Short-Time Electron Transfer Processes in Ionic Aqueous Solution

Aug 15, 1996 - Counterion and H/D isotope substitution effects on early electron transfer steps in aqueous sodium chloride solution (X2O/NaCl = 55, X ...
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J. Phys. Chem. 1996, 100, 13993-14004

13993

Short-Time Electron Transfer Processes in Ionic Aqueous Solution: Counterion and H/D Isotope Effects on Electron-Atom Pairs Relaxation H. Gelabert and Y. Gauduel* Laboratoire d’Optique Applique´ e, CNRS URA 1406, INSERM U451, Ecole Polytechnique-ENS Techniques AVance´ es, 91125 Palaiseau ce´ dex, France ReceiVed: March 6, 1996; In Final Form: May 29, 1996X

Counterion and H/D isotope substitution effects on early electron transfer steps in aqueous sodium chloride solution (X2O/NaCl ) 55, X ) H,D) have been investigated by femtosecond absorption spectroscopy of short-lived electron-chlorine atom pairs in the 18500-8000 cm-1 spectral range. The frequency dependence of the overall signal rise time and early signal decay (geminate recombination process) from 1.77 to 2.29 eV has been analyzed in the framework of inhomogeneous populations of hydrated electron for which the absorption bands overlap in the visible spectral region. By use of a kinetic model, two well-defined electron hydration channels have been discriminated. The spectral signature of a delayed hydration channel (e-hyd′) is fully developed 4.5 ps after the initial energy deposition in the sample against 1.4 ps for the fastest electron hydration channel (e-hyd). Compared to the absorption band of the hydrated electron ground state (e-hyd), which peaks around 1.7 eV, the e-Hyd′ population exhibits a spectral blue shift of about 0.15 eV. This electronic ground state is assigned to a polaron-like state, i.e., an electron localized in the vicinity of sodium ion: {e-‚‚‚Na+}hyd. The time-resolved spectroscopic experiments demonstrate that this electron localization process can be triggered by short-range counterion effects on transient electron-chlorine atom pairs ({Cl:e-}pair:Na+). The existence of selective H/D isotope substitution effects on short-lived electronic configurations of aqueous Cl- (electronCl atom)pairs provides direct evidence that solvent cage dynamics around Cl- can differently interfere with multiple electron transfer channels. Contrary to infrared presolvated electron relaxation (p-like excited hydrated electron), early electron-chlorine atom pair relaxation (Cl:e- f Cl-) or 1D geminate recombination of fully hydrated electron with chlorine atom (Cl + e-hyd f Cl-) for which no significant isotope effect is observed, the electron transfer process involving the deactivation of {Cl:e-}pair:(Na+) population is drastically influenced by a change of intramolecular OH/OD vibrational mode frequency. In aqueous NaCl solution, the electron photodetachment from these transient electronic configurations [{Cl:e-}pair:(Na+)] f {Cl}, {e-‚‚‚Na+}hyd] takes place with a characteristic time of 750 fs in H2O and 980 fs in D2O. The complete buildup of a polaronlike state (second electron hydration channel) is delayed by a factor of 1.6 (4.5 ps in H2O and 7.4 ps in D2O) when the energetic vibrational modes of water molecules (OH vs OD) are decreased by x2. The conjugate effects of very fast responses and slower cooperative motions of water molecules during short-range (electronatom)pairs-Na+ couplings and a second electron hydration channel are discussed.

1. Introduction Charge transfers are involved in numerous processes of chemical or biological interests and represent a complex domain of reaction dynamics when occurring in liquid environments.1-6 Elementary steps of a univalent reduction or oxidation reaction can be assisted or hindered by solvent dynamics, and one of the most fundamental questions on reaction dynamics in solutions concerns the role of solvent structure and time dependence of energy reorganization during an intra- or intermolecular charge transfer. During the last decade, numerous efforts devoted to neutron or X-ray diffraction investigations and ultrashort spectroscopic studies in liquids have been performed in order to enhance our knowledge of solution structures, microscopic description of solute-solvent interactions, rotational dielectric friction, and solvation phenomena.7-16 In solution chemistry, electronic and reaction dynamics may be investigated on the time scale of molecular motions, i.e., on a subpicosecond time scale by using ultrafast spectroscopic techniques and computer molecular dynamics simulations.5,6,17-19 Indeed, the experimental and theoretical investigations of the * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 15, 1996.

S0022-3654(96)00684-3 CCC: $12.00

time dependence of excess electron absorption spectra in water or alcohols push ahead our understanding of ultrafast electronic dynamics in molecular polar liquids.20-29 The direct discrimination of a nonequilibrium state of excess electron absorbing in the infrared has permitted the clarification of the electron solvation process in pure water, aqueous solutions, and organized media.30 These experimental investigations show that dielectric relaxation is not the correct description of solvent energy reorganization in the vicinity of excess electron. Results of quantum molecular dynamic simulations of excess electron in pure water and hole burning experiments largely agree with this pioneering femtosecond absorption spectroscopic research in pure liquid water showing that the subpicosecond relaxation dynamics of an infrared electron (presolvated electron) would be dominated by a nonadiabatic transition of excited p-states of hydrated electron toward a s-like ground state.22,27,28,31 However, IR femtosecond absorption spectroscopy of pure water also shows that electron hydration via a two-state model is not the unique mode of electron-water coupling because concerted electron-hydrated proton transfers represent an alternative electron relaxation process in a protic liquid.32 The presence of a molecular cation (H2O+) immediately after the primary water ionization step © 1996 American Chemical Society

13994 J. Phys. Chem., Vol. 100, No. 33, 1996 makes it necessary to consider the influence of ultrafast ionmolecule reaction and neoformed protonic radicals on complex electronic dynamics phenomena. This point is particularly important when the initial distribution function of prototropic radical populations can influence the escape probability of an excess electron within a hydrogen-bonded liquid. In pure water, the early absorption signal decay, which is observed following the femtosecond UV excitation, has been analyzed in the framework of a finite proton jump between the protonated radicals and the hydrated electron. When water molecules are excited with a femtosecond UV pulse at 310 nm, this 1D geminate recombination process is characterized by a time Td of 1.2 ps at 294 K.33 Experimental picosecond spectroscopic studies performed with various excitation wavelengths (248-390 nm) show an energy dependence on the percentage of electron-prototropic radicals geminate recombination yield and on its dynamics.34-36 The early steps of geminate recombination reaction are governed by the primary energy deposition channels and the subsequent electron photodetachment processes: autoionization process of excited states of water, electron photodetachment via optical charge transfer, or direct photoinduced electron transfer into the conduction band of liquid water. As previously discussed in recent publications, the early geminate recombination reaction influences the shorttime dependence of the fully hydrated electron population20,32 and complicates the discrimination of multiple nonequilibrium electronic states in solution. For instance, after photoexcitation of pure water, protonated radicals can trigger short-lived couplings with localized electrons and favor near-infrared absorption bands that overlap with those of the presolvated electron and the fully hydrated state.37 Counterions effects on electronic dynamics in aqueous solutions make the investigation of electron transfer processes in ionic solutions of considerable interest because transient electronic states of solute (electron donor) can be affected by modifications of structural and dynamical properties of hydrated ions. Previous pulse radiolysis and flash photolysis experiments performed with very concentrated alkaline aqueous solutions have suggested the existence of couplings between the hydrated electron and alkali metal cations.38,39-42 In electrolyte solutions, the mean force potential, including solute-solute and solutesolvent interactions, plays an important role in chemistry of solutions, mainly during SN1 reactions or back electron transfer.43,44 The distribution functions of solvent molecules around the anion and cation influence the profile of the mean force potential of ion pairs such as contact ion pairs (CIP) or solventseparated ion pairs (SSIP).45-47 The presence of ionic entities affects the orientational properties of water molecules, and the two-phase model has been used to describe the specific correlation time of anionic or cationic hydration shells and the remaining bulk water.48-50 Flash photolysis of halide ions in aqueous solutions provides the opportunity for investigation of electron transfer steps via direct electron photodetachment or charge transfer to solvent states (CTTS states) and for discussion of the results in the framework of quantum MD simulations of transient electronic configurations of ionic solute.51-57 Recent UV, visible, and IR femtosecond photophysical investigations on electron detachment dynamics via one- (4 eV) or two-photon (2 × 4 eV) excitation of aqueous chloride ions have permitted the identification of multiple electronic states within the first 2 ps following energy deposition, in particular, high- and low-excited CTTS states, infrared prehydrated electron (excited hydrated electron), and hydrated electron ground states (eq 1).56,57

