Problems in the description of the intensities of electronic spectra of diatomic molecules in the adiabatic approximation

Abstract

The local single-particle correlation function for the symmetric Wolff model is evaluated at zero temperature in the weak-coupling-longtime limit by a stationary-phase argument. The leading behaviour is given by an asymptotic expansion in the coupling strength. The correlation function furthermore has a weak essential singularity at vanishing coupling strength, corresponding to the exponential decay of transient Fermi surface effects, and is scale invariant under a simultaneous change of time scale and coupling-strength scale. In the intermediate and strong coupling regime the calculation breaks down due to manybody effects. Comments are made on some frequently used operator representations and some recent work by Mattis.