Coherence in nonradiative transitions: internal conversion in Rydberg-excited N-methyl and N-ethyl morpholine.
The survival of coherent wavepacket motion during internal conversions is observed in relatively large molecules, N-methyl morpholine and N-ethyl morpholine, where standard models imply fast decoherence in a statistical limit. Optical excitation in the region of 194 to 230 nm excites the molecule to 3s or 3p Rydberg states, launching a wavepacket motion in the umbrella mode of the tertiary amine chromophore. In the short wavelength range, <214 nm, the molecules are excited to the 3p Rydberg state, which decays by internal conversion on a time scale of about 100 fs to the lower-lying 3s Rydberg state. Time-resolved photoionization photoelectron experiments reveal that the coherent wavepacket motion survives the internal conversion with oscillations continuing in the 3s state for several 650 fs periods before the phase lock is lost due to dephasing into the dense bath of vibrational modes of the molecule.