Geminate Charge Recombination in Liquid Alkane with Concentrated

ARTICLE pubs.acs.org/JPCA

Geminate Charge Recombination in Liquid Alkane with Concentrated CCl4: Effects of CCl4 Radical Anion and Narrowing of Initial Distribution of Cl
Akinori Saeki,*,† Naoto Yamamoto, Yoichi Yoshida, and Takahiro Kozawa* The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

bS Supporting Information ABSTRACT: Dynamics of radical cations and electrons in an admixture of a linear saturated hydrocarbon (n-dodecane) and halocarbon (carbon tetrachloride, CCl4) were investigated by picosecond electron beam pulse radiolysis. The decay of thermalized electrons (eth
) observed in infrared transient photoabsorption were simply accelerated by the addition of CCl4, giving a high rate constant of 2.3  1011 mol
1 dm3 s
1. The decrease of the initial yield of eth
was quantified by C37 (50 mmol), which is linked to the reaction of epithermal electrons (e
) with CCl4. In contrast, the n-dodecane radical cation (RH2•+) monitored in the near-infrared indicated a convex-type dependence of the decay rate on CCl4 concentration, although the initial yield of RH2•+ remained almost constant up to a much higher CCl4 concentration. The decay of RH2•+ was analyzed by Monte Carlo simulations of geminate ion recombination with eth
, chlorine anion (Cl
) formed via dissociative electron attachment, and CCl4 radical anion. The results showed a good agreement with the experiments by considering two assumptions: (1) CCl4 radical anion formed via eth
attachment and (2) narrowing of the initial distribution of Cl
. The decrease in the initial yield of RH2•+ at high CCl4 concentration was well explained by immediate decomposition of CCl4•+ to CCl3+ and hole transfer from CCl4•+ to adjacent RH2 without diffusive motion of the reactants. Time-dependent density functional theory supported the spectroscopic assignment of intermediate species in the n-dodecane/CCl4 system. The present results would be of help in understanding the electron capture reaction in multicomponent systems such as a chemically amplified resist in lithography.

1. INTRODUCTION Geminate charge recombination is a fundamental process involved in the early stage of radiation-induced and photoinduced charge separation in organic materials. In the bulk heterojunction framework of organic photovoltaic cells,1 geminate recombination of charges, which are generated via exciton dissociation at the donor and acceptor interface, may compete with efficient charge collection in devices, resulting in the decrease of device performance.2 In electron beam or extreme ultraviolet lithographies,3 positive and negative charges generated by ionization of the resist matrix undergo a considerable amount of deactivation of deposited energy through the geminate recombination.4 Besides, the ionization and resultant recombination processes cause a spatial blur of the energy deposition profile, which might affect the spatial resolution of nanometer-scale fine patterning of ultrasmall electric circuits.5 Kinetics of geminate recombination measured by transient photoabsorption spectroscopy are well documented with nonpolar solvents such as alkanes (RH2).6
8 As the first event of radiation chemical reactions, alkane radical cations (RH2•+) and electrons (e
) are formed upon exposure to radiation. The electron is quickly thermalized by the loss of its kinetic energy, converting to a thermalized electron (eth
). Geminate charge recombination takes place immediately after the irradiation especially in a low dielectric matrix, because most eth
are within r 2011 American Chemical Society

the Onsager length [rc = e2/(4πεkBT), where e is the elementary charge, kB is the Boltzmann constant, and T is absolute temperature]. RH2 ' RH2 •
þ e


ð1Þ

e
f eth


ð2Þ

RH2 •
þ eth
f RH2 

ð3Þ

Much effort has been devoted to understanding the fundamental aspects of this reaction from the theoretical9 and experimental6
8 viewpoints; however, geminate recombination in a multicomponent system, i.e., in the presence of highly concentrated electron scavengers, has not been well surveyed. Halocarbon is a representative of electron scavengers that have been assumed to capture electrons efficiently and release halogen anions promptly via dissociative electron attachment (DEA). Carbon tetrachloride (CCl4) has been used as a typical electron scavenger, and the radiation chemistry of CCl4 itself has attracted considerable attention due to the complexity and reactive species generated by radiation.10
12 In the presence of CCl4, the following schemes Received: June 25, 2011 Revised: August 2, 2011 Published: August 24, 2011 10166

