  A microscopic understanding of molecules is essential for many fields of
natural sciences but their tiny size hinders direct optical access to their
constituents. Rydberg macrodimers - bound states of two highly-excited Rydberg
atoms - feature bond lengths easily exceeding optical wavelengths. Here we
report on the direct microscopic observation and detailed characterization of
such macrodimers in a gas of ultracold atoms in an optical lattice. The size of
about 0.7 micrometers, comparable to the size of small bacteria, matches the
diagonal distance of the lattice. By exciting pairs in the initial
two-dimensional atom array, we resolve more than 50 vibrational resonances.
Using our spatially resolved detection, we observe the macrodimers by
correlated atom loss and demonstrate control of the molecular alignment by the
choice of the vibrational state. Our results allow for precision testing of
Rydberg interaction potentials and establish quantum gas microscopy as a
powerful new tool for quantum chemistry.
