oxygen on platinum
Image: S. BACKUS/Univ. of Colorado

Most talk of movies at this time of year centers on big-budget Hollywood blockbusters. Now researchers at University of Colorado at Boulder and the University of Michigan at Ann Arbor have made a schematic movie that they say is the first of its kind: one showing the course of a reaction directly monitored from start to finish. Their results appear in this week's issue of Physical Review Letters.

In the experiment, Michael Bauer and his colleagues placed oxygen molecules on an extremely cold platinum surface. Femtosecond bursts of light from an infrared laser excited the oxygen molecules and caused them to rotate. Changes to the shared arrangement of the electrons between the oxygen molecules and the platinum surfacethe chemical bondoccurred in approximately half a picosecond. By monitoring changes in the energy spectra of the oxygen molecules, the researchers were able to follow the entire process of the reaction.

They saw that initially an oxygen molecule in what is known as the superoxo state lay with its bond axis nearly parallel to the platinum surface. When energy from the laser light heated the platinum's electrons, though, an excited electron then tunneled into an orbital on the oxygen molecule, causing it to rotate. Afterwards the molecule relaxed to its original position. The researchers made a schematic movie that illustrates how the reaction proceeds (see image).

Surface reactions are important in many industry and catalytic processes. "This work," the authors write, "demonstrates a powerful new technique for probing changes of local order on surfaces on their fundamental timescales."