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are appended to the geminate recombination in pure nonpolar solvent. CCl4 þ eth
ðor e
Þ f CCl4 •
f CCl3 • þ Cl


ð4Þ

RH2 •þ þ Cl
f products

ð5Þ

RH2 •þ þ CCl3 • f CCl3 þ þ RH2

ð6Þ

Equation 4 stands for DEA, but we indicate the intermediate of CCl4 radical anion (CCl4•
) in the equations (vide infra). RH2•+ decays via geminate recombination with chlorine anion (Cl
) and trichloromethyl radical (CCl3•) formed upon dissociation of CCl4•
. Equation 5 would be much more dominant than eq 6, because of the strong Coulombic attractive force of the charges. As a consequence, the reactant of geminate decay of RH2•+ is converted from eth
to Cl
, which might decelerate the decay rate and change the initial distribution function of anionic species. In a chemically amplified resist (CAR)13 of extreme ultraviolet lithography, a large amount of photoacid generator (PAG; e.g., 10 wt %) is involved to generate as much acid as possible that acts as a catalyst of deprotection of the resist polymer during a postexposure bake. PAG is also a strong electron scavenger and suppresses the deactivation path of geminate recombination between the radical cation of the resist polymer and eth
in CAR. Precise knowledge of geminate recombination in a model system with a highly concentrated electron scavenger will facilitate a better understanding of the charge deactivation path of CAR.14 Along with the incentive to clarify the fundamental radiation chemical reaction, we investigate the geminate recombination in a mixture of n-dodecane and CCl4 by using picosecond pulse radiolysis. They are mixed well at the desired fraction, and their dielectric constants are almost the same (2.01 and 2.24, respectively). Moreover, RH2*, RH2•+, and eth
in n-dodecane can be monitored separately by transient photoabsorption in the visible, near-infrared, and infrared regions, respectively, which allows us to discuss the geminate decay of RH2•+ and electron capture by CCl4 .

2. EXPERIMENTAL SECTION Picosecond pulse radiolysis was performed using a 27 MeV L-band electron linear accelerator (linac) at the Institute of Scientific and Industrial Research, Osaka University.15 The samples were irradiated by an electron pulse (ca. 15 ps duration) from the linac. A femtosecond laser pulse from a Ti:sapphire laser system (Tsunami 3941C-S1W, Spectra Physics Inc.) with a regenerative amplifier (Spitfire, Spectra Physics Inc.) and an optical parametric amplifier (OPA-800HG-UR, Spectra Physics Inc.) was used as a probe and detected by a Si or InGaAs pin photodetector. For the measurements of transient photoabsorption spectra in the visible wavelength region, a femtosecond white-light continuum ranging from ca. 450 to 900 nm was produced by focusing a fundamental oscillation (800 nm) into a sapphire plate. The continuum after passing a sample was separated into spectrum components by a monochromator (SP-2358, Princeton Instruments) and monitored by a thermoelectrically cooled CCD camera (PIXIS400BR, Princeton Instruments) which has 400  1340 CCD elements and a 16-bit dynamic analog-to-digital converter. The transient spectroscopy near 800 nm was not possible using the white-light continuum, because of the remaining strong fundamental oscillation and

Figure 1. Transient absorption spectra from the peak to 800 ps delay (blue to red) after electron pulse irradiation observed in Ar-bubbled ndodecane with (a) 0 and (b) 0.1 mol dm
3 CCl4. The solid black line in (a) is a differential spectrum of (a) and (b) at 800 ps, which can extract the contribution of n-dodecane excited state (RH2*).