Gelabert and Gauduel t < 500 fs

(Cl-)hyd + 1.2hν 98 t < 1500 fs

{CTTS**, CTTS*, (Cl), (e-)hyd*} 98 (e-)hyd + (Cl)hyd (1) Additional near-infrared spectroscopic investigations of twophoton excitation of aqueous ionic solute (Cl-) have permitted us to clearly identify electron-chlorine atom pairs.57 In aqueous 1 M sodium chloride solution, the formation of electron-atom pairs occurs with a rate of 3.7 × 1012 s-1 (eq 2). T3 ) 270 fs

(Cl-)hyd‚‚‚(Na+)hyd + 2hν 98 (Cl:e-)pairs‚‚‚(Na+)hyd (2) Our femtosecond near-infrared spectroscopic investigations underline the existence of inhomogeneous populations of electron-atom pairs that exhibit biexponential decay in water. The fast component (T4 ) 330 fs) has been assigned to an ultrafast electron-atom reaction within the solvation area of chlorine atoms (eq 3). This relaxation step occurs on a time scale that is short compared to the residence time of water molecules in the vicinity of the chloride ion (τSion ) 4.5 ps). On this time scale, the water molecules of the first coordination shell cannot exchange significantly with molecules of the bulk. T4 ) 330 fs

T4′ ) 750 fs

(Cl-)hyd, (Na+)hyd 79 (Cl:e-)hyd‚‚‚(Na+)hyd 98 electron detachment? (3) Experimentally, a slower deactivation rate of electronchlorine atom pairs (T4′ ) 750 fs) has also been identified by femtosecond near-infrared spectroscopy experiments of aqueous sodium chloride solution (eq 3). This process is tentatively assigned to a specific electron transfer channel controlled by aqueous positive ions (Na+).57 Up to now, the influence of a positive counterion on ultrafast behavior of transient electronatom pairs in aqueous sodium chloride solution has not been investigated in detail. Some aspects of femtosecond investigations of high- and lowexcited CTTS states and electron-atom pairs in aqueous halide solutions qualitatively agree with predictions of the quantum MD simulation of multiple electronic dynamics in aqueous Ior Cl- systems.58-60 The existence of branching between shortlived electron photodetachment pathways would be dependent on inhomogeneous solvent cage effects (solvent electronic polarization response, for instance) around neoformed excited electronic configurations of the chloride ion. Considering the treatment of solvent electronic polarization around chloride ions, recent quantum molecular dynamics simulations of electron photodetachment from the 3p to 4s transition of Cl- predict the existence of metastable electron-atom pairs whose significant fraction (∼20%) would relax toward the Cl- ground state via a nonadiabatic geminate recombination on the picosecond regime. Energetically, these electron-atom pairs can be understood as semi-ionized or metastable states for which an excited electron fluctuates in the vicinity of the hydrated chlorine atom core. In photoexcited aqueous halide solutions, a branching from electron-atom pairs would lead to the hydration of the electron through an adiabatic electron detachment process.58,60 The aim of the present paper is to extend our understanding of alternative electron transfer channels in aqueous sodium chloride solution by considering the role of short-range counterion effects on electron-chlorine atom pairs dynamics. We have focused our attention on the discrimination of inhomoge-

Electron Transfer Processes neous hydrated electron populations in the visible spectral range. Aqueous halide solution offers the opportunity to push ahead the investigation of electron photodetachment processes from transient electronic configurations of the ionic solute (Cl-), taking into account the effect of short-range ion-ion interactions such as contact ion pairs (CIP) or solvent-separated ion pairs (SSIP). In this way, considering short-time electron transfer steps in sodium chloride solution, we have investigated the influence of the positive counterion (Na+) atmosphere on the dynamics of an electron detachment process from transient electron-chlorine atom pairs. Isotopic H/D substitution studies have been performed in order to determine whether the time scales of the early steps of electron photodetachment processes are dependent on the microscopic density fluctuation of solvent molecules. 2. Experimental Section The femtosecond spectroscopy of aqueous sodium chloride solutions has been performed in the spectral range 3.44-0.99 eV (360-1250 nm) by using a pump-probe configuration. The experimental setup has been described in detail previously.57 Nonamplified femtosecond pulses, centered at 620 nm and as short as 70-80 fs, are generated by a passively mode-locked CW dye ring laser (CPM). The pulses are amplified at a 20 Hz repetition rate in a five-stage dye amplifier pumped by a Q-switched frequency-doubled Nd:YAG laser. The compression of amplified beams through a four-prism arrangment allows output pulses of energy above 1 mJ and typically of 80-90 fs duration. The photoexcitation of aqueous chloride ions is initiated by pumping with amplified femtosecond ultraviolet pulses of typically 90-100 fs duration. The pump beam (310 nm, E ) 4 eV) is generated by frequency doubling of amplified pulses in a 1 mm KDP crystal and focused on the quartz Suprasil cell, which contains the aqueous salt solution. For the timeresolved UV, visible, and IR spectroscopy, the energy of the excitation pulse (310 nm, 4 eV) has been adjusted around 7 ( 0.5 µJ. The test beam is selected from a continuum generation and is split in to two parts: one probes the excited region of the sample while the other is focused on the sample and used as a reference. Time-resolved experiments are performed at 294 K on samples with a 2 mm optical path length, continuously moved to avoid local heating. The transient-induced absorption signals obtained in the pump-probe configuration are transferred to computers and stored on a hard disk for further processing. The time dependence of an induced absorption is investigated either with silicon photodiodes (visible spectral range) or germanium photodiodes for the near-infrared. A key point for short-time resolution in molecular liquids is the exact knowledge for each probe wavelength of both (i) the pulse shape, (ii) the position of the zero time delay (i.e., the position of the delay line for which the pump and probe pulses coincide exactly), and (iii) the instrument response function. These optical aspects take into account the geometrical parameters of the experimental device and are of particular importance for careful spectroscopic investigations within a narrow temporal window (t , 4 ps). Regarding the specific case of the present study, the different procedures we use to analyze femtosecond UV, visible, or IR spectroscopic data require direct measurements of the instrumental response over a wide spectral range (360-1250 nm) and determination of the position of the zero time delay with an accuracy of 20 fs over a temporal window of 2 ps. At a given test wavelength, the instrumental response is determined on a nonpolar liquid (n-heptane) for which we investigate the

