<2μs

Edwin J. Hart and J. W. Boag Absorption Spectrum of the Hydrated Electron in Water and in Aqueous Solutions J. Am. Chem. Soc. 1962, 84, 21, 4090–4095; doi.org/10.1021/ja00880a025

J. W. BOAG & EDWIN J. HART Absorption Spectra in Irradiated Water and Some Solutions: Absorption Spectra of ‘Hydrated’ Electron’ Nature 1963, 197, 45–47; doi.org/10.1038/197045a0

The articles were not devoted to electron solvation dynamics, but as can be seen from the titles, they were about hydrated electron spectra at different conditions. However, the pulse duration of their source of radiation (~2μs) - an electron accelerator  puts a limitation on its formation lifetime <2μs. 
 
It would be false to say, that they did not understand that time, that solvation of excess electron in water must ~10-11 s (R. L. Platzman, “Basic Mechanisms in Radiobiology,” U. S. at. Acad Sci. Pub. h-0.305, 1953, p. 34), what is more than 1.5 order overestimation. The real value of electron solvation lifetime is 300 fs. 

These articles represent facinating research effort of the past. The light source THEY USED is just impressive: Electrical arc between Uranium electrodes in water  

<5ns

J. K. Thomas and R. V. Bensasson Direct Observation of Regions of High Ion and Radical Concentration in the Radiolysis of Water and Ethanol J. Chem. Phys. 1967, 46, 4147; https://doi.org/10.1063/1.1840498

Developed by Thomas and Hunt nanosecond pulse radiolysis setup allowed to investigate fast nanosecond relaxation of a solvated electron, namely reactions occuring in the spurs. However, this also implied that the solvated electron formation rate lies in the time range <5ns. At that time, there was no technique allowing to time-resolve electron solvation.

<23ps

R. K. Wolff, M. J. Bronskill, and J. W. Hunt Picosecond Pulse Radiolysis Studies. I. The Solvated Electron in Aqueous and Alcohol Solutions J. Chem. Phys. 1970, 53, 4201; https://doi.org/10.1063/1.1673922

R. K. Wolff, M. J. Bronskill, and J. W. Hunt Picosecond Pulse Radiolysis Studies. II. Reactions of Electrons with Concentrated Scavengers J. Chem. Phys. 1970, 53, 4211 (1970); https://doi.org/10.1063/1.1673923

J. E. Aldrich, M. J. Bronskill, R. K. Wolff, and J. W. Hunt Picosecond Pulse Radiolysis. III. Reaction Rates and Reduction in Yields of Hydrated Electrons J. Chem. Phys. 1971, 55, 530; doi.org/10.1063/1.1675784

Even though the pulse radiolysis setup that was used had a time-resolution of 23ps in these series of works, it became clear from the experiments that the time of solvation from excess electron should be shorter than 1 ps. However, I will use value of 23ps, since if was technical  limitation and value of <1ps, was determined from the yields of solvated electron.

<5.5ps

Geraldine Kenney‐Wallace and David C. Walker Photoexcitation of Hydrated Electrons Using a Q‐Switched Ruby Laser J. Chem. Phys. 1971, 55, 447; doi.org/10.1021/j100593a007

Super exciting work, first I have seen on re-excitation of the solvated electron. Using different power of Ruby laser, they were capable of using two-state Jortner model for the solvated electron to estimate upper edge value for solvation time, that was <5.5ps.

<4ps

P.M.Rentzepis, R.P.Jones, J. Jortner Relaxation of excess electrons in a polar solvent Chemical Physics Letters 1972, 15, 1972, 480-482; doi.org/10.1016/0009-2614(72)80353-1

P. M. Rentzepis, R. P. Jones, J. Jortner Dynamics of solvation of an excess electron J. Chem. Phys. 1973, 59, 766; doi.org/10.1063/1.1680087

These are ps laser based time-resolved works, where excess electrons have generated by photolysis of the ferrocyanide ion in aqueous solution. They found that the solvation process is completed within the time resolution of their Nd : glass picosecond laser system.

