This paper presents a discussion of Rydberg atom techniques applied to the study of electron‐molecule collisions. These techniques present new opportunities for study of ultra‐low‐energy electron collisions and for investigation of chemical dynamics.
In the present work very-high-n Rydberg-atoms (nsim100−nsim400) are used to probe electron-molecule collisions at ultra-low electron energies. Based on the essentially-free-electron model, for sufficiently high n, Rydberg atom-molecule collisions can be described in terms of a binary interaction between the target molecule and the essentially-free Rydberg electron. Since the average kinetic energy of the Rydberg electron is ultra-low rm(sim85mueV−sim1.4meV), analysis of the very-high-n Rydberg-atom collision data can provide information on electron-molecule scattering at electron energies corresponding to electron temperatures of only $\\sim1\\sp\\circ$K, which are far below those accessible using any alternate approach.\nRate constants for destruction of very-high-n Rydberg atoms in collisions with various target molecules have been measured. This study focuses on Rydberg electron transfer to an electron-attaching molecule which results in negative ion formation via the reactions rmK(itnp/)+XYtoKsp++(XY)sp−∗ortoKsp++Xsp−+Yeqno(1)and on rotational energy transfer from polar molecules which leads to Rydberg atom ionization rmK(itnp/)+XY(itJ/)toKsp++itesp−+XY(itJ/−1)eqno(2)\nThe n dependence of the rate constants for Rydberg-atom destruction depends on the reactions involved. In reaction (1) the rate constant is independent of n whereas in reaction (2) it increases nearly linearly with n. When both reactions are possible, the measured n dependence can be explained in terms of contributions from each process.\nThe essentially-free-electron model suggests that the n dependence of the rate constants for Rydberg-atom destruction reflects the energy dependence of the cross sections of the corresponding free electron-molecule collision processes which are $$\\eqalignno{&e\\sp- + \\rm XY \\to (XY)\\sp{-*}\\ or\\ \\to X\\sp- + Y&(1\\sp\\prime)\\cr &e\\sp- + \\rm XY({\\it J\\/}) \\to {\\it e}\\sp- + XY({\\it J\\/}-1)&(2\\sp\\prime)\\cr}$$Analysis of the data therefore provides the behavior of cross sections for these processes at ultra-low electron energies.
Recent investigations of Rydberg atoms are described in an effort to give a picture of their properties, many of which can be understood from the behaviour of the Coulomb radial functions. The authors begin by a discussion of Coulomb wavefunctions, and progress to a description of the energy level structure. The behaviour of Rydberg atoms in electric fields is discussed to provide an understanding of the phenomenon and to show how electric field ionisation may be used as a tool in a wide variety of experiments. Unique opportunities to study the interaction with black-body radiation and vacuum fluctuations are discussed. A survey of the broad range of Rydberg atom collisions, with ground-state atoms and molecules, charged particles and other Rydberg atoms, is presented to show the different physical interactions responsible. This is followed by a brief summary of the study of autoionising Rydberg states, a stepping stone toward the elusive 'double Rydberg' atom. Finally typical experimental procedures are described.