Characterization of Polycyciic Aromatic Compounds on Surfaces Using Ion-Beam-Induced Desorption and Multiphoton Resonance Ionization

Multiphoton resonance ionization (MPRI) has been combined with ion-beam-induced desorption to examine polycyclic aromatic compounds (PAC’s) present on surfaces. The results show that it is possible to obtain extremely high-quality mass spectra of these compounds with subfemtomole sensitivity limits. Although the degree of ionization selectivity of atomic species is much lower than for MPRI of molecules, we show that it is possible to control the degree of photofragmentation by varying the laser intensity. Spectra obtained by MPRI, by nonresonant multiphoton ionization (MPI), and directly by secondary ion mass spectrometry (SIMS) are quantitatively compared. For pyrene, the molecular ion signal is more than 10 times larger for MPRI than for MPI and more than 100 times larger than with SIMS. From the measured intensities, we estimate the fraction of pyrene molecular ions formed in the SIMS mode is on the order of 10⁻⁴. Finally, we demonstrate that mixtures of PAC’s are readily detected by examining a cocktail of codeposited benzo[a]pyrene, dlbenz[ac]-anthracene, triphenylene, and pyrene. In general, we propose that this approach is a powerful new method for detecting low concentrations of complex molecules adsorbed on surfaces.