J. Phys. Chem., Vol. 100, No. 33, 1996 13995 rise time of the molecular cation (heptane+) and short-lived excited states of hydrocarbon molecules. The femtosecondinduced absorption obtained following the excitation of pure n-heptane by ultraviolet pulse is the fastest we ever found in our pump-probe configuration, taking into account the convolution functions of biexponential pump and probe beams and a time-broadening factor due to the group velocity dispersion within the sample. For each test wavelength, we have computed this broadening to be 40-50 fs in the green spectral region and 90-100 fs in the near-infrared. The experimental determination of the zero time delay is performed from femtosecond spectroscopic signals obtained with a hydrocarbon sample. Subsequent spectroscopic investigations on polar solutions require determination of the position of the zero time delay, taking into account the refractive index effect between polar and nonpolar liquids at different test wavelengths. This refractive index effect being dependent on geometrical parameters, each measurement is performed on polar and nonpolar samples with similar experimental conditions for the laser beams. In the pump-probe configuration, the time dependence of the induced signal in an aqueous sample is analyzed by using a normalized correlation function Corrω(t) between the excitation beam IEx and the test beam IT separated by the time delay τ. This correlation function takes into account the pump-probe pulse duration and the overall optical broadening factor due to group velocity dispersion in liquid samples. For a given test wavelength, the correlation function is defined as the overall instrument response. This response is directly measured on nonpolar samples for which an instantaneous signal rise time has been identified.20 n Corrω(t) ) ∫-∞ IT(t + τ)IEx (τ) dτ +∞

(4)

In ionic aqueous solutions, the time-resolved analysis of absorption spectroscopy data S(t) triggered by a biphotonic interaction process is performed by the convolution between pump and test beams and photophysical phenomena A(t).

S(t) ) A(t) × Corrω(t)

(5)

The overall induced absorption signal obtained at a specific test frequency (ω) is expressed as the linear sum of absorption transients of electronic states. These populations of electronic states (Ci) are characterized by a cross section ω: i)n

i)n

i)1

i)1

∆Aω(τ) ) ∑Aiω(τ) ) ∑Ciiωl

(6)

The computational analysis of femtosecond and picosecond dynamics of early photophysical and photochemical steps has been performed on a Sparc-station Sun C1+ (Sun OS 4-0-3). The kinetic model is presented in the next section. Aqueous salt solutions were produced by dissolving sodium chloride (purity ) 99.999%) from Alfa Produckte in light water at a final concentration of 1 M. Water is twice distilled in a quartz distillator with KMnO4. Its resistivity is greater than 19 MΩ at 294 K and the pH equals 6.5. Molecular oxygen was removed from the sample by tonometry using pure nitrogen gas. Deuterated samples are obtained by using heavy water (isotopic purity > 99.95%) produced by Centre d’Etudes Nucle´aires of Saclay. 3. Results and Discussion 1. Electron Photodetachment Process from Short-Lived Electron-Atom Pairs. A comparative set of induced absorp-

13996 J. Phys. Chem., Vol. 100, No. 33, 1996

Figure 1. Time dependence of visible induced absorption signals in pure water and ionic aqueous solution (H2O/NaCl ) 55) following the photoexcitation by femtosecond ultraviolet pulses (excitation ) 310 nm) at 294 K. Upper part shows the apparent signal rise time remains independent of the probe wavelength (2.29-1.72 eV) in pure water (Amax1.72 eV ≈ 0.015). Early signal decay observed at short time (t ≈ 3 ps) is due to a geminate recombination between hydrated electron ground state and protonated radicals (H3O+, OH).32 Lower part represents the test wavelength (1.77, 2.29 eV) dependence of the overall signal rise time in ionic aqueous solution ((Amax1.77 eV ≈ 0.070). At 2.29 eV, a contribution (δ ≈ 0.2) due to low excited CTTS state (CTTS*) is considered. This state occurs within the pulse and exhibits a monoexponential decay with a characteristic time of 190 fs.57 The resting signals discriminated between experimental data and best computed fits at 1.77 and 2.29 eV are shown in the insert.

tion signal rise times obtained following femtosecond UV excitation of solvent molecules (H2O) or aqueous ionic solute (Cl-) is reported in Figure 1. In pure liquid water, the overall signal rise time in the visible is independent of the probe wavelengths (1.72-2.29 eV). As previously discussed, in pure water this signal is due to excess electron hydration dynamics triggered by biphotonic ionization of water molecules.20,61 The early signal decay we observe within a short temporal window (t < 4 ps) is assigned to a geminate recombination between the hydrated electron ground state and the neoformed radicals (H3O+, OH). In aqueous sodium chloride solution ([H2O]/ [NaCl] ) 55), femtosecond visible spectroscopic data exhibit a frequency dependence of the overall signal rise time along the high-energy tail (1.77-2.29 eV) of the hydrated electron ground state (Figure 1). The additional set of normalized curves obtained at a longer time (t ≈ 30 ps) exhibits a frequency dependence on the early signal decay (Figure 2). This signal decay is assigned to a recombination reaction between the fully hydrated electron and the aqueous chlorine atom. Qualitatively, the frequency dependence between 1.77 and 2.29 eV is drastically marked, both on the overall visible signal rise time and on the apparent fraction of the early signal decay at 30 ps: (S30ps/Smax)1.77 eV ) 0.81, (S30ps/Smax)2.29 eV ) 0.88.

Gelabert and Gauduel

Figure 2. Early behavior of induced absorption signal at 2.29, 1.88, and 1.77 eV following the femtosecond UV excitation of an aqueous sodium chloride solution (H2O/NaCl ) 55). Signal decay is expressed as a 1D geminate recombination reaction between hydrated electron ground state and the chlorine atom (γ erf(Tr/t)1/2). Long-lived signal is due to the contributions of more stable hydrated electrons ground state populations (e-hyd, {e-‚‚‚Na+}hyd) whose spectral bands overlap in the visible. Lower part shows computed relative spectral contributions of two well-defined populations of hydrated electron ground states (e-hyd, {e-‚‚‚Na+}hyd) following the femtosecond electron photodetachment in aqueous chloride ions (H2O/NaCl ) 55). The frequency dependence on spectral contributions underlines the signatures of two related populations whose maximum of {e-‚‚‚Na+}hyd band is blue shifted ∆E ≈ 0.15 eV by comparison with that of the e-hyd band.

At this stage of our analysis, we should address three questions. (i) Does the visible broad band characterize a homogeneous hydrated electron ground state population? (ii) Does a non-negligible fraction of near-infrared electron-atom pairs (eq 3) contribute to a specific electron photodetachment channel? (iii) Does the presence of positive counterions (Na+) influence the electronic dynamics of transient states? 1.a. Kinetic Model of Time-ResolVed Electronic States. The analysis of spectroscopic data in electrolyte solution requires the development of a kinetic model describing the time dependence of multiple photophysical processes. In the present section, we focus mainly on femtosecond visible spectroscopy of relaxed electronic states whose short-lived precursors, triggered by a two-photon process, have been previously identified by femtosecond infrared spectroscopy. Some important aspects of this model devoted to femtosecond infrared spectroscopy of aqueous sodium chloride solution have been described in detail in a recent paper.57 From normalized populations of excited hydrated Cl- ions, a first electron photodetachment channel leads to a hydrated electron population whose kinetics of [(e-)hyd(t)] takes into account the early geminate recombination reaction between (e-)hyd and the chlorine atom (eq 7).

