The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DEVELOPMENT OF SIMULTANEOUS MEASUREMENT OF
TIME-RESOLVED MICROWAVE CONDUCTIVITY AND LIGHT ABSORPTIONSPECTROSCOPY USING ELECTRON BEAM LINEAR ACCELERATOR
A. Saeki *A), S. Seki A), T. Sunagawa B), K. Ushida C), Y. Koizumi A), T. Kozawa A) and S. Tagawa A)
A)

The Institute of Scientific and Industrial Research,
Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapanB)
Fukui University of Technology, 3-6-1 Gakuen, Fukui, Fukui 910-8505, JapanC)
Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0106, Japan
Abstract
In order to perform conductivity experiments ofisolated conductive polymer in non-polar liquid, wedeveloped “time-resolved microwave conductivity”
measurement system using X-band microwave.
Homodyne system consisting of main and reference arms
was adopted, so that good S/N ratio was achieved. Lightabsorption spectroscopy utilizing nanosecond streak
camera is installed into the same geometric set up, whichhelps us assign the conductivity signals to an intermediate
species and evaluate their concentration. After system
optimization and development of automatic dataacquisition software, typical conductivity kinetics ofstandard sample was obtained. Charged carrier dynamics
and the evaluation of mobilities in σ- and π- conjugatedpolymers such as polysilane will be investigated bymeans of dielectric absorption technique. In the future, theionization source will be replaced by electron beam fromL-band linac and pulse radiolysis will be performed
simultaneously with the conductivity measurement. The
combination of microwave conductivity measurement andlight absorption spectroscopy enables us to study thecharge transfer dynamics, the conformational effects ofpolymer on the charge localization, mobility evaluationand so on.

1. INTRODUCTION
Dielectric absorption of microwave[1] occurs owing to
the interaction with dipole moment and/or charged carries
produced by photo-ionization or direct ionization via
radiation such as electron beam. The real and imaginaryparts of dielectric constant in dielectric media relate withpassed or reflected microwave power and its phase shift,respectively.
Time-Resolved Microwave Conductivity (TRMC)
technique[2,3] was developed and has served for theinvestigation of charged carrier mobility at pulse end.
This technique has also been applied for the studies ofsinglet, triplet, radical species and CT state in condensed,
liquid and gaseous phase. The signal detection in theseexperiments was based on a reflection cell and direct
pick-up of reflected microwave from the cell, in order toobtain high time resolution. However in our new system,
resonant cavity and homodyne circuit were adopted, so* saeki03@sanken.osaka-u.ac.jp

that intense signal and phase change detection of reflected
microwave became available.
Target structure in future nanotechnology, whether thetechnique is bottom-up or top-down, aims at the size ofone molecule or one atom. For example, carbon nanotube,
which has been extensively investigated worldwide, isexpected to serve as a basic element in single electrontransistor, wiring and so on. Considering the scale, theintrinsic mobility of charged carrier will play animportant role in realizing such a device. Requirement of
direct observation of intrinsic mobility in σ- and πconjugated polymer arises for material screening andevaluation of polymers which is used for organic
electro-luminescent device and so on.
Nano-scale gap bridged by carbon nanotube which isfabricated by photo or EB lithography seems to be goodsolution for the evaluation of conductivity in single
molecule, but has problems like contact between themolecule and electrode. The conductivity of the molecule
may show several values, depending on the electrodewriting process, contact condition such as ohmic or not.
Time-of-flight experiments also need samples without
pin-hole and impurity.
To address these issues, TRMC is utilized to measure
the conductivity in condensed matter. This techniqueenables us to observe the intrinsic conductivity along an
isolated polymer chain without fabricating electrodes via
lithographic process.

2. MICROWAVE CIRCUIT FOR TRMC
AND SIMULTANEOUS LIGHT
ABSORPTION SPECTROSCOPY
Figure 1 shows the microwave circuit for the dielectric
absorption measurement. The circuit made by Micro
Electro Co. Ltd. was designed for X-band radio frequency
of around 9 GHz, and the microwave from Gunn
oscillator is divided and guided to the main and signal
arms. The microwave in the main arm goes through thecirculator and into the resonant cavity. Microwave powerin the resonant cavity is adjusted about 3 mW via
attenuator in the main arm. Reflected microwave from theresonant cavity is amplified by FET amp. and merges
with the microwave from the reference arm in magic tee(mixer).

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

AFC unit
Att. 3

Freq. Meter

Att. 2P.S.
MagicTeeDiff. Amp
Osc.