extremely low stability of the light. The time difference between the electron beam pulse and probe laser was changed by an optical delay. Infinity purity grade CCl4 and the highest purity grade ndodecane (>99.8%) were purchased from Wako Corp. and Aldrich, respectively, and used as received. The samples were loaded in a quartz cell whose optical path was 2 cm and bubbled by Ar gas for at least 10 min. The pulse radiolysis experiments were performed at room temperature. A Monte Carlo simulation was performed according to a previous report.16 The motion of an ion under an electric field for time step Δt is given by Δr = (6DΔt)1/2n + μEΔt, where D, μ, n, and E represent the diffusion coefficient of an ion, the mobility of an ion, a uniform random variable from
1 to 1, and an electric field generated with opposite charge, respectively. The simulation was based on a single spur model. A pair of charges recombined when the separation distance became less than a reaction radius. The Δt was taken as 0.1 ps, which was confirmed to be small enough to not mislead the decay kinetics. In the presence of an electron scavenger, the electron was converted to another species according to the given chemical reaction and the reaction rate constant. If the product was an anionic species, the motion of the charge was simulated further until recombination occurred. The survival probability was calculated by averaging of the recombination time records. More than 2.0  104 routines were performed to obtain one kinetic trace, so that the calculation error was decreased to less than ca. 2%. The kinetics was convoluted by a Gaussian response function (σ = 19 ps).

3. RESULTS AND DISCUSSION Transient Absorption Spectra. Figure 1 shows transient absorption spectra and kinetic traces in n-dodecane with and 10167

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in nonpolar solvent. The black solid lines are the geminate recombination decay reproduced by the Monte Carlo simulation. We used the following equation as an initial distribution function f(r) of charge separation distance r:      r
R r
R exp
ð7Þ f ðrÞ ¼ 1
exp
r1 r2

Figure 2. (a) Kinetic decay of electrons at 1600 nm in Ar-bubbled ndodecane solutions of CCl4. The concentrations of CCl4 are 0, 0.01, 0.02, 0.05, 0.01, 0.02, 0.03, 0.05, and 0.1 mol dm
3 (blue to red). The black lines are electron decays simulated by considering geminate recombination and scavenging (eqs 3 and 4). (b) Observed rate constants (kobs) as a function of CCl4 concentration. The solid line is a least-squares fit of a linear function, which gave a pseudo-first-order rate constant k of 2.3  1011 mol
1 dm3 s
1. (c) Plot of peak intensity of optical density against [CCl4]. The solid line is exp(
[CCl4]/C37), where C37 was 50 mmol dm
3.

without 100 mmol dm
3 CCl4. In the absence of CCl4, the absorption at 850 nm decays rapidly while those at 500 and 600 nm display slower decay (Figure S1 in Supporting Information). The former is ascribed to RH2•+ and the latter is due to excited state (RH2*) formed by geminate recombination of eq 3. A gradual blue shift of transient absorption spectra can be seen from the pulse end to 800 ps delay, in accordance with the formation of RH2*. In contrast, the spectrum shapes do not change significantly in the presence of 100 mmol dm
3 CCl4. The decay of 850 nm is decelerated and identical to those at 600 and 500 nm over the whole time range (Figure S1 in the Supporting Information). CCl4 scavenges e
and eth
with a high rate constant (vide infra) and perturbs the formation of RH2*. Thus the absorption spectrum is readily attributed to RH2•+, in good coincidence with previous reports7 and timedependent density functional theory (TD-DFT) calculations (Table S1 in the Supporting Information). The difference of the absorption spectra at 800 ps between with and without CCl4 is drawn in Figure 1a, where the peak is located around 600 nm and does not extend to the near-infrared region. This differential spectrum is attributed to RH2*, in good agreement with our nanosecond pulse radiolysis (Figure S2 in the Supporting Information) and previous reports.7 Decay of Electron at 1600 nm. For the direct observation of reaction between eth
and CCl4, electrons in n-dodecane with 0
0.1 mol dm
3 CCl4 were measured in the infrared region. Figure 2a displays the kinetic decays of eth
at 1600 nm. Note that this absorption is often referred to as a solvated electron even

