The Realm of Cold and Ultracold

Abstract

Chemistry, as we know and experience it every day, mostly occurs at room temperature (T = 298 K). At lower temperatures, the molecules in a gas have lower velocities, and hence, for the same density, collisions are less frequent. Then, one may think that as the temperature drops, the reaction rate will follow the same fate. This classical vision is partially true. However, one needs to keep in mind that as the temperature of a gas drops, quantum mechanics takes over, and phenomena insignificant at room temperature will dominate the physics and chemistry in this regime. One of these quantum phenomena is the onset of resonances [1–10], which are washed out at room temperature. Other interesting quantum phenomena are the threshold behaviors of elastic and inelastic collisions, also known as the Wigner threshold laws [11]. These predict that the elastic cross section tends toward a constant value as the temperature approaches zero, whereas the inelastic cross section increases with $1/\sqrt {T}$, where T is the temperature of the gas, as we will explain in this book.