<3ps

W. John Chase and John W. Hunt Solvation time of the electron in polar liquids. Water and alcohols J. Phys. Chem. 1975, 79, 26, 2835–2845; doi.org/10.1021/j100593a007

In this pulse radiolysis work, many questions of solvation dynamics are raized, the important references are widely represented. The discrepancies are discussed. 3ps was an upper edge value for water.

<0.3ps

Jay M. Wiesenfeld and Erich P. Ippen Dynamics of electron solvation in liquid water Chemical Physics Letters 1980, 73, 47-50; doi.org/10.1016/0009-2614(80)85199-2

This is an example of another photolysis work, whereby varying the scavengers' concentration affecting the yield of the solvated electron the upper limit of electron solvation was determined.

They conluded, that the upper limit (<0.3ps) for the electron solvation time in water suggested a mechanism involving preexisting deep traps rather than the mechanisms requiring molecular reorientation. 

G. A. Kenney-Wallace and C. D. Jonah Picosecond spectroscopy and solvation clusters. The dynamics of localizing electrons in polar fluids J. Phys. Chem. 1982, 86, 2572–2586; doi.org/10.1021/j100211a007

It is a critical review of the Solvation Cluster model, based on experimental results in alcohol-alkanes mixtures. This paper also raises the question of deep trap existence, e.g., for water case. They propose an explanation of water ultrafast relaxation not due to trapping of the electron in deep traps but due to proton jumps between water molecules.

A. Migus, Y. Gauduel, J. L. Martin, and A. Antonetti Excess electrons in liquid water: First evidence of a prehydrated state with femtosecond lifetime Phys. Rev. Lett.  1987, 58, 1559; doi.org/10.1103/PhysRevLett.58.1559

Y. Gauduel, S. Pommeret, A. Migus, and A. Antonetti Femtosecond dynamics of geminate pair recombination in pure liquid water J. Phys. Chem. 1989, 93, 10, 3880–3882; doi.org/10.1021/j100347a002

Real observation for the first time

The localization and solvation of excess electrons in water was observed for the first time. Before excess electron becomes solvated, it thermalizes and reaches a localized state absorbing in the infrared within ~100 fs. The presolvated electron has a lifetime 240 fs. The spectral shift from presolvated electron to solvated one was not observed.

Important observation: All authors were researchers; there are no PhD students on the paper. The First three authors were younger than 40; only Dr. Antonetti was >50. 

Soon enough, Kenneth B. Eisenthal confirmed the results of Antonetti (see work below).

Hong Lu, Frederick H. Long, Robert M. Bowman, and Kenneth B. Eisenthal Femtosecond studies of electron-cation geminate recombination in water J. Phys. Chem. 1989, 93, 1, 27–28; doi.org/10.1021/j100338a010

There were plenty of interesting and important publications during 90s, I will list them here:

1990

Frederick H. Long, Hong Lu, Xuelong Shi, Kenneth B.Eisenthal Femtosecond studies of electron photodetachment from an iodide ion in solution: The trapped electron Chemical Physics Letters 1990, 169, 165-171; doi.org/10.1016/0009-2614(90)85182-C

Wet electron term never settled in the literature, presolvated electron is knowdays is widely accepted.

Y. Gauduel, S. Pommeret, A. Migus, N. Yamada, and A. Antonetti Femtosecond spectroscopy of an encounter pair radical (H₃O⁺..e^–)hyd in concentrated aqueous solution J. Am. Chem. Soc. 1990, 112, 2925–2931; doi.org/10.1021/ja00164a013

Y. Gauduel, S. Pommeret, A. Migus, A. Antonetti Some evidence of ultrafast H₂O⁺-water molecule reaction in femtosecond photoionization of pure liquid water: Influence on geminate pair recombination dynamics Chemical Physics 1990, 149, 1-10; doi.org/10.1016/0301-0104(90)80126-I

Frederick H. Long, Hong Lu, and Kenneth B. Eisenthal Femtosecond Studies of the Presolvated Electron: An Excited State of the Solvated Electron? Phys. Rev. Lett. 1990, 64, 1469; doi.org/10.1103/PhysRevLett.64.1469

Resume : These are the first genuine attempts to understand the electron solvation mechanism using time-resolved femtosecond spectroscopy, reconstructing the absorption spectra of presolvated and solvated electrons.

