Metastable Metal Ion Production in Sputtering dc Glow Discharge Plasmas:  Characterization by Electronic State Chromatography

Abstract

The sputtering dc glow discharge is a stable and intense source of a wide variety of metal ions for use in ion/molecule studies. We have examined factors influencing the formation of metastable excited states of sputtered species in this device using the electronic state chromatography (ESC) technique. These determinations are essential if the glow discharge is to be used as a source of metal ions for subsequent reaction, since the electronic configuration of the metal can dramatically affect its reactivity in the gas phase. Further, the ability to manipulate excited-state production provides the means for the study of state-specific behavior of these species. We have obtained evidence in a series of experiments that indicates that the glow discharge is capable of producing several metal ions in metastable excited states as well as the ground state. Identification of specific configurations for several first-, second-, and third-row metal ions has been made on the basis of reduced zero-field mobilities in He. Results suggest that significant population of excited states higher than approximately 3.4 eV above the ion ground state does not occur in an Ar discharge for the metals examined here. Examination of the dependence of configuration distributions for several metal ions has revealed that excited-state production is sensitive to certain discharge parameters including pressure, distance to sampling orifice, and working gas composition. These results suggest that production of excited metal ions in the discharge involves energetic electrons. Further, some degree of deactivation of excited states occurs prior to extraction for ions whose excited states are collisionally coupled to lower states via interaction with the discharge gas. Metastable deactivation is enhanced in the third-row ions as a result of large spin−orbit effects.

Ion mobility studies of electronically excited states of atomic transition metal cations: Development of an ion mobility source for guided ion beam experiments

“…Initial attempts using a number of experimental conditions resulted in the observation of only one peak in our drift studies. This is consistent with the results of Taylor et al who used a glow discharge metal ion source [6]. More recent studies have produced the results shown in Figure 5.…”

“…Some of the tailing may be a result of the pulsing where ions at the end of the pulse will be accelerated by the return of the pulsing lens to the more positive trapping potential. Such asymmetry has been observed previously for all third-row transitionmetal ions (including W ϩ ) studied by Taylor et al [6]. They discuss the phenomenon extensively and note that the tails correlate with the observation of other species (metal oxides or metal hydrates), which have lower mobilities.…”

“…The use of the smaller apertures was investigated to further increase the pressures in the drift cell, which facilitate quantitative low-field mobility measurements. Such measurements [42] will be valuable as the mobilities of most second-row transition-metal ions have not been measured, and only those for the ground states of most third-row transition-metal ions are available [6]. However, for chemical studies using the GIBMS, maximum ion throughput combined with good separation is the primary requirement.…”

“…This observation can be rationalized by a stronger interaction with He for the configurations having an empty 4s orbital [5]. Later, Taylor et al [6] reproduced these results and extended them to second-row (Pd ϩ and Ag ϩ ) and third-row (Hf ϩ , Ta ϩ , W ϩ , Re ϩ , Ir ϩ , Pt ϩ , Au ϩ , and Hg ϩ ) transition-metal ions. Although they found separations of configurations only for Pd ϩ , Pt ϩ , and Au ϩ , the conditions intrinsic to the glow discharge source used in these studies are likely to deactivate many excited states.…”

“…Furthermore, this method is reliable when the electronic states are well separated, such as those for the first-row transition-metal ions, but cannot be applied confidently for many heavier elements where the states lie much closer together or overlap, as occurs when spin-orbit coupling becomes more important, i.e., especially for third-row transition-metal ions [31]. These various limitations have meant that very few state-specific studies have been performed for second-and third-row transitionmetal ions either in our laboratories [32,33] or others [6].…”