  We propose to measure the surface charge accumulating at the interface
between a plasma and a dielectric by infrared spectroscopy using the dielectric
as a multi-internal reflection element. The surplus charge leads to an
attenuation of the transmitted signal from which the magnitude of the charge
can be inferred. Calculating the optical response perturbatively in first order
from the Boltzmann equation for the electron-hole plasma inside the solid, we
can show that in the parameter range of interest a classical Drude term
results. Only the integrated surface charge enters, opening up thereby a very
efficient analysis of measured data.
