The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Picosecond Pulse radiolysis Study on Primary Processes of Radiation Chemistry
Yoichi YOSHIDA, Yoshida, Yasuhiro MIZUTANI, Takahiro KOZAW A, Miyako MIKI, Shuhei SEKI, Tamotsu
YAMAMOTO, Katsunori HORI, Yoshinobu IZUMI*, kiminori US HIDA**, Seiichi TAGAW A
The Institute of Scientific and Industrial Research, Osaka University
Mihogaoka 8-1, Ibaraki, Osaka 567, Japan*Department of Nuclear engineering, Osaka university
Yamadaoka 2-1, Suita, Osaka, 565, Japan**The insti~te of Physical and Chemical Research (RIKEN)
ffirosawa 2-1, Wako-shi, Saitama, 351-01 Japan

Abstract

The primary processes of the radiation chemistrywere studeid by a new picosecond pulse radiolysis
system for the absorption spectroscopy at ISIR,
Osaka University. The system was composed of a38 MeV picosecond electron linac for theirradiation source and a femtosecond laser
system for the analyzing .light. The processes ofgeminate ion recombination in liquid alkanes wasobserved on the picosecond time-scale and wasanaylzed base on the diffusion theory.
1. Histry·of Picosecond Pulse Radiolysis
Primaryprocesses
ofradiation
chemistry is very important to know thewhole reaction processes induced by highenergy radiation. Pulse radiolysis technique
is one of the most powerful method toresearch the primary processes, because very
fast reaction can be detected directly.
There are two method, the time
resolved
emissionandabsorption
spectroscopy, to measure the very fastreaction. The · studies on excited states andenergy transfer were done by using
picosecond pulse radiolysis for emission
spectroscopy 1 · 2 ·3· 4 ·5·6wi~h a very fast responselight detector such as a streak camera. In
absorption spectroscopy, the streak camera 7·8
and very fast response detectors also wereused. However, the. experiment had thedifficulty to make very intense analyzing
light.
The stroboscopic method does not
required the very fast light detector. The
time resolution is mainly decided by thepulse width of the radiation source and theanalyzing light. The several storoboscopic
picosecond pulse radiolysis systems weredeveloped since the first stroboscopic.picosecond pulse radiolysis by Hunt et al.1°in 1970. Argonne group 11 developed thepicosecond pulse radiolysis by using a single
electron pulse from a L-band linac in 1980.
Later so called twin-linac system 12,l3,l 4 weredeveloped at Tokyo univ. in 1985. In thesesystems, the Cherenkov light pulse producedby the high energy electron was used for the
analyzing light.
Although the primaryprocesses of radiation chemistry werestudied by using these pulse radiolysis, they
were not good at the infrared spectroscopy,
because \\of the low intensity of theCherenkov light in the infrared region.
Apicosecond
pulse
radiolysis
systeml5,16 by using the laser diode instead ofthe Cherenkov light was developed in 1991.
The system enabled theabsorption
spectroscopy from visible to near infrared
region. However, it was difficult to obtained\L-band
Linac t - - - - - - - 1..

~~syn<h~JI

Femtosecond

SHG

me
OPO
WHITE
250 nm - 2 JLDl
Fig.l Concept of a new picosecond pulse
radiolysis system.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Fig.l Block diagram of the new picosecond
pulse radiolysis system.

complete spectra, because the laser diodewas not tunable light source.
Recently, a new picosecond pulse
radiolysis system has been developed in
Osaka university. Figure 1 shows theconcept of the new system. The femtosecond
laser which is synchronized with thepicosecond electron pulse is used for theanalyzing light. The tunable laser pulse
could cover the wide wavelength region from .
ultra violet to infrared by the non-linear
optical effect. The study on the primaryprocess of the radiation chemistry has beenstatred by using the new system.
2. Geminate Ion Recombination
The most important process ofradaiation chemistry in liquid alkanes is thegeminate ion recombinationl7,18 of alkaneradiacal cations and electrons which areproduced by the ioniziation. Figure 2 shows
the behavior of the geminate process. Afterthe thermalization of the electron emitted
from the parent alkane by ionization, aparent radical cation and an electron diffuse
under the Coulomb interaction. Most pairs
recombinaed very quickly on the picosecond
time-scale, due to the long-range Coulombinteraction in non-polar liquid. Small
fraction of the geminate pairs escapes therecombination and becomes the free ion:
The geminate ion recombination is aninhomogenious reaction and the kinetics is
described the diffusion theory under thethecalled,
sopotential,
Coulomb
Smoluchowski equation19,20, as follows.

