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280 Charged Particle and Photon Interactions with Matter

the electrostatic repulsion between

−

anions. On the other hand, the mobility of

−

may be reduced

by asymmetric effects of ion solvation around

−

. Because the smallest cation among the salts used

is Li

, the Li

cation may have the strongest electrostatic interaction with the

−

anion. This may

lead to a decrease in the mobility of

−

. The effect of salt on the diffusion coefcient of the hydrated

electron has been studied previously, and the diffusion rate of the hydrated electron decreased with

the

addition of salt.

notable contrast in the kinetic salt effects for

TMPA N(SO CF )

2 3 2

−

and

PP13 N(SO CF )

2 3 2

−

higher salt concentrations (0.5–1.5M) is that the values of log(k/k

) decrease with increasing salt

concentration, whereas no such decrease was observed for

Li N(SO CF )

2 3 2

−

in the same concentra-

tion range. Since the ionic liquids are miscible with methanol, it is possible to explore the entire

range of mole fraction. The solution viscosity rises signicantly at higher proportions of ionic liquid.

Accordingly, the rate constants were plotted against the inverse of the viscosity, η, in Figure 11.12.

The solid line in the gure is the expectation according to Equation 11.15 for diffusion-limited rate

constants of reactions between neutral molecules. In the case of ionic liquids, there is a signicant

change in the reaction rate with salt addition under low-viscosity conditions (0.5 < η < 1mPa s). The

solution viscosities barely increased upon the addition of ionic liquids up to almost 1M, while greater

screening of the electrostatic repulsion between diiodide anions caused the reaction rate constants to

increase over one order of magnitude from 3 × 10

to 5 × 10

−1

. However, at higher viscosities

(η> 2mPa s), there was no further increase of the reaction rates, which remained close to the behav-

ior of the diffusion-limited rate constant, as predicted by Equation 11.15. This transition represents a

change in regimes between a methanol solution of a salt and an ionic liquid containing methanol as

a cosolvent. The behavior shown in Figure 11.12 indicates that the diffusion coefcients of diiodide

anions in the ionic liquid/methanol mixtures follow those of the constituent ions of the ionic liquid.

For the

Li N(SO CF )

2 3 2

−

salt, in the lowest viscosity region (η < 0.7mPa s) there is no signi-

cant difference between

Li N(SO CF )

2 3 2

−

and the ionic liquids. However, at a slightly higher vis-

cosity region (0.7 < η < 2.0 mPa s), the viscosity dependence of the reaction rate is different from

that for the ionic liquids. At the same viscosity level, the reaction rate for the

Li N(SO CF )

2 3 2

−

solution is slower than those for ionic liquid salts. As the reaction rate constant is very close

to the diffusion limit for neutral molecules calculated from Equation 11.15 at η = 2.0 mPa s,

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Log (k/k

)

5432

1/2

Figure 11.11 Relation between log(k/k

) and the square root of the ionic strength. (

⦁

) TMPA N(SO CF )

2 3 2

−

(

⚬)

PP13 N(SO CF )

2 3 2

−

, (▴)

Li N(SO CF )

2 3 2

−

. (Reproduced from Takahashi, K. etal., Chem. Lett., 38, 236,

2009a.

With permission.)

Radiation Chemistry and Photochemistry of Ionic Liquids 281

it can be said that the electrostatic repulsion between

−

anions is almost entirely screened by

Li N(SO CF )

2 3 2

−

at this concentration.

11.6.3 reactionS of excited-State radicalS in ionic liQuidS

The photochemistry and radical reactions of benzophenone have been widely investigated in con-

ventional solvents over the past 50 years. One can obtain insights into the characteristics of ionic

liquids as solvents by comparing benzophenone reaction dynamics in ionic liquids with those in

various molecular solvents. Recently, Wakasa has studied the magnetic eld effects on photo-

induced hydrogen abstraction reactions of triplet benzophenone with thiophenol in ionic liquids

(Wakasa, 2007; Hamasaki etal., 2008). The yield of the ketyl radical decreased even with small

magnetic eld intensities. The high viscosity of ionic liquids could not be the reason of such a large

magnetic eld effect. He speculated that the cage effect could be important because a conned sys-

tem is necessary for the spin conversion. This speculation is consistent with recent ndings on the

local

structure or domains in ionic liquids.

this section, excited-state reactions of benzophenone ketyl radical (BPH) are investigated using

a two-color, twin-pulse laser. Because the absorption maximum of BPH is located around 540 nm, it

is easy to excite BPH using the second harmonic of an Nd-YAG laser. In molecular solvents, a con-

siderable number of studies of the excited benzophenone ketyl radical (

BPH*) have been carried

out with absorption and emission spectroscopies (Nagarajan and Fessenden, 1984; Cai etal., 2003,

2005; Sakamoto etal., 2004). The dipole moment of

BPH* was estimated to be 7D, indicating that

BPH* is highly polarized. It is reported that

BPH* decays via radiative and nonradiative relax-

ations, and unimolecular and bimolecular chemical reaction processes. For the chemical reaction

processes, O–H bond cleavage of

BPH* and electron transfer from

BPH* to benzophenone have

been reported (Nagarajan and Fessenden, 1984; Cai etal., 2003, 2005; Sakamoto etal., 2004, 2005).