Electron Transfer Processes

J. Phys. Chem., Vol. 100, No. 33, 1996 13997

T1 ) 130 fs

T2 ) 300 fs

(Cl-)hyd + 2hν 98 {(Cl), (e-)hyd*} 98 (e-)hyd + (Cl)hyd (γ), Tr f (Cl )hyd (7)

In this first photophysical channel, the electron transfer involves a two-state phenomenon via the relaxation of prehydrated electron (e-)prehyd, i.e., p-like excited hydrated electron absorbing in the infrared (eq 7). By solving the differential equations for electronic kinetics in bulk water, we obtain the time dependence of (e-)prehyd and (e-)hyd populations:

P(e-)prehyd(t) ) P0[T2/(T2 - T1)][exp(-t/T2) - exp(-t/T1)] (8) N(e-)hyd(t) ) P0[1 - (1/(T2 - T1))][(T2 exp(-t/T2) - T1 exp(-t/T1))] (9) Considering the high polarizability of the aqueous chlorine atom, a geminate recombination reaction is analyzed in the picosecond regime. This process is understood as a finite oriented diffusive phenomenon within transient distributions of Cl and hydrated electron ground states. The time dependence of this fast reaction characterized by an activated process between two reactants (e-hyd, Clhyd) is described in terms of a 1D diffusion process. The determination of the kinetics of this recombination step requires the solution of a differential equation describing a 1D diffusive process: ∂C/∂t(x,t) ) D(∂2C/∂t2). This kinetic approach accounts for the effect of dispersion on the energetics of electron transfer in solution. The analytical solution corresponds to a nonexponential decay (eq 10) and is characterized by the asymptotic time dependence (1/xt), in agreement with the behavior recombination observed at longer time.33 By computation of fits to the experimental data, this equation permits us to define two adjustable parameters: the recombination fraction γ and the 1D diffusion rate (kr ) 1/Tr).

{

+t dNhyd(t)

Pe-hyd(t) ) ∫-∞

dt

( )}

1 - γ + γ erf

Tr

(t - t′)

1/2

dt′

(10)

In the present spectroscopic work, we focus on a second photophysical channel whose transient electronic states (electronchlorine atom pairs) have been investigated by near-infrared spectroscopy.57,62 The deactivation rate of hydrated electronchlorine atom pairs {(Cl:e-)pairs} far from the counterion corresponds to an ultrafast geminate recombination, leading to the relaxed byproduct (Cl-) which does not absorb in the visible (eq 11). T3

T4

(Cl-)hyd‚‚‚(Na+)hyd + hν 98 {(Cl:e-)pair‚‚‚(Na+)hyd} 98 (Cl-)hyd‚‚‚(Na+)hyd (11) The spectral contribution of the transient state {(Cl:e-)pair‚‚‚ (Na+)hyd} to the total absorption signal is given by the following expression:

P(Cl:e-)pair‚‚‚(Na+)hyd(t) ) P′0[T4/(T4 - T3)][exp(-t/T4) - exp(-t/T3)] (12) On the other hand, we have investigated the hypothesis that the relaxation of transient electron-Cl atom pairs leads to a second electron hydration channel in the vicinity of the sodium ion: {Cl:e-}pairs:{Na+} f (Cl)hyd, {e-‚‚‚Na+}hyd (eq 13). Equation 13 represents the specific electron transfer mechanism

for which an electron photodetachment from electron-atom pairs would be driven by electric field effects of Na+, leading to a polaron-like state ({e-‚‚‚Na+}hyd). T3

T4′

(Cl-)hyd:(Na+)hyd + hν 98 {(Cl:e-)pair:(Na+)hyd} 98 (Cl)hyd, {e-‚‚‚Na+}hyd (13) In this expression, {e-‚‚‚Na+}hyd denotes the (e-)hyd′ population. The short-range interaction of (Na+) with electron-atom pairs would favor an electron detachment in sodium chloride solution. The slow deactivation rate of electron-atom pairs we have measured by near-infrared spectroscopy (k4′ ) 1/T4′ ) 1.33 × 1012 s-1) would correspond to a lower limit of a second electron hydration channel. In the framework of a twostate model, the time evolution of these transient and fully relaxed electronic states is expressed by the following equations:

P(Cl:e-)pair:(Na+)hyd(t) ) P′′0[T4′/(T4′ - T3)][exp(-t/T4′) - exp(-t/T3)] (14) P{e-‚‚‚Na+}hyd(t) ) P′′0[1 - (1/(T4′ - T3))][(T4′ exp(-t/T4′) - T3 exp(-t/T3)] (15) The time-resolved spectroscopic curves obtained in the nearinfrared and in the visible are analyzed by using a linear combination of transient and relaxed electronic contributions. For each test wavelength, the dynamical model is convoluted with the instrumental response function: ω ω ∆Aω(t) ) ∆Aωehyd(t) + ∆A ehyd′(t) + ∆A eprehyd(t) +

∆Aω(e-:Cl)pairs(t) ) RPe-hyd(t) + βPe-hyd′(t) + δCTTS*(t) + aPe-prehyd(t) + bP(Cl:e-)pairs (16) with R + β + δ + a + b ) 1 and b ) b′ + b′′. In expression 16, the adjusted fit parameters R and β correspond to the contributions of the two prevailing equilibrated electronic states (e-hyd, {e-‚‚‚Na+}hyd), which account for 90% of the signal at 1.88 eV. In the red spectral region, the other minor contributions δ, a, b′, b′′ are due to nonequilibrium UV and infrared electronic states: low-excited CTTS state (CTTS*), e-prehyd, (Cl:e-)pairs‚‚‚Na+, and (Cl:e-)pair:Na+, respectively. 1.b. Identification of a Relaxed Polaron-Like State: {e-‚‚‚Na+}hyd. The results of femtosecond spectroscopic experiments performed in aqueous sodium chloride solution are presented in the Figures 1 and 2 and Table 1. The 1.77 eV signal rise time is dominated by the buildup of a hydrated electron ground state population (e-hyd). Its short-lived infrared precursor (e-prehyd) appearing within 130 fs exhibits an ultrafast relaxation dynamics (T2 ) 300 ( 20 fs). The electronic ground state (e-hyd) is fully developed 1.4 ps after the initial electron photodetachment process. At longer times (Figure 2, top), the signal decay is characterized by the fraction γ of hydrated electron population that recombines with the chlorine atom according to 1D diffusive kinetics (Tr) of 1.3 ( 0.1 ps. In aqueous sodium chloride solution ([H2O]/[NaCl] ) 55) at 294 K, γ equals 0.38 ( 0.02. The two parameters (Tr, γ) are independent of the test wavelength in the visible spectral range (Table 1) and characterize a homogeneous e-hyd population whose spectral band is reported in Figure 2, bottom. This population of e-hyd is never at equilibrium because the fast geminate recombination reaction competes with the hydration

13998 J. Phys. Chem., Vol. 100, No. 33, 1996

Gelabert and Gauduel

TABLE 1: Dynamics of Electron Photodetachment and Subsequent Relaxation Steps in Ionic Aqueous Solution ([H2O]/[NaCl] ) 55) at 294 Ka parameters

2.29 eV

1.88 eV

1.77 eV

1.41 eV 0.99 eV

T1 T2 γ Tr R

130 ( 10 fs 300 ( 20 fs 0.38 ( 0.02 1.3 ps 0.55 ( 0.02

e-hyd Population 130 ( 10 fs 130 ( 10 fs 300 ( 20 fs 300 ( 20 fs 0.38 ( 0.02 0.38 ( 0.02 1.3 ps 1.3 ps 0.65 ( 0.02 0.71 ( 0.05

130 ( 10 fs 300 ( 20 fs 0.38 1.3 ps

T3 T4 β

{e-‚‚‚Na+}hyd Population 250 ( 20 fs 250 ( 20 fs 250 ( 20 fs 750 ( 30 fs 750 ( 30 fs 750 ( 30 fs 0.23 ( 0.01 0.25 ( 0.01 0.12 ( 0.01

250 ( 20 fs 750 ( 30 fs

β/R

0.42

Ratio 0.38

0.17

a The characteristic parameters of two localized electron ground states (e-hyd and {e-‚‚‚Na+}hyd) are extracted from experimental curves reported in Figures 1 and 2.

