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(S-1) + H-2 (X-1 Sigma(g)+) channel following vacuum ultraviolet (VUV) photodissociation of H2O in the wavelength range of lambda = similar to 100-112 nm are vibrationally excited. In particular, more than 90% of H-2(X) fragments populate in a vibrational state v= 3 at lambda similar to 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.

Automated summary

The study reveals a new pathway to produce vibrationally excited H-2 through water photochemistry, complementing the previous understanding that vibrationally excited H-2 was mainly achieved by shock wave or far-ultraviolet fluorescence pumping. Experimental evidence from vacuum ultraviolet photodissociation of H2O shows that vibrationally excited H-2 can play a significant role in the interstellar space, indicating a need for recognition in appropriate interstellar chemistry models.