These

characteristics of

BPH* are interesting enough to be studied in ionic liquids.

In Figure 11.13, transient kinetics of the excited state of BPH were examined in (a) methanol

and (b)

Bmim N(SO CF )

2 3 2

−

, respectively. The transient kinetics were monitored at 500nm, and

0.01

2 4 6 8

0.1

2 4 6 8

1/viscosity (mPa

–1

)

TMPA-TFSI

PP13-TFSI

Li-TFSI

k (M

–1

)

Figure 11.12 Plots of rate constants for reaction between anion radicals as a function of the inverse of viscosity:

(

•)

TMPA N(SO CF )

2 3 2

−

, (○)

PP13 N(SO CF )

2 3 2

−

, (▴)

Li N(SO CF )

2 3 2

−

. (Reproduced from Takahashi, K. etal.,

Chem. Lett., 38, 236, 2009a. With permission.)

282 Charged Particle and Photon Interactions with Matter

CCl

was added as a reaction partner of the excited state of BPH. The BPH formed by irradiation

with a 355 nm pulse was excited with a 532 nm pulse at 20 ms after the rst 355nm pulse. When

the sample containing CCl

was irradiated with the 532nm laser pulse, the transient absorption sig-

nal displayed an instrument-limited decrease, and the absorption decrease did not recover within

the lifetime of BPH. As the concentration of CCl

increased, the amount of permanent bleaching

was increased. Therefore, the bleaching can be attributed to the reaction of the excited state of

BPH (

BPH*) with CCl

. It has been reported that

BPH* decays via both radiative and nonradia-

tive relaxations, and unimolecular and bimolecular chemical reaction processes. The unimolecular

chemical reaction process has been assigned to the O–H bond cleavage. For the bimolecular reac-

tion of

BPH*, electron transfer from

BPH* to BP has been suggested. By considering the previ-

ously suggested reaction paths, the following reactions may be considered to explain the present

permanent bleaching:

BPH BPH

 →

(11.16)

BPH

BP H

 → + (11.17)

BPH

BP BPH BP+  → +

+ −

(11.18)

Abs

500

(–)

Abs

500

(–)

Time (μs)

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.5

0.4

0.3

0.2

0.1

0.0

(a)

(b)

806040200

43210

Single pulse

Double pulse

Figure 11.13 Transient absorption signal for BPH in (a) methanol and (b) Bmim

N(SO CF )

2 3 2

−

with the

second

532

pulse.

Radiation Chemistry and Photochemistry of Ionic Liquids 283

BPH

CCl P

+  →

(11.19)

Because the chemical reactions (11.16) through (11.19) cannot reproduce BPH, the following rela-

tion

may be established between the observed bleaching and the above reactions:

OD OD

BPH* BPH* BP BPH* CCl

−

+ +

k k k

[ ] [ ][ ] [ ][ ]

[ HH* BPH* BPH* BP BPH* CCl

] [ ] [ ][ ] [ ][ ]+ + +k k k

(11.20)

where OD

and OD

are the optical densities immediately before and after the 532nm pulse, respec-

tively. F is a constant that depends on the experimental conditions such as laser intensity and quan-

tum efciency for the formation of

BPH* by a 532nm irradiation to BPH. When the concentration

of CCl

is high enough for the condition k

[CCl

] >> (k

+ k

[BP]) to be satised, Equation 11.20 may

written as follows:

OD OD CCl

4 4

−

= +

F k [ ]

(11.21)

Therefore, at high CCl

concentrations, the value of F and the ratio of k

can be determined

from a plot of OD

/(OD

− OD

) versus 1/[CCl

]. Consequently, the factor F in both ethanol and

Bmim N(SO CF )

2 3 2

−

was determined to be 0.7 ± 0.02. Once factor F is determined, the ratio k

can

be elucidated by a least squares tting of the data. The tting results are k

= 7.8 × 10

−2

(inmetha-

nol) and 9.5×10

−2

(in

Bmim N(SO CF )

2 3 2

−

). k

was estimated to be 4 × 10

and 2 × 10

−1

from the

lifetime of BPH(D

) in methanol and Bmim

N(SO

)

