Ultrafast electron transfers in aqueous electrolyte solutions

journal of MOLECULAR

LIQUIDS ELSEVIER

Journal of Molecular Liquids 78 (1998) 111-122

U l t r a f a s t e l e c t r o n t r a n s f e r s in a q u e o u s e l e c t r o l y t e

solutions.

Y. Gauduel, M. Sander*, H. Gelabert Laboratoire d'Optique Apphqu~e, CNRS URA 1406, INSERM U451, Ecole Polytechnique-ENS Techniques Avanc~es, 91761 Palaiseau cedex (France) * I n s t i t u t fiir Physikalische Chemie, Universit~it GSttingen, G e r m a n y

A b s t r a c t . Femtosecond spectroscopy of aqueous ionic solutions represents a good

tool for the study of e l e m e n t a r y chemical steps at the microscopic level. Ultrafast electron t r a n s f e r processes are investigated at 294K. © 1998 Elsevier Science B.V. All rights reserved. 1. I N T R O D U C T I O N The microscopic investigation of e l e m e n t a r y radiationless electron transfer in liquids represents a f u n d a m e n t a l challenge in physical chemistry and biochemistry. It would enhance our u n d e r s t a n d i n g of oxydo-reduction reactions at the a n g s t r f m or s u b a n g s t r 5 m level. An i m p o r t a n t need is to study ultrafast processes on the time scale of molecular motions in order to describe solvent cage dynamics in the vicinity of a r e a c t a n t or during competitive electron transfer branchings [1,2]. Femtosecond spectroscopy of p r i m a r y events, triggered by u l t r a s h o r t optical pulses, provides unique informations about e l e m e n t a r y chemical processes in liquids and solutions [3-6]. Ionic solutions represent a p a r a d i g m for the investigation of short-lived states in complicated many-body systems (figure 1). Time-resolved spectroscopy of t r a n s i e n t species in a reaction can be performed with high power optical pulses in the time window 10 -t4 -10 H s. The nonlinear interaction of UV pulses, typically of 80-90 fs duration and an energy power of 1012 W cm -2, with aqueous electrolytes yields multiple electronic configurations of ionic solutes and non-equilibrium states of a photodetached electron. The p r i m a r y steps of an electron transfer reaction are investigated by a p u m p - p r o b e configuration with tunable test wavelengths (COT) from the UV to n e a r IR (3.26 - 0.93 eV). The time-dependence of small t r a n s i e n t induced absorption signals S °'r (~) are expressed by the physical response (( R ~) of the sample and the normalized correlation function between the excitation pulse (Ip) and the probe b e a m fIT) s e p a r a t e d by a time delay v (equation 1). This correlation function t a k e s into account the propagation of the p u m p - p r o b e pulses and the 0167-7322/98/$19.00 © 1998ElsevierScience B.V. All rights reserved PII S0167-'7322(98) 00086-5

112 overall optical broadening factor due to the group velocity dispersion. The femtosecond spectroscopic procedures developed for a very large spectral range (26 000-7500 cm 1) have been published in recent papers [7].

®

(,>

The time-dependence of the measured absorption signal s'~r(z) is defined by the following expression:

S°~(z) = ~

{ a/° ~ n i ( t - r )

® C°rr~°P'~°T(t)

(2)

When the computed analyses are performed from normalized signals (s°'r.or (~)), the expression (2) becomes: Sf°nor (~)

=

Zait°.ni("C) i

(3)

o~i . n i ( V S m ~ )

....................... ~-.. ~ /"

41

i "'-.. /

,,, ~,:'

{e-} Cl. l

, ,

'.,.

j

)

./

A-" ',

Figure 1. Aqueous electrolyte solutions represent a paradigm for the investigation of elementary electron transfer reactions. These complicated many body systems involve anion-solvent, electron-solvent, electron-counterion, solvent-solvent and cation-solvent couplings. 2. UV-IR S P E C T R O S C O P Y OF E L E C T R O N T RA N SFE R CHANNELS Electrolyte solutions containing aqueous halide ions are useful systems for the investigation of solvent dynamics and electric field effects (cations) on elementary chemical steps. Extensive experimental and theoretical researches have been devoted particularly to the role of non-radiative processes during oneelectron transfer [8-12]. These non-radiative relaxations can assist an electron

113

solvation process or a deactivation of excited charge transfer to solvent states (excited CTTS). Figure 2 shows a 3D representation of transient electronic spectra involved in the photoexcitation of a halide ion (CI), the complete electron photodetachment in the thermal bath (water molecules) or within the solvation shells of the alkali metal ion.

