  We use time-resolved hard x-ray diffraction to investigate the structural
dynamics of the multiferroic insulator TbMnO$_3$ in the low temperature
antiferromagnetic and ferroelectrically ordered phase. The lattice response
following photoexcitation at 1.55 eV is detected by measuring the (0 2 4) and
(1 3 -5) Bragg reflections. A 0.02% tensile strain, normal to the surface, is
seen to arise within 20 - 30 ps. The magnitude of this transient strain is over
an order of magnitude lower than that predicted from laser-induced heating,
which we attribute to a bottleneck in the energy transfer between the
electronic and lattice subsystems. The timescale for the transient expansion is
consistent with that of previously reported demagnetization dynamics. We
discuss a possible relationship between structural and demagnetization dynamics
in TbMnO$_3$, in which photoinduced atomic motion modulates the exchange
interaction, leading to a destruction of the magnetic order in the system.
