We study theoretically and experimentally the electronic relaxation of NO_2 molecules excited by absorption of one ～400 nm pump photon. Semiclassical 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 occurring on the same time scale. 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 NO_2, high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation.
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