1991

Y. Gauduel, S. Pommeret; A. Migus and A. Antonetti Hydrogen/Deuterium Isotope Effects on Femtosecond Electron Reactivity in Aqueous Media J . Phys. Chem. 1991, 95, 533-539; doi.org/10.1021/j100155a010

S. Pommeret, A. Antonetti and Y. Gauduel Electron Hydration in Pure Liquid Water. Existence of Two Nonequilibrium Configurations in the Near-Infrared Region J . Am. Chem. Soc. 1991, 113, 9105-9111; doi.org/10.1021/ja00024a012

Frederick H.Long, Hong Lu, Xuelong Shi and Kenneth B.Eisenthal Intensity dependent geminate recombination in water Chemical Physics Letters 1991, 185, 47-52; doi.org/10.1016/0009-2614(91)80137-M

Articles propose more complicated schemes of excess electron solvation, including encounter pairs, different excess electrons, geminate recombination…The attention towards the difference between the photo and high-energy particles ionization.

1992-1994

C. Pepin, D. Houde, H. Remita, T. Goulet and J.-P. Jay-Gerin Evidence for resonance-enhanced multiphoton ionization of liquid water using 2 eV laser light: Variation of hydrated electron absorbance with femtosecond pulse intensity Phys. Rev. Lett. 1992, 69, 3389; doi.org/10.1103/PhysRevLett.69.3389

The actual process of water photoionization was raised. 

Y. Gauduel, S. Pommeret and A. Antonetti Early formation of electron-radical pairs in a polar protic liquid: evidence of ultrafast concerted electron-proton transfers J. Phys. Chem. 1993, 97, 134–142; doi.org/10.1021/j100103a024

C. Pepin, T. Goulet, D. Houde and J.-P. Jay-Grin Femtosecond Kinetic Measurements of Excess Electrons in Methanol: Substantiation for a Hybrid Solvation Mechanism J. Phys. Chem. 1994, 98, 7009–7013; doi.org/10.1021/j100079a020

Hybrid mechanism: the continuous shift is proposed for the methanol solvation case.

Y. Kimura, Joseph C. Alfano, P. K. Walhout and Paul F. Barbara Ultrafast Transient Absorption Spectroscopy of the Solvated Electron in Water J. Phys. Chem. 1994, 98, 3450–3458; doi.org/10.1021/j100064a029

Barbara proposes two-state electronic relaxation for excess electron but adds ground-state relaxation dynamics. Well written description of electron solvation. Blue-shift observed.

J.L. McGowen, H.M. Ajo a, J.Z. Zhang and Benjamin J. Schwartz Femtosecond studies of hydrated electron recombination following multiphoton ionization at 390 nm Chemical Physics Letters 1994, 231, 504-510; doi.org/10.1016/0009-2614(94)01281-4

1995 doi.org/10.1016/0009-2614(94)01314-L; doi.org/10.1021/j100018a024
Electron Solvation in Neat Alcohols.

1996

Xuelong Shi, Frederick H. Long, Hong Lu and Kenneth B. Eisenthal Femtosecond Electron Solvation Kinetics in Water J. Phys. Chem. 1996, 100, 11903–11906; doi.org/10.1021/jp961261r

A. Reuther, A. Laubereau and D. N. Nikogosyan Primary Photochemical Processes in Water . Phys. Chem. 1996, 100, 16794–16800; doi.org/10.1021/jp961462v

Models of solvated electron formation in water, geminate recombination, etc.

L. Turi, P. Holpár and E. Keszei Alternative Mechanisms for Solvation Dynamics of Laser-Induced Electrons in Methanol J. Phys. Chem. A 1997, 101, 5469–5476; doi.org/10.1021/jp970174b

C. Pépin, T. Goulet, D. Houde, and J.-P. Jay-Gerin Observation of a Continuous Spectral Shift in the Solvation Kinetics of Electrons in Neat Liquid Deuterated Water J. Phys. Chem. A 1997, 101, 4351–4360; doi.org/10.1021/jp970354l

Three things to mention:
 1) No-isosbestic point
 2) Continuous blue-shift during presolvated electron relaxation, or in other words, the formation of the solvated electron
 3) Constructive critics of earlier works, e.g., Gauduel et al.

