Photodissociation reactions are elementary chemical reactions showing extraordinarily rich dynamics that reflect a great variety of detail mechanisms. Very sophisticated experimental tools are needed to unravel and understand the ensuing processes in details.
Photolysis of carbonyl compounds
The Walsh type I C-C bond fission reaction of ketones is a classic example of a photochemical reaction. It is of great imprtance in the Earth's atmosphere and as a source of methyl radicals in organic chemistry. At shorter photolysis wavelengths, the acetyl co-product may carry enough internal energy to undergo secondary unimolecular dissociation. The wavelength-dependent branching ratio between formation of CH3 + CH3CO and CH3 + CH3 + CO has long been debated. Since the two methyl fragments are formed with different kinetic energies, the method of velocity-mapped photofragment imaging used in our laboratory provides valuable insight into the dynamics of acetone and many acetone derivatives.
With the advent of first femtosecond time-resolved data on acetone, severe questions arose about the previously accepted photodissociation mechanism via the triplet state of acetone. Using femtosecond photoelectron imaging, we have recently been able to demonstrate the involvement of Rydberg state.
Dynamics of pyrrole via the πσ* state
The photodissociation of pyrrole via the πσ* state has become a prototypical case. Following theoretical predictions by Domcke and Sobolewski, our seminal papers on pyrrole were the first that established this novel - and by now widely recognized - photodissociation pathway.
J. Wei, J. Riedel, A. Kuczmann, F. Renth, F. Temps, "Photodissociation of Pyrrole: Evidence for Mode Specific Dynamics from Conical Intersections," Faraday Discussion 127 - Non-Adiabatic Effects in Chemical Dynamics 127, 267 - 282 (2004).
- J. Wei, A. Kucmann, J. Riedel, F. Renth, F. Temps, "Photofragment Velocity Map Imaging of H Atom Elimination in the First Excited State of Pyrrole", Phys. Chem. Chem. Phys. 5, 315 - 320 (2003).
Photodissociation of alkylnitrites
The photodissociation mechanism of methyl- and other alkylnitrites via the S1 state has been studied in great detail in the 1990s by theory, especially by the Schinke group. In principle, the reaction may proceed following a vibrationally adiabatic route (ΔvNO = 0) by tunneling across a small potential energy barrier or following a vibrationally nonadiabatic route(ΔvNO = -1). A clear interpretation of older experimental results obtained by Doppler spectroscopy of the NO fragment has, however, been hampered by uncertainty of the correct assignment of the vibrational band progression in the absorption spectrum of methylnitrite. Using velocity-mapped photofragment imaging, we are able to solve this problem without ambiguity in a very straightforward way.