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Physicians find surprising effect:

Fast heating makes gold harder

Experimental apparatus for femtosecond electron diffraction

11.02.2009, Press releases

It is common sense that metals become softer if they are heated up. Physicist Ralph Ernstorfer now demonstrated the exact opposite: When he heated gold with a strong but very short laser pulse the metal got harder rather than softer. The results of his work have just been published in "Science".

Blacksmiths do it every day: they heat up metal to make it workable. But when a group of scientists at the University of Toronto heated up gold atoms with ultra short laser pulses, they saw the exact opposite: instead of becoming softer the gold crystal got harder.

The explanation: The gold was heated at a rate of a billion degrees per second, too fast for the electrons absorbing the energy to collide with surrounding atoms and distribute energy. This means the electrons are on average further away from the atomic nucleus and there is less screening of the positive nuclear charge by these heated electrons. So the bonds between atoms actually get stronger.

"A gold crystal consists of gold ions and weakly bound electrons which screen the repulsive forces between the ions," explainsd lead author Ralph Ernstorfer. As a result, there are attractive forces between the ions. In contrast to many other materials, heating the electrons in gold with an ultra short laser pulse strengthens the forces between the ions, resulting in a harder lattice with an increased melting point.

"The effect of bond hardening in gold has been theoretically predicted. Now we have actually observed it for the first time," said Ernstorfer, who was a post-doctoral research fellow in the group of professor R. J. Dwayne Miller at the University of Toronto at the time of the experiments.

The researchers employed a technique called "femtosecond electron diffraction" to make the observation. This technique can be described as a camera for making atomic-level movies. By sending femto-second pulses of electrons through the thin gold crystal, the atomic motions of the ions were recorded in real time while heating the material with lasers. By measuring the speed of heating, amplitude of the atomic motions and ultimate melting of the crystal, the laser-induced change of the lattice stability could be inferred.

The physicians call this rarified state of matter „warm dense matter“. With their experiments they were able to relate the observed liquid structure to the increased lattice stability for the first time.

Ralph Ernstorfer meanwhile moved back to Europe. Since October 2008 he works in the group of Professor Reinhard Kienberger at the Physics Department of the Technische Univer-sitaet Muenchen. Now he studies even faster phenomena: by using atto-second laser pulses at the Max-Planck-Institute of Quantum Optics he explores the motion of electrons.

Original publication:

The Formation of Warm Dense Matter: Experimental Evidence for Electronic Bond Hardening in Gold; Ralph Ernstorfer, Maher Harb, Christoph T. Hebeisen, Germán Sciaini, Thibault Dartigalongue, R. J. Dwayne Miller
Science, Published Online, January 22, 2009 – DOI: 10.1126/science.1162697


Dr. Ralph Ernstorfer
Technische Universitaet Muenchen
department of Physics, Chair E11
James Franck Str. 1, 85748 Garching
Tel.:    +49.89.3290 5732
E-Mail: ralph.ernstorfer@mpq.mpg.de

Kontakt: presse@tum.de

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