The energy dependence of the photodesorption cross-section in the OCS/Ag(111) system is interpreted in terms of the hot electron attachment model including the electron cascade process and the substrate absorbance. An electron attachment level 3 eV above the Fermi level gives the best fit of the experimental action spectrum. The sensitivity of the fit to a variety of parameters is analysed. The fit is a strong function of attachment level energy. The weight of other parameters (attachment level width, surface-adsorbate barrier, secondary electron cascade profile) is less significant. It is shown that the model may reproduce the reported universal wavelength dependence of photodesorption on different metal substrates only if the attachment level is located a constant distance from the metal Fermi level.