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We study the photo-desorption occurring in H$_2$O:CO:NH$_3$ ice mixturesirradiated 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-promoteddesorption dominates the photo-desorption yield inducing the release of weaklybound (to the surface of the ice) species; as the number of weakly boundspecies declines, the photo-desorption yield decrease about one order ofmagnitude, until porosity effects, reducing the surface/volume ratio, produce afurther drop of the yield.  We derive an upper limit to the CO photo-desorption yield, that in ourexperiments varies from 1.4 to 0.007 molecule photon$^{-1}$ in the range $\sim10^{15} - 10^{20}$~absorbed photons cm$^{-2}$. We apply these findings to aprotoplanetary disk model irradiated by a central T~Tauri star.

Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment