the Q values of the collisions. ..... This value implies an approximate lower limit on ... shaded regions are gates used to obtain gated Rvalue spectra. Inset: MCBM ...
New Insights into Multielectron Processes in Slow Collisions of Highly Charged Ions with Many-Electron Neutral Targets Kami All, Ahmad A. Hasan, Erik D. Emmons, and Guillermo Hinojosa Department of Physics, University of Nevada, Reno, Nevada 89557-0058 Abstract. Recent results from triple-coincidence measurements of Auger electrons, scattered projectiles, and atomic recoil ions are reported. The studies employed supersonically cooled targets, position imaging detectors, and time-of-flight coincidence techniques; making it possible to simultaneously perform Auger-electron and cold-target recoil-ion momentum spectroscopic (COLTRIMS) measurements. The measurements provide subpartial Auger-electron spectra corresponding to specific final projectile and recoil ion charge states, as well as information on the Q values of the collisions. The measurements provide new insights into the collision dynamics as well as the relaxation pathways of multiply excited states populated during the collisions.
INTRODUCTION Collisions of slow (v(l), y: (2,4)-»(l), z: (2,5)-»(l). The electron TOP ranges, in the main spectra, are shown in solid lines for the first autoionization steps and in dashed lines for the second autoionization steps.
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spectrum, both subpartial spectra are dominated by electrons associated with the initial population of the (3,3,n3=3-5) triply excited configurations. Auger electrons resulting from the autoionizing (3,3,n3=3-5)->(2,n=3-5) transitions are common to both subpartial spectra. Whether an event results in the (Ar3+,N6+) ion pair or the (Ar +,N5+) ion pair is then determined by the competition between the radiative and autoionization decay modes of the resulting doubly excited states. The (Ar3+,N5+) spectrum is therefore a pure spectrum involving one autoionization step only, while the (Ar3+,N6+) spectrum is still a composite one resulting from the autoionization of the triply and subsequent doubly excited configurations. While autoionization of the (2,n=3-5) doubly excited configurations involves the emission of jT-Auger electrons that are well separated in energy from the autoionization electrons of the triply excited configurations, the (3,3) configuration autoionizes with the emission of electrons that overlap those from the (3,3,n3=3-5) configurations, thus further complicating the analysis of the (Ar3+,N6+) spectrum. A1Auger electrons are essentially absent in the (Ar3+,N5+) spectrum, indicating that K -Auger electrons are not emitted in the first autoionization step which is consistent with the widely adopted assumption of the dominance of autoionization to the nearest continuum limits. In the first autoionization step, the (3,3,3) configuration autoionizes to the (2,3) continuum limits, while the (3,3,n3=4,5) can autoionize to the (2,3) or the (2,n=4,5) continuum limits. It is clear that autoionization of the (3,3,n3=4,5) configurations to the associated (2,n=4,5) continuum limits is more probable than to the (2,3) limits. In autoionizing to both continuum limits, the (3,3,n3=4,5) configurations give rise to electrons that overlap in energy. Furthermore, the (3,3,n3=4,5)-»(2,n=4,5) transitions give rise to electrons that overlap those from the (3,3,3)-»(2,3) transitions, thus rendering the relative initial populations impossible to obtain from the (Ar3+,N5+) spectrum. Examination of the inset of Fig. 4, which is a moderate resolution K-Auger-electron spectrum obtained in coincidence with the combination (Ar3+,N6+), reveals that both the (2,4) and (2,5) configurations have been populated following the first autoionization step. However, due to the limited resolution and statistical precision, we refrain from attempting to obtain the relative initial populations from this spectrum. Instead, we will discuss the radiative and nonradiative properties of the combined (3,3,n3=4,5) configurations. Accurate branching ratios for autoionization of the (3,3,n3=4,5) configurations to the different continuum limits are difficult to obtain. We can, however, try to estimate approximate ratios. The right-hand shoulder of the electron distribution under the labels #, b, and dm the (Ar3+,N5+) spectrum seems to result from the (3,3,3)^(2,3) transitions. If we assume that the actual contribution of the (3,3,3)-»(2,3) transitions is roughly twice the area under the shoulder, and then subtract that contribution from each subpartial spectrum, we obtain the contributions of the other transitions. Equal (3,3,3)^(2,3) contributions can be assumed for both spectra because aK~0.5 for the (2,3) doubly excited daughter configuration [4]. By then taking the ratio of the remaining intensity under the labels b, d, and h in both subpartial spectra to the remaining intensity under b, c, 6). Some MCBM reaction windows for double-electron capture are shown in the inset of Fig. 5. The reaction windows for the strings jl and j2 overlap the (4,4), (4,5),
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Figure 5. The subpartial Auger-electron spectrum in coincidence with the (Ar2+,N6+) ion pair. The shaded regions are gates used to obtain gated Rvalue spectra. Inset: MCBM reaction windows for some strings j giving rise to DC. Other windows that lie in between the shown reaction windows are not shown. Binding energies for various N5+ doubly excited configurations are also shown.
and (3,n>6) configurations. The window for j3 overlaps (3,4) and (3,5), while (3,3) can only be accounted for by a 4-electron string such as j4. According to the MCBM, the population of (3,3), (3,4), and (3,5) must be double-electron capture accompanied with target excitation (DCX). The shaded regions in Fig. 5 are Auger-electron gates representing electrons that originate in the different initial configurations. Gated Rvalue spectra can be obtained by demanding that events give rise to electrons in the different gates. Figure 6 shows such Rvalue spectra. We note a nearly perfect match between the experimental and the Rvalue ranges for a pure population, i.e. no TX, of (4,5), (4,4), and (3,n>6), in agreement with the MCBM predictions. The Rvalue spectra for the (3,n=3-5) configurations, however, exhibit significant shifts toward smaller Rvalues. These shifts constitute a direct and unequivocal evidence for outer-shell TX, again in agreement with the MCBM predictions. Clearly, DCX is a significant fraction of DC into (3,n=3,4), but most dramatic is the population of (3,3) where essentially all events are DCX.
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Q(eV) Figure 6. Gated Rvalue spectra corresponding to the different gates in Fig. 5. The spectra are identified by the initial populations of the gates. The labeled ranges assume DC without TX.
CONCLUSIONS We have presented recent results from triple-coincidence measurements of Auger electrons, scattered projectiles, and atomic recoil ions. The measurements provided subpartial Auger-electron spectra corresponding to specific final projectile and recoil ion charge states, as well as information on the Q values of the collisions. Relaxation pathways of triply excited states have been discussed, and an unequivocal evidence for significant TX accompanying DC has been presented.
ACKNOWLEDGMENTS This work was supported by the NSF under Grant No. PHY-9732614.
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