Ultrafast Internal Conversion and Solvation of Electrons in Water

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4499−4504

pubs.acs.org/JPCL

Ultrafast Internal Conversion and Solvation of Electrons in Water, Methanol, and Ethanol Shutaro Karashima, Yo-ichi Yamamoto, and Toshinori Suzuki* Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-Ku, Kyoto 606-8502, Japan

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S Supporting Information *

ABSTRACT: Ultrafast internal conversion from the first excited state of a solvated electron in water, methanol, and ethanol is investigated using time-resolved photoelectron spectroscopy of liquid microjets and a spectral retrieval method. Photoelectron spectra corrected for inelastic scattering clearly reveal well-separated signals from the excited and ground states, and the latter enables us to analyze the solvation dynamics in the ground state after internal conversion. Measurements with 25 fs time resolution identify a rapid increase in the vertical electron binding energy of the solvated electron owing to nuclear wave packet motions in the excited state and allow us to precisely determine the internal conversion time.

he hydrated electron e−aq,1 an electron solvated in liquid water, is the most fundamental chemical species in radiation chemistry and biology.2 However, its structure and dynamics are elusive and remain under intense debate.3 A widely accepted structure of e−aq is a bubble-like electron cloud with an approximate radius of 0.26 nm, created by Pauli repulsion between the excess electron and valence electrons of liquid water.3 However, a completely different noncavity structure has also been proposed.4 To deepen our understanding of solution chemistry, it is essential to elucidate solute−solvent many-body interactions, nuclear quantum effects, and nonadiabatic dynamics. e−aq is a benchmark system that includes all of these major issues. Regarding the electronic relaxation dynamics of e−aq, Barbara and colleagues pioneered transient absorption spectroscopy (TAS) studies of internal conversion from the first electronically excited state,5−7 which inspired a number of subsequent theoretical and experimental studies.3,4,8−16 In the TAS experiments, e−aq is photoexcited to the first electronically excited state through the visible absorption band centered at 720 nm, and the electron cloud of e−aq instantaneously changes from a nodeless pseudospherical shape to one with a single node. The hydration shell of the electron rapidly responds to this change, and it stabilizes the excited state while the ground state destabilizes in an ultrashort time, facilitating a nonadiabatic transition (internal conversion). A number of researchers employed TAS to elucidate the nonadiabatic dynamics of e−aq; however, TAS of this system was highly challenging in that at least four different photoabsorption and emission processes contribute to the spectra, making the interpretation ambiguous. Another difficulty was that the transient absorption spectrum rapidly shifted from the infrared

T

© XXXX American Chemical Society

to visible region in an ultrashort time, which was difficult to follow spectroscopically. The analysis of TAS data by Barbara and colleagues suggested two different kinetics models for the internal − conversion of eaq , because these two models provided comparable root-mean-square errors in the least-squares fitting.7,17 One is termed an adiabatic model, which implies that the solvation dynamics in the excited state precedes internal conversion, which takes place in a picosecond. The other is a nonadiabatic model, which indicates that internal conversion occurs in a fraction of a picosecond without significant solvation dynamics in the excited state. Although nearly all theoretical simulations conducted to date have supported the adiabatic model,3 time-resolved photoelectron spectroscopy (TRPES)18 of liquid water clearly points to nonadiabatic dynamics.19−21 TRPES probes nonadiabatic dynamics using photoexcitation to unoccupied electron scattering states, from which signals originating from different electronic states are clearly separated. Furthermore, TRPES does not require a widely tunable probe laser to observe the dynamics. Thus, TRPES seems free from the difficulties encountered when employing TAS; however, this is not entirely true. It is noted that inelastic scattering of an electron in the conduction band of a liquid alters the electron kinetic energy (eKE) distribution,22,23 causing the experimentally observed photoelectron kinetic energy (PKE) distribution to be blurred and energy-shifted from the initial eKE distribution. Consequently, the excitedReceived: June 17, 2019 Accepted: July 23, 2019

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DOI: 10.1021/acs.jpclett.9b01750 J. Phys. Chem. Lett. 2019, 10, 4499−4504

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Schematic diagram of three-pulse photoelectron spectroscopy of liquid. (b−d) Photoelectron spectra of e−solv in (b) H2O, (c) CH3OH, and (d) C2H5OH measured using 700 nm pump and 270 nm probe pulses with a time resolution of 80 fs. The spectral retrieval method has been applied, and the ground-state bleach signal has been removed by adding the photoelectron spectrum measured separately. The delay axes are plotted on a linear scale up to 0.4 ps and on a logarithmic scale afterward. The eBE plotted on the vertical axes is calculated as hυprobe − eKE.

