Time-resolved high-harmonic spectroscopy of nonadiabatic dynamics in NO2

Time-resolved high-harmonic spectroscopy is an emerging approach to measuring coupled electronic and nuclear dynamics in photochemical reactions. A general conceptual and theoretical model for the technique is derived from first principles and applied to study the sensitivity of the technique to nonadiabatic dynamics. By comparing the model with detailed experimental data on photoexcited NO2 molecules, we find that time-resolved high-harmonic spectroscopy is primarily sensitive to electronic population dynamics. The coordinate dependence of the vertical ionization potential and photorecombination matrix elements contribute also, but much less significantly, to the observed dynamics because of the rapid spreading of the wave packet in the excited state. We discuss the extension of the method to larger polyatomic molecules in light of this insight.