Fifty Years of Atomic and Molecular Collision Theory

Atomic, Molecular and Optical (AMO) Physics is about photons, electrons and positrons, positive and negative ions, atoms and molecules and their mutual interactions. There are two major strands of inquiry in AMO Physics: studies of spectra and studies of collisions. In spectroscopic measurements, ions, atoms and molecules emit or absorb electromagnetic radiation. Electromagnetic radiation consists of light waves and waves have wavelengths that can be measured by spectroscopic instruments. Our eyes are spectroscopic instruments with which we can distinguish colours increasing in wavelength from the violet to the red. Butterflies see in the ultraviolet which lies at the short wavelength side of the violet. At much shorter wavelengths are X-rays and At wavelengths longer than red are the infrared, millimetre and radio waves. The major impetus for the revolution in scientific thought that culminated in the formulation of quantum mechanics was driven by measurements of the spectrum of radiation emitted by hydrogen atoms. To explain the observation that narrow features appeared at specific wavelengths, Niels Bohr in 1913 gave up classical mechanics and in its place he invoked the idea that the electron in a hydrogen atom occupies only certain discrete orbits in its motion about the central nucleus, in contrast to classical mechanics according to which all orbits are allowed. Then radiative transitions between these stationary states of the electron produce spectral lines at specific wavelengths as observed. Earlier, Planck’s theory of the thermal radiation emitted by a hot object had invoked the idea of discrete quantized energy modes but no one, least of all Planck, envisaged the theory as more than a temporary empirical fix that was needed to reproduce the measurements that had been made at long wavelengths. However, Planck’s quantum theory of radiation is correct and it helped to establish a fundamental concept of quantum mechanics, the duality of particles and waves; in the case of electromagnetic waves, the particles are photons. Spectroscopy is the major diagnostic probe in science. The wavelength of a spectral line is a unique property of the emitting or absorbing ion, atom or molecule and its presence in the spectrum identifies definitively the species. By measuring the spectra of an object, near or remote, the elements of which the object is made can be determined. The Astrophysical Journal was launched in 1895 with the subtitle “An International Review of Spectroscopy and Astronomical Physics”. The spectral line positions depend on the velocity of the emitter or absorber. Measurements of spectral lines emitted by stars in external galaxies reveal shifts in the wavelengths. The lines are red-shifted from which we conclude that all galaxies are moving away from us and the space of the Universe is expanding. The other major strand of research in AMO Physics and indeed other disciplines of Physics is the study of the collisions of beams of particles. One of the most famous experiments in physics was the measurement by Rutherford of the angular distribution of a beam of scattered by a thin foil. It demonstrated that at the centre of an atom lies a