Gunn
Oscillator
Att. 1

Excitation
Laser

Circulator
FET Amp.

Coupl. Iris

CavityFigure 1 Microwave circuit for the dielectric absorption measurement.
The sample is placed in the position of strongest
electric filed in the cavity (Figure 2).
Sample Cell

Analyzing light

Excitation
Laser

adjusted π/2 in order for the detection of resonant
frequency shift.
As described above, our microwave circuit consistingof the main and reference arms is homodyne. For theachievement of long term stabilization of the microwave,
a portion of microwave guided into reference arm is also
divided and led to automatic frequency controller (AFC)arm. The closed loop from Gunn oscillator, AFC arm,AFC unit and Gunn oscillator has a feedback, but is not
used up to date because of an experimental inconvenience.
In addition, since TRMC measurement is carried out by
one laser pulse and its decay curve falls to base line
within usually about 1 ms, AFC feedback loop is notnecessary.
Relationship between reflected microwave power andconductivity is described as follows,

Figure 2 Set up of sample in the resonant cavity.
Resonant cavity has a literal property that it confines
microwave of certain frequency (resonant frequency). Themicrowave frequency is adjusted to the resonant
frequency before the transient charged carriers are
generated by the excitation laser. Therefore in the casesthat the frequency of incident microwave changes fromthe resonant frequency and/or dielectric absorption(microwave power loss) occurs, the Q value changes,
leading to the reflection of the microwave from theresonant cavity. Q value is defined by the following
equation.

Q0 = f 0 ∆ f 0(1)

The phase difference of microwaves from the main andreference arm is adjusted zero, so that the differentialchange of Q value is detected. On the other hand, it is∆σ = e¦ µ i N i =i

1 ∆PrA Pr(2)where ∆σ, e, µi, Ni, Pr, ∆Pr are conductivity, theelectronic charge, mobility of i species, number of ispecies, reflected microwave power and change of
microwave power reflected. A is sensitivity factor anddecided from measurable parameters such as resonant
frequency, fraction of reflected and incident microwave
power, static dielectric constant of resonant cavity
including sample for each experimental condition.
While the conductivity can be measured by TRMC, it
appears as the sum of mobility and number of charged
carrier. If one wants to obtain the value of mobility itself,
the number of charged carrier must be assigned
independently.
Simultaneouslightabsorption
spectroscopy helps us assign the conductivity kinetics to a

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Streak trig.
Grating

Gate trig.
CCD

Streak camera

DG535
DG535

Bias

Nd:YAG laser
DG535

Microwave
circuit

ArF, KrF
excimer laser

P.C.
DG535

CounterH.V.

Ext. trig.

Xe Lamp.
OscilloscopeFigure 3 Schematic drawing of simultaneous TRMC and light absorption spectroscopy experiment.
certain species and estimate its concentration. The
schematic drawing of simultaneous TRMC and lightabsorption spectroscopy experiment is illustrated in
Figure 3. The nanosecond excitation laser pulses fromArF excimer (193 nm), KrF excimer (248 nm), THG (355nm) and SHG (532 nm) of Nd:YAG are available up todate.
25(a)Intensity [arb. unit]20

3. USE OF ELECTRON BEAM FROM
LINAC AS AN IRRADIATION SOURCE
The use of electron beam from liniac as an irradiation
source has advantages compared to photo-ionization.
High-energy electron beam (EB) homogeneously ionize
the sample. Namely, high-energy EB shows goodtransparency for many kinds of organic compounds.
Besides, the G value (number of produced species /100
eV absorbed dose) of primary species can be estimatedeasily.

151050-5- 0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

REFERENCES

Time [µs]575 nm
0 ns

Typical dielectric absorption kinetics and absorption
spectrum of poly-dioctylfluorene film were observed byusing TRMC as shown in Figure 4. The analysis andsystematic investigation of this π-conjugated polymer will
be performed in the near future.

710 nm(b)

500 nsFigure 4 (a)Dielectric absorption kinetics and (b) lightabsorption spectrum of poly-dioctylfluorene film

[1] H. Shimamori, “Microwave Dielectric Absorption
Studies of Photochemical Transient”, Progress in
Photochemistry and Photophysics, Vol VI, CRC
Press[2] J.M.Warman et al., J. Phys. Condens. Matter 14(2002) 9935
[3] P.P. Infelta et al., Radiat. Phys. Chem., 10 (1977) 353