Technical Comments

Response to Comment on "Characterization of Excess Electrons in Water-Cluster Anions by Quantum Simulations"

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Science  16 Dec 2005:
Vol. 310, Issue 5755, pp. 1769
DOI: 10.1126/science.1119314

Abstract

We reiterate that the conclusions of our original report are based on identifiable characteristic trends in several observables with cluster size. The numerical comparison between simulated and experimental vertical detachment energies emphasized by Verlet et al. reflect quantitative limitations of our atomistic model, but, in our opinion, do not undermine these conclusions.

The comment by Verlet et al. (1) challenges the strength of our simulation-based conclusion (2) that all experimental spectral and energetic data on water-cluster anions to date are attributable to surface-bound electronic states. However, our comparison of the calculated trends in properties to the data of Ayotte and Johnson (3) and Coe et al. (4) is consistent in all respects with surface states, and the measured data are not overall consistent with the characteristics calculated for interior states. This is evident for the vertical detachment energies (VDEs) shown in figure 1 in (1), manifesting linear variation for surface, but not interior states, but it is more dramatic for the spectral moments [reflected in the radii and kinetic energies; figure 4 in (2)]. We therefore conclude that the states previously denoted as isomer I by Verlet et al. (5), and equivalent to those measured by these other authors (3, 4), are also surface states. Furthermore, we argue that if isomer I is indeed a surface state, so are isomers II and III (5), which bind the electron considerably more weakly. This logic is explicit in our study (2). Our report also acknowledged the numerical issues pointed out in the comment, noting that the calculated VDE values were closer to those measured by Neumark (5) for the identified surface states than to the Coe data (4). However, as we stated, “the surface and interior electron binding morphologies lead to distinctly different trends in measured physical properties” and “the comparison of the trends to the corresponding published experimental data strongly supports the conclusion that the available experiments reporting these results reflect only clusters characterized by electronic surface states.”

We agree with the Verlet et al. discussion (1) of temperature effects on morphology. It is reasonable that the coldest clusters manifest nonequilibrium factors, for both the experiment and simulation, as we suggested as a possibility (2). In the simulation, we start the electron internally, and in the experiment, it is attached to a preformed water cluster, i.e., initially externally, so nonequilibrium can lead to differences. Johnson and co-workers (6) have provided a rationale for the difference between isomer I and the more weakly binding isomer II that is consistent with our conclusions. Based on detailed spectral analysis of relatively small clusters, they attribute the difference to the presence (isomer I) or absence (isomer II) of proximal water molecules with both molecular hydrogen atoms oriented toward, and penetrating into, the electronic distribution. Whether this conjecture is manifest in simulations of larger anionic clusters is being investigated.

We also agree with Verlet et al. (1) that there is an important place for new experiments and for further calculations to fill in the picture robustly. It would certainly be of interest to extend the photoelectron spectra measurements into the range predicted for the stable interior states of larger clusters. As shown in figure 1 in (1), these are predicted to lie at deeper energies than the bulk hydrated electron. At the same time, the beautifully executed excited-state dynamics experiments reported by Verlet et al. (5) provide a rich set of data, which strongly motivates theoretical study of dynamics. These excited-state dynamics studies are more challenging for the theoretical community, paralleling the challenges now overcome by the experimental community. These challenges form the next hurdle for simulation.

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