Coherence in nonradiative transitions: internal conversion in Rydberg-excited N-methyl and N-ethyl morpholine

Significance 

Nonradiative transitions, which are transitions between molecular energy levels without release or absorption of radiation, are important in chemical reactions as they are responsible for energy flow between non-reactive and reactive modes. They are also important in photochemically induced reactions. Therefore, non-radiative transitions can be analyzed by adopting photochemistry toolset.

A molecule, upon photon absorption, can reemit the energy as radiation or redistribute it in a number of non-reactive and reactive pathways. The non-radiative de-excitation processes tend to compete with photo-emission processes, thereby limiting the photoemission lifetimes. Examples of where non-radiative mechanisms influence photochemical mechanisms are light harvesting in nanocrystals, photosynthesis, and photo-protective processes in DNA.

Radiationless de-excitation processes include: (1) internal conversion, which is a transition from a higher electronic state to a lower electronic state without a change of the spin state, (2) intersystem crossing coupling a transition between electronic states with a spin flip, and (3) intramolecular vibrational relaxation.

The description of internal conversion in the statistical limit has the notion that the receiver states are statistically distributed and cover the entire energetically available phase space. However, recent research has challenged this description of the internal conversion processes. Researchers have explored the dependence of time constants of internal conversion mechanisms on the initially excited states, and have found that only a limited portion of the total phase space may be active in the course of internal conversion.

In a recent article published in the journal Physical Chemistry Chemical Physics, Yao Zhang and Professor Peter M. Weber at Brown University in collaboration with Professor Hannes Jónsson at the University of Iceland sought to explore the ergodicity of internal conversion by investigating the nature of both the initial as well as the receiving states of the nonradiative transition. Their study aimed also to observe molecules in their dark states. The researchers are pioneers in studying molecular reactions in real time and in the computational description of the experiments. They previously developed two highly time-resolved techniques to explore molecular dynamics: the pump-probe photoelectron spectroscopy that was used in the investigation described here, and a complementary time-resolved diffraction technique that follows the molecular dynamics from a structural point of view.

The research team presented time-resolved binding energy spectra of Rydberg-excited N-methyl morpholine and N-ethyl morpholine, which captured the coherent structural dynamics in real time before and after internal conversion from the initially excited 3p levels to the lower 3s Rydberg states. Combined with the newly developed computational tools, this experiment revealed the nature of the final state, after the internal conversion, from a structural perspective.

The authors also used optical excitation in the 194-230 nm region to excite the molecule to 3s or 3p Rydberg states, thereby launching wavepacket motions in the umbrella mode of the tertiary amine chromophore. For very short excitation wavelengths, less than 214 nm, the excitation leads to the 3p Rydberg state, which decays by internal conversion on a time scale of approximately 100 fs to the lower 3s Rydberg state. The authors discovered coherent dynamical motions with a few 650 fs periods on the 3s surface and thus observed that the coherence survived the nonradiative transition, persisting into the picosecond time regime.

In summary, the time-resolved photoionization photoelectron experiments revealed that the coherent wavepacket motion survived the internal conversion, with oscillations that persist for several periods prior to dephasing into the dense bath of vibrational modes of the molecule.

Reference

Yao Zhang, Hannes Jónsson and Peter M. Weber. Coherence in nonradiative transitions: internal conversion in Rydberg-excited N-methyl and N-ethyl morpholine. Physical Chemistry Chemical Physics 2017,19,26403.

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