  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
to 26 eV is mainly sensitive to two among the three active Raman modes in SF6,
i.e. the strongest and fully symmetric nu 1-A1g mode (774 cm-1, 43 fs) and the
slowest mode nu5-T2g (524 cm-1, 63 fs). A time-frequency analysis of the
harmonic emission reveals additional dynamics: the strength and central
frequency of the nu 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 behavior, oscillating in 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 molecularvibrations, associated to the high sensitivity
of transient grating spectroscopy, make their combination a unique tool to
probe vibrational dynamics.

Chemical Physics;Atomic Physics