  We have deduced the structure of the \ce{bromobenzene}--\ce{I2} heterodimer
and the \ce{(bromobenzene)2} homodimer inside helium droplets using a
combination of laser-induced alignment, Coulomb explosion imaging, and
three-dimensional ion imaging. The complexes were fixed in a variety of
orientations in the laboratory frame, then in each case multiply ionized by an
intense laser pulse. A three dimensional ion imaging detector, including a
Timepix3 detector allowed us to measure the correlations between velocity
vectors of different fragments and, in conjunction with classical simulations,
work backward to the initial structure of the complex prior to explosion. For
the heterodimer, we find that the \ce{I2} molecular axis intersects the phenyl
ring of the bromobenzene approximately perpendicularly. The homodimer has a
stacked parallel structure, with the two bromine atoms pointing in opposite
directions. These results illustrate the ability of Coulomb explosion imaging
to determine the structure of large complexes, and point the way toward
real-time measurements of bimolecular reactions inside helium droplets.
