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 t o 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 th in films. Since postionization for these systems yields ionization eff iciencies that are considerably higher than those achieved with second ary ion mass spectrometry (SIMS), spatial resolution of 2600 Angstrom for molecular species could be achieved while static conditions were m aintained. Cooling of the sample to <210 K was shown to be essential f or molecular systems since the number of thermally evaporating molecul es overwhelms the number of sputtered molecules at room temperature. O ther details necessary to incorporate laser postionization into a time -of-flight SIMS instrument using a pulsed LMIG source are also discuss ed.