state signal of e−aq partially overlaps with the ground-state signal, an effect that has been neglected in previous TRPES studies.19,20 In order to overcome this obstacle, we have recently developed a spectral retrieval method to account for these inelastic scattering effects.21 In the present study, we apply this method to TRPES of ultrafast internal conversion of solvated electrons (e−solv) in water, methanol,24−26 and ethanol from the first excited state to further elucidate the dynamics. We also improve the time resolution to 25 fs to accurately determine the dynamical time constants and examine wave packet dynamics prior to internal conversion. Our experiment is illustrated in Figure 1a. A continuous liquid microjet of 0.09−0.3 mol/L NaBr solution in water, methanol, or ethanol was introduced into a 1.2 m long time-offlight (TOF) photoelectron spectrometer through a fused silica capillary with a 15 μm inner diameter. The microjet was illuminated with 200 nm pulses 1 mm downstream from the capillary nozzle to create e−solv by the charge-transfer-to-solvent − (CTTS) reaction from Br− to solvent. The esolv was thermalized for 500 ps and excited with 700 nm pump pulses to the first electronically excited state. The subsequent electronic relaxation dynamics of e−solv were interrogated using photoelectron spectroscopy with time-delayed 270 nm (4.6 eV) or 350 nm (3.5 eV) probe pulses. A magnetic bottle was used to collect photoelectrons emitted from the liquid surface in a solid angle greater than 2π steradian; therefore, photoemission angular anisotropy was not resolved in this study. The linear polarization vector of the 200 nm and pump pulses was parallel to the axis of TOF spectrometer, while that of the probe pulses was at the magic angle. The spectral retrieval method is described elsewhere.21 The method utilizes comparison of the electron binding energy (eBE) distributions of e−solv in water, methanol, and ethanol measured using photoelectron spectroscopy with UV and extreme UV pulses. With extreme UV pulses, the influence of inelastic scattering

on the observed photoelectron spectrum is negligible, and the eBE distributions have been determined to be an almost perfect Gaussian shape G(E) for water, methanol, and ethanol, where E is energy. On the other hand, the spectrum g(E) measured with UV pulses deviates from the Gaussian distribution owing to inelastic scattering. 22 Therefore, comparison of G(E) and g(E) reveals how the Gaussian distribution is transformed under the influence of inelastic scattering. From a number of pairs of these G(E) and g(E) measured with different UV photon energies, one can determine a linear transformation from G(E) to g(E) by inelastic scattering. Thus, its inverse transformation can be employed to retrieve the eKE and eBE distributions with minimal influence from inelastic scattering.21 The details of the spectral retrieval method, including its numerical tests, are explained in the Supporting Information. Panels b, c, and d of Figure 1 show the pump−probe photoelectron spectra of e−solv in water, methanol, and ethanol, respectively, measured using 700 nm pump and 270 nm probe pulses with a time resolution of 80 fs. The spectral retrieval method has been applied, and the ground-state bleach signal was removed to clearly present the pump−probe signal from the excited state and nonequilibrated ground state. The figures present well-resolved excited- and ground-state signals. They are most clearly separated in the spectra of e−solv in ethanol presented in Figure 1d; a round shaped distribution appearing at short time delay in the eBE region of 1−2 eV is of the excited state, and a gradually shifting distribution in the 2−4 eV region is of the ground state. These results unambiguously show that the excited state undergoes ultrafast internal conversion to the ground state in about 100 fs. The decay time constants of the integrated excited-state signals of e−solv in water, methanol, and ethanol are estimated to be 60, 130, and 160 fs, respectively; however, we will present more precise values later based on measurements with 25 fs time resolution. 4500


