Novel chemistry induced by ultashort laser pulses

August 13, 1999

Berlin, Germany; 06/08/99 -- In the last decades, few scientific fields have advanced so dramatically and contributed so much to transforming our everyday life as the science of solid surfaces, crucial, for example for the semiconductor and catalysis industries. However, when it comes to an understanding of the ultrafast temporal evolution of surface processes, insights are just starting to emerge. This knowledge of the time-scales and pathways of energy transfer between the surface and the reactants is essential to understand how and why surface reactions occur.

This is exactly where researchers in the group of Gerhard Ertl at the Fritz Haber Institute of the Max Planck Society in Berlin have succeeded ('SCIENCE', Aug. 13). Using ultrashort laser pulses, they were able to 'switch on' an important model surface reaction (which does not occur by heating the surface: a novel reaction pathway is thus opened with the laser). They also unraveled the ultrafast speeds and mechanisms of energy flow for this reaction, the catalytic oxidation of carbon monoxide to form carbon dioxide on transition metal (ruthenium) surfaces: This prototype surface reaction is of interest from a technological point of view, as the key reaction in automotive exhaust catalysis, and also from a fundamental point of view, being a model reaction for understanding heterogeneous catalysis.

LASER PULSES AND THEIR INTERACTION WITH METALS
To be able to investigate the ultrafast time evolution of the reaction, femtosecond (fs) laser pulses are employed (these are a controlled sequence of well-defined flashes of light, each lasting only 100 fs = 1*10E-13 s; for comparison, in this time span light (travelling at 300.000 km/s) only traverses 30 um, roughly the diameter of a human hair). After the first of these pulses initiates the reaction, its ultrafast time evolution can be monitored using the subsequent pulses, which take 'snapshots' of the reaction as it evolves on the surface.

The laser pulses are absorbed by the metal, which can be represented by two heat reservoirs: One consists of the metal electrons, the other of the vibrations of the metal atoms (also called lattice vibrations). Only the first reservoir, the metal electrons, initially absorbs the laser energy, thereby becoming very hot (several thousands of degrees Kelvin above the metal's melting point). However, it takes only about 2 picoseconds (twenty times the laser pulse duration) until the electrons and the lattice vibrations have the same (much lower) temperature again. The energy transfer from the two heat reservoirs to reactants on the surface determines how and why a chemical reaction occurs. During the extremely short time of different temperatures the researchers were able to determine whether the metal electrons or the lattice vibrations initiate the reaction.

CONVENTIONAL THERMAL CHEMISTRY
In contrast to the excitation by a laser pulse, during conventional thermal heating (a classical picture for this would be a flame), the temperatures of the metal electrons and the lattice vibrations are always equal, so that there is no way of distinguishing which reservoir provides the energy to induce the reaction. When the ruthenium surface with CO and O is conventionally heated, no reaction between O and CO, i.e. no oxidation of CO molecules takes place. Instead the CO molecules are found to leave the surface at acertain temperature; the more strongly bound O atoms do not desorb (as depicted to the left in the animation).

NOVEL CHEMISTRY INDUCED BY ULTRASHORT LASER PULSES
Remarkably, by exciting the same surface with ultrashort laser pulses, the reaction between O and CO to CO2 does take place: The energy from the excited hot electrons is transferred into the oxygen-metal bond. The strong bond is weakened so much that the oxidation reaction with the neighboring CO molecule becomes possible and CO2 is formed and leaves the surface (as depicted on the right in the animation). Additionally, also desorption of CO molecules occurs, as in the case of conventional heating. Therefore the oxidation reaction has to compete with the desorption (when all CO has desorbed, none is left to be oxidized). It is by the ultrafast heating of the electrons using laser pulses and their very rapid energy transfer (on a 500 fs time-scale) into the oxygen-metal bond, that the desorption process can be outpaced (the desorption is much slower, because here the energy comes from the lattice vibrations, which have to be heated by the electrons first). Hence, novel chemistry comes into play: With the laser pulse, the system is rapidly steered into reactive regions that are normally inaccesible.

ENERGY TRANSFER MECHANISMS
From the experimental data and with help of advanced modeling, the researchers deduced exactly how the energy transfer from the hot electrons to the oxygen-metal bond takes place: An electron actually 'hops' from the metal onto the oxygen atom, for a very short time (~10e-15 s). An astonishing consequence of this mechanism is that using oxygen atoms of slightly different mass (different isotopes), increasing the mass by a factor of only 1.25, the CO2 yield was observed to drop by a factor of 2.2. A more detailed explanation on these issues and further facts will be available at http://www.fhi-berlin.mpg.de/pc/femtos by Friday, Aug. 13, 1999.

CONCLUSION AND OUTLOOK
This work clearly shows that the study of ultrafast chemical reaction dynamics can be taken into the area of surface chemistry. In particular, it is demonstrated that hot metal electrons can chemically activate reactants adsorbed on the metal surface, in contrast to the traditional picture of chemical reactions, where activation is occuring exclusively through excitation of the reactants by the lattice vibrations. The researchers in Berlin point out that in metal-catalyzed surface reactions, the energy transfer by electrons may be underestimated. They even show that hot electrons can open up a new reaction pathway. Further studies should help to obatin a deeper understanding of chemical surface reactions on the time-scales of the atomic motion, laying the foundation for more efficient (or even new) chemical processes of technological importance in the future.
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Max-Planck-Gesellschaft

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