The team used a "tunneling noise spectroscopy" technique to determine how long the atom stays in one place. This measurement method was developed by two of the authors based on their 2004 discovery that an atom emits a characteristic scratching sound when an STM is used to move the atom between two types of bonding sites on a crystal** (see www.nist.gov/public_affairs/releases/hiphopatoms.htm).
"The two most important new findings," Stroscio says, "are an increased understanding of the science behind atomic switching and the development of a new measurement capability to spatially map the probability of an electron exciting the desired atom motion."
The scientists analyzed what happened to the atom switching rate as changes occurred in the STM voltage and in the current between the STM tip and surface. Above a threshold voltage of about 15-20 millivolts, the probability for switching per electron is constant, meaning that the electrons contain sufficient energy to move the cobalt atom. Higher currents result in faster switching.
The data suggested that a single electron boosts the molecule above a critical energy level, allowing a key bond to break so the cobalt atom can switch positions. The cobalt atom was less likely to switch as the molecular chain was extended in length from two to five copper atoms, demonstrating that the atom switching dynamics can be tuned through changes in the molecular architecture.