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CTTS reaction from I− to these solvents. The results indicate that the structural change of the solvation shell in CTTS is more significant than in internal conversion of e−solv. Because the time for internal conversion from the first excited state of e−aq is the key quantity that has been intensely debated, we performed TRPES with 25 fs time resolution using the fundamental (700 nm) of a noncollinear optical parametric amplifier and its second harmonic (350 nm) as the pump and probe pulses, respectively. Panels a, b, c, and d of Figure 3 show the photoelectron time-energy maps of e−solv in H2O, D2O, methanol, and ethanol, respectively. Weak probe−pump signals are present in the negative delay time region, especially for methanol and ethanol. Because the probe photon energy of 3.54 eV is smaller or comparable to VBEs of e−solv (3.8, 3.4, and 3.2 eV for water, methanol, and ethanol, respectively),21 the photoelectron signals are mainly from the excited state. Figure 3e−h are the time profiles of the integrated signal intensities of the excited state. Plotted are the signals with eBE less than 2.9 eV for water, 2.3 eV for methanol, and 2.2 eV for ethanol, in which the threshold energies were carefully selected to clearly separate the excited- and ground-state signals. When we compress the time axis of the D2O plot by a factor of 2 , the result falls into remarkable agreement with that of H2O, as shown in Figure 3e. This finding agrees very well with the TAS results of Wiersma and colleagues.11 The time profiles were quantitatively analyzed using least-squares fitting by assuming a single exponential decay for the probe−pump signal and biexponential decay for the pump−probe signal. The fit provided excited-state lifetimes of 50 ± 5, 77 ± 5, 111 ± 10, and 128 ± 20 fs, for H2O, D2O, methanol, and ethanol, respectively (Table 2). The errors were estimated by considering the influence of the selection of eBE region over which the signal intensity is integrated, as described in detail in the Supporting Information. The values for H2O and D2O are in reasonable agreement with our previous report but are somewhat shorter.20 As for e−solv in methanol and ethanol, the experimental intensity profiles are flatter near the time origin. This presumably means that these systems do not undergo internal conversion in the vicinity of their Franck−Condon state. The least-squares fits for methanol and ethanol were slightly improved by assuming an induction period (tconst) during which a constant population is maintained prior to exponential decay, and the exponential decay times in methanol and ethanol were estimated to be 91 ± 10 fs (tconst = 31 ± 1 fs) and 114 ± 20 fs (tconst = 25 ± 5 fs), respectively. Now we examine the excited-state signal in further detail. We have carefully subtracted the probe−pump signal from the experimental data to isolate the pump−probe signal, as shown in Figure 4. The circle markers in the panels indicate the peak positions of eBE (VBE), and they clearly show that VBE progressively increases with time in all cases. Further discussion on the VBE, including the consistency with our previous report,20 is presented in the Supporting Information.

It is noted that the ground-state bleach signal is also influenced by inelastic scattering, so the raw experimental data do not exhibit correct eBE distributions; thus, the retrieval method is indispensable for the spectral analysis of the groundstate signal. From the retrieved ground-state spectra in Figure 1b−d, one can extract the time-correlation function c(t) defined as c(t ) =

⟨eBE(t )⟩ − ⟨eBE(∞)⟩ ⟨eBE(0)⟩ − ⟨eBE(∞)⟩

for the nonequilibrated ground electronic state. ⟨eBE(t)⟩ is the first moment of the eBE distribution at time t. In our previous study,21 c(t) was calculated from time dependence of the vertical electron binding energy (VBE), while we considered that ⟨eBE(t)⟩ is more appropriate for c(t) in the present study, though the results do not differ greatly.27 The extracted c(t) are shown in Figure 2a−c along with their least-squares fits, from which the solvation time constants are

Figure 2. Time correlation function c(t) in the ground state of e−solv in (a) H2O, (b) CH3OH, and (c) C2H5OH calculated from panels b, c, and d of Figures 1, respectively. The experimental data are shown with square markers, and the least-squares fits are shown as solid lines.

estimated to be 0.43 ± 0.17 and 1.0 ± 0.6 for water, 0.71 ± 0.03 and 12.9 ± 0.8 for methanol, and 0.69 ± 0.03 and 15 ± 1 for ethanol. The results for H2O exhibit larger errors than those for alcohols; similar analysis using ⟨eBE(t)⟩ on the experimental data reported in our previous study21 provides the solvation time constants of 0.22 ± 0.02 and 1.0 ± 0.3 ps with clearly smaller errors. As summarized in Table 1, the time constants determined for internal conversion of e−solv are generally smaller than those obtained previously21 for the

Table 1. Solvation Time Constants Obtained Using the Least Squares Fit of Time Correlation Function in the Ground State of Solvated Electronsa CTTS reaction from I−b

internal conversion of e−solv c

solvent

H2O

CH3OH

C2H5OH

H2O

CH3OH

C2H5OH

τ1 (ps) τ2 (ps)

