The ORNL Multicharged Ion Research Facility (MIRF)

High Energy Grazing Surface Scattering
Study of ions transmitted by an anodic nanocapillary array:

Objects having nanometer to micron dimensions, such as nanoparticles, nanopores and nanotubes, provide a bridge between the atomic quantum and macroscopic classical worlds. By studying multiply charged ions transmitted through aligned nanopores, for example, we can investigate fundamental atomic collision processes such as charge exchange, angular scattering and energy loss phenomena in a mesoscopic ultra-grazing ion-surface setting under a wide variety of conditions. This knowledge may help to characterize the inside of nanopores or carbon nanotubes and lead to atomic physics–based diagnostics or future applications for the rapidly expanding field of nanotechnology.

To this end, we have studied a nanocapillary array consisting of a dense distribution (>3x10⁹ pores/cm²) of cylindrical nanopores typically 100 nm in diameter and 60 m in length formed by anodic oxidation of aluminum. The Al₂O₃ array, a rigid and self-supporting insulator, was extensively characterized previously by other researchers using techniques such as scanning electron microscopy, gas absorption, deuterium-based nuclear magnetic resonance, and small-angle neutron scattering [Crawford92, Marchal03]. The nanopores, which are smooth, straight and hexagonally arranged, are six times longer than those used previously to guide 3-keV Ne⁷⁺ ions through insulating PET or mylar films [Stolterfoht02]. Heretofore, experimental high-resolution angular scattering studies have not been reported for any nanocapillary target.

The transmission of incident 10- to 20-keV/q Ar⁺, Ar³⁺, Ne³⁺, and Ne⁷⁺ ions through the array was investigated [Krause06]. Charge-state-selected angular distributions were studied using a two-dimensional position sensitive detector (TDPSD). The principal transmitted charge state was found to be the incident charge state in all cases. Yields in lower charge states and the neutral channel formed by electron capture are typically below 3% of the entrance q-state yield. The transmitted fraction of incident beam, 210-8, is many orders of magnitude smaller than the array’s surface porosity ( 40%). No evidence of significant energy loss was observed for the transmitted ions.

Figure 1. Q-state selected angular distribution for incident 140-keV Ne⁷⁺ ions (q’=7 out) transmitted by the nanocapillary array. The nanopores are tilted 0.1 Deg. from the incident beam direction (along the X axis) where observed structure is enhanced.

The observed angular distribution shown in Figure 1, for example, consists of two-dimensional structures sitting on a continuum distribution. The distribution and sharp angular structures can be steered in the direction of the pores within about ± 0.5° without a significant loss of transmitted intensity by rotating the sample with respect to the incident beam. Analysis of the structure has allowed the identification of single, double and triple collisions inside the nanopores. All data suggest that the structure observed in the scattered-ion angular distributions arises when ions bounce at ultra-low grazing angles in very large impact parameter Coulomb collisions with electrically charged nanopore walls. Arrays with and without deposited 7- and 15-nm Au films have been investigated [Krause06a]. These films increased the transmitted ion beam fraction 10 to 20 times. Some of our results contrast sharply with the experimental results obtained in short (10 mm) PET film target studies using much slower incident 3-keV Ne7+ ions (0.35 keV/u), where the high-transmission “ion guidance” mechanism was identified and studied [Stolterfoht02]. For example, the PET film studies found a large transmitted fraction (0.5 vs < 2 x 10^-7 here), broad unstructured angular distributions (vs highly structured distributions here), and a strong observed transient effect (vs. a weak effect here). In addition, the PET angular distribution centroid moved differently than present experiments when the target is tilted ( vs 2 here, where is the tilt angle).

References:

[Crawford92] G.P. Crawford, L.M. Steele, R. Ondis-Crawford, G.S. Iannacchione, C.J. Yeager, J.W. Doane, and D. Finotello, J. Chem. Phys. 96, 7788 (1992).

[Marchal03] D. Marchal and B. Demé, J. Appl. Cryst. 36, 713 (2003).

[Krause06] [5] H.F. Krause, C.R. Vane and F.W. Meyer, Accepted for publication in Physical Review A, (2006).

[Krause06a] H.F. Krause, C.R. Vane, F.W. Meyer, and H. M. Christen, invited presentation, 13th International Conference on the Physics of Highly Charged Ions, Queens University Belfast (2006).

[Stolterfoht02] N. Stolterfoht , J.-H. Bremer, V. Hoffmann, R. Hellhammer, D. Fink, A. Petrov, and B. Sulik, “Transmission of 3 keV Ne7+ Ions through Nanocapillaries Etched in Polymer Films: Evidence for Capillary Guiding,” Phys. Rev. Lett. 88, 133201 (2002); N. Stolterfoht et al., NIM. B 203, 246 (2003).

New high energy grazing beam line:

The new high-voltage platform installed at the ORNL multi-charged ion research facility (MIRF) has extended the energy range of ions available for experimental investigations of their interactions with electrons, atoms, molecules and solid surfaces [Meyer06]. We have built a new grazing surface beam line that is located after the second hemispherical analyzer downstream from the HV platform ECR source (see Figure 2).

Figure 2: The new ultra high vacuum grazing beam line showing the surface scattering chamber (left) and the projectile charge state analyzer (right).

Sufficient beam intensity and quality were transmitted to the end of the grazing line in initial tests using an 80 keV H2+ beam from the platform. The surface scattering chamber and charge state analyzer that will host a variety of different experiments will enable multiple-parameter coincidence grazing surface studies to be performed at energies up to 250 keV/q. It will be possible to uniquely identify scattered charge states, emitted electrons, states involved in Auger processes, photons and products sputtered from the surface that arise in the same collision event. The investigation of ions transmitted by targets of aligned nanopore or carbon nanotubes will also be possible using the new grazing end station.

Reference:

[Meyer06] F.W. Meyer et al., Nucl. Instru. And meth. In Phys. Res. B 242, 71 (2006).

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