Abstract
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 tilde B and tilde X states of water at collinear O–H–H geometries following fast internal conversion from the initially excited tilde 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.
Highlights
Hydroxyl radicals (OH) play a central role in the interstellar medium
OH radical production routes in the interstellar medium (ISM) include the dissociative recombination of electrons and molecular cations formed by ion-neutral reactions, photodesorption from the surface of icy grains and, at much higher temperatures, gas-phase atom-molecule collisions, for example, O + H2 and H + H2O3
The spectra obtained with the polarization vector (ε) of the VUV-free electron laser (FEL) radiation aligned perpendicular to the a
Summary
The most intense peak in the Eint(OH) spectrum involves contributions from the ν = 0, N = 49, ν = 1, N = 47, and ν = 2, N = 45 levels of OH(X), with internal energies ~4.41 eV, that is, above the bond dissociation energy of the OH radical (D0(O–H) = 4.39 eV27) We term such extremely rotationally excited OH(X) products “super rotors,” and note that vibrationally excited super rotors are even more pronounced (see Fig. 2). The similarities between the OH (X, v = 0) rotational population distributions observed following excitation to the D~ð100Þ and D~ð000Þ states (peaking at N = 47 and N = 45, respectively) suggest that the quantum of symmetric stretch vibration partitions into the total energy available to the dissociating H2O molecules. The OH(X) super rotors, with an energy above the dissociation limit, are only stable by virtue of the associated centrifugal barriers, that is, by the additional contribution to the potential energy of the form N(N + 1)h2/8π2μR2, where N is the rotational
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