Microscopic cluster formation during the laser desorption of chrysene-d(12)

The laser ablation of chrysene-d(12) has been investigated by laser desorpt ion/post-ionization time-of-flight mass spectrometry (L2ToFMS). The early s tages of plume expansion were probed by locating the focus of the post-ioni zation laser to within 50 mu m 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 ioni zation laser focus was moved to-within 50 mu m of the surface the appearanc e of the chrysene-d(12) parent ion signals changed from being sharply defin ed with a mass resolution of similar to 700 to broad signals of reduced int ensity and a significantly lower resolution. Furthermore, the broad ionizat ion signals were observed at longer delay times on increasing the desorptio n-laser power. We attribute the observed phenomena to the laser desorption of molecular clusters, their transient survival, and ultimate evaporation t o discrete molecules.