Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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310 Charged Particle and Photon Interactions with Matter

with the published experimental and theoretical studies discussed above. The changes observed

in the 0–50 ps timescale are supported according to the authors by ultrafast photolysis experi-

ments that also show an initial decay assigned to a “dispersive dynamics of ultrafast radicals

events.” The results obtained by Oulianov etal., (2007) are also reported in Figure 12.17 (right):

in contrast, as expected for the solvated electron, a quasi-at absorption signal is observed during

the rst tens of picoseconds. To obtain these results, the authors conducted the experiments with

10 Hz LWA systems, and combined their pulse-probe setup with the measurement of hydrated

electron decay at the nanosecond timescale as a normalization source to correct the dose uc-

tuations shot to shot. Theauthors also outline the difculty to keep this normalization constant

during the series of experiments because of laser pointing instabilities.

12.4.2.2 solvent

ffect

on the t

ime-dependent

G-

value

of s

olvated

electron

Tetrahydrofuran

(THF) is an aprotic solvent that is largely used in organic chemistry. Considering

the spurs reactivity, the interest of THF comes from its dielectric constant that is 10 times lower

than that of water. In THF, the Coulombic eld is very strong and favors long-range interactions

between charged reactive centers. It is also known that ionized THF formed by irradiation is

rapidly deprotonated and that solvated electrons can recombine with both the proton adduct and

the residual C-centered radical (Salmon etal., 1974). Based on a systematic study of solvated

electron scavenging by methyl bromide and ethyl bromide with different concentrations in THF,

it was concluded (Kadhum and Salmon, 1984) that the initial formation yield of solvated electron

in THF is around (4.2 ± 0.4) ×10

−7

mol J

−1

and that, after spur reactions, the value is decreased to

(0.36 ± 0.02) × 10

−7

mol J

−1

at microsecond timescale (or (0.4 ± 0.02) × 10

−7

mol J

−1

depending on

data treatment). The transient absorption signals of solvated electron in water and THF recorded

at 790 nm after radiolysis by a picosecond electron pulse are reported in Figure 12.18 (De Waele

et al., 2006, 2007). The ratio between the initial absorbance (at 30 ps) and the absorbance at

Normalized signal

25 50

Time (ps) Time (ps)

Time (ps)

ΔOD ΔOD

Low energy radiation

(UV photons: 2 × 4 eV)

Pure liquid water

294 K

Femtolysis

high energy radiation

(relativistic electrons)

10×10

–3

15×10

–3

–20(a)

(b)

0 20

2000 400

1 M

5 M

10 mm

5 mm

2 mm

Probe:820 nm

Figure 12.17 Pulse-radiolysis experiments performed using relativist electron bunches generated by laser-

wakeeld acceleration as irradiation beam and a laser pulse around 800 nm as a probe beam. Left: comparison

of the decays of solvated electrons formed by two-photon ionization, and pulse radiolysis of water. (Reprinted

from Brozek-Pluska, B. etal., Radiat. Phys. Chem., 72, 149, 2005.) Right: decay of solvated electrons formed

by pulse radiolysis of water measured for different cell thicknesses (a) and in the presence of 1 and 5 M acid

(b). (Adapted from Oulianov, D.A. etal., J. Appl. Phys., 101, 053102, 2007.)

Time-Resolved Study on Nonhomogeneous Chemistry Induced in Polar Solvents 311

200 ps is 0.95 and 0.79 in water and in THF, respectively. Figure 12.19 (inset) presents the decay

of solvated electrons in THF at longer times, up to 2.5 ns. The G-value of solvated electrons in

THF is much more time dependent than those in polar solvents like water and alcohols. From

the t of solvated electron decay, it was found that the initial separation distance between sol-

vated electrons and the cation is around 4 nm (De Waele etal., 2007). This distance is shorter

than the Onsager radius for this solvent (7 nm). The low yield of solvated electron in THF can be

100

THF

–50 0 50

Pulse-probe delay (ps)

Absorbance (mOD)

100 150 200

Absorbance (mOD)

Figure 12.18 Decay of solvated electrons measured in water (right scale) and in THF (left scale), see

(De Waele etal., 2006) for details. (Reprinted from De Waele, V. etal., Chem. Phys. Lett., 423, 30, 2006.)

