Isotopic fractionation of sputtered anions: C− and C2-

The isotopic fractionation of C− and C2- anions sputtered from graphite by 14.5-keV Cs+ bombardment was investigated by high-resolution mass spectrometry. Specifically, the 13C−/12C− and the 13C12C−/12C12C−ratios were monitored under steady-state sputtering conditions for different emission energies, ranging from ∼ 0 eV to about 55 eV. Generally, the heavier ions are depleted in the flux relative to the standard isotopic abundance of carbon. Over a wide range of emission energies this fractionation amounts to about 6% for 13C−/12C− and for 13C12C−/12C12C−; however, for very low energies (< 3 eV) the magnitude of fractionation increases strongly, reaching values of 30-40% at the lowest energies. Several possible sources for the observed fractionation are identified and discussed, namely fractionation in the sputtering process, in the ionization event, and due to mass-dependent transmission and detection efficiencies of the mass spectrometer. From a comparison with the data it is concluded that the pronounced isotopic fractionation found at low emission energies is primarily due to the velocity dependence of the ionization probability that favors the formation/emission of the swifter (i.e. the lighter) anions. In this range, the fractionation scales with the inverse emission velocity, v−1, in agreement with models predicting an exponential dependence of the ionization probability on v−1.