The first lesson was an introduction to the physics of Rydberg atoms, atomic species in which an electron is carried in a highly excited state, giving them "exaggerated" properties very different from those of ordinary atoms. These states are manifested by the existence of a series of absorption and emission lines whose wavelengths (and frequencies) are defined by the formula that the Swedish physicist Rydberg empirically established at the end of the 19th century - hence the name given to these states. They are characterized, among other parameters, by a principal quantum number n, which appears in Rydberg's formula and identifies the atom's level of energetic excitation. The size of these atoms, measuring the dimensions of the excited electron's orbit or the extension of its wave function, increases as the square of the principal quantum number and becomes of the order of a thousand to ten thousand times that of an atom in its ground state for n of the order of 50 to 100, which goes a long way towards explaining the exaggerated properties of these atoms.
The lesson began with a brief historical account recalling that these states played an important role in the reflections that led Niels Bohr in 1913 to establish his famous model of the atom, the precursor of the description given by modern quantum physics. Rydberg atoms then made their appearance in physics in the 1930s with the study of the shift in atomic lines of the Rydberg series when atoms are excited in the presence of rare gas atoms (argon, xenon or helium). Fermi and his collaborators studied this problem in the 1930s, both experimentally and theoretically, and interpreted the displacement of the lines by the effect of collisions of the excited electron of Rydberg atoms, described by its quasi-free particle wave function, on noble gas atoms entering the Rydberg orbit. In this study, Fermi introduced for the first time the notion ofscattering length, which was later to play an important role in theory, in nuclear physics and then in cold-gas atomic physics. Rydberg atoms first appeared in astrophysics in the 1960s, when they were detected in the millimeter-wave spectrum emitted by interstellar gas from the recombination of ions and electrons. The transient excitation of Rydberg atoms with principal quantum numbers in the hundreds was observed. But it wasn't until the 1970s that the experimental study of these atoms could really begin in the laboratory, with the advent of frequency-tunable lasers, essential for preparing them efficiently and selectively in an atomic jet.
