  The evolution of the electronic absorption edge of type I, II and III kerogen
is studied by diffuse reflectance UV-Visible absorption spectroscopy. The
functional form of the electronic absorption edge for all kerogens measured is
in excellent agreement with the "Urbach tail" phenomenology. The Urbach decay
width extracted from the exponential fit within the visible range is strongly
correlated with the aliphatic/aromatic ratio in isolated kerogen, regardless of
the kerogen type. No correlation is found between the decay width and the
average size of aromatic clusters, which is explained in terms of a non-linear
increase in optical absorption with increasing size of the aromatic clusters
determined by 13C NMR. Further, absorption spectra calculated with density
functional theory calculations on proxy ensemble models of kerogen are in
excellent agreement with the experimental results. The correlation of the decay
width with conventional maturity indicators such as vitrinite reflectance is
found to be good within a particular kerogen type, but not consistent across
different kerogen types, reflecting systematic variations in bulk composition
for different type kerogen types with the same vitrinite reflectance. Thus,
diffuse reflectance visible absorption spectroscopy is presented as a rapid,
calibrated and non-destructive method to monitor both the maturity and the
chemical composition of kerogen. The chemical insight of kerogen in relation to
its optical absorption provided by this methodology may serve for rapid
screening of kerogen for electronics and optical devices in place of
functionalized produced carbon.
