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.

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.

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).

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.

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 by Friday, Aug. 13, 1999.

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.


Related Electrons Articles from Brightsurf:

One-way street for electrons
An international team of physicists, led by researchers of the Universities of Oldenburg and Bremen, Germany, has recorded an ultrafast film of the directed energy transport between neighbouring molecules in a nanomaterial.

Mystery solved: a 'New Kind of Electrons'
Why do certain materials emit electrons with a very specific energy?

Sticky electrons: When repulsion turns into attraction
Scientists in Vienna explain what happens at a strange 'border line' in materials science: Under certain conditions, materials change from well-known behaviour to different, partly unexplained phenomena.

Self-imaging of a molecule by its own electrons
Researchers at the Max Born Institute (MBI) have shown that high-resolution movies of molecular dynamics can be recorded using electrons ejected from the molecule by an intense laser field.

Electrons in the fast lane
Microscopic structures could further improve perovskite solar cells

Laser takes pictures of electrons in crystals
Microscopes of visible light allow to see tiny objects as living cells and their interior.

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.

Flatter graphene, faster electrons
Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel developed a technique to flatten corrugations in graphene layers.

Researchers develop one-way street for electrons
The work has shown that these electron ratchets create geometric diodes that operate at room temperature and may unlock unprecedented abilities in the illusive terahertz regime.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

Read More: Electrons News and Electrons Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to