Angular Distribution of Molecules Sputtered by Gas Cluster Ion Beams and Implications for Secondary Neutral Mass Spectrometry

We present experimental data on the angular distribution of Irganox 1010 organic molecules sputtered by large argon gas cluster projectiles (E/n = 5 eV, 10 keV Ar-2000). Ejection probability distributions as derived from deposit patterns on planar collector surfaces were recorded at various angles of incidence of the primary cluster ion beam. The sputtered material is ejected at polar angles, on average, greater than 45 degrees from the surface normal. At normal incidence there is no azimuthal dependence in the ejecta distribution, but the ejecta are forward directed even at incidence angles as low as 15 degrees. After this initial rapid change, the ejecta distribution shows a rather weak dependence on the incidence angle of the primary ion beam and the polar, and azimuthal angles of preferred ejection remain relatively constant. Ejecta distributions agree with previously published results from molecular dynamics simulations for organic molecules sputtered with large argon gas cluster projectiles and are consistent with the picture derived from experimental data for metal target species. The close chemical resemblance of collector and target materials as identified by secondary ion mass spectrometry, and the large total volume of deposits accounting for over 75% of the sputtered material as inferred from symmetry considerations, indicate that a large fraction of the sputtered material is intact molecules. Findings are discussed with respect to the utilization of large cluster projectiles as primary ion beams in secondary neutral mass spectrometry.