  We produce YbF molecules with a density of 10^18 m^-3 using laser ablation
inside a cryogenically-cooled cell filled with a helium buffer gas. Using
absorption imaging and absorption spectroscopy we study the formation,
diffusion, thermalization and optical pumping of the molecules. The absorption
images show an initial rapid expansion of molecules away from the ablation
target followed by a much slower diffusion to the cell walls. We study how the
time constant for diffusion depends on the helium density and temperature, and
obtain values for the YbF-He diffusion cross-section at two different
temperatures. We measure the translational and rotational temperatures of the
molecules as a function of time since formation, obtain the characteristic time
constant for the molecules to thermalize with the cell walls, and elucidate the
process responsible for limiting this thermalization rate. Finally, we make a
detailed study of how the absorption of the probe laser saturates as its
intensity increases, showing that the saturation intensity is proportional to
the helium density. We use this to estimate collision rates and the density of
molecules in the cell.
