Observation of Structural Wavepacket Motion: The Umbrella Mode in Rydberg-Excited N-Methyl Morpholine

Abstract

We have observed time-resolved, structural dynamics of a coherent vibrational wavepacket in Rydberg-excited N-methyl morpholine, a molecule with 48 internal degrees of freedom. The molecular structure was established by associating the time-dependent Rydberg electron binding energy, obtained from time-resolved photoionization-photoelectron spectroscopy, to the molecular structure using self-interaction corrected density functional calculations. Optical excitation at 226 nm launches an oscillatory wavepacket in the amine umbrella coordinate with a 650 fs period. Even though the Franck-Condon excitation is at an angle of 17°, the wavepacket settles into an oscillation between 4° and -10° within a fraction of a vibrational period and then dephases with a time constant of 750 fs.

Ultrafast X-ray scattering reveals vibrational coherence following Rydberg excitation

“…In ultrafast photoelectron spectroscopy studies, excitation to the 3p z state has been seen to launch a coherent structural oscillation that persists following internal conversion to the 3s state with a 106 fs time constant 20 . Electronic structure calculations indicate that the molecular structure in the electronic ground state is a chair-equatorial structure 21,22 (shown in Figure 1), and the observed photoelectron signals indicate that the structural motion that is primarily responsible for the observed oscillation is a coherent vibration in a mode that involves the planarization of the amine, consistent with related computational studies on the same molecule.…”

Transient Symmetry Controls Photo Dynamics near Conical Intersections

“…Intermode coupling can be reasonably expected to dissipate excess potential energy, resulting in a rapid loss of the coherence that is intrinsic to the initially prepared wavepacket. Nevertheless, preservation of vibrational coherence has recently been observed in cyclic tertiary aliphatic amines (CTAAs) , and associated with symmetry restrictions of the available vibrational modes that can dissipate the energy . We demonstrate now that even the mere possibility of a symmetric conformation can control the fate of a photoexcited system to the extent that it is coherently funneled through a CI.…”

Reply to: “The diamine cation is not a chemical example where density functional theory fails”

“…Without it we would not be able to identify the localized state. While the possibility of coherent dynamics prevailing over kinetics is real and has been observed in other systems 3,4 , the time resolved spectra displayed in Fig. 1 of ref.…”