Molecular depth profiling and imaging using cluster ion beams with femtosecond laser postionization

Abstract

The emergence of cluster ion sources as viable SIMS probes has opened new possibilities for detection of neutral molecules by laser postionization. Previous studies have shown that with atomic bombardment multiphoton ionization using high-power femtosecond pulses leads to photofragmentation. The large amount of photofragmentation can be mostly attributed to high amounts of internal energy imparted to the sputtered molecules during the desorption process. Several pieces of preliminary data suggest that molecules subjected to cluster beam bombardment are desorbed with lower internal energies than those subjected to atomic beam bombardment. Lower energy molecules may then be less likely to photodissociate creating less photofragments in the laser postionization spectra. Here we present data taken from coronene films prepared by physical vapor deposition comparing a 40 keV C 60 + ion source with a 20 keV Au + ion source, which supports this hypothesis. Furthermore, the depth profiling capabilities of cluster beams may be combined with laser postionization to obtain molecular depth profiles by monitoring the neutral flux. In addition, imaging and depth profiling may be combined with atomic force microscopy (AFM) to provide three-dimensional molecular images.