title: On the SIMS Ionization Probability of Organic Molecules.
abstract: The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α+) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10-5. Our lab has developed a method for the direct determination of α+in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (C24H12), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C60cluster projectiles is of the order of 10-3, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event. Graphical Abstract <!-- [INSERT GRAPHICAL ABSTRACT TEXT HERE] -->.
authors.affiliation: Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA, 16802, USA. nicholasp@psu.edu.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Popczun
authors.affiliation: Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA, 16802, USA.
authors.forename: Lars
authors.initials: L
authors.lastname: Breuer
authors.affiliation: Fakultät für Physik, Universität Duisburg-Essen, 47048, Duisburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Wucher
authors.affiliation: Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA, 16802, USA.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Winograd
keywords: Coronene; Femtosecond; Ionization; Ionization probability; LPI; Organic; Post-ionization; SIMS; SNMS; Sputtering