0.22 ± 0.02 1.0 ± 0.3

0.71 ± 0.03 12.9 ± 0.8

0.69 ± 0.03 15 ± 1

0.39 ± 0.02 2.7 ± 1.6

0.83 ± 0.02 19.5 ± 0.7

1.09 ± 0.02 45 ± 2

The errors are standard deviation of the fitting. bThese values are taken from ref 21. cThese values are calculated from the data reported in ref 21.

a

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Figure 3. Photoelectron spectra of e−solv in (a) H2O, (b) D2O, (c) CH3OH, and (d) C2H5OH measured using 700 nm pump and 350 nm probe pulses with a time resolution of 25 fs. The spectral retrieval method has been applied, and the ground-state bleach signal has been removed. Time profiles of the integrated excited-state signal intensities of (e) H2O, (f) D2O, (g) CH3OH, and (h) C2H5OH. The least-squares fits are shown as solid lines. The profile of the D2O signal is also plotted as gray circles in panel e using a time axis compressed by a factor of 2 . The excellent agreement with the H2O time profile indicates that the difference between their excited-state dynamics can be primarily ascribed to the mass effect.

Table 2. Lifetimes (fs) of the Excited State of e−solv in TRPESa this work cross correlation (fwhm) H2O D2O CH3OH C2H5OH

ref 20

ref 28

25

55−80

150b

50 ± 5 77 ± 5 111 ± 10 128 ± 20

60 ± 10 100 ± 20

75 ± 12 102 ± 8

to strong correlation between the fitting parameters.6,7 The essential difficulty of TAS is that the ground-state bleach signal, stimulated emission and absorption from the excited state, and transient absorption from the nonequilibrated ground state overlap in the same wavelength region. Multiphoton excitation can also occur at high pump pulse energy.7 Wiersma and colleagues determined the lifetimes of e−aq in H2O and D2O to be 50 and 70 fs, which are in remarkable agreement with our results.11 The difference between their experiment and those of others was that the entire spectrum in the visible region was simultaneously measured with broad-band probe pulses with a low energy of 5 nJ.11 Nonetheless, it is impressive that they were able to draw their conclusion based on solely the visible data, where four decay-associated spectra heavily overlapped. In the analysis of TAS by Barbara and co-workers, the two models provided similar root-mean-square errors of the fits.7 Schwartz and colleagues argued that these two models can be differentiated using the temperature dependence of the dynamics, and they concluded the lifetime of the excited state of e−aq to be 137 ± 40 fs.17 This value differs from our TRPES result, but it also supports the nonadiabatic model. As for e−solv in methanol, Barbara and colleagues estimated the excited-state lifetime to be longer than 300 fs, which seems incorrect.24 Laubereau and co-workers reported the solvation time in the excited state, internal conversion time, and the solvation time in the ground state to be 0.105, 0.67, and 5.3 ps, respectively.25 Interestingly, these values are similar to 0.111 (or 0.091), 0.7, and 13 ps obtained in the present study. Therefore, it is likely that TAS extracted accurate dynamical time constants for liquid methanol, while the spectral assignments were incorrect. TRPES by Elkins et al. estimated the lifetime to be 130 ± 40 fs, which is in agreement with our value within the experimental error.26 In conclusion, TRPES with a 25 fs time resolution and our spectral retrieval method unambiguously show that e−solv in H2O and D2O undergo ultrafast internal conversion in 50 and 80 fs, respectively. e−solv in methanol and ethanol also exhibit ultrafast

ref 26 160

130 ± 40

a

The errors were estimated by taking into account the influence of the threshold energy to separate the excited- and ground-state signals (see the Supporting Information). bCalculated from Gaussian width of 65 fs.

