  Analysis of the Cassini Ultraviolet Imaging Spectrometer (UVIS) stellar and
solar occultations at Titan to date include 12 species: N$_{2}$ (nitrogen),
CH$_{4}$ (methane), C$_{2}$H$_{2}$ (acetylene), C$_{2}$H$_{4}$ (ethylene),
C$_{2}$H$_{6}$ (ethane), C$_{4}$H$_{2}$ (diacetylene), C$_{6}$H$_{6}$
(benzene), C$_{6}$N$_{2}$ (dicyanodiacetylene), C$_{2}$N$_{2}$ (cyanogen), HCN
(hydrogen cyanide), HC$_{3}$N (cyanoacetylene), and aerosols distinguished by a
structureless continuum extinction (absorption plus scattering) of photons in
the EUV. The introduction of aerosol particles, retaining the same refractive
index properties as tholin with radius $\sim$125 \AA and using Mie theory,
provides a satisfactory fit to the spectra. The derived vertical profile of
aerosol density shows distinct structure, implying a reactive generation
process reaching altitudes more than 1000 km above the surface. A photochemical
model presented here provides a reference basis for examining the chemical and
physical processes leading to the distinctive atmospheric opacity at Titan. We
find that dicyanodiacetylene is condensable at $\sim$650 km, where the
atmospheric temperature minimum is located. This species is the simplest
molecule identified to be condensable. Observations are needed to confirm the
existence and production rates of dicyanodiacetylene.
