  Pure acetonitrile (CH3CN) and mixed CO:CH3CN and H2O:CH3CN ices have been
irradiated at 15K with Vacuum UltraViolet (VUV) photons in the 7-13.6 eV range
using synchrotron radiation. VUV photodesorption yields of CH3CN and of
photo-products have been derived as a function of the incident photon energy.
The coadsorption of CH3CN with CO and H2O molecules, which are expected to be
among the main constituents of interstellar ices, is found to have no
significant influence on the VUV photodesorption spectra of CH3CN, CHCN, HCN,
CN and CH3. Contrary to what has generally been evidenced for most of the
condensed molecules, these findings point toward a desorption process for which
the CH3CN molecule that absorbs the VUV photon is the one desorbing. It can be
ejected in the gas phase as intact CH3CN or in the form of its
photo-dissociation fragments. Astrophysical VUV photodesorption yields,
applicable to different locations, are derived and can be incorporated into
astrochemical modeling. They vary from 0.67(+/-0.33).10^{-5} to
2.0(+/-1.0).10^{-5} molecule/photon for CH3CN depending on the region
considered, which is high compared to other organic molecules such as methanol.
These results could explain the multiple detections of gas phase CH3CN in
different regions of the interstellar medium and are well-correlated to
astrophysical observations of the Horsehead nebula and of protoplanetary disks
(such as TW Hya and HD 163296).