,_ =vv(v w

1 vv+ wkTI)(1)

where ro, D, k, and V are the survival
probability of the geminate pairs, the sum of
the diffusion coefficients of the pairs, theHoltzman constant, and ·the Coulomb
potential, respectively. The equation shows
the rapidity of the geminate ion
recombination depends on the D.
The geminate ion recombination has
andtheoreticallystudiedbeen
experimentall y by many researchers.
However, it had been difficult for a long time
to observation of the geminate decays in neatalkanes by using pulse radiolysis, because
Usually, thethe decays are so quick.
geminate decay of solute pairs instead of thealkane radical cations and electrons had
been studied21, because the solute geminate
decays were slow due to the large diffusion
coefficients.
3. Primary processes of radiation chemistry of
liquid alkanes
Figure 3 shows the typical timedependent behavior of transient absorptionof n- dodecane radical cations obtained in thepicosecond pulse radioly$is liquid n-dodecane
monitored at 830 nm22. The cation radicals
formed within the time resolution of thesystem. The decay showed a typicalgeminate decay. The theoretical decay line
was obtained by the Smoluchowski equation.
In the analysis, the sum of the diffusion
coefficients of 6 x 10·4 cm2Js for the electron
and _the radical cations and the exponential
initial distribution with the initial.
separation of 6.6 nm were used. Theseparameters agreed with reported values15,l6.

0.40
0.30
!2'c::1t0.20

ci 0.10

d0.00
-o.10
00.2

0.4

0.8
0.6Time [ns)

1.2

1.4

Fig. 3 Typical time-dependent behavior . of transientabsorption of n- dodecane radical cation obtained in thepicosecond pulse radiolysis liquid n-dodecane monitored at
830 om. ( • : experimatal data, line : theoretical decay baseon diffusion theory which is deconvoluted with the response
function.)-523-

The lith Symposium onAccelerator Science and Technology, Harima Science Garden City, 1997

Table 1 Mobilities of electrons, characteristi c lifetimes,initial spatial distributions and initial separation.
Liquids
Mobilities
lifetime
distributions(ps)(cm2Ns)trans-decalin
0.013
35
exponential
methylcyclohexane
0.044
16
exponential
n-hexane
0.071
9.4
exponential
cis-decalin
0.10
5.6
exponential
cyclohexane
0.23
3.2
exponential
isooctane
5.3
0.43
exponential
neopentane
50
0.63
gaussian
TMS
100
0.31
gaussian
exponential R(r,ro)=(l/ro)exp(-r/ro)
gaussian:

Initial separation
(nm)
6.1
6.2
6.0
5.6
6.1
9.3
13.2
13.2

R(r,ro)=(4r2/rr 0.5ro3)exp( -r/ro)

In other alkanes, such as cyclohexane
and n-hexane, the decays of the electrons
agree with the Smoluchow ski equation. The
mobilities of the electrons, the characterist ic
lifetimes, initial spatial distribution s, andinitial separation are summarize d in Table
121,23.
The difference of the initial
distribution depends on the slow down
processes of electrons at ionization.
Acknowledg ment
The Autors are grateful to Prof. S. Okuda,Dr. Y. Honda and Dr. N. Kimura, ISIR, Osaka
University, for helpful discussions and Mr. S.
Suemine, Japan Unikon Corp. for the operation ofthe linac. Also, we are thankful to Mr. H. Ookawa
and Mr. T. Kojima for their experimenta l support.
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