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 26eV is mainly sensitive to two among the three active Raman modes in SF₆, i.e. the strongest and fully symmetric υ_1-A_1g mode (774cm^-1, 43 fs) and the slowest mode υ_5-T_2g (524cm^-1, 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of the υ₁ mode oscillate with a frequency of 52cm^-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 (774cm ^-1) and slow (52cm^-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.