  We study theoretically and experimentally the electronic relaxation of NO2
molecules excited by absorption of one ~400 nm pump photon. Semi-classical
simulations based on trajectory surface hopping calculations are performed.
They predict fast oscillations of the electronic character around the
intersection of the ground and first excited diabatic states. An experiment
based on high-order harmonic transient grating spectroscopy reveals dynamics
occuring on the same timescale. A systematic study of the detected transient is
conducted to investigate the possible influence of the pump intensity, pump
wavelength, and rotational temperature of the molecules. The quantitative
agreement between measured and predicted dynamics shows that, in NO2, high
harmonic transient grating spectroscopy encodes vibrational dynamics underlying
the electronic relaxation.
