  UV-induced photodesorption of ice is a non-thermal evaporation process that
can explain the presence of cold molecular gas in a range of interstellar
regions. Information on the average UV photodesorption yield of astrophysically
important ices exists for broadband UV lamp experiments. UV fields around
low-mass pre-main sequence stars, around shocks and in many other astrophysical
environments are however often dominated by discrete atomic and molecular
emission lines. It is therefore crucial to consider the wavelength dependence
of photodesorption yields and mechanisms. In this work, for the first time, the
wavelength-dependent photodesorption of pure CO ice is explored between 90 and
170 nm. The experiments are performed under ultra high vacuum conditions using
tunable synchrotron radiation. Ice photodesorption is simultaneously probed by
infrared absorption spectroscopy in reflection mode of the ice and by
quadrupole mass spectrometry of the gas phase. The experimental results for CO
reveal a strong wavelength dependence directly linked to the vibronic
transition strengths of CO ice, implying that photodesorption is induced by
electronic transition (DIET). The observed dependence on the ice absorption
spectra implies relatively low photodesorption yields at 121.6 nm (Ly-alpha),
where CO barely absorbs, compared to the high yields found at wavelengths
coinciding with transitions into the first electronic state of CO (singulet Pi
at 150 nm); the CO photodesorption rates depend strongly on the UV profiles
encountered in different star formation environments.
