It Crackles in Platinum. Hot Electrons Create Pressure in Metal

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A team of researchers was able to show that ultrashort optical laser pulses can trigger extremely fast lattice vibrations in periodically layered metals ...

In the study conducted by European XFEL, the University of Potsdam (Germany), and other participating institutions, platinum and copper layers only a few nanometers thick were stacked into an artificial metal lattice. When stimulated by a laser pulse, the artificial crystal lattice starts oscillating at about one terahertz, as was discovered. About a trillion times per second, the platinum nanosheets expand while simultaneously compressing the copper layers. This immediately triggered oscillation is far too quick to be explained by the transfer of thermal energy through electrons—and the resulting expansion of the crystal lattice. "This surprised us," commented Jan-Etienne Pudell from European XFEL. The oscillation, therefore, does not arise from the pressure of the heated lattice but rather from electronic pressure—particularly in the platinum component.

"We are not simply seeing a metal getting warm and expanding," explains Matias Bargheer, spokesperson for the Collaborative Research Center SFB 1636 "Elementary Processes of Light-Driven Reactions at Nanoscale Metals" at the University of Potsdam. Rather, it is observed that the electrons themselves exert pressure within less than a trillionth of a second, practically pelting the metal surface from the inside. This is highly exciting for the chemistry of metals that are only a few nanometers thick, as it sheds new light on the relationship between hot electrons, heat, and atomic motion, all the way to chemical reactions. The results also demonstrate that such processes can be tailored by material selection and layer thickness, the researchers further elaborate.

Better Understand Terahertz Oscillations with the MID System

For the measurements, the international team used the MID (Materials Imaging and Dynamics) experiment station at European XFEL. The platinum-copper lattice was excited there with very short laser pulses of the same order of magnitude and examined with equally short but highly energetic X-ray pulses. The X-ray pulses can directly resolve the structural changes in the material. This experiment provides both material-sensitive and depth-sensitive information. This allows the experts to visualize how the different metal layers shift after the laser excitation. Incidentally, the MID experiment station was specifically built to answer this question: How do atoms and electrons move in complex materials when they are driven out of equilibrium by light? Here, not only was it possible to observe the emergence of a terahertz vibration, but also to determine which physical mechanism drives it.

Opened New Opportunities for Chemistry and Physics

For SFB 1636, the results are particularly relevant because the pressure of the hot electrons in the platinum arises from the reflection at the surface and at the interfaces between two metals. This pressure is a measure of the electrons pelting the surface of the platinum layer, through which energy can be transferred to molecules bound to the surface. This creates a new experimental and conceptual link between plasmonic chemistry (which studies quantum changes in charge density in solids), the dynamics of energetic electrons, heat flow, and ultrafast structural changes.