!

[H20] / [NaCI] = 55 /

- - - ~/~0 /-520 Ground states: {e-}hyd {e..... Ion}pa/rs J

~, 0.8 E= o.s •~

I .,f-.,~J

~

~

//~7~.

./

/

~

CTTS'*

/280

{e-}hyd*

,,,e

'

0.4

0.2 0

/// I 3.44

I 229

I 1.77 1.88

I

J 1.41

I

~'~-J'/2 I 0.99

000

1.51 1 24 Energy (eV)

Figure 2. Computed 3D spectrum of transient (UV, IR bands) and fully relaxed electronic states (visible) in aqueous sodium chloride solution (294 K) photoexcited by femtosecond UV pulses [15]. Two ultra-short-lived transitions have been discriminated by UV and IR spectroscopy. A low lying excited CTTS state (UV signature) is triggered by direct pumping via a monophotonic process (3p -> 4s transition) or an ultrafast deactivation of a IR high excited CTTS (t < 50 fs). These well-separated electronic states (- 2 eV) are fully relaxed in less than 1.5 10~2 s. The sub-picosecond stabilization of these electronic states by water molecules represents a typical solvent effect. In ground CTTS, an electron is partially delocalized between the atomic core and the first hydration shells [13] and during the photoexcitation, short-lived couplings take place between a newly created electronic state of the solute (excited CTTS states of a halide ion) and polar solvent molecules. These couplings can be controlled by librational motions, short-range polarization effects, molecular reorganization of solvent in the vicinity of newly created electronic configurations of the solute. The behavior of low and high lying excited CTTS states are dependent on energy levels of the anion-solvent

114

couplings and anion-cation interactions. Within the Franck-Condon approximation the early steps of a photo-induced electron detachment from an aqueous halide is governed by the solvent density states fluctuation of excited CTTS states. The existence of ultrafast electron transfers from aqueous chloride ions is underlined by the spectral signatures of h y d r a t e d electron and electroncation pairs (figure 2). An asymmetric broad b a n d centered at 1.77 eV is due mainly to a s -> p transition of a fully relaxed electron [14,15]. Two ultrafast electron t r a n s f e r channels h a v e been indentified by femtosecond IR spectroscopy of aqueous NaC1 solution (equations 4,5).

Electron trapping

(Ct')hy d + 2 h v

t<<50 f i

Electron

e'prehyd (Cl')hy d + 2hv

) (C1-)hyd,

)

(t=130~)

(CI) + e'prehyd

(4a)

hydration

(t=300 fi)

(4b)

) (e')hy d

Electron-atom pair formation (t=270 fi)

(C1.)hy d ( Ultrafastrelaxation (t=330fs)

(Cle)pairs

)

(C1 " e')pairs E.T. (t=750Js))(e.)hyd,

(5a) (5b)

One of these channels is characterized by a t r a n s i e n t b a n d p e a k i n g around 1.41 eV, wholly built in 500 fs a n d containing two subbands due to inhomogeneous electron-chlorine atom pairs ({Cl:e}:Na ÷, {Cl:e'}...Na+). These pairs lead either to a complete electron d e t a c h m e n t (formation of polaron-like state {Na+:e'}hyd) or an ultrafast electron-atom recombination. The dual deactivation processes of t r a n s i e n t electron-chlorine atom pairs are understood as different solvent cage effects in the vicinity of electron-chlorine atom pairs [8]. The ultrafast deactivation of IR high excited CTTS states (CTTS**) can compete with an electron p h o t o d e t a c h m e n t process. The i n t e r m e d i a t e step corresponds to an excited h y d r a t e d electron (infrared e'prohya). This IR electronic state exhibits a nonradiative relaxation towards the ground state of the h y d r a t e d electron. A sub-picosecond IR relaxation h a s been also evidenced by femtosecond IR spectroscopy of liquid water, aqueous micelles or ionic solutions [16,17] and r e p r e s e n t s a general aspect of the electron hydration. Computational q u a n t u m MD work h a s established t h a t the infrared electron is a p-like orbital and t h a t solvent modes would participate to the p -> s state transition [18,19,20]. In aqueous NaC1 solution (figure 3), the balance between two electron hydration channels is dependent on inhomogeneous configurations of ion-solvent and ion pairs (CIP a n d SSIP states).

