Vibrationally excited molecular hydrogen production from the water photochemistry

Vibrationally excited molecular hydrogen has been commonly observed in the dense photo-dominated regions (PDRs). It plays an important role in understanding the chemical evolution in the interstellar medium. Until recently, it was widely accepted that vibrational excitation of interstellar H(2) was achieved by shock wave or far-ultraviolet fluorescence pumping. Here we show a further pathway to produce vibrationally excited H(2) via the water photochemistry. The results indicate that the H(2) fragments identified in the O((1)S) + H(2)(X(1)Σ(g)(+)) channel following vacuum ultraviolet (VUV) photodissociation of H(2)O in the wavelength range of λ = ~100-112 nm are vibrationally excited. In particular, more than 90% of H(2)(X) fragments populate in a vibrational state v = 3 at λ~112.81 nm. The abundance of water and VUV photons in the interstellar space suggests that the contributions of these vibrationally excited H(2) from the water photochemistry could be significant and should be recognized in appropriate interstellar chemistry models.