process (eq 7). As previously shown,57 7% of this ground state population does not contribute to the direct measurement of a transient absorption signal within the first 2 ps after the energy deposition. Figures 1 (bottom) and 2 (top) emphasize that the apparent signal rise time and early decay in the visible are both dependent on the probe wavelength. In our photophysical model, these spectroscopic effects are interpreted in terms of the existence of a second electronic relaxation channel ({e-‚‚‚Na+}hyd) whose transient step involves nonequilibrium electronic configuration of the ionic solute: {Cl:e-}pairs:Na+. The time dependence of these inhomogenous electron-atom pairs has been investigated by infrared femtosecond spectroscopy, and the experimental data are analyzed by using the eqs 14 and 15. The contribution of a second relaxed electronic state (e-hyd′) in the visible is given by the eq 15. The kinetic model considers that the rise time of the {e-‚‚‚Na+}hyd ground state is driven by the time-dependence of nIR transient electron-atom pairs, {Cl:e-:hyd‚‚‚(Na+)hyd pairs, with the appearance time (T3) of 250 ( 20 fs and the deactivation time scale (T4′) of 750 ( 30 fs. The spectral contribution (β) of the second fully relaxed electronic state {e-‚‚‚Na+}hyd represents less than 10% of the total induced absorption signal at 1.77 eV. Figure 2, bottom, gives the relative spectral contributions of two prevailing equilibrated hydrated electrons in the visible. The relative spectral contribution of each electronic ground state at different test wavelengths is obtained from the adjusted parameters R and β we obtained in expression 16. Owing to the fact that the time dependence of (e-)hyd and {e-‚‚‚Na+}hyd populations remains independent of the probe wavelength, the adjusted parameters R and β in the red and green spectral regions would change in the same way as the cross section of the hydrated electron ground state. The first broad band peaking around 1.7 eV is assigned to the well-known hydrated electron ground state [(e-)hyd] in bulk water.63 This band overlaps another one that we attribute to a second hydrated electron ground state population {e-‚‚‚Na+}hyd. A key result of the present study performed in aqueous 1 M NaCl solution concerns the relative spectral contribution of this stable electronic state (Figure 2, bottom). The {e-‚‚‚Na+}hyd band exhibits a slight blue shift of about 0.15 eV compared to the (e-)hyd one. Considering the results of previous studies in ionic solutions,64 the existence of a blue shifted {e-‚‚‚Na+}hyd band is understood as a change in the electron hydration energy induced by the presence of positive counterions (Na+). According to polaron theory in electrolyte solutions,41 the total potential energy of

an electron is determined by polar solvent and ionic interactions. For the initial electron-ion separation distance less than the Onsager radius, the potential energy of an electron in the vicinity of an alkali metal ion is given by eq 17:

Ze2 r

U ) -φ

(17)

with φ being the fraction of time spent in the vicinity of the cation. The polaron theory predicts that the time fraction spent by the excess electron in the vicinity of the chemically inert cation influences the spectral shift value of the hydrated electron. This effect can be estimated by the energy of the light absorption for a polaron trapped by the cation:41 np - E1s} ) hνtr ) {E(1s)

{

(

)

(

)}

1 µe4 φZ 4 2 φ2Z2 1 1 2 (1 φ) C + + 1C 2 2 3   3 n  2hh  opt opt (18)

(

)

In this expression, C ) (1/opt - 1/st) and n is the refraction index. On the basis of our experimental results, we conclude that an electron photodetachment from transient electron-atom pairs can be driven by the electric field effect of positive counterions (Na+) and that solvent cage effects assist electron hydration in the vicinity of the sodium ion (eq 13). This charge transfer process is fully developed within 4.2 ps (Figure 2, bottom). The influence of different ionic concentrations on the spectral shift of hydrated electrons has been investigated and will be described in a forthcoming paper. In 1 M NaCl solution, the partial overlap observed between two broad bands of hydrated electrons (Figure 2, bottom) leads to the frequency dependence (i) of the overall signal rise time in the visible (Figure 1, bottom) and (ii) of the apparent signal decay within the first 30 ps after the energy deposition in the sample (Figure 2, top). Within the framework of our computed fits at 2.29, 1.88, and 1.77 eV (Figure 2, top), the frequency dependence of the overall signal decay is explained by the combined influence of these two hydrated electron ground state populations only one of which (e-hyd) exhibits a partial recombination process with the neighboring chlorine atom (eqs 7 and 10). The second equilibrated hydrated electron population ({e-‚‚‚Na+}hyd), being more stabilized by the presence of Na+, doesn’t contribute to the early signal decay in the visible. The present results demonstrate that in a complex situation such as electronic dynamics in ionic solutions, the percentage of the apparent signal decay at a given test wavelength is not a direct parameter characterizing an early geminate recombination process because inhomogenous populations of the hydrated electron ground states contribute to the total signal. The inhomogeneous electron hydration channels in aqueous sodium chloride solution are sensitive to the early photophysical events, which take place in different ion-solvent configurations and initial ion pairs (CIP and SSIP states). These conclusions are strengthened by the results obtained at very short times (t < 4 ps). If we examine the relative contributions of the two equilibrated hydrated electrons at 1.77, 1.88, and 2.29 eV (Table 1), it is clear that the second electron relaxation channel contributes to the delay of the overall signal rise time as shown in Figure 1, bottom. Regarding the short-time experimental data in the visible, it is clear that the initial distributions of excess electron through localization and hydration steps are governed by multiple energy-scattering processes in the vicinity of the chloride or the sodium ion. Moreover, field effects due to chemically

Electron Transfer Processes inactive counterions (Na+) can influence the electron transfer mechanism in ionic solutions. Our results suggest that the microscopic structure of the reaction area would modify the electronic dynamics within the solvation shells of chlorine atom or cation (Na+). Specific interconversion processes of the contact and the solvent-separated ion pairs (CIP T SSIP) have been observed in ionic solutions.65-68 In sodium chloride solution, the ionic motion within the transient state of ion pairs occurs with a velocity of 4 Å ps-1.48 Several computer simulations including MD and Monte Carlo methods have predicted that structural characteristics for the chloride ionwater complex or sodium-water complex can exist over 5-6 Å.69-73 These short-range solute-water molecules interactions influence the transient potential of mean force profile of the solute-solute interactions (ion pairs).47,49,74 Considering the microscopic aspects of electron solvation in an ionic environment, the electron localization dynamics in the atmosphere of the positive counterion (Na+) is faster than the NMR relaxation rate of strongly bound water around this ion or the formation time of the Na+ atmosphere around the hydrated electron ground state. The time of the relaxation of the Na+ atmosphere in the vicinity of the hydrated electron can be estimated by the equation of Coyle et al.:75 Ti ) 3.55 × 10-9 ∑Zi/∑µΛi. In this expression, Λ is the equivalent conductance of each type of ion in solution. By use of the limiting equivalent conductance of the hydrated electron (170 cm2 Ω-1 equiv-1)76 and of Na+ (45 cm2 Ω-1 equiv-1),77 the relaxation time of the Na+ atmosphere of the hydrated electron is estimated to be about 33 ps for 1 M NaCl. We cannot entirely exclude that such ionic dynamics partially contributes to a rearrangement of medium energy during the delayed electron hydration channel ({e-‚‚‚Na+}hyd). Let us focus on the short-time data identified at 2.29 eV (540 nm). In this spectral region, the β/R ratio indicates an important contribution of {e-‚‚‚Na+}hyd in the induced absorption signal rise time. By comparison of the experimental traces with the best computed fits obtained with our kinetic model, a significant residual signal is observed (Figure 1, bottom). This resting signal exhibits an oscillatory shape within the temporal window (-1-+1 ps). We tentatively attribute this residual component to nonexponential dynamical responses of the solvent molecules during the electron detachment and relaxation of {e-‚‚‚Na+}hyd: ultrafast inertial solvent response (negative part of the resting signal) and collective motions of the water shell around the alkali metal (positive part of the resting signal) and collective motions of the water shell around the alkali metal (positive part of the resting signal). Regarding this positive residual signal, we cannot exclude that diffusive reorientations and translation of water molecules in the Na+ neighborhood favor a spectral shift and/or a band narrowing of the {e-‚‚‚Na+}hyd population. Within this hypothesis, the second hydration channel leading to the {e-‚‚‚Na+}hyd ground state will not be equivalent to the nonadiabatic electron transfer as for the (e-)* hyd f (e )hyd radiationless transition. 2. H/D Isotope Substitution Effects on Electron-Chlorine Atom Pairs Dynamics. In order to determine whether the second hydration channel is partially governed by the dynamical solvent cage effects, we have investigated the effect of the H/D isotope substitution on the early steps of the electron photodetachment from electron-atom pairs. From classical and quantum simulations, heavy water is considered to be more structured than normal water and exhibits longer longitudinal relaxation time than H2O.78 Moreover, the frequency of the vibrational modes are 21/2 larger in the normal than in heavy water.79,80 Significant experimental results obtained by fem-