We now consider why TAS studies on the nonadiabatic dynamics of e−aq reached a different conclusion from that of TRPES. The early TAS experiment by Barbara was performed with a time resolution of 300 fs,5,29,30 which was perhaps insufficient to elucidate the ultrafast dynamics in water. Laubereau et al. paid much attention to transient absorption in the infrared region and devoted efforts to extend the probe wavelength to the infrared; however, their pulse duration was 150 fs in the visible and 300 fs at 5 μm, which presumably hampered precise measurements.12 It is likely that the transient absorption in the near-infrared predominantly resulted from the nonequilibrated ground state e−aq created by ultrafast internal conversion. The subpicosecond and picosecond time constants extracted by these measurements are attributed to the diffusional solvent response of liquid water. Nonetheless, transient absorption in the region of the OH stretching vibration by Laubereau and colleagues may involve information on the coupling of an excess electron cloud with the OH bond in the first hydration shell;12 therefore, further detailed investigation may possibly provide valuable information on the nonadiabatic transition. Barbara’s subsequent experiment performed at 35 fs time resolution was not conclusive owing 4502


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Figure 4. Photoelectron spectra from the excited electronic state of e−solv in (a) H2O, (b) D2O, (c) CH3OH, and (d) C2H5OH calculated from panels a , b, c, and d of Figure 3, respectively. The probe−pump signals have been subtracted. The circle markers show the VBEs at each delay time. (2) Alizadeh, E.; Sanche, L. Precursors of Solvated Electrons in Radiobiological Physics and Chemistry. Chem. Rev. 2012, 112, 5578− 5602. (3) Turi, L.; Rossky, P. J. Theoretical Studies of Spectroscopy and Dynamics of Hydrated Electrons. Chem. Rev. 2012, 112, 5641−5674. (4) Larsen, R. E.; Glover, W. J.; Schwartz, B. J. Does the Hydrated Electron Occupy a Cavity? Science 2010, 329, 65−69. (5) Kimura, Y.; Alfano, J. C.; Walhout, P. K.; Barbara, P. F. Ultrafast Transient Absorption Spectroscopy of the Solvated Electron in Water. J. Phys. Chem. 1994, 98, 3450−3458. (6) Silva, C.; Walhout, P. K.; Yokoyama, K.; Barbara, P. F. Femtosecond Solvation Dynamics of the Hydrated Electron. Phys. Rev. Lett. 1998, 80, 1086−1089. (7) Yokoyama, K.; Silva, C.; Son, D. H.; Walhout, P. K.; Barbara, P. F. Detailed Investigation of the Femtosecond Pump-Probe Spectroscopy of the Hydrated Electron. J. Phys. Chem. A 1998, 102, 6957− 6966. (8) Borgis, D.; Rossky, P. J.; Turi, L. Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif. J. Phys. Chem. Lett. 2017, 8, 2304−2309. (9) Assel, M.; Laenen, R.; Laubereau, A. Femtosecond Solvation Dynamics of Solvated Electrons in Neat Water. Chem. Phys. Lett. 2000, 317, 13−22. (10) Vilchiz, V. H.; Kloepfer, J. A.; Germaine, A. C.; Lenchenkov, V. A.; Bradforth, S. E. Map for the Relaxation Dynamics of Hot Photoelectrons Injected into Liquid Water via Anion Threshold Photodetachment and above Threshold Solvent Ionization. J. Phys. Chem. A 2001, 105, 1711−1723. (11) Pshenichnikov, M. S.; Baltušska, A.; Wiersma, D. A. HydratedElectron Population Dynamics. Chem. Phys. Lett. 2004, 389, 171−175. (12) Thaller, A.; Laenen, R.; Laubereau, A. Femtosecond Spectroscopy of the Hydrated Electron: Novel Features in the Infrared. Chem. Phys. Lett. 2004, 398, 459−465. (13) Hammer, N. I.; Shin, J. W.; Headrick, J. M.; Diken, E. G.; Roscioli, J. R.; Weddle, G. H.; Johnson, M. A. How Do Small Water Clusters Bind an Excess Electron? Science 2004, 306, 675−679. (14) Coe, J. V.; Lee, G. H.; Eaton, J. G.; Arnold, S. T.; Sarkas, H. W.; Bowen, K. H.; Ludewigt, C.; Haberland, H.; Worsnop, D. R.

internal conversion in about 100 fs. The results indicate that a nonadiabatic model is more appropriate for describing these internal conversion processes. The VBE value of the excited state increases within this ultrashort time owing to ultrafast solvent response. The ground-state solvation times extracted in this study are generally in good agreement with similar studies.31,32

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01750.

