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 was achieved by shock wave or far-ultraviolet fluorescence pumping. Here we show a further pathway to produce vibrationally excited H via the water photochemistry. The results indicate that the H fragments identified in the O(S) + H(XΣ) channel following vacuum ultraviolet (VUV) photodissociation of HO in the wavelength range of λ = 100-112 nm are vibrationally excited. In particular, more than 90% of H(X) fragments populate in a vibrational state v = 3 at λ112.81 nm. The abundance of water and VUV photons in the interstellar space suggests that the contributions of these vibrationally excited H from the water photochemistry could be significant and should be recognized in appropriate interstellar chemistry models.