J. Phys. Chem., Vol. 100, No. 33, 1996 13999

Figure 3. Upper part: H/D isotope effect on overall signal rise time at 1.77 and 2.29 eV following femtosecond UV excitation of ionic aqueous solution (X2O/NaCl ) 55). Lower part: time dependence of resting signal between experimental curves and computed best fits obtained with the kinetic model. For explanation see the text.

TABLE 2: H/D Isotope Substitution Effects on Dynamics of Two Electron Hydration Channels and on Electron-Atom Pairs Deactivation Processes in [X2O]/[NaCl] ) 55, X ) H, D parameters (P)

H2O

D2O

P(D)/P(H)

T1 ) 1/k1 T2 ) 1/k2 a0.99 eV

{e-hyd}* population 130 ( 10 fs 130 ( 10 fs 300 ( 20 fs 300 ( 20 fs 0.43 0.46

∼1 ∼1 1.07

T3 ) 1/k3 T4 ) 1/k4 T4′ ) 1/k4′ b1.41 eV ) b′ + b′′

{Cl:e-}pairs 250 ( 20 fs 330 ( 20 fs 750 ( 30 fs 0.49

250 ( 20 fs 330 ( 20 fs 980 ( 30 fs 0.58

∼1 ∼1 1.31 1.18

Tr R1.77 eV R1.88 eV

[e-hyd] Population 1.3 ( 0.1 ps 1.4 ( 0.1 ps 0.71 0.76 0.65 0.68

1.08 1.07 1.05

β1.77 eV β1.88 eV

{e-‚‚‚Na+}hyd Population 0.12 0.14 0.25 0.30

1.17 1.20

Ratio β1.77 eV/R1.77 eV β1.88 eV/R1.88 eV T′4/T4

0.17 0.385 2.27

0.185 0.44 2.97

1.09 1.14 1.31

tosecond visible and infrared spectroscopy of X2O/NaCl ) 55 (X ) H, D) are presented in Figures 3-6 and in Table 2. Femtosecond investigations in the visible demonstrate a complex test wavelength dependence of the rise time and the behavior of induced absorption signals (Figures 3 and 4). As in normal water solution, the frequency dependence of the overall signal rise time and behavior from 1.77 to 2.29 eV have been analyzed within the framework of two spectral bands overlapping in the visible and assigned to e-hyd and {e-‚‚‚Na+}hyd

14000 J. Phys. Chem., Vol. 100, No. 33, 1996

Gelabert and Gauduel

Figure 5. Compared H/D isotope effects on the apparent percentage of absorption signal decay at 1.88 eV and 1.77 eV. This picosecond signal decay is due to a 1D recombination process of hydrated electron population (e-hyd) with the chlorine atom (eqs 7 and 10).

Figure 4. H/D isotope substitution effect on the signal rise time and early induced absorption behavior at 1.41 eV (880 nm), 1.21 eV (1000 nm), and 0.99 eV (1250 nm) following femtosecond UV excitation of aqueous sodium chloride solution (X2O/NaCl ) 55). For these different test wavelengths, the signal decay is nonexponential and includes multiple electronic dynamics processes. Smooth lines represent the best computed fits of signal decay by using the kinetic expression A(t) ) (1 - (a + b′ + b′′)) + a exp(-t/T2) + b′ exp(-t/T4) + b′′ exp(-t/T4′). A significant H/D isotope effect is observed on the relaxation dynamics T4′ of electron-chlorine atom pairs (see also Table 2).

populations. At 1.77 eV, there is no significant H/D isotope effect on the signal rise time dominated by the (e-hyd) population (Table 2). The signal decay that occurs on the picosecond time scale is assigned to the early geminate recombination reaction between the fully hydrated electron (e-hyd) and the aqueous chlorine atom (eqs 7 and 10). In deuterated solvent, this reaction exhibits a characteristic time Tr of 1.4 ( 0.1 ps (Table 2) and the fraction of the initial population that will recombine with chlorine atom is not significantly influenced by an H/D substitution (γ ) 0.36 ( 0.02). The lack of a significant isotope effect on Tr allows us to exclude a role of prototropic configurations of water during this electron transfer process. Although the recombination yield of the apparent signal decay remains similar in H2O and D2O at 1.77 eV (Figure 5), this test wavelength represents an intermediate spectral region for which the H/D effects on the electron-atom pairs are balanced by H/D isotopic effects on the ground state hydrated electron populations, mainly on {e-‚‚‚Na+}hyd population (Table 2). On the other hand, significant H/D isotope effects have been observed within different temporal windows (Figures 3-5). When the probe tests the hydrated electron high-energy tail (1.88 or 2.29 eV), a significant H/D isotope effect is observed on the overall signal rise time and signal decay. Short-time curves exhibited in Figure 3, top, particularly illustrate this fact: heavy water causes a significant slowdown of the dynamics of the second electron hydration process, leading to {e-‚‚‚Na+}hyd. Moreover, the apparent recombination yield of the signal decaying at longer time is more important in H2O than in D2O

(Figure 5). At this stage of the investigation, let us focus our attention on the important results we have obtained in the infrared (1.41, 1.24, 0.99 eV) following H/D isotope substitution and their implications on the two-electron hydration channels dynamics investigated in the visible. The results are presented in Figures 4 and 5. In ionic solution [D2O]/[NaCl] ) 55 and the infrared electronic dynamics exhibits multiexponential behavior. At 0.99 eV, the transient signal is dominated by ultrafast transitions involving high chloride ion excited states (CTTS**) and the excited p-state of the hydrated electron (e-hyd*). The relaxation dynamics (T2) of this excited hydrated electron state toward the ground state (eq 1) is not modified by a H/D isotope substitution (Table 2). This result agrees with previous investigations on IR electron relaxation in pure water or diluted sodium chloride aqueous solutions.33 The incomplete recovery of the induced absorption signal at 0.99 eV is due to the small contribution of the long-lived fully hydrated electron which still absorbs in this spectral region (Figure 6). The IR spectroscopic data obtained with aqueous sodium chloride solutions confirm that the electron hydration dynamics via a two-state model in bulk water (eqs 7-9) remains weakly influenced by the frequency of energetic vibrational modes of water molecules. In the near-infrared spectral range (1.24-1.41 eV) where the electron-atom pairs absorb, the predictions of the kinetic model permit us to identify a selective H/D isotope substitution effect on the electronic dynamics (Figure 4). The overall signal appearance time does not provide fundamental information by itself because multiple transient electronic components contribute to this signal rise time. This explains why a careful analysis of spectroscopic experiments with different temporal windows (2 ps < t < 20 ps) is performed to clearly discriminate the H/D isotope substitution effects on electron photodetachment from metastable states of aqueous ionic solute. From the adjusted parameters reported in Table 2, several important points should be emphasized. The appearance time of inhomogeneous (Cl: e-)pairs remains independent of the H/D isotope substitution. As in H2O, these transient electronic states exhibit a dual behavior defined by two deactivation rates in D2O. The fraction of electron-Cl atom pairs ((Cl:e-)pairs‚‚‚Na+) characterized by the deactivation rate T4 (T4 ) 330 fs) is not modified by the H/D substitution. This relaxation channel is assigned to the ultrafast nondiffusive recombination of an excess electron within the solvation shells of chlorine atoms and leads to the aqueous halide ion ground state (eq 3). The main point we emphasize here is that the deactivation kinetics of {Cl:e-}pairs:(Na+)hyd population (eq 14) is consider-