115 Contact

(e-atom) pairs

CTTS**

I

Solvent separated

270

t

{e-:CI atom} pairs fs

"\

/

(2x4eV) l ( 33Ors I " /

CI- (4s)

{CTTS*} - ~ "'..... CI-(3p)

'~ i j~

..-

I ( I i 190 fs I / i/ .... I ? L ~ j z /

Electron detachment

{Cl:e-}

~Ofs

~_~_ ~ " {CI} r " t

Solvent (H20)

C o u n t e r i o n (X+)

assist e-transfer Coordinates

Figure 3. Ultrafast photoinduced electron transfers in aqueous sodium chloride solution following the femtosecond UV mono or two-photon excitation of a halide ion (C1-) at 294K ([H20]/[NaC1] = 55).

3. IONIC S T R E N G T H AND ULTRAFAST NONRADIATIVE RELAXATION The femtosecond UV-IR spectroscopy of electron transfer in ionic solutions allows investigation of the r61e of inhomogeneous microscopic structures on early branchings of short-lived states. These microscopic effects are particularly evident on IR electronic trajectories in sodium chloride solution [21]. Monte Carlo calculations and MD simulations of ionic solutions have established that the microscopic structure of ion hydration shells, and the solvent properties, are connected to each other [22-26]. In aqueous sodium chloride solutions, the rate constant for the transition between contact and solvent separated ion pairs (CIP SSIP states) has been estimated to be within 50-200 ps [23]. The lifetime of aqueous {Cl:Na+}pairs (CIP, SSIP states) is long enough for an electron photodetachment to take place without significant timescale changes of the mean force potential profile of {Cl':Na+}pairs (Wr). Experimental studies of specific counter-ion effects during femtosecond electronic dynamics underline the complex role of inhomogeneous ion-ion distributions on ultrafast electron transfer processes. In this way, counterion effects cannot be discussed only in the framework of ion-pairs dynamics and need to consider the influence of ion-solvent correlation functions, short-range ordering water molecules and anisotropic electric field effects [27-29]. The effects of molecular ratio on primary IR electron transfer steps in aqueous solutions are reported in figure 3.

116

Test: 0.99 eV (1250 nm) 1

Test: 1.24 eV (1000nm) ~

e

~[

•~

~I',

[H20] / [NaCI] = R

.......... H 2 0

'll

[N20] ! [NaCI 1: R i.~

-~ 04 ._N

"

\R=55

~"W ' ?~iV~'~ H 2 0 :~ _

.R=55 R=135

.

T I M E / ps

T I M E / ps -1

0

1

2

3

I

I

1

2

3

Figure $. Infrared electronic dynamics in pure water and aqueous NaC1 solutions (R: 9-135) following an electron photodetachment by femtosecond UV excitation at 294K (1 or 2 x 4 eV). At 0.99 eV, the nonexponential relaxations involve a p -> s transition and a high excited CTTS deactivation. The ionic strength effect on the p -> s transition is clearly observed. At 1.24 eV, the contribution of an additional near-infrared electron-chlorine atom pairs is discriminated in moderately (R = 55) and highly concentrated (R = 9) solutions. These data emphasize an electric field effect of Na ÷ on ultrafast electron transfer and provide direct evidence for the role of inhomogeneous ion pair distributions on elementary electron transfers. A specific counter-ion effect on the p -> s transition of the h y d r a t e d electron is observed at 0.99 eV and analyzed in the framework of solvent motion dynamics. In dilute ionic solution (R ~ 135), the nonradiative back relaxation of the IR excited electron to the ground state (t ~ 260 fs) remains similar to electronic dynamics in pure liquid water (equation 2). At high ionic strengh (R ~ 9), this radiationless transition is slowed down by about 35% [21]. This agrees with the fact that the rotational correlation time in ionic solution is longer than in pure water whereas the 1H NMR relaxation time is significantly reduced by slow motions of water molecules [30]. In saturated NaC1 solution at 294K (R = 9), Na ÷ and Cl contain bounded water molecules and contribute to changes in the dynamics of electron-atom pairs. The nonexponential relaxation of nIR pairs (figure 3) emphasize that N a ÷ increases the probability of transient {Cl:e}p,~s:Na ÷ configurations (figure 4). This imbalance in favor of CIP states will assist the appearance of relaxed polaron type states (equation 6). (Cl')hy,t ( 330 fi"

(C! : e') p a l"r s : N a + = 7 5 0 f s => (c)hyd ' i.e. {e..Na " *}hyd

(6)