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Further information on experimental details, wave packet motion on the excited-state potential surface, and retrieval method (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Toshinori Suzuki: 0000-0002-4603-9168 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by JSPS KAKENHI Grant Number 15H05753. REFERENCES

(1) Hart, E. J.; Boag, J. W. Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions. J. Am. Chem. Soc. 1962, 84, 4090−4095. 4503


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The Journal of Physical Chemistry Letters Photoelectron Spectroscopy of Hydrated Electron Cluster Anions, (H2O)−n=2−69. J. Chem. Phys. 1990, 92, 3980−3982. (15) Turi, L.; Sheu, W. S.; Rossky, P. J. Characterization of Excess Electrons in Water-Cluster Anions by Quantum Simulations. Science 2005, 309, 914−917. (16) Verlet, J. R. R.; Bragg, A. E.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Observation of Large Water-Cluster Anions with Surface-Bound Excess Electrons. Science 2005, 307, 93−96. (17) Farr, E. P.; Zho, C. C.; Challa, J. R.; Schwartz, B. J. Temperature Dependence of the Hydrated Electron’s Excited-State Relaxation. II. Elucidating the Relaxation Mechanism through Ultrafast Transient Absorption and Stimulated Emission Spectroscopy. J. Chem. Phys. 2017, 147, 074504. (18) Suzuki, T. Ultrafast Photoelectron Spectroscopy of Aqueous Solutions. J. Chem. Phys. 2019, in press. (19) Elkins, M. H.; Williams, H. L.; Shreve, A. T.; Neumark, D. M. Relaxation Mechanism of the Hydrated Electron. Science 2013, 342, 1496−1499. (20) Karashima, S.; Yamamoto, Y.; Suzuki, T. Resolving Nonadiabatic Dynamics of Hydrated Electrons Using Ultrafast Photoemission Anisotropy. Phys. Rev. Lett. 2016, 116, 137601. (21) Nishitani, J.; Yamamoto, Y.; West, C. W.; Karashima, S.; Suzuki, T. Binding Energy of Solvated Electrons and Retrieval of True UV Photoelectron Spectra of Liquids. Sci. Adv. 2019, in press. (22) Yamamoto, Y.; Karashima, S.; Adachi, S.; Suzuki, T. Wavelength Dependence of UV Photoemission from Solvated Electrons in Bulk Water, Methanol, and Ethanol. J. Phys. Chem. A 2016, 120, 1153−1159. (23) Luckhaus, D.; Yamamoto, Y.; Suzuki, T.; Signorell, R. Genuine Binding Energy of the Hydrated Electron. Sci. Adv. 2017, 3, No. e1603224. (24) Silva, C.; Walhout, P. K.; Reid, P. J.; Barbara, P. F. Detailed Investigations of the Pump-Probe Spectroscopy of the Equilibrated Solvated Electron in Alcohols. J. Phys. Chem. A 1998, 102, 5701− 5707. (25) Thaller, A.; Laenen, R.; Laubereau, A. The Precursors of the Solvated Electron in Methanol Studied by Femtosecond PumpRepump-Probe Spectroscopy. J. Chem. Phys. 2006, 124, 024515. (26) Elkins, M. H.; Williams, H. L.; Neumark, D. M. Dynamics of Electron Solvation in Methanol: Excited State Relaxation and Generation by Charge-Transfer-to-Solvent. J. Chem. Phys. 2015, 142, 234501. (27) Mosyak, A. A.; Prezhdo, O. V.; Rossky, P. J. Solvation Dynamics of an Excess Electron in Methanol and Water. J. Chem. Phys. 1998, 109, 6390−6395. (28) Elkins, M. H.; Williams, H. L.; Neumark, D. M. Isotope Effect on Hydrated Electron Relaxation Dynamics Studied with TimeResolved Liquid Jet Photoelectron Spectroscopy. J. Chem. Phys. 2016, 144, 184503. (29) Alfano, J. C.; Walhout, P. K.; Kimura, Y.; Barbara, P. F. Ultrafast Transient-Absorption Spectroscopy of the Aqueous Solvated Electron. J. Chem. Phys. 1993, 98, 5996−5998. (30) Walhout, P. K.; Alfano, J. C.; Kimura, Y.; Silva, C.; Reid, P. J.; Barbara, P. F. Direct Pump/Probe Spectroscopy of the near-IR Band of the Solvated Electron in Alcohols. Chem. Phys. Lett. 1995, 232, 135−140. (31) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Femtosecond Solvation Dynamics of Water. Nature 1994, 369, 471− 473. (32) Bagchi, B. Molecular Relaxation in Liquids; Oxford University Press: New York, 2012.

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Ultrafast Internal Conversion and Solvation of Electrons in Water

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