Working with a unique scanning tunneling microscope (STM), a team led by Miquel Salmeron, a physicist with Berkeley Lab's Materials Sciences Division, cooled the surface of a single crystal of palladium, a good catalyst for reactions involving hydrogen and water, to a temperature of about 40 Kelvins(-233 degrees Celsius) in an ultrahigh vacuum. Water molecules were then introduced onto this surface and their motion was tracked with the STM. As expected from previous studies, single molecules migrated across the surface to aggregate into clusters of two (dimers), three (trimers), four (tetramers) five (pentamers) and six (hexamers). The surprise came when the scientists were able to watch the molecules as they moved.
"Isolated water molecules moved by hopping from one lattice point (on the substrate's crystal) to the nearest neighboring point whereupon if they collided with another water molecule they began to form clusters," says Salmeron. "The speed with which the molecules moved increased by four orders of magnitude when dimers were formed. The mobility of trimers and tetramers was also very high compared to the isolated molecules."
This ran contrary to the usual storyline in which single molecules diffuse or move across a surface more rapidly than clusters. Salmeron likens the situation to pulling either one skater across the ice or a group of skaters connected by a line.
"Since each skater rubs against the surface of the ice, to pull them all together means a lot of rubbing," he says. "The situation can be quite different, however, when the sliding takes place over a corrugated surface, like atoms sliding over the atomic landscape of a surface."