117

The effects of chemically inert cations on the spectral shift of hydrated electron can be carefully investigated at very short times [21]. An electron photodetachment from transient electron-atom pairs is made easier by the electric field of positive alkali metal ions and solvent cages assist electron stabilization. In electrolyte solutions the {e..Na÷}hydband exhibits a slight blue shift compared to that of the hydrated electron (figure 5). This shift is understood as a change of the electron hydration energy due to sodium ions and is analyzed in the framework of polaron theory of electrolyte solutions [31,32]. The potential energy of an excess electron is governed by the polar solvent molecules and ionic interactions. When the initial electron-ion distance (e-..Na÷) is less than the Onsager radius the potential energy of the electron depends on the fraction of the time spent in the vicinity of the alkali metal ion. The theory predicts that this fraction of time affects the e n e r g y of a polaron light absorption trapped by the chemically inert cation.

Electron transfer at 1.24 eV R=55R=9

(e-)hyd s state 0,8 0,6 0,4

CTTS~, e-IR

,

, [e-..Na+]hyd

Electron transfer at 1.88 eV R=55R=9 (e-)hyd s state 0,8

\0.6

{Cl:e-}...Na+ pair

{Cl:e-}:Na+ pair

'~ {e-}lR ,

{Cl:e-}..Na+ pair

[e-..Na+]hyd

{Cl:e-}:Na+ pair

Figure A(. Ionic strength effects (R = [H20]/[NaC1]) on the relative spectral contributions of femtosecond electron transfers. The test wavelengths (1.24 and 1.88 eV) show the discrimination of transient electronic states (electron-C1 atom pairs) and relaxed hydrated electron and polaron like state.

118

12

II

1

q8

I

o=

i/

k

~Q6

I/

~',

;

/

~ " ~ J H20.'-O. 6 e V "~ ',,.4t,--NaCI: - O. 72 e ~ \ • n c NaCI6M H20"~ •

<

~a Q4 -g

n

Q2

IE

2ps

1

2ps

[~

I

I

[7

I D

1,2

I

1,4

1,6

D

1,8

2

22

24

26

Energy / eV

Figure,~. Left: 3D spectra of short-lived IR excess electron in p u r e w a t e r and concentrated aqueous sodium chloride solution ([H20]/[NaC1] = 9) at 294K. Right: Picosecond absorption spectra of h y d r a t e d electron. In ionic solution, a blue shift (AE ~ 0.10 eV) a n d b a n d broadening (W1~2 ~ 0.12 eV) are due to polaron states.

4. E A R L Y G E M I N A T E R E C O M B I N A T I O N P R O C E S S E S

Short-time spectroscopy of ionic solutions allows investigation of the effect of microscopic inhomogeneity of ion-ion distributions on early electron tranfer branchings (figure 6). The short-range effects of an alkali ion (X"÷, n =1, 2) on u l t r a f a s t electron-chlorine recombination have been investigated by IR spectroscopy of electron-chlorine atom pairs. These short-range effects are i m p o r t a n t in the charge switching process between an electron a t t a c h m e n t or formation of a polaron-type state [8,21]. Considering an interconversion between different distributions of electron-atom pairs, attractive modes would govern u l t r a f a s t barrierless electron-atom reactions and repulsive modes favor electronatom detachments. Short-range effects are also influenced by the dynamics of w a t e r molecules to a change of the charge distribution in the vicinity of halide ions [33]. Long-range couplings between the h y d r a t e d electron and the chlorine atom (C1) participate in picosecond recombination processes (figure 6,equation 7).

1t9

Y=CI

S : solvent

)

S

n=l

,2

0

Short-range couplings Transient pairs t--

LU

Long-range couplings 1D Geminate Recombination

2"

Figure ~. Sub-picosecond (short-range couplings) and picosecond (long-range couplings) electron-atom recombination trajectories in aqueous ionic solutions. (C1) + (e)hyd

1D Gem. recombination ('rr, p) )

(Cl')hyd

(7)

Considering the polarisability of C1 atoms, the picosecond recombination is understood as a finite oriented phenomenon within transient distributions of C1 atom and hydrated electron [8,11]. This electron transfer (equation 7) is analyzed in terms of an anisotropic mechanism (unidirectional process): ~__

c)2c (8)

(x,t) = D ~Ox

The analytic solution of a 1D diffusion model is given by the expression:

The time-dependence of the ground state {e}solis expressed by the equation:

ne,ol_ ( t ) = i +,_~ d[e-~].ppdt {1 - p + O

[l- l

err t~-~t'}

(lOa)

dr'

