Counting Atoms and Molecules - A Proposed End-User Facility for LEUTL

Michael J. Pellin, Materials Science / Chemistry Division, ANL

The combination of two unique ANL capabilities will produce a world-leading instrument for quantitative trace elemental and molecular analysis of samples whose dimensions can be nanoscale laterally and monolayer in depth. During the course of this work, the SPIRIT instrument (single-photon ionization or resonant ionization to threshold), will be used to demonstrate minimum detection limits for important light elements (C, O, N) over two orders of magnitude lower than currently available surface analytical techniques.  SPIRIT will also demonstrate for the first time quantitative molecular surface analysis - a particularly vexing problem currently for the analyst with significant implications for the study of problems ranging from molecular self-assembly, DNA analysis, and combinatorial chemistry. Finally, SPIRIT will be used to study electron induced desorption from cryogenic targets, a problem related to next generation accelerator development.

There are three significant advantages of using tunable VUV from the Argonne FEL for ionization.   First, excellent trace quantification will be possible when the energy/pulse is sufficient to saturate the ionization of each species.  Saturation minimizes fluctuations due to pulse instability and eliminates the need for absolute ionization cross sections.  The FEL, with an estimated 0.1mJ pulse energy in the VUV, will therefore make possible accurate, sensitive quantification for many species (molecules and light elements) for the first time.  Second, large molecules that make up soft materials (e.g. polymers, DNA) are often difficult to ionize without significant fragmentation.  We have demonstrated previously soft ionization of such molecules that is free of fragmentation, but at very low achievable laser power and without tunability, greatly limiting the applicability of the technique.  The FEL will allow tuning the VUV energy to just above a particular ionization potential for high sensitivity measurements, and just below the IP for low backgrounds.   Third, since all known molecules, atoms, and clusters can be ionized in the region accessible to the FEL (190nm to 50nm), when it is coupled with a carefully designed mass spectrometer, we can have a universal detector.   Such an instrument would allow a direct comparison of the quantities of different species in a material.

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