Measurement of reflectance by gas ionization

In an attempt to improve the accuracy of reflectance determinations, particularly from high vapor pressure samples, we have built and tested a double ionization chamebr which measures the ionization from a photon beam in the vapor or added gas before and after reflection. The photon beam strikes the sample at nearnormal incidence after passing through an oblong ionization chamber containing two collector wires. These are connected to one side of an electrometer-ratiometer of new design. A similar geometry is used for the specularly reflected beam, with the wires here connected to the other side of the ratiometer. A potential of 20 V on the sample is found to be sufficient to prevent the large photoelectric current from the sample from entering the chambers for photon energies to 26 eV. A sitnple calculation shows that the current ratio should decline exponentially with pressure in the operating pressure range from 100 microm to 5000 microm. Extrapolation of the data to zero pressure yields the sample reflectance. In testing, we found the reflectance to be 12.47% at 841 A for an evaporated gold film using a filling gas of air.