Conical Intersections and Slopes Nearby, Inferred From Coherent Oscillations and From Photoproducts

Conical intersections of potential surfaces control photochemical reactions. There has been only limited success to determine their structures in ultrafast spectroscopy, including time-resolved diffraction, despite recent progresses. It is here pointed out that directions of slopes on potentials can sometimes be easily recognized from coherent oscillations. They not only help to find conical intersections but sometimes play a central role, e.g., for controlling a wavelength dependence of a photoreaction. Slopes on potentials are forces. The idea that slopes in the Franck-Condon region often precede reactive forces (e.g., those of pericyclic reactions) rationalizes a number of empirical rules with wide application, especially if combined with the idea that a slight distortion makes some coordinates Franck-Condon active. Examples considered are cis-trans isomerization (with competing reactions) of polyenes and of alkenes including ethylene, ring opening of cyclohexadiene and cyclobutenes, intramolecular charge transfer (dimethylamino-benzonitrile), and photodissociation of metal carbonyls. Based on paths and their intersections, a solution is also suggested for the conflict between ultrafast singlet and triplet channels in metal carbonyls. In times below a few picoseconds, all processes are coherent, there is no thermalization and no vertical relaxation, and the wave packet stays compact all along the path.