  We investigate molecular adsorption doping by electron withdrawing NO2 and
electron donating NH3 on epitaxial graphene grown on C-face SiC substrates.
Amperometric measurements show conductance changes upon introduction of
molecular adsorbents on epitaxial graphene. Conductance changes are a trade-off
between carrier concentration and scattering, and manifest at direct current
and optical frequencies. We therefore investigate changes in the infrared (IR)
reflection spectra to correlate these two frequency domains, as reflectance
changes are due to a change of epitaxial graphene (EG) surface conductance. We
match theory with experimental IR data and extract changes in carrier
concentration and scattering due to gas adsorption. Finally, we separate the
intraband and interband scattering contributions to the electronic transport
under gas adsorption. The results indicate that, under gas adsorption, the
influence of interband scattering cannot be neglected, even at DC.