M. Assel, R. Laenen and A. Laubereau Dynamics of Excited Solvated Electrons in Aqueous Solution Monitored with Femtosecond-Time and Polarization Resolution J. Phys. Chem. A 1998, 102, 2256–2262; doi.org/10.1021/jp981809p

The negligible anisotropy <0.01 of the probe absorption measured during and after the excitation process indicates that the observed distribution of solvent cavities of hydrated electrons is close to spherical symmetry.
See in more details paper published in 2000: 

M. Assel, R. Laenen and A. Laubereau Femtosecond solvation dynamics of solvated electrons in neat water Chemical Physics Letters 1999, 317, 13-22; doi.org/10.1016/S0009-2614(99)01369-X

Carlos Silva, Peter K. Walhout, Kazushige Yokoyama and Paul F. Barbara Femtosecond Solvation Dynamics of the Hydrated Electron Phys. Rev. Lett. 1998, 80, 1086; doi.org/10.1103/PhysRevLett.80.1086

The first femtosecond measurements on the hydrated electron with sufficient
time resolution to observe inertial solvation dynamics.

Kazushige Yokoyama, Carlos Silva, Dong Hee Son, Peter K. Walhout and Paul F. Barbara Detailed Investigation of the Femtosecond Pump−Probe Spectroscopy of the Hydrated Electron J. Phys. Chem. A 1998, 102, 6957–6966; doi.org/10.1021/jp981809p

The extensive study of the ∼35fs resolved dynamics of the hydrated electron in H2O and D2O at more probe wavelengths and as a function of pump-pulse intensity.

A. Kummrow, M. F. Emde, A. Baltuška, M. S. Pshenichnikov and D. A. Wiersma Wave Packet Dynamics in Ultrafast Spectroscopy of the Hydrated Electron J. Phys. Chem. A 1998, 102, 4172–4176; J. Phys. Chem. A 1998, 102, 4172–4176

Michel F. Emde, Andrius Baltus̆ka, Andreas Kummrow, Maxim S. Pshenichnikov and Douwe A. Wiersma Ultrafast Librational Dynamics of the Hydrated Electron Phys. Rev. Lett. 1998, 80, 4645; doi.org/10.1103/PhysRevLett.80.4645

Andrius Baltuška, Michel F. Emde, Maxim S. Pshenichnikov and Douwe A. Wiersma Early-Time Dynamics of the Photoexcited Hydrated Electron J. Phys. Chem. A 1999, 103, 10065–10082; doi.org/10.1021/jp992482a

Extreme resolution of 5fs using photon echo spectroscopy. Wave packet dynamics.

See as well Ultrafast Phenomena XI Proceedings of the 11th International Conference, Garmisch-Partenkirchen, Germany, July 12–17, 1998 

M. Assel, R. Laenen and A. Laubereau Retrapping and solvation dynamics after femtosecond UV excitation of the solvated electron in water J. Chem. Phys. 1999, 111, 6869; https://doi.org/10.1063/1.479979

4-level scheme. On the nature of so-called wet electron, that we call presolvated.

T. Goulet, C. Pépin, D. Houde and J.-P. Jay-Gerin On the relaxation kinetics following the absorption of light by solvated electrons in polar liquids: roles of the continuous spectral shifts and of the stepwise transition Radiation Physics and Chemistry 1999, 54, 441-448; doi.org/10.1016/S0969-806X(97)00316-2

Comprehensive analysis of 3-pulse and 2-pulse experiments on electron solvation in polar liquids. Debate on the mechanism is still going on.

Andreas Hertwig, Horst Hippler and Andreas-N. Unterreiner Transient spectra, formation, and geminate recombination of solvated electrons in pure water UV-photolysis: an alternative view Phys. Chem. Chem. Phys. 1999, 1, 5633-5642; doi.org/10.1039/A906950J

The alternative view of electron solvation, without considering excited states.

