Photoelectron spectrum of ammonia, a test case for the calculation of Franck-Condon factors in molecules undergoing large geometrical displacements upon photoionization.

The vibrational structure of the photoelectron spectrum of ammonia, the simplest molecule undergoing a large displacement of its equilibrium geometry upon photoionization, is analyzed by evaluating the Franck-Condon integrals at the anharmonic level of approximation. It is shown that if the rectilinear Cartesian representation of normal modes is adopted Duschinsky's transformation yields a too large displacement of the bond distance coordinate, with the appearance of several progressions which are not observed in the experimental spectrum. This apparent failure is completely corrected by the inclusion of anharmonic couplings between the principal active mode, the out of plane bending of the planar cation, and the totally symmetric stretching mode, leading to a satisfying reproduction of the observed spectrum and to a more convincing assignment of the weaker progression observed in the high-resolution spectrum.