  Extreme ultraviolet frequency combs are a versatile tool with applications
including precision measurement, strong-field physics, and solid-state physics.
Here we report on an application of extreme ultraviolet frequency combs and
their driving lasers for studying strong-field effects in molecular systems. We
perform field-free molecular alignment and high-order harmonic generation with
aligned molecules in a gas jet at a repetition rate of 154 MHz using a
high-powered optical frequency comb inside a femtosecond enhancement cavity.
The cavity-enhanced system provides a means to reach suitable intensities to
study field-free molecular alignment and enhance the observable effects of the
molecule-field interaction. We observe modulations of the driving field,
arising from the nature of impulsive stimulated Raman scattering responsible
for coherent molecular rotations. We foresee the impact of this work on the
study of molecule-based strong-field physics, with improved precision and a
more fundamental understanding of the interaction effects on both the field and
molecules.
