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Photochemical reactions in solution often proceed via competing reaction pathways
comprising intermediates that capture a solvent molecule. A disclosure of the underlying
reaction mechanisms is challenging due to the rapid nature of these processes and the
intricate identification of how many solvent molecules are involved. Here combining
broadband femtosecond transient absorption and quantum mechanics/molecular mechanics
simulations, we show for one of the most reactive species, diphenylcarbene, that the
decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding
dynamics determine which reaction channels are accessible in binary solvent mixtures at
room temperature. In-depth analysis of the amount of nascent intermediates corroborates
the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking
analogy to complexes found at cryogenic temperatures. Our results show that adjacent
solvent molecules take the role of key abettors rather than bystanders for the fate of the
reactive intermediate.
Coherent two-dimensional (2D) optical spectroscopy has revolutionized our ability to probe many types of couplings and ultrafast dynamics in complex quantum systems. The dynamics and function of any quantum system strongly depend on couplings to the environment. Thus, studying coherent interactions for different environments remains a topic of tremendous interest. Here we introduce coherent 2D electronic mass spectrometry that allows 2D measurements on effusive molecular beams and thus on quantum systems with minimum system-bath interaction and employ this to identify the major ionization pathway of 3d Rydberg states in NO2. Furthermore, we present 2D spectra of multiphoton ionization, disclosing distinct differences in the nonlinear response functions leading to the ionization products. We also realize the equivalent of spectrally resolved transient-absorption measurements without the necessity for acquiring weak absorption changes. Using time-of-flight detection introduces cations as an observable, enabling the 2D spectroscopic study on isolated systems of photophysical and photochemical reactions.