Status of atomic and molecular physics (theoretical chemistry) in 1969 (in brief)*

Physics

In the late sixties physics was dominated by elementary particles, nuclei, solid state and atoms (in that order of interest). By atomic physics it was essentially meant study of electronic structure and interaction of electrons with atoms. Study of molecules, if it was not in the same context as the atomic physics, was considered as of part of chemistry, more specifically physical chemistry (there is no unique consensus on the proper name for the study of individual molecules in relation to chemistry, the latter being essentially the study of reactions among molecules in the milieu of their huge numbers). In few instances, however, molecular physics was/is incorporated in solid state physics, for example in the treatment of the (molecular) origins of magnetism, or in understanding of the specific heats of solids. That was perhaps the reason why at a much later date my application for the professorship in the physics department at the University of Zagreb was turned down with note that the subject of atomic and molecular physics is already part of curricula of solid state physics. An indication of the status of molecular physics is the classic book on the atomic physics by Mott and Massey where molecules are treated with less interest than nuclei, and when they were analyzed it is done with relative superficiality. In conclusion one can safely say that atomic and molecular physics was not existent as part of physics curricula, it was considered to be chemistry. As an illustration of this when I started the work on my PhD at the Sussex University I was given to study the three atom molecule systems (a three body problem), the research that in the nuclear physics produced recognizable results. Without having much experience in that I approached a very distinguished nuclear physicist, but after presenting him with the problem he refused to discuss it on the grounds that it is classical mechanics (read - chemistry).

Besides in solid state physics molecules are essential bases of statistical physics, and in the treatment of diffusion. However, in these fields they are treated as a step towards achieving other goals and not understanding properties and interactions of molecules per se.

Chemistry

Chemistry is a vast subject, which is dominated by two central questions: given the species A and B (molecules, in general) what is the result of their interaction and how fast it occurs? This (chemical) reaction occurs under specific circumstances, in a mixture of huge number of these species. The role of physical, in particular theoretical, chemistry is to answer these questions, and in the late sixties there were two major thrusts to do that. One was based on the statistical theory of chemical reactivity, which in essence says that between the reactants and the products there is a barrier of certain energy. This energy determines the speed of chemical reaction, and the purpose of the theory is to calculate it. This can be done if the potential between the species is known, and quantum mechanics enables this. Calculation of the potential is a very demanding task that requires very powerful computing devices, which in the late sixties where far from being adequate. Much of effort was devoted to various ways of calculating molecular integrals and optimizing computer codes to extract as much as possible information on small molecules. This part of research is later to be known as the quantum chemistry.

The other approach to the chemical reactivity problem was to analyze encounters of two isolated species and then applies statistical averaging to obtain the answer of the chemical interest. In the late sixties there were few experimental setups that used the crossed molecular beam technique to study these individual processes, but on the simple systems that could be hardly labeled as of chemical interest. Already at that time two aims of these experiments crystallized: one is to obtain the cross sections for individual reactions under controlled conditions, and the other was to obtain potential between the species from the experimentally available information. The latter in particular is very important because it charts the way how to obtain the same information in, say, nuclear or elementary particle physics. The theory that would accompany this research was in the cradles, no powerful computing devices were available (based on today standards) and no theoretical methods were developed. As to the latter, the only strides made at that time were copies of nuclear collision theory. The only novelty was attempt at merging classical and quantum mechanics into what is referred to as the semi classical mechanics.

Another major field of study was electron, photoionisation and rotational and vibrational molecular spectroscopy, which produced important information about the molecules and were quite developed at that time. However, slowly a new and powerful technique was on the horizon - laser spectroscopy. The subject of spectroscopy was of interest to chemists for identifying molecules and also to astrochemists, an emerging field that will soon become exceptionally important for understanding the working of the Universe. In all these the main object of study were stable molecules, and therefore the subject is of not direct relevance to chemical reactivity (it was much, much later that time resolved spectroscopy would produce this connection). But there was one problem that brought together reactions and spectroscopy - predissociation. Broadening of spectroscopy lines is influenced by several factors, amongst them due to the finite lifetime of the molecule before it splits apart.

* Thanks are to Prof. Sydney Leach for useful additions and comments.