Non-Born-Oppenheimer electronic wave packet in molecular nitrogen at 14 eV probed by time-resolved photoelectron spectroscopy

We report the observation of a $4.1\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}0.2$ THz quantum beat in the photoelectron spectrum of molecular nitrogen following coherent optical excitation by 14 eV photons and probed by two photons at 800 nm. The intermediate states in the two pathway interference are the valence state ${b}^{\ensuremath{'}}\phantom{\rule{4pt}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}\phantom{\rule{0.16em}{0ex}}v=13$ and Rydberg state ${c}_{4}^{\ensuremath{'}}\phantom{\rule{4pt}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}\phantom{\rule{0.16em}{0ex}}v=4$. The amplitude of the coherent oscillations decays in about 3 ps and does not revive. Opposite phases in the photoelectron yield corresponding to different vibrational levels of the final cationic states $X^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}\phantom{\rule{0.16em}{0ex}}v=0,\phantom{\rule{4pt}{0ex}}A{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Pi}}}_{u}\phantom{\rule{0.16em}{0ex}}v=1$, and $X^{2}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Sigma}}_{g}^{+}\phantom{\rule{0.16em}{0ex}}v=\phantom{\rule{0.16em}{0ex}}2,3,4$ are reported. Simulation results show that the interference originates from the strongly mixed character of the two simultaneously excited eigenstates, causing large population oscillations in the zeroth-order Born-Oppenheimer diabatic basis. No quantum beat was observed when probing with single photon at 400 nm, suggesting a resonance with a state just below the ionization potential which acts as a filter, improving the quantum beat contrast.