Photodissociation of water clusters doped with , molecules has been studied in a molecular beam experiment. The clusters are dissociated with laser pulses, and the H fragments are ionized at and their time-of-flight distributions are measured. Experiments with deuterated species and suggest that the photodissociation signal originates from the presence of the molecule on the water cluster, but does not come directly from a photolysis of the molecule. The H fragment is proposed to originate from the hydronium molecule . Possible mechanisms of the production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the ion remains rather immobile.
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The background signal corresponds to the photolysis of rest gas molecules, namely, pump fluid hydrocarbons, and the photolysis of HBr molecules diffused from the pickup cell.
The absolute signal intensities depend on many factors, e.g., laser power and its focusing into the TOF chamber, the pickup cell pressure, etc. Since the spectra are quite sensitive to some of these parameters, the relative intensities should be regarded as approximate. However, a ratio of would be expected between the H-fragment signals from and , if the H atom were “diluted” by charge transfer process in the cluster. In fact, the measured ratio is orders of magnitude smaller.
It should be noted that various structures of proton transfer states ranging from hydronium to a strongly stretched molecular halide can occur on the ice particle surface due to a range of microsolvation environments. The hydrogen in the intact molecule and in the hydronium ion represent two extreme cases. A high asymmetry in the hydronium structure would lead to a deposition of the excitation energy into a dissociative mode for the bridging hydrogen between halogen and oxygen atoms. This would lead to fast departing hydrogens and less isotopic mixing which are both not observed in the experiment. Therefore the structures close to hydronium are expected to dominate.