−

, respectively. Therefore, k

in methanol

and Bmim

N(SO

)

−

is found to be 5.1 × 10

and 2.1 × 10

−1

, respectively, suggesting that

the reaction of BPH(D

) with CCl

may be a diffusion-limited reaction. However, despite the fact

that the viscosity of

Bmim N(SO CF )

2 3 2

−

is about 100 times higher than that of methanol, the rate

Bmim N(SO CF )

2 3 2

−

is only 20 times slower than that in methanol. Therefore, this result suggests

that the diffusion of

BPH* cannot be described by the hydrodynamic approximation using the

viscosity

of ionic liquid

Bmim N(SO CF )

2 3 2

−

, as discussed earlier in this chapter.

11.7 summary

This chapter provides an overview of current topics in the radiation chemistry and photochem-

istry of ionic liquids. The photochemistry of benzophenone in ionic liquids was also covered, but

a complete review of photochemical investigations in ionic liquids is well beyond the scope of

this chapter.

The ionic liquid radiation chemistry described here has focused on electron dynamics and reac-

tivity. However, the reactions of hole species and radiation-induced radicals are equally important,

although they are often harder to study due to weak or nonexistent optical spectroscopic features.

Recently, EPR studies of irradiated and photolyzed ionic liquid glasses revealed important infor-

mation about the initial distributions of holes in several classes of ionic liquids, and their implica-

tions for pathways of radiolytic damage accumulation (Shkrob etal., 2007; Shkrob and Wishart,

2009; Wishart and Shkrob, 2009). The results are consistent with the ndings of the elegant and

detailed radiolytic product studies using electrospray mass spectroscopy by the Moisy group at

CEA Marcoule (Berthon etal., 2006; Bosse etal., 2008; Le Rouzo etal., 2009). Nevertheless, fur-

ther time-resolved investigations of hole and radical chemistry in ionic liquids are sorely needed. It

is expected that transient EPR and vibrational spectroscopic radiolysis studies on ionic liquids will

be conducted in the near future. Since ionic liquids show potential as a medium for the processing

of spent nuclear fuel, further understanding of the interactions of charged particles and photons with

ionic

liquids could make a signicant contribution to future world energy supplies.

284 Charged Particle and Photon Interactions with Matter

aCknowledgments

This work was supported in part at the Brookhaven National Laboratory (BNL) by the U.S.

Department of Energy Ofce of Basic Energy Sciences, Division of Chemical Sciences, Geosciences,

and Biosciences under contract # DE-AC02-98CH10886. Kenji Takahashi acknowledges the Ministry

of Education, Culture, Sports, Science and Technology (MEXT) of Japan for a Grant-in-Aid for

Scientic Research (Priority Area 452 “Science of Ionic Liquids”).

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289

Time-Resolved Study on

Nonhomogeneous

Chemistry

Induced

by Ionizing Radiation

with

Low Linear Energy

Transfer

in Water and Polar

Solvents

at Room Temperature

Vincent De Waele

Université Paris-Sud

Orsay,

France

Isabelle Lampre

Université Paris-Sud

Orsay,

France

Mehran Mostafavi

Université Paris-Sud

Orsay,

France

Contents

12.1 Introduction ..........................................................................................................................290

12.2 Solvent

Decomposition.........................................................................................................292

12.3

Solvation

Processes of Electrons in Polar Solvents..............................................................294

12.4

Spur

Reactions and Time-Dependent Radical Yields ..........................................................300

12.4.1

Ultrafast

Pulse-Radiolysis Setups for the Study of Spur Reactions ......................... 301

12.4.1.1

Goals of Ultrafast Pulse-Radiolysis Measurements .................................. 301

12.4.1.2 Picosecond

Pulse-Radiolysis Systems: A Short Story and the State

ofthe

Art

....................................................................................................302

12.4.1.3 Picosecond

Pulse-Radiolysis Experiments Using Laser-Driven

Photocathode

RF Gun

................................................................................303

12.4.2 Recent Advances in Spurs Chemistry Obtained by Picosecond Pulse-Radiolysis ....... 307

12.4.2.1 Time-Dependent

G-Value of Solvated Electron in Water..........................307

12.4.2.2

Solvent

Effect on the Time-Dependent G-Value of Solvated Electron...... 310

12.4.2.3

Time-Dependent

G-Value of OH

•

Radical................................................. 312

12.4.2.4 Time-Dependent

G-Value of H

•

Atom....................................................... 314

12.5 Efcient

System to Scavenge All Primary Radicals in Spurs.............................................. 315

12.6

Concluding

Remarks ............................................................................................................ 317

References...................................................................................................................................... 318