Variation in optical constants between photocathodes

The complex permittivity values of four S20 photocathodes are reported and contrasted. Values were obtained indirectly by measuring the reflectance of the cathode and then using non-linear fitting software to obtain the best estimates of the real and imaginary parts of the complex permittivity, together with the thickness of the cathode layer. The permittivity values span the wavelength range from 250 to 950 nm. The values are broadly similar for the 4 sets of data. However, the variations may reflect the fact that the S20 is believed to consist of alkali compounds with at least two different stoichiometric compositions and the stoichiometry may not be identical between cathodes. For comparison purposes data were recorded for a simpler composition bi-alkali cathode and compared with earlier data derived from an alternative analysis and there was good agreement between the values from the two methods.