M. Assel, R. Laenen and A. Laubereau Femtosecond solvation dynamics of solvated electrons in neat water Chemical Physics Letters 2000, 317, 13-22; doi.org/10.1016/S0009-2614(99)01369-X

5-level scheme

R. Laenen, T. Roth, and A. Laubereau Novel Precursors of Solvated Electrons in Water: Evidence for a Charge Transfer Process Phys. Rev. Lett. 2000, 85, 50; doi.org/10.1103/PhysRevLett.85.50

6-level scheme

Michael J. Tauber and Richard A. Mathies Fluorescence and Resonance Raman Spectra of the Aqueous Solvated Electron J. Phys. Chem. A 2001, 105, 10952–10960; doi.org/10.1021/jp012184p

Michael J.Tauber and Richard A. Mathies Resonance Raman spectra and vibronic analysis of the aqueous solvated electron Chemical Physics Letters 2002, 354, 518-526; doi.org/10.1016/S0009-2614(02)00203-8

Fluorescence and resonance Raman spectroscopies are used to probe the solvent structure and dynamics of the aqueous solvated electron.

Misao Mizuno and Tahei Tahara Novel Resonance Raman Enhancement of Local Structure around Solvated Electrons in Water J. Phys. Chem. A 2001, 105, 8823–8826; doi.org/10.1021/jp0119173

Patanjali Kambhampati, Dong Hee Son, Tak W. Kee, and Paul F. Barbara Solvation Dynamics of the Hydrated Electron Depends on Its Initial Degree of Electron Delocalization J. Phys. Chem. A 2002, 106, 2374–2378; doi.org/10.1021/jp014291p

Critical paper: relaxation of different precursors of solvated electrons. 2 and 3-pulse experiments.

Andreas Hertwig, Horst Hippler and Andreas-N. Unterreiner Temperature-dependent studies of solvated electrons in liquid water with two and three femtosecond pulse sequences Phys. Chem. Chem. Phys. 2002, 4, 4412-4419; doi.org/10.1039/B204530N

Molly C. Cavanagh, Ignacio B. Martini and Benjamin J. Schwartz Revisiting the pump–probe polarized transient hole-burning of the hydrated electron: Is its absorption spectrum inhomogeneously broadened? Chemical Physics Letters 2004, 396, 359-366; doi.org/10.1016/j.cplett.2004.07.109

The absorption spectrum of solvated electron is homogeneously or inhomogeneously broadened?

M. S. Pshenichnikov, A. Baltuška and D. A. Wiersma  Hydrated-electron population dynamics Chemical Physics Letters. 2004, 389, 171 – 175; doi.org/10.1016/j.cplett.2004.03.107

Comparison of electron solvation in H2O with D2O case.

D. Hern Paik, I-Ren Lee, Ding-Shyue Yang, J. Spencer Baskin, Ahmed H. Zewail Electrons in Finite-Sized Water Cavities: Hydration Dynamics Observed in Real Time Science  2004,
306, 672-675; doi.org/10.1126/science.1102827

The femtosecond dynamics of electrons in water was studied in water clusters varying in size up to 35 molecules. The 2-state model is discussed.  
 

A Thaller, R Laenen, A Laubereau The precursors of the solvated electron in methanol studied by femtosecond pump-repump-probe spectroscopy J. Chem. Phys. 2006, 124, 024515; doi.org/10.1063/1.2155481

Methanol case: detailed pump-repump-probe studies of solvated electron formation. 

Madeline H. Elkins, Holly L. Williams, Alexander T. Shreve, Daniel M. Neumark Relaxation Mechanism of the Hydrated Electron Science  2013, 342, 1496-1499; 10.1126/science.1246291

Experiments support the non-adiabatic model of electron solvation (fast inter-system crossing between p and s state).

T. KONDOH, J. YANG, K. KAN, M. GOHDO, H. SHIBATA and Y. YOSHIDA Femtosecond Pulse Radiolysis Electronics and Communications in Japan 2016, 99, 664–669; 10.1002/ecj.11787

For the first time, formation of the solvated electron was observed using pulse radiolysis technique with 240fs resolution.