Paper AS-TuM10
Strong Field Laser Postionization Imaging and Depth Profiling Using C₆₀ Cluster Ion Beams

Tuesday, October 21, 2008, 11:00 am, Room 207

Laser post-ionization (LPI) of sputtered neutral molecules has been achieved by combining strong field ionization (SFI) techniques with newly developed methods for generating C₆₀ cluster ion beams. LPI has been a long desired addition to cluster bombardment experiments for its ability to further enhance the sensitivity of surface measurements. In previous studies, LPI has proven to be beneficial when ionizing atomic species but, problematic when ionizing molecular species.¹ The source of this dichotomy is photodissociation resulting in highly fragmented molecular species. Here we show that by implementing SFI at longer wavelengths the photodissociation of molecular species is greatly reduced, thus vastly improving the efficacy of LPI of sputtered neutral molecules. An important application of LPI is that of chemical imaging using secondary ion mass spectrometry (SIMS). SIMS provides a method for high resolution chemical imaging without the need for sample modification. LPI provides several benefits to SIMS imaging including increased sensitivity, elimination of matrix ionization effects, and an insight into the fundamental properties of the generally undetected sputtered neutral molecules. In addition, SIMS not only provides chemical information in the x and y directions, but due to the development of cluster ion beams may provide chemical information as a function of depth. In past years, sputtering surfaces with C₆₀ primary ion beams has proven to be a highly effective method of depth profiling.² These advances in cluster ion beams have led to new frontiers for the SIMS community as a whole; however, experimental results are still hampered by matrix ionization effects as well as a fundamental lack of understanding of certain aspects of the sputtering physics associated with depth profiling. LPI finds application here as well providing depth profiles indicative of sputtering dynamics without interference from matrix ionization effects occurring at both the surface and interface regions; and allows for an experimental look at the fundamental physics underlying the sputtering process.

¹V. Vorsa, et al., "Femtosecond Photoionization of Ion-Beam Desorbed Aliphatic and Aromatic Amino Acids: Fragmentation via a -Cleavage Reactions", J. Phys. Chem. 103(37), 7889 (1999).
² J. Cheng and N. Winograd, "Depth Profiling of Peptide Films with TOF-SIMS and a C60 probe", Anal. Chem. 77, 3651-3659 (2005).