Resonant Auger decay of above-threshold core-excited ${\mathrm{H}}_2\mathrm{O}$

The decay properties of an above-threshold resonance in core-ionized water, which is difficult to identify with usual spectroscopic methods, have been analyzed by resonant Auger spectroscopy. The resonance is shown to correspond to a doubly excited neutral state embedded in the O $1s$ ionization continuum. The main result of the experiments is the assessment on the dissociative nature of the intermediate state, with a consequent fragmentation in H neutral plus $\mathrm{O}*\mathrm{H}$ core excited. This dissociation takes place on the same time scale as electron decay, and therefore is categorized as an ultrafast dissociation process.

Figure 1

(a) Total ion yield spectrum of ${\mathrm{H}}_2\mathrm{O}$ in the O $1s$ excitation region measured with $\sim 50\mathrm{meV}$ photon bandwidth. (b) Negative ion yields for the ${\mathrm{H}}^{-}$ and ${\mathrm{O}}^{-}$ fragments (curves from Ref. 3).

Figure 2

A schematic and simplified representation of the ultrafast dissociation process in water (from Ref. 4). Note that this is a cut through the three-dimensional ${\mathrm{H}}_2{\mathrm{O}}^{+}$ potential-energy surface along the $\mathrm{H}{\mathrm{O}}^{+}\text{−}\mathrm{H}$ bond direction. For the OH fragment the potential energy will vary along the $\mathrm{O}\text{−}\mathrm{H}$ coordinate, as indicated by the OH potential-energy curve (broken line).

Figure 3

Resonant Auger spectrum recorded on top of the resonance at $\sim 550\mathrm{eV}$ photon energy [see Fig. 1b].