Temperature dependence of the hydrated electron’s excited-state relaxation. I. Simulation predictions of resonance Raman and pump-probe transient absorption spectra of cavity and non-cavity models

Abstract

We use one-electron non-adiabatic mixed quantum/classical simulations to explore the temperature dependence of both the ground-state structure and the excited-state relaxation dynamics of the hydrated electron. We compare the results for both the traditional cavity picture and a more recent non-cavity model of the hydrated electron and make definite predictions for distinguishing between the different possible structural models in future experiments. We find that the traditional cavity model shows no temperature-dependent change in structure at constant density, leading to a predicted resonance Raman spectrum that is essentially temperature-independent. In contrast, the non-cavity model predicts a blue-shift in the hydrated electron's resonance Raman O-H stretch with increasing temperature. The lack of a temperature-dependent ground-state structural change of the cavity model also leads to a prediction of little change with temperature of both the excited-state lifetime and hot ground-state cooling time of the hydrated electron following photoexcitation. This is in sharp contrast to the predictions of the non-cavity model, where both the excited-state lifetime and hot ground-state cooling time are expected to decrease significantly with increasing temperature. These simulation-based predictions should be directly testable by the results of future time-resolved photoelectron spectroscopy experiments. Finally, the temperature-dependent differences in predicted excited-state lifetime and hot ground-state cooling time of the two models also lead to different predicted pump-probe transient absorption spectroscopy of the hydrated electron as a function of temperature. We perform such experiments and describe them in Paper II [E. P. Farr et al., J. Chem. Phys. 147, 074504 (2017)], and find changes in the excited-state lifetime and hot ground-state cooling time with temperature that match well with the predictions of the non-cavity model. In particular, the experiments reveal stimulated emission from the excited state with an amplitude and lifetime that decreases with increasing temperature, a result in contrast to the lack of stimulated emission predicted by the cavity model but in good agreement with the non-cavity model. Overall, until ab initio calculations describing the non-adiabatic excited-state dynamics of an excess electron with hundreds of water molecules at a variety of temperatures become computationally feasible, the simulations presented here provide a definitive route for connecting the predictions of cavity and non-cavity models of the hydrated electron with future experiments.

Temperature dependence of the hydrated electron’s excited-state relaxation. II. Elucidating the relaxation mechanism through ultrafast transient absorption and stimulated emission spectroscopy

“…The p-state lifetimes extracted from the model are strongly temperature dependent, varying by a factor of roughly two over the experimental temperature range of 0-45 • C. At room temperature, the best-fit lifetime is 137 ± 40 fs. Our temperature-dependent lifetime results are in excellent agreement with what is predicted by noncavity hydrated electron models in Paper I, 18 and the roomtemperature lifetime is in good accord with that estimated from recent TRPES experiments. 29,[45][46][47] Overall, our results make clear that any model of the hydrated electron should predict a significant temperature dependence to the excited-state lifetime, something that the current generation of cavity models does not do.…”

“…Finally, although Barbara and co-workers also attempted to model the electron's excited-state absorption contribution, there is, unfortunately, no simple way that we are aware of to make physically reasonable simplifying assumptions about this component. However, we know from both previous experimental 35,40,59 and simulation 18,43 studies (including that in Paper I 18 ) that the photoexcited hydrated electron's ESA lies primarily to the red of its ground-state absorption and thus there should be little ESA contribution to the blue of 700 nm. Therefore, in this work, we focus only on this spectral region as a means to neglect the ESA component and thus further simplify the kinetic model used to fit the data.…”

“…As discussed in Paper I, 18 our suggestion of a non-cavity structure for the hydrated electron has been controversial. [19][20][21][22][23][24] Indeed, our potential, dubbed Larsen-Glover-Schwartz (LGS) in the literature, predicts that the electron is overly bound energetically 19,20,25 and has a negative molar volume of solvation, 26 in contrast to what is known from experiment.…”

“…52 Large shifts in the ground-state spectra take place as the temperature (and pressure) of water is varied, [53][54][55] however, and Paper I makes clear that at least for the non-cavity picture, the ground-state structure of the hydrated electron could change significantly with temperature. 18 If there are temperaturedependent changes in the equilibrium structure, this suggests that the excited-state relaxation dynamics of the hydrated electron could also be temperature dependent. Indeed, in Paper I, 18 we found a large temperature dependence of the p-state relaxation dynamics for photoexcited simulated noncavity hydrated electrons, although simulated cavity hydrated electrons showed little change in structure with temperature and thus almost no temperature dependence to their p-state lifetime.…”

“…18 If there are temperaturedependent changes in the equilibrium structure, this suggests that the excited-state relaxation dynamics of the hydrated electron could also be temperature dependent. Indeed, in Paper I, 18 we found a large temperature dependence of the p-state relaxation dynamics for photoexcited simulated noncavity hydrated electrons, although simulated cavity hydrated electrons showed little change in structure with temperature and thus almost no temperature dependence to their p-state lifetime.…”