0.01

0.008

0.006

AbsorbanceResidues

0.004

0.002

–0.002

0 500 1000

Time (ps)

1500 2000 2500

1.0

1.5

2.0

G (10

–7

mol J

–1

)

2.5

3.0

500 1000 1500

Time (ps)

2000 2500

Figure 12.19 Comparison of the time-dependent G-value of solvated electrons in THF, determined by

scavenging method and direct measurement of the time-dependent decay. The dashed line corresponds to the

time-dependent G-value recalculated from the scavenging data from Salmon etal. (1974), the dotted-line gives

the data uncertainty on these values. The grey area is the direct time-dependent G-value obtained from the

tted experimental decay given in inset and represents the 99% condence interval, given data uncertainty.

Details are given in De Waele etal. (2006).

312 Charged Particle and Photon Interactions with Matter

understood in terms of the short thermalization distance of the electron compared to the Onsager

radius in this solvent.

The time-dependent G-value obtained from the scavenging data of (Kadhum and Salmon, 1984)

is compared with the t of the time-resolved measurement in Figure 12.19. A very good agree-

ment between the kinetics decay and the value obtained by the scavenging method is obtained

between 500ps and 2.5ns. At short times (less than 500ps), there is a discrepancy, which is under-

standable because the scavenging method is not appropriate at a very short time. According to the

time-resolved measurements in THF, the G-value at 30ps is around 2.7 × 10

−7

mol J

−1

and from

the deconvolved signal at zero time it is around 3 × 10

−7

mol J

−1

that is noticeably smaller than the

value reported by scavenging method (4 × 10

−7

mol J

−1

). This suggests that either the initial yield in

THF (at zero time) is lower than that in water or that a very fast decay occurs within the electron

pulse. Results reported by Scwartz etal. (Martini etal., 2000) and obtained by femtosecond laser

photolysis show that the decay in the rst hundred picoseconds is indeed very fast. Recently, the

reactivity between Biphenyl molecules and the precursors of the solvated electron in THF was stud-

ied by picosecond pulse-radiolysis (Saeki etal., 2007). The results also support an initial G-value of

solvated

electrons equal to 2.7 × 10

−7

mol J

−1

at 30ps in THF.

The yield of solvated electron was also measured in different alcohols by picosecond pulse-

radiolysis (Lin etal., 2006; Muroya etal., 2008). The data are summarized in Table 12.2, together

with the values for water and THF. Compared to the situation in water, the initial G-values in

alcohols are also found close to the value of 4.0 × 10

−7

mol J

−1

. However, the decays are more

pronounced than that in water, in good agreement with the dielectric constant lower in alcohols

than in water.

12.4.2.3 time-dependent

G-

value

of oh

•

radical

The G-value of OH

•

after 100 ns is well known. Nevertheless, its initial G-value is still contro-

versial. Direct measurements of the G-values of the OH

•

radical were performed (Jonah and

Miller, 1977). The authors reported the time dependence of G(OH

•

) by following the decay of

its weak absorption band in the UV. A value of 6.1 × 10

−7

mol J

−1

was estimated at 200 ps. Other

evaluations were obtained from scavenging studies using formate or hexacyanoferrate ions.

These data are limited to a scavenging power of 0.7 × 10

−1

, and different stochastic and deter-

ministic modelings of water radiolysis were performed (Figure 12.20). The often-cited value of

G(OH

•

) = 6.1 × 10

−7

mol J

−1

for OH

•

radical at 200ps was based on an estimated G

200ps

(

−

) = 4.7 ×

−7

mol J

−1

for solvated electrons. The OH

•

yield was reduced to G

200 ps

(OH

•

) = 5.3 × 10

−7

mol J

−1

the basis of the recent G-value of hydrated electron. Jay-Gerin and Ferradini reported that the

G-value for OH

•

at 100 ps is 4.6 × 10

−7

mol J

−1

(Jay-Gerin and Ferradini, 2000). This controversy

table 12.2

G-values

easured

for the s

olvated

electron in a

lcohols,

ater

and thF

25°C

(mpa · s)

dielectric

Constant λ

max

(nm)