Vibrationally excited molecular hydrogen production from the water photochemistry

Abstract

Abstract 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.