Organic Molecules on the Rebound

Scientists at the International University of Bremen and the University of Bonn have recently determined the precise structure of a large organic molecule after its interaction with a metal surface. The group of scientists also used the structure information to decipher clues about the chemical bond between the molecule and the surface. The organic-metallic interface is very important in science, especially in the fields of catalysis (chemical reactions between two species proceeding in the presence of a third species), bio-sensing, and molecular electronics (where signals are processed through circuit elements consisting, in some cases, of single molecules or arrays of molecules).

In this regard, larger molecules are harder to study because of their size, their tortuous shape, and many internal modes of vibration. In the Bremen-Bonn experiment the starting point is a super-clean silver surface in ultrahigh vacuum. Next the molecule is allowed to fall onto the surface where it reacts chemically with surface atoms and is slightly distorted thereby. Next, x rays from a synchrotron are brought to bear on the adsorbed molecule. By the scattering of the x rays the researchers can deduce, in some cases atom for atom, where the component parts of the molecule are relative to the nearby metal surface.

The worked-out structure of the reacted molecule can then be compared to the structure for the same type of molecule in the free (gaseous) state. In this way the distortion of the molecule, whose full name is perylene-tetracarboxylic-dianhydride (PTCDA), can be worked out. It is notable that the x-ray scattering technique used here was not the normal Bragg scattering in use for decades. Because the sample was so thin, the approach employed here was based on standing x-ray waves. The x rays reflected from the silver crystal formed standing waves when they interfered with incoming x rays.

The ensuing atomic-scale "ruler" can be used to map the organic molecule by slightly grading the energy of the incoming x rays. This normal incidence x-ray standing wave technique has been used before but very rarely on large organic adsorbates where it has great potential. What happened as the normally planar molecule approached the surface? Surprisingly, there was some bending, mostly because of the readiness of some oxygen atoms (which weren't supposed to play much of a chemical role) to form bonds with the surface silver atoms.

Another discovery: the molecule forms not a single bond but a hierarchy of two types of bonds. (Hauschild et al., Physical Review Letters, 28 January 2005; contact Stefan Tautz, 49-421-200-3223, s.tautz@iu-bremen.de; lab website http://imperia.iu-bremen.de/ses/physics/tautz/30797/ )