  We study the photo-desorption occurring in H$_2$O:CO:NH$_3$ ice mixtures
irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV)
soft X-rays generated at the National Synchrotron Radiation Research Center
(Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen
(m/z = 1) to complex species with m/z = 69 (e.g., C$_3$H$_3$NO, C$_4$H$_5$O,
C$_4$H$_7$N), supporting the enrichment of the gas phase.
  At low number of absorbed photons, substrate-mediated exciton-promoted
desorption dominates the photo-desorption yield inducing the release of weakly
bound (to the surface of the ice) species; as the number of weakly bound
species declines, the photo-desorption yield decrease about one order of
magnitude, until porosity effects, reducing the surface/volume ratio, produce a
further drop of the yield.
  We derive an upper limit to the CO photo-desorption yield, that in our
experiments varies from 1.4 to 0.007 molecule photon$^{-1}$ in the range $\sim
10^{15} - 10^{20}$~absorbed photons cm$^{-2}$. We apply these findings to a
protoplanetary disk model irradiated by a central T~Tauri star.
