Slower Collisions Enhance Excitation

In a study that bucks conventional wisdom, researchers have found that in collisions of molecules with metal surfaces, slow-moving molecules are more likely than speedy ones to undergo electronic excitation (Science 2008, 321, 1191). Until now, experimental and theoretical studies have indicated that the reverse trend is universal. The study suggests that theories of surface processes may need to be modified to account for the role of energetic electrons in surface reactions. In addition, the investigation may lead to new types of chemical sensors. To look into the relationship between electron emission and molecular velocity, Alec M. Wodtke of the University of California, Santa Barbara, and coworkers used molecular beam methods to control the velocity of highly vibrationally excited NO molecules and measured electron emission as the molecules collided with a gold surface. To explain their observations, the researchers propose that an incoming molecule can capture a surface electron while the molecule is stretched. As it contracts, that anionic configuration becomes unstable, leading to electron excitation and emission. According to that model, slow-moving molecules spend more time than fast ones in the "sweet spot" above the surface on which these electron-transfer processes occur.