Lack of proportionality of total electron yield and soft x-ray absorption coefficient

For many applications, the total electron yield (TEY) eta is assumed to be proportional to the product of the linear absorption coefficient and the photon energy. To test this model we measured the total electron yield of copper and gold at the (111) surface of single crystals in the photon energy range between 50 eV and 1500 eV with relative uncertainties lower than 3.6%. In addition, the data for the absorption coefficient were improved by measuring the transmittance of polyimide films covered either with thin gold or copper layers. The experiments were performed at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY I. For photon energies below 150 eV, the total electron yield is proportional to the absorption coefficient, provided the saturation effects as determined here, by measurements of the TEY as a function of the angle of incidence of the radiation, are taken into account. At higher photon energy, the ratio between the TEY and the product of absorption coefficient times the photon energy decreases continuously by about 30% down to 1500 eV. We present a new analytical model for the total electron yield describing the contributions of primary and secondary electrons. Input parameters such as the electron escape depth and the reflection of the soft x-rays has been experimentally determined. When these parameters are used, our model achieves good agreement with the measured TEY, when the only free parameter, i.e., the efficiency with which the absorbed photon energy is converted into secondary electrons, is adjusted. (C) 2000 American Institute of Physics. [S0021-8979(00)04301-2].