Electron Transfer Processes

J. Phys. Chem., Vol. 100, No. 33, 1996 14001

Figure 6. Relative spectral distributions of electronic states involved in two electron hydration processes after a femtosecond UV excitation of an aqueous sodium chloride solution (D2O/NaCl ) 55). Fastest channel (e-hyd) exhibits an infrared precursor assigned to an excited state of hydrated electron as in pure water (e-prehyd or {e-}hyd*). Second delayed hydration channel ({e-‚‚‚Na+}hyd) is governed by an electron localization in the vicinity of the counterion (Na+). Near-infrared band peaking around 1.41 eV is assigned to transient electron-chlorine atom pairs ({e-:Cl}pair:Na+ and {e-:Cl}pair‚‚‚Na+).

ably longer in D2O than in H2O (T4′ ) 980 vs 750 fs). This result is interpreted as a direct solvent cage effect on electron photodetachment dynamics from electron-Cl pairs. The calculated spectral contribution of nIR electron-atom pairs in D2O (Figure 6) is in agreement with femtosecond spectroscopic investigations using an optical multichannel analyzer (OMA4) equipped with a cooled CCD detector.62 Table 2 shows that the spectral contribution of transient electron-chlorine atom pairs is slightly higher in D2O than in H2O. Consequently, the fraction of nIR signal decaying within the first 15 ps is increased in D2O compared to that in H2O (Figure 4). This can be due either to the direct H/D isotope effect on the cross section of electron-atom pairs as for the hydrated electron63 or to the effect of stronger hydrogen bonds on a larger concentration of electron-atom pairs in D2O. Within the latter hypothesis, the hydrogen bond network can assist or impede an electron photodetachment from transient electron-atom pairs, contributing to the confinement of the electron in the vicinity of the chlorine atom and/or influencing local anisotropic effects near the counterion (Na+). According to the time-resolved spectroscopic data of Figures 3 and 5 and the adjusted parameters reported in Table 2, the isotope effects we observed are in agreement with the infrared spectroscopic results shown in Figure 4. The H/D isotope substitution increases both the spectral contribution of e-hyd′ population (i.e., {e-‚‚‚Na+}hyd) and the β/R ratio. As previously discussed, in light water, considering that the long-lived electronic population ({e-‚‚‚Na+}hyd) does not participate in the early geminate recombination reaction with the chlorine atom, the apparent fraction of visible signal decaying within the first 20 ps is reduced in D2O (Figure 5). This effect is more evident at 1.88 eV for which the relative spectral contribution of {e-‚‚‚Na+}hyd population is higher. In the same way, the overall signal rise time at 1.88 eV (result not shown) and 2.29 eV is largely dominated by the {e-‚‚‚Na+}hyd population dynamics (Figure 3). In this green spectral region, a similar oscillating residual signal between the experimental curve and the best computed fits of the kinetic model is observed in light and heavy water. It is the first time we observe such subpicosecond oscillatory components during solvation dynamics in aqueous solution. Experimental work is in progress to extend our understanding of this oscillatory phenomenon.

4. General Discussion and Conclusions Intensive experimental and theoretical studies of ionic solutions establish the relation between the microscopic structure of hydration shells around the anion or cation and the dynamical solvent properties.70,71,74,81-87 The present work devoted to UV-IR spectroscopic investigations of charge transfer in aqueous sodium chloride solutions emphasizes (i) the influence of local effects in the vicinity of the chloride ion and alkali ion (Na+) and (ii) the possible role of dynamical Cl--Na+ distribution function or the potential of mean force profile during the electron detachment process from excited electronic configurations of aqueous chloride ion. Our experimental results allow us to conclude that after a two-photon excitation of aqueous chloride ions, a second electron hydration mechanism proceeds via the electron detachment from transient electronchlorine atom pairs: {Cl:e-}pairs:Na+. This electron photodetachment is in qualitative agreement with predictions of recent quantum molecular dynamics simulations on the 3p f 4s transition of aqueous chloride ion for which an adiabatic electron detachment from the electron-chlorine atom pair is considered.60 However, our femtosecond spectroscopic experiments in sodium chloride solution allow us to establish that this second electron hydration pathway is influenced by the presence of counterions (Na+) in the vicinity of electron-atom pairs and leads to a polaron-like state, i.e., a relaxed electron exhibiting short-range interactions with sodium ion {e-‚‚‚Na+}hyd. Quantum MD calculations on electronic dynamics in aqueous halide solutions do not take into account the presence of alkali ions during the ultrafast relaxation of electron-chlorine atom pairs or the subsequent electron detachment process.58-60 This is the reason why the direct comparison between our femtosecond spectroscopic data and available computer quantum MD simulations on electron photodetachment in aqueous sodium chloride solution remains difficult. An important aspect of aqueous sodium chloride solution concerns the microscopic structure and relaxation dynamics of solvent shells that contribute to the total solvation energy of Cl- and Na+. Counterion effects we have observed in the present study can be discussed in the framework of ion pair dynamics and ion-solvent correlation functions, considering that ionic solutions are characterized by short-range ordering of water molecules around isolated anions, cations, or ion pairs. In aqueous sodium chloride solution, the rate constant for the transition between contact and solvent-separated ion pairs (CIP-

