Femtosecond dynamics and coherence of retro-Diels-Alder reactions in radical cations*

Ultrafast tunnel ionization enables femtosecond time-resolved dynamics measurements of the retro-Diels-Alder reactions of positively charged cyclohexene, norbornene, and dicyclopentadiene as they likely occur under electron-ionization mass spectrometry. Unlike the sub-200 fs reaction times that have been observed following UV excitation of the neutral species, on the ionic potential energy surfaces, these reactions occur on the picosecond timescale and in some cases exhibit vibrational coherence that is likely associated with a concerted reaction mechanism.