It is known that high-molecular-weight, thermally labile molecules can be desorbed intact using keV ion beams. This knowledge has led to numerous applications of fast atom bombardment and secondary ion mass spectrometry (SIMS) by mass spectrometric detection of the desorbed ions. Here we show that these measurements can be enhanced significantly by using resonance-enhanced laser ionization to softly ionize the neutral component of the desorbed flux. This experimental configuration can produce sensitivity improvements of several orders of magnitude over SIMS while adding a certain degree of selectivity to the ionization process itself. Examples of this performance will be presented using a wide variety of molecules, including polycyclic aromatic hydrocarbons, organic polymers, molecular salts and biologically important molecules. Results from model systems to complex samples are discussed, along with their implications for submicron molecular imaging using this technique.
I N T R O D U C T I O NIt has been 24 years since Benninghoven introduced to us the concept of static SIMS.' From this initial idea we have, of course, expanded our vision of ion/solid interactions manyfold. The journey through organic SIMS, liquid matrix SIMS, time-of-flight SIMS and angleresolved SIMS has indeed been a fascinating one. The work emanating from Benninghoven's laboratory has exerted a central influence upon those of us working in the field and has helped to establish the SIMS methodology as a core surface characterization t.echnique.Can static SIMS measurements be improved? It is a difficult challenge because primary ion doses are necessarily ions cm-', suggesting that the methodology must work exceedingly well right from the start. Obviously, however, improvements have been steady and rapid over the last 22 years. In fact, it is the implementations of new instrumental tools that have allowed this field to retain its vigor through such an extended period.Possibly the most difficult issue to deal with in static SIMS experiments involves matrix ionization effects. The ion yield of molecular species, in particular, can vary over many orders of magnitude. These yields are also strongly dependent upon the surface electronic properties. Moreover, it is not possible to enhance molecular ion yields by 0'' or Cs+ bombardment as is traditionally done by dynamic STMS users. This approach is clearly in violation of the basic principles of static SIMS. Cationization or anionization rarely mitigates these problems.Since 1982,' our group has focused on the use of laser techniques to post-ionize neutral species that desorb from the surface. Not only can resonance-enhanced laser ionization frequently improve sensitivity by several * Author to whom correspondence should be addressed. orders of magnitude by efficiently sampling the more abundant neutral flux, but it can also produce soft and selective ionization.Initially, our experiments concentrated on the study of atomic photoionization using multiphoton resonance schemes.2 In this experiment a p...
