Microscopic cluster formation during the laser desorption of chrysene-d₁₂

The laser ablation of chrysene-d₁₂ has been investigated by laser desorption/post-ionization time-of-flight mass spectrometry (L2ToFMS). The early stages of plume expansion were probed by locating the focus of the post-ionization laser to within 50 µm of the surface. A spatial and temporal study of the desorption plume was carried out by recording positive ion time-of-flight mass spectra as a function of delay time and position. When the ionization laser focus was moved to within 50 µm of the surface, the appearance of the chrysene-d₁₂ parent ion signals changed from being sharply defined with a mass resolution of ~700 to broad signals of reduced intensity and a significantly lower resolution. Furthermore, the broad ionization signals were observed at longer delay times on increasing the desorption laser power. We attribute the observed phenomena to the laser desorption of molecular clusters, their transient survival, and ultimate evaporation to discrete molecules. © 1999 American Chemical Society.