14002 J. Phys. Chem., Vol. 100, No. 33, 1996 SSIP states) is estimated to be within the range 50-200 ps.48 This time scale is long compared to the deactivation dynamics of electron-atom pairs in the aqueous Na+ atmosphere (0.75 ps in H2O). In other words, the lifetime of aqueous Cl-:Na+ pairs (CIP or SSIP states) is long enough so that electron photodetachment from transient electron-chlorine atom pairs takes place without significant temporal change of the mean force potential profile of Cl--Na+ pairs (Wr). In SSIP configurations, ion-ion interactions can prevail until ∼8 Å.66 MD simulations have established that the dynamics of water molecules is dependent on the angular distribution of solvent molecules within the internal solvation shells entrapped between Na+ and Cl- (solvent bridge bonding) and external solvation shells around Cl- or Na+.49 When ion pairs are analyzed in dilute solution, computer simulations predict that the solvent cage effect can exist on the subpicosecond time scale.50 It is interesting to note that the experimental ratio we determine from the two relaxation times of electron-Cl atom pairs (r ) T4′/T4 ) 2.27 in H2O) varies in the same way as the estimate of the reorientation dynamics ratio, defined as T(H2O)Na+/T(H2O)Cl) 2.17. This value is obtained from computer simulations of first solvation shell dynamics around Cl- and Na+ by using a flexible SPC model for the water potential.50 Our spectroscopic data on electronic dynamics in sodium chloride solution allow us to suggest that the two relaxation times of transient electronatom pairs are dependent on the existence of solvent cage effects in the vicinity of two well-defined configurations of electronchlorine atom pairs: {Cl:e-}pairs‚‚‚Na+ and {Cl:e-}pairs:Na+. In this way, the short-range effects of the alkali ion (Na+) on electron detachment and localization would be governed by the dynamical response of solvent molecules to a change of charge distribution in the vicinity of the sodium ion. We tentatively suggest that such solvent motions assist the electron detachment and the subsequent relaxation steps toward a second electron hydration channel: ([{Cl:e-}pair:(Na+)] f {Cl}, {e-‚‚‚Na+}hyd). This second electron localization process leading to a polaronlike state cannot entirely be described by a two-state model and apparently involves subpicosecond oscillatory components during the signal rise time of the fully relaxed electronic state ({e-‚‚‚Na+}hyd). Regarding these oscillations, we should wonder whether the relaxation of water molecules around {e-‚‚‚Na+} states involves two temporal regimes as predicted by the recent classical molecular dynamics simulations of ion solvation in liquids.88 Indeed, we cannot exclude that the second electron hydration channel governed by the presence of a positive counterion (Na+) would involve a spectral shift due to the adiabatic electron transfer and solvent molecule reorganization around the neoformed ground state {e-‚‚‚Na+}hyd. In order to extend our understanding of the counterion effects on electron transfer in ionic solutions, experiments with different water molecular ratio or counterion valence are in progress. They will be discussed in forthcoming papers. A key point raised by our study concerns the selective H/D isotope effects we observe on specific electron relaxation channels in aqueous sodium chloride solution. Figures 7 and 8 represent a comparative analysis of the time dependence of multiple electron detachment pathways following UV excitation of the aqueous halide ion (Cl-). In aqueous sodium chloride solution, the first electron hydration channel in ionic solution involves a relaxation of the e-prehyd or p-like excited e-hyd state for which the dynamics remains similar in normal and heavy water. The electron-chlorine atom pairs population [{Cl: e-}pair‚‚‚Na+] characterized by the deactivation time (T4) and the 1D recombination between the hydrated electron ground state and the chlorine atom (Tr) on the picosecond time scale are not

Gelabert and Gauduel

Figure 7. Computed analysis of H/D isotope substitution on the time dependence of electron photodetachment leading to two populations of hydrated electron ground states (e-hyd and {e-‚‚‚Na+}hyd, i.e., (e-)hyd′) following the photoexcitation of an aqueous chloride ion. Time dependence of two transient electronic precursors (e-prehyd and {e-: Cl}pairs) are also reported. In deuterated ionic solution, a specific electron transfer channel is significantly delayed as indicated by the circles.

Figure 8. Selective isotope effect on two electron relaxation channels leading to fully relaxed hydrated electron populations ({e-}hyd,{e-‚‚‚Na+}hyd}) in aqueous sodium chloride solution (1 M). For a complete analysis, the ultrafast electron hydration channel in pure water is also considered. On the x axis are reported the formation times of different electronic ground states. Relaxation dynamics of electronic precursors is indicated on the ordinate. Experimental points are represented by squares. Lines are used as a guide for eyes.

significantly modified by a H/D isotopic substitution. The lack of an experimental isotope effect on the ultrafast electronic dynamics represents interesting situations for which a change of the intramolecular vibrational modes OX (X ) H, D) does not influence the primary steps of electron transfer processes in sodium chloride solution. Previously, an insignificant isotope effect (∼4 %) on electronic dynamics in pure water has been established by performing femtosecond infrared spectroscopy of the presolvated electron (e-prehyd or p-like excited e-hyd)33 and has been recently confirmed by hole-burning experiments.22 With regard to the results of quantum or classical simulations of electron solvation dynamics in water, the lack of significant experimental H/D isotope effect remains an intriguing aspect of electronic dynamics in hydrogen-bonded liquids.27,89,90 A very slight H/D isotope effect on electron solvation dynamics is predicted by a very recent hydrodynamical model,89 and the lack of an isotope effect on a nonadiabatic simulation of excited hydrated electron relaxation (p f s radiationless transition) has been recently discussed in the framework of quantum decoherence dynamics.90 The present work establishes that a significant experimental H/D isotope effect is observed during electronchlorine atom pairs [{Cl:e-}pair:Na+] deactivation (T4′) via an

Electron Transfer Processes electron photodetachment and localization within an aqueous Na+ atmosphere (Figure 8). Indeed, the T4′(D2O)/T4′(H2O) ratio equals 1.31 and the T4′/T4 ratio increases from 2.27 in normal water to 2.95 in heavy water. This selective isotope effect on the relaxation of specific {Cl:e-}pair:Na+ configurations allows us to suggest that the dynamics of water molecules that are tightly bound to Na+ assists the electron detachment process from transient electron-atom pairs and governs the second electron hydration channel ({e-‚‚‚Na+}hyd). With regard to the similar oscillatory residual signals discriminated during the second electron hydration channel in H2O and D2O, it can be suggested that inhomogeneous solvent dynamics including ultrafast electronic solvent response, librational motions, and slower solute/solvent frictional couplings would induce a slight spectral shift due to dispersive responses of solvent molecules during charge redistribution around the neoformed {e-‚‚‚Na+}hyd state. In conclusion, the observation of specific H/D isotope effects on electronic dynamics in aqueous sodium chloride solution provides direct evidence for the role of complex solvent responses on early branchings between multiple short-lived electronic configurations of aqueous ionic solute. The present results establish that during relaxation of dual short-lived electron-atom pair configurations, {Cl:e-}pairs‚‚‚Na+ and {Cl: e-}pairs:Na+, the charge-switching process between the Cl atom (ultrafast electron-chlorine atom recombination) and counterion (second electron hydration channel) is dramatically sensitive to the frequency of the intramolecular vibrational modes of solvent molecules. Aqueous sodium choride solution represents an interesting archetype for the investigation of early branching trajectories between ultrafast nonadiabatic/adiabatic electron transfers in liquid phase. Regarding the conclusions of recent quantum MD simulations on the role of orbital symmetry during electron detachment process from low excited CTTS states of aqueous halide,58 we should wonder whether, in competition with an ultrafast electron-atom recombination, Na+ effects on electron detachment process are due to anisotropic influences on orbitals of transient electron-atom pairs. The experimental identification of complex primary electron transfer steps in aqueous sodium chloride solution provides guidance for further theoretical developments on short-range interaction effects during electron transfer in molecular liquids, as well as on the influence of solvent electronic polarization during charge distribution and understanding of elementary chemical steps in complicated many-body systems. Acknowledgment. This work has been performed with the support of the Groupement de Recherche n°1017 of the CNRS (France). We thank Professor M. Holz and D. Borgis for fruitful discussions before the preparation of the manuscript. References and Notes (1) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J. Phys. Chem. 1991, 95, 2068. (2) Newton, M. D. Chem. ReV. 1991, 91, 767. (3) DeVault, D. Quantum mechanical tunneling in biological systems; Cambridge University Press: Cambridge, UK, 1984. (4) Van der Zwan, G.; Hynes, J. T. Chem. Phys. 1991, 152, 169. (5) Ultrafast Dynamics of Chemical Systems; Simon, J. D., Ed.; Kluwer Academic Publishers: Dordrecht, 1994. (6) Ultrafast Reaction Dynamics and SolVent Effects, AIP Conference Proceedings, Gauduel, Y., Rossky, P. J., Eds.; AIP Press: New York, 1994. (7) Franks, F., Ed. In Water. A ComprehensiVe Treatrise; Plenum Press: New York, 1979; Vol. 6, Chapter 1. Water and Solutions; Neilson, G. W., Enderby, J. E., Eds.; Adam Hilger: Bristol, 1986. Enderby, J. E. Annu. ReV. Phys. Chem. 1983, 34, 155. (8) Engstro¨m, S.; Jo¨nsson, B.; Impey, R. W. J. Chem. Phys. 1984, 80, 5481. Bopp, P. In The Physics and Chemistry of Aqueous Ionic Solutions;

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