Strong nonadiabatic effects and conical intersections in molecular spectroscopy and unimolecular decay: C2H4+

Publisher Website
Google Scholar
Abstract
The importance of nonadiabatic effects in small polyatomic molecules is discussed. It is pointed out that the interaction between different molecular electronic states can in general not be described in terms of a single vibrational mode. Rather, totally symmetric modes which modulate the electronic energy separation must also be taken into account. The inclusion of these modes leads to a multidimensional intersection of the adiabatic potential energy surfaces and to a dramatic enhancement of the nonadiabatic effects. In the presence of several totally symmetric modes, those modes that have a minor influence on the vibronic coupling problem by themselves can still strongly enhance the nonadiabatic effects. The importance of the multimode effects is demonstrated for the second band in the photoelectron (PE) spectrum of ethylene. This band is well separated energetically (2 eV and more) from all other bands in the spectrum. It is found that in this band none of the ∼1000 calculated lines can be understood within the adiabatic approximation. The line structure is highly erratic and cannot be explained by any decoupling of the modes nor by a ’’broadening’’ of the adiabatic vibrational levels. It is concluded that strong nonadiabatic effects constitute a more common phenomenon than is usually believed.

This publication has 51 references indexed in Scilit: