Coherent Motion Reveals Non‐Ergodic Nature of Internal Conversion between Excited States

Abstract

We found that specific nuclear motion along low-frequency modes is effective in coupling electronic states and that this motion prevail in some small molecules. Thus, in direct contradiction to what is expected based on the standard models, the internal conversion process can proceed faster for smaller molecules. Specifically, we focus on the S(2) →S(1) internal conversion in cyclobutanone, cyclopentanone, and cyclohexanone. By means of time-resolved mass spectrometry and photoelectron spectroscopy the relative rate of this transition is determined to be 13:2:1. Remarkably, we observe coherent nuclear motion on the S(2) surface in a ring-puckering mode and motion along this mode in combination with symmetry considerations allow for a consistent explanation of the observed relative time-scales not afforded by only considering the density of vibrational states or other aspects of the standard models.

Quantum-dynamical Modeling of the Rydberg to Valence Excited-State Internal Conversion in Cyclobutanone and Cyclopentanone

“…The decay of the (n,3s) population can for both molecules be adequately fitted by a decaying biexponential function with time-constants of 0.95 ps and 6.32 ps for cyclobutanone and 3.99 ps and 81.5 ps for cyclopentanone. The short time-constants are in good correspondence with the ones obtained from time-resolved mass spectrometry of 0.74 ps and 5.30 ps for cyclobutanone and cyclopentanone respectively [2]. A significant difference between the two molecules can be found in the symmetries of their excited states.…”

“…Contrary to the expectation of a rise in the rate of transition as ρ m (E) is increased, the exact opposite trend was observed for the cycloketones [2]. This observation points to a significant coordinate dependence of the coupling matrix element in Eq.…”

“…Processes in excited states initiated by short laser pulses can exhibit significant non-ergodic behavior as exemplified by the cycloketones [1,2]. The direct time-scale for the (n,3s) to (n,π*) transition in these molecules exhibit such effects [2].…”

“…The direct time-scale for the (n,3s) to (n,π*) transition in these molecules exhibit such effects [2]. A perturbative approach, i.e., Fermi's golden rule, should be able to describe the rate of such a transition …”

Symmetry, vibrational energy redistribution and vibronic coupling: The internal conversion processes of cycloketones

“…1,[11][12][13][14][15][16][17] Initial processes involve internal conversion from a Rydberg state to the (n, π *) state, the lowest lying excited singlet state. This is sometimes referred to as predissociation dynamics due to the eventual fate of the molecules.…”