Imaging with Ion Beams and Laser Postionization

We investigate the use of a liquid metal ion gun (LMIG) combined with multiphoton resonance ionization of sputtered atoms and/or molecules to acquire mass-selected images with submicrometer spatial resolution. Images from several model systems are presented including those of an In grid, and of patterned surfaces of benzo[a]pyrene and tryptophan thin films. Since postionization for these systems yields ionization efficiencies that are considerably higher than those achieved with secondary ion mass spectrometry (SIMS), spatial resolution of 2600 Å for molecular species could be achieved while static conditions were maintained. Cooling of the sample to <210 K was shown to be essential for molecular systems since the number of thermally evaporating molecules overwhelms the number of sputtered molecules at room temperature. Other details necessary to incorporate laser postionization into a time-of-flight SIMS instrument using a pulsed LMIG source are also discussed.