1934 Edoardo Amaldi and Emilio Segr´e1 observed that Rydberg atoms could be excited even when immersed in a dense gas of other atoms (\perturbers"), although at an energy shifted from that of an isolated Rydberg Trilobites, butterflies, and other atom [3]. Curiously, depending on the species of per- turber, these line shifts could be either blue or red exotic specimens of long-range detuned from the atomic line. This behavior quali- Rydberg molecules tatively contradicted the classical model, which treats the surrounding gas of polarizable atoms as a dielectric material and predicts only a red shift. Fermi resolved Matthew T Eiles1;2 this mystery by developing a quantum scattering the- 1Department of Physics and Astronomy, Purdue University, ory which introduced foundational concepts like the West Lafayette, Indiana, 47907, USA. scattering length and zero-range pseudopotential [4]. 2 Max-Planck-Institut f¨urPhysik komplexer Systeme, His model recognized that a ground state atom's po- N¨othnitzerStr. 38, 01187 Dresden, Germany larization potential extends over only a fraction of the E-mail:
[email protected] Rydberg electron's enormous de Broglie wavelength, and thus it only adds a scattering phase shift to the Abstract. This Ph.D. tutorial discusses ultra-long-range Rydberg wave function2. The resulting energy shift is Rydberg molecules, the exotic bound states of a Rydberg atom and one or more ground state atoms immersed in the Rydberg equivalent to that provided by a delta function poten- electron's wave function. This novel chemical bond is distinct tial located at the perturber and proportional to the from an ionic or covalent bond, and is accomplished by a very electron-atom scattering length.