Phase-matched high harmonic generation for the study of rotational coherence molecular dynamics

We report the observation of periodic modulations in the high order harmonic radiation from diatomic molecules in a semi-infinitive gas cell when a preceding femtosecond laser beam, which is off-axis to the high-harmonic generation beam, modifies the phase-matching condition through the field-free alignment of the molecules. The observed modulations of the high harmonic radiation versus the time delay between the pulses result from periodic changes in the nonlinear refractive index associated with rotational Raman coherence. This opens up a new potential technique for studying rotational coherence dynamics in the ground states of molecules