Gordon, Michael J. (2004) Low-energy ion beamline-scattering apparatus with application to charge exchange collisions at surfaces. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01022004-085659
Ion-surface interactions are important in a variety of fields such as plasma physics, surface analysis, and semiconductor manufacturing. However, the low-energy regime (50 eV - 1 keV) has been generally avoided by the research community because of the experimental challenges associated with providing sufficient ion beam current at low impact energy to conduct surface scattering studies. This energy regime is a useful range to study because threshold physical and chemical processes occur at low energies. We have set out to probe this neglected energy range by developing an ion scattering system to investigate a wide variety of ion-surface interaction phenomena below 1 keV. This thesis describes the design and construction of our system and its application to charge exchange collisions at surfaces.
Our design philosophy has been to take an inductively coupled plasma (ICP) source and couple it to a high-voltage ion beam transport line with magnetic mass- filtering to provide a clean ion beam surface probe with high current (>100 uA/cm2) and tunable energy (~50 eV - 1 keV). Space charge repulsion between the ions, which usually precludes high current at low energy, is circumvented using the accel-decel scheme for transport. In this arrangement, ions are created at the desired collision energy in the plasma source, extracted and accelerated to high transport energy (to fight space charge forces), and then decelerated back down to their original creation potential right before impacting the grounded target. In this way, the beam current is high, the collision energy is easily tunable (just by floating the whole plasma source above ground), and the target is always kept grounded. The ICP-based beamline is a generic and robust system because any ion created in the plasma can be individually singled out and delivered to the target as a clean surface probe composed of only one species and one charge state.
The particle flux leaving the target surface is analyzed with a hybrid scattered product detector which allows simultaneous mass and energy filtering. The detector combines an electron-impact ionizer, hemispherical electrostatic sector, and quadrupole mass filter in series with single ion detection capabilities so that small signals of both ions and neutrals can be analyzed. Energy dispersion, followed by mass dispersion, is an effective combination because overlapping signals can be separated easily (i.e., multiple charge states or a mix of ion species leaving the target).
The performance of the entire scattering system has been evaluated in an investigation of Ne+ scattering off lighter target materials (Mg, Al, Si , and Ti), where the scattered particle flux can contain inelastic Ne+ and Ne++ exit channels as a result of charge exchange between the projectile and target nuclei. Specifically, we have seen a sudden "turn-on" in Ne++ generation as the collision energy is raised above a threshold value. This turn-on seems indicative of inelastic loss channels that open up as the distance-of-closest-approach gets smaller during the hard collision. Values for the inelastic loss in the center-of-mass frame for Ne+ and Ne++ have been evaluated with our system for collision energies up to 1.3 keV and compared with literature data at higher energies. The inelasticity values we see in the threshold region are too small to be readily explained by the mechanisms proposed for higher collision energies in the literature for both Ne+ and Ne++. Finally, a simple orbital overlap model is presented which suggests that Ne++ generation is coincident with a required atomic orbital overlap between the projectile and target atom L-electron shells, which signify that the Ne 2p orbital is promoted through the 4f sigma molecular orbital at some threshold internuclear distance.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||charge exchange; ion scattering|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Major Option:||Chemical Engineering|
|Awards:||Constantin G. Economou Memorial Prize, 1997|
|Thesis Availability:||Mixed availability, specified at file level|
|Defense Date:||1 October 2003|
|Non-Caltech Author Email:||gordon (AT) its.caltech.edu|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||27 Jan 2004|
|Last Modified:||11 Dec 2014 00:06|
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