Hydroxyl radicals (OH) play a central role in the interstellar medium.
Here, we observe highly rotationally excited OH radicals with energies
above the bond dissociation energy, termed OH “super rotors”, from the
vacuum ultraviolet photodissociation of water. The most highly excited
OH(X) super rotors identified at 115.2 nm photolysis have an
internal energy of 4.86 eV. A striking enhancement in the yield of
vibrationally-excited OH super rotors is detected when exciting the
bending vibration of the water molecule. Theoretical analysis shows that
bending excitation enhances the probability of non-adiabatic coupling
between the B~ and X~ states of water at collinear O–H–H geometries following fast internal conversion from the initially excited D~ state. The present study illustrates a route to produce extremely rotationally excited OH(X)
radicals from vacuum ultraviolet water photolysis, which may be related
to the production of the highly rotationally excited OH(X) radicals observed in the interstellar medium.