discovered_01_2015

discovered 01.15 FOCUS WWW.HZDR.DE When it comes to ultra-thin membranes, statistics fail ‘Normally, the energy loss of ion beams increases with the thickness of the penetrated material,’ the physicist explains. If one material is twice as thick as another, the ions also lose twice as much energy and are decelerated at twice the rate. Researchers have been able to confirm this relationship time and again, and it is a simple enough notion: ions lose a tiny fraction of their energy and speed as they pass each layer of atoms. After passing a million atomic layers, they will have lost this fraction of energy a million times and will thus have been decelerated accordingly. That sounds alluringly simple. Yet when Richard Wilhelm, in cooperation with Friedrich Aumayr from Technische Universität Wien, shoots extremely highly charged xenon ions at ultra-thin membranes, the assumption does not hold true. ‘A certain portion of the ions lose a great deal of energy while the rest continue to fly at almost unchecked speeds,’ the physicist ascertained to his amazement. What happened in his experiment? What did the researcher do differently from his colleagues? Stripped atoms First of all, Richard Wilhelm uses extremely thin membranes with a thickness of one nanometer, which is merely three layers of atoms. This extremely thin membrane is not a standard product, but rather a specialty of Armin Gölzhäuser from Bielefeld University. And secondly, only few other institutes in the world are able to shoot slow highly charged ions onto surfaces as the HZDR researcher has done. Initially, the HZDR facility generates electrons, which in turn knock some electrons out of the noble gas xenon. Each single xenon atom has 54 electrons in its shell, 44 of which the facility is able to remove – in theory. In practice, this process works well up to 40 electrons. The xenon atoms thus lose their electron coat, one could say that they are stripped down to their atomic underwear, their inner layer. The atoms not only lose their shells, but with each electron, also a negative electric charge. What remains are stripped atoms that possess as many positive charges as they have lost electrons. Scientists call atoms that are charged this way ‘ions’. At 30, 35 or even 40 positive units, these xenon ions are extremely highly charged. These ions are generated with a variety of different charges and are accelerated with a voltage of 4,500 Volts. Subsequently, a bending magnet diverts the generated xenon-ion beam at a 90-degree angle. Researchers can now adjust the strength of the magnet in such a way that, for example, only ions with a charge of 35 are diverted at exactly 90 degrees. All other ions are diverted a little more or a little less. What remains is a beam of ions which each carry 35 positive charges. Using such homogeneous ion beams, researchers often obtain much clearer results than with a mix of differently- charged ions. Before the beam hits the nanomembrane, however, the ions are significantly decelerated. A certain portion of these slow ions then passes the three atomic layers of the ultra-thin membrane without losing much energy at all, picking up only two or three electrons from the membrane as they pass. The remaining xenon ions, however, are decelerated significantly and pick up a lot of electrons from the membrane, thus losing their extremely high positive charge down to a mere two or three units. Gaps for ions There is actually quite a simple explanation for this surprising discovery: A single atomic layer is not a solid wall, but contains relatively large gaps through which ions can pass. There is a pretty good chance that an ion can pass unimpeded through the gaps of an ultra-thin membrane with only three atomic layers. If on the other hand – as is the case almost everywhere else in the world – the experiment is conducted Richard Wilhelm Hailing from Erfurt, Richard Wilhelm first became acquainted with the HZDR Ion Beam Center while taking his degree in physics at TU Dresden. After a research stay at the University of Stockholm, he returned to Dresden where he completed his doctorate on the topic of ‘Interaction of slow, highly-charged ions with surfaces and membranes’. In 2014, he and two colleagues received the HZDR Research Award for outstanding research. Today, the 28-year- old is helping to build a new nano-engineering facility with low-energy ions. Another theory assumes that the intense energy contained in the extremely highly-charged ions heats up the small area.

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