  DEAP-3600 uses liquid argon contained in a spherical acrylic vessel as a
target medium to perform a sensitive spin-independent dark matter search. Argon
scintillates in the vacuum ultraviolet spectrum, which requires wavelength
shifting to convert the VUV photons to visible so they can be transmitted
through the acrylic light guides and detected by the surrounding
photomultiplier tubes. The wavelength shifter 1,1,4,4-tetraphenyl-1,3-butadiene
was evaporatively deposited to the inner surface of the acrylic vessel under
vacuum. Two evaporations were performed on the DEAP-3600 acrylic vessel with an
estimated coating thickness of 3.00 $\pm$ 0.02 $\mu$m which is successfully
wavelength shifting with liquid argon in the detector. Details on the
wavelength shifter coating requirements, deposition source, testing, and final
performance are presented.
