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

Abstract

The structure of the hydrated electron, particularly whether it exists primarily within a cavity or encompasses interior water molecules, has been the subject of much recent debate. In Paper I [C.-C. Zho et al., J. Chem. Phys. 147, 074503 (2017)], we found that mixed quantum/classical simulations with cavity and non-cavity pseudopotentials gave different predictions for the temperature dependence of the rate of the photoexcited hydrated electron's relaxation back to the ground state. In this paper, we measure the ultrafast transient absorption spectroscopy of the photoexcited hydrated electron as a function of temperature to confront the predictions of our simulations. The ultrafast spectroscopy clearly shows faster relaxation dynamics at higher temperatures. In particular, the transient absorption data show a clear excess bleach beyond that of the equilibrium hydrated electron's ground-state absorption that can only be explained by stimulated emission. This stimulated emission component, which is consistent with the experimentally known fluorescence spectrum of the hydrated electron, decreases in both amplitude and lifetime as the temperature is increased. We use a kinetic model to globally fit the temperature-dependent transient absorption data at multiple temperatures ranging from 0 to 45 C. We find the room-temperature lifetime of the excited-state hydrated electron to be 137±40 fs, in close agreement with recent time-resolved photoelectron spectroscopy (TRPES) experiments and in strong support of the "non-adiabatic" picture of the hydrated electron's excited-state relaxation. Moreover, we find that the excited-state lifetime is strongly temperature dependent, changing by slightly more than a factor of two over the 45C temperature range explored. This temperature dependence of the lifetime, along with a faster rate of ground-state cooling with increasing bulk temperature, should be directly observable by future TRPES experiments. Our data also suggest that the red side of the hydrated electron's fluorescence spectrum should significantly decrease with increasing temperature. Overall, our results are not consistent with the nearly complete lack of temperature dependence predicted by traditional cavity models of the hydrated electron but instead agree qualitatively and nearly quantitatively with the temperature-dependent structural changes predicted by the non-cavity hydrated electron model.

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

“…In Paper II, 39 we perform temperature-dependent transient absorption measurements and find results that are in qualitative disagreement with the traditional cavity picture, but in excellent agreement with our non-cavity model. This reinforces the notion that a picture of the hydrated electron as a strongly repulsive, hydrophobic object that resides in a cavity is not consistent with its known temperature-dependent properties; hopefully temperature-dependent resonance Raman and time-resolved photoelectron spectroscopy experiments will be performed in the near future to further test the differing predictions of cavity and non-cavity hydrated electron models.…”

“…For LGS, in addition to the fact that the early time bleach is red-shifted, the kinetics at which the bleach blue-shifts and recovers are faster at higher temperatures, in reasonable agreement with the experiments in Paper II. 39 For TB, the calculated transient absorption signals at the three different temperatures are the same within the error, which along with the very rapid blue-shift of the maximum bleach is in contrast to experiment; see Paper II 39 for details.…”

“…6, we show the calculated transient absorption spectra at different temperatures for the LGS non-cavity (left panels) and the TB cavity (right panels) models in the energy range of the transient absorption spectra that we study experimentally in Paper II. 39 For the spectral region plotted in Fig. 6, the calculated transient absorption spectra consist mostly of the ground-state bleach, along with contributions from stimulated emission at early times, and the recovering hot ground state absorption that fills in the bleach at later times.…”

“…Over the range of temperatures investigated, the average excited-state lifetime for the TB model changes by ∼30%, while that for the LGS noncavity model changes by over a factor of three: the two models thus predict a temperature dependence that differs by an order of magnitude. For a temperature difference of 45 • C, which we were able to explore experimentally in Paper II, 39 the TB model predicts a ≤20% change in the excited-state lifetime, while the LGS model predicts a lifetime change of just under a factor of two.…”

“…Time-resolved photoelectron spectroscopy experiments can directly measure this lifetime, 12,57,58 and it should be readily possible to do these experiments over at least a limited temperature range. Even if the experiments only can be done between temperatures of 0 and 45 • C, as with the transient absorption experiments described in Paper II, 39 the experiments should still easily be able to distinguish the ∼20% change in lifetime predicted by the cavity model versus the factor of ∼2 change predicted by the non-cavity model. It should also be possible to observe the lifetime change by pump-probe transient absorption spectroscopy, which we turn to in Sec.…”