High-order harmonic transient grating spectroscopy of SF6 molecular vibrations

Strong field transient grating spectroscopy has shown to be a very versatile tool in time-resolved molecular spectroscopy. Here we use this technique to investigate the high-order harmonic generation from SF6 molecules vibrationally excited by impulsive stimulated Raman scattering. Transient grating spectroscopy enables us to reveal clear modulations of the harmonic emission. This heterodyne detection shows that the harmonic emission generated between 14 and 26 eV is mainly sensitive to two among the three active Raman modes in SF6, i.e. the strongest and fully symmetric upsilon 1-A(1g) mode (774 cm(-1), 43 fs) and the slowest mode upsilon 5-T-2g (524 cm(-1), 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of upsilon 1 mode oscillate with a frequency of 52 cm(-1) (640 fs). This could be a signature of the vibration of dimers in the generating medium. Harmonic 11 shows a remarkable behaviour, oscillating in the opposite phase, both on the fast (774 cm(-1)) and slow (52 cm(-1)) timescales, which indicates a strong modulation of the recombination matrix element as a function of the nuclear geometry. These results demonstrate that the high sensitivity of high-order harmonic generation to molecular vibrations, associated to the high sensitivity of transient grating spectroscopy, make their combination a unique tool to probe vibrational dynamics.