  We investigate the influence of thermally activated internal molecular
dynamics on the phase shifts of matter waves inside a molecule interferometer.
While de Broglie physics generally describes only the center-of-mass motion of
a quantum object, our experiment demonstrates that the translational quantum
phase is sensitive to dynamic conformational state changes inside the
diffracted molecules. The structural flexibility of tailor-made hot organic
particles is sufficient to admit a mixture of strongly fluctuating dipole
moments. These modify the electric susceptibility and through this the quantum
interference pattern in the presence of an external electric field. Detailed
molecular dynamics simulations combined with density functional theory allow us
to quantify the time-dependent structural reconfigurations and to predict the
ensemble-averaged square of the dipole moment which is found to be in good
agreement with the interferometric result. The experiment thus opens a new
perspective on matter wave interferometry as it demonstrates for the first time
that it is possible to collect structural information about molecules even if
they are delocalized over more than hundred times their own diameter.
