  Microscopic understanding of low-frequency molecular motions in liquids has
been a longstanding goal in soft-matter science. So far, such low-frequency
motions have mostly been accessed indirectly by off-resonant optical pulses. A
more direct approach would be to interrogate the dynamic structure of liquids
with terahertz (THz) radiation. Here, we provide evidence that resonant
excitation with intense THz pulses is capable of driving
reorientational-librational modes of aprotic polar liquids through coupling to
the permanent molecular dipole moments. We observe a hallmark of this enhanced
coupling: a transient optical birefringence up to an order of magnitude higher
than obtained with optical excitation. Our results open up the path to
applications such as efficient molecular alignment and systematic study of the
coupling of rotational motion to other collective motions in liquids.
