  We present the first detection of gas phase S2H in the Horsehead, a
moderately UV-irradiated nebula. This confirms the presence of doubly
sulfuretted species in the interstellar medium and opens a new challenge for
sulfur chemistry. The observed S2H abundance is ~5x10$^{-11}$, only a factor
4-6 lower than that of the widespread H2S molecule. H2S and S2H are efficiently
formed on the UV-irradiated icy grain mantles. We performed ice irradiation
experiments to determine the H2S and S2H photodesorption yields. The obtained
values are ~1.2x10$^{-3}$ and <1x10$^{-5}$ molecules per incident photon for
H2S and S2H, respectively. Our upper limit to the S2H photodesorption yield
suggests that photo-desorption is not a competitive mechanism to release the
S2H molecules to the gas phase. Other desorption mechanisms such as chemical
desorption, cosmic-ray desorption and grain shattering can increase the gaseous
S2H abundance to some extent. Alternatively, S2H can be formed via gas phase
reactions involving gaseous H2S and the abundant ions S+ and SH+. The detection
of S2H in this nebula could be therefore the result of the coexistence of an
active grain surface chemistry and gaseous photo-chemistry.
