ON THE USE OF TERM VALUES IN MOLECULAR SPECTROSCOPY

Author Institution: The Royal Institute of Technology, Stockholm 70Term values, calculated by least squares methods, have been used frequently in atomic spectroscopy during recent years. A system of linear equations is set up for the unknown term values. To make the system have a unique solution the value of at least one of the unknowns is preset, i.e. the term value scale is referred to some zero point, which may be chosen arbitrarily. The system is generally strongly over-determined. Different weights may be attributed to the different equations, which are solved numerically by least-squares methods. In atomic spectroscopy it has sometimes been possible to carry through the required calculations using a desk calculator. In molecular spectroscopy this would in most cases be impossible since both the number of equations to be solved and the number of unknowns are quite large, often to be counted in thousands. The calculation problem becomes still more intricate if one also wants to estimate the precision of the determined quantities. Thus it is through the advent of the electronic computer that a method similar to the one used in atomic spectroscopy has become conceivable in molecular spectroscopy. The introduction of the method proves to have significance to several well-known analysis problems. In particular the term values thus derived may be used to determine molecular constants. The orthodox methods of determining such constants are generally based on combination differences. These methods, carried out by hand or by computer, frequently lead to a loss of accuracy, contrary to methods based on term values. A report will be given on the application of term value methods to a number of studies of diatomic molecules. The importance of attributing appropriate weights to the measured lines will be stressed