a time-of-flight (ToF) mass spectrometer. The electron emission occurs near the kinetic emission threshold, yet yields are notable (>3) for all investigated targets. A key observation for the projectile-target combinations studied is the absence of correlation between the electron emission and the number and type of co-emitted secondary ions for flat and homogeneous samples. This observation validates a novel concept of “positional mass spectrometry”. In this approach a surface is probed in the event-by-event bombardment detection mode. Impacts of an individual C60 projectile are localized via electron emission. The location combined with the corresponding secondary ion information allows to map the distribution of surface molecules. The unique feature of positional mass spectrometry is the ability to identify co-emitted ions from a single projectile impact. To test the concept an electron emission microscope has been combined with a ToF mass spectrometer; the device operates with synchronized detection of electrons and ions. The spatial resolution of the method depends on the kinetic energy and angular distribution of the secondary electrons and the aberrations of the electron optics. Initial tests of positional mass spectrometry showed a spatial resolution of 1.2 μm. Progress is anticipated with improvements in the electron optics used and application of projectiles generating more prolific electron emission.