LAUSR

Abstract Effects of electronic orbital angular momentum in triatomic molecules are reviewed, known collectively under the name Renner-Teller effect. The focus is on the years between 1960 and 1980, when numerous examples of the Renner-Teller effect became known experimentally. In the same years, Renner’s original theory was refined and extended in various groups in order to account for the new observations. The method developed in A. J. Merer’s research group in Vancouver is discussed in some detail. This effort, based on the theory of Hougen, Bunker, and Johns for large amplitude nuclear bending motion (1970), led to the first quantitative interpretation of the quasilinear bending level structure of the NH 2 radical - itself the first known example of the Renner-Teller effect observed by Dressler and Ramsay in 1958. Alternative approaches, refinements, and extensions of the theory implemented after 1980 are also described, the most important extension being the inclusion of the stretching vibrational degrees of freedom, which allows the description of anharmonic Fermi interactions. Finally, the manifestations of the Renner-Teller effect expected in Rydberg states are discussed, and a sketch of the corresponding theory is presented. It is shown that the Renner-Teller effect can also play an important role in continuum processes, such as the recombination of electrons with linear ions.