}

12o In equations 10a, 10b, ~r is the time-dependence of a finite 1D electron transfer, p the fraction of electrons involved in the geminate recombination and T1, T2 the t r a p p i n g and solvation times of excess electron [8]. The analytical solution accounts for the dispersive electron transfer in liquids, including the asymptotic time-dependence (1/~ft) of a geminate recombination and contains two adjustable p a r a m e t e r s (1/~r a n d p). Femtosecond data on X20/NaC1 and X20/MgCl= solutions (X -- H, D) are reported in figure 7. In ionic solutions the picosecond signal decay is mainly assigned to the early recombination of the h y d r a t e d electron (s state) and the chlorine atom (C1). In H20 this reaction exhibits a characteristic time (~r) of 1.3 _+ 0.1 ps. Contrary to results obtained in p u r e w a t e r [34] the lack of a significant H/D substitution effect on ~r allows us to exclude the influence of protonated radicals ((XzO+),x20, (OX),xzo) during the geminate recombination of an h y d r a t e d electron with a chlorine atom.

1.77eV (700 n m )

1.77eV (700 n m )

A

_~ o.~ ¢o

oo ~oE 7,

B

~

1

: 11C ix20] / [Naa] = 55

.~} 0,(

"~ a4

1: [,~:~O]/[NaCI]= 137

N

X=D

2 [Y,20],[NaC_~]= 55 3: Pure wat e r

o Z

~.0 TIME k

-~10

0

lo

2o

[(:i-] = 1 Eq C

/ ps

3o

TIME /ps -10

0

10

20

30

Fignre"~'. A: H/D isotope effects on picosecond geminate recombination in aqueous solutions at 294K. B: Counterion effect (Na + vs Mg ++) on the 1D recombination between h y d r a t e d electron and C1 atom. In D20: Zr = 1.4 ps with Na + or Mg ++, p -- 0.38 + 0.02 for Na+ against 0.30 _+0.2 for Mg ++. Although the recombination yield (p) a n d the reaction dynamics (zr) are i n d e p e n d e n t of R (R = [X20]/[NaC1]), the ionic s t r e n g h t effect on the signal decay is due to different spectral contributions of electron-C1 atom pairs and polarontype states [8]. Indeed, the recombination yield (p) cannot be directly extracted from e x p e r i m e n t a l curves a n d needs complex analysis of time-resolved electronic states [8]. In NaC1 solutions, the ion-ion pair distribution influenced by the molecular ratio, or the counterion valence, governs geminate recombinations (figure 7). For a given concentration of Cl, a counterion effect (Na + vs Mg ++) is clearly observed. The different absorption signal decays are due to a slight effect of Mg +÷ on the recombination yield (p = 0.38 _+ 0.02 with N a ÷ against 0.30 + 0.2

121 with Mg++). The differences observed in figure 7 do not correspond to a direct counterion effect on the recombination dynamics (xr = 1.4 ps with N a ÷ or Mg +÷ in DzO), but r a t h e r to the spectral contributions of two electronic states: an electron h y d r a t i o n with a p -> s transition (equation 2) a n d electron-cation pairs relaxation towards polaron-type states ({Cl:e}Mg+÷hya -> {e:Mg++}hya). The spectral contribution of this polaron type states is higher with Mg +÷ t h a n N a ÷ [33]. 5. C O N C L U S I O N S Investigation of solvent cage effects on e l e m e n t a r y chemical processes in liquids is f u n d a m e n t a l to the u n d e r s t a n d i n g of u l t r a f a s t ion-molecule reactions, electron-ion or ion-ion pairs interconversions a n d p r i m a r y recombination processes. It is suggested t h a t the density state fluctuations of solutions influence the energy of early electron-solvent couplings (electron a t t a c h m e n t or localization) and would govern competitive u l t r a f a s t electronic channels. The characterization of electron-atom reactions in polar solutions provides a f u r t h e r basis for (a) the investigation of ultrashort-lived solvent cage effects at the microscopic level, Co) a better u n d e r s t a n d i n g of b r a n c h i n g processes during u l t r a f a s t electron transfers a n d (c) a knowledge of the rble of reorientational correlation function of solvent molecules around semi-ionized states a n d m e t a s t a b l e electronic states. Counterion effects also r e p r e s e n t a challenge for q u a n t u m simulations of reaction dynamics in liquids a n d solutions.

Acknowledgments. This work was supported by the Chemical D e p a r t m e n t of CNRS, GDR 1017 (France) a n d Commission of the E u r o p e a n Communities.

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