1. Contribution from the Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892, Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251-1892, Soc. Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy, Institute of Mass Spectrometry, University of Amsterdam, 1018 WS Amsterdam, The Netherlands, and Department of Physics and Astronomy, Free University, 1081 HV Amsterdam, The Netherlands