where r1, r2, and R are fitting parameters of the first and second exponential functions and a reaction radius of charge recombination, respectively. A single exponential function has been often assumed as f in alkane, showing a better agreement with free ion yield17 and decay kinetics on the long time scale8 than a Gaussian function. However, it deviates from the experimental decay on the short time scale (faster than ca. 50 ps), which has remained controversial. The exponential distribution function might suggest that an ejected electron with kinetic energy loses its energy at one collision like photon absorption obeying the Lambert
Beer law, while a Gaussian function implies energy loss by multiple interactions with media. We therefore incorporated another exponential term (r1) that reduces the density of eth
near the parent RH2•+ like the Gaussian function and mitigates the difference of experimental and simulated curves. The simulation curve is in good coincidence with the experiments using r1 = 4 nm, r2 = 8 nm, R = 0.5 nm, and diffusion constants of RH2•+ (6  10
6 cm2 s
1)18 and eth
(6.4  10
4 cm2 s
1),8 although the theoretical free ion yield was about twice that of the experiments.17 This overestimate would be acceptable as the theoretical value is based on charge escape of infinite distance at infinite time scale, while the experiment was performed by charge transfer to solute at finite time scale, or charge collection under an applied electric field of less than the dielectric breakdown limit. Impurities might reduce the experimental free ion yield. The scavenging reaction of eth
by CCl4 was simulated by multiplying the decay of pure n-dodecane by exp(
kobst). As shown in Figure 2b, a least-squares fit gave the observed decay rates (kobs) for each CCl4 concentration, [CCl4], and exhibited a good linearity up to 0.05 mol dm
3. The saturation of kobs at 0.1 mol dm
3 is a result of the response function limit. The pseudo-first-order rate constant k was estimated in the linear region to be as large as (2.3 ( 0.1)  1011 mol
1 dm3 s
1, in good accordance with another report.19 From an exponential fitting to the fractional decrease of peak optical density, C37, defined by a scavenger concentration that reduces the peak intensity to 37%,20 was obtained as well, to be 50 ( 3 mmol dm
3. These small C37 and large k values demonstrate a high reactivity of CCl4 with e
and eth
. Decay of n-Dodecane Radical Cation at 850 nm. Hereafter we focus on the kinetic decay of RH2•+. Figure 3 shows the kinetic traces of n-dodecane radical cation observed at 850 nm in the presence of various concentrations of CCl4 from 0 mol dm
3 to pure CCl4. With an increase of [CCl4], the decay rate decreased at low [CCl4] and turned to increase at high concentration accompanied by a sudden drop in the peak of the optical density. We plotted the half-lifetime of the decay and peak optical density as a function of [CCl4] in Figure 4. The longest lifetimes lie in the range 0.03
0.2 mol dm
3, while the drop of the peak intensity appears at a quite high concentration of 9 mol dm
3 (94 wt % CCl4). The almost flat tendency of absorption intensity even at high [CCl4] clearly indicates that the absorption of eth
is negligible at 850 nm. Also, the decays at 850 and 1600 nm without CCl4 are identical, so RH2•+ disappears mainly via 10168

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In highly concentrated CCl4 solution, the following reactions should be considered in addition to eqs 1
6. CCl4 ' CCl4 •þ þ e
CCl4 •þ þ RH2 f RH2 •þ þ CCl4

Figure 3. Kinetic traces of n-dodecane radical cation monitored at 850 nm. The concentration of CCl4 was changed from 0 to 10.4 (pure CCl4), color coded from blue to orange in the direction of the arrow.

Figure 4. (a) Plot of optical density peak of n-dodecane radical cation observed at 850 nm as a function of CCl4 concentration. The solid line is an analysis curve based on sphere of action model of static quenching. See the text for details. The inset is the magnification of high [CCl4] region in linear scale. (b) Half-lifetimes of geminate recombination decay of n-dodecane radical cation at 850 nm in the presence of CCl4. They are indicated by closed circles and the dotted line. The inset is the magnification of high [CCl4] region in linear scale. The colored solid lines are simulated curves of half-lifetimes using DCl
= 5  10
5 cm2 s
1 with the initial distribution length (r2) of Cl
changing from 2 to 9 nm (solid black arrow). See Figure S4 in the Supporting Information for the other DCl
cases. Note that CCl4•
is not incorporated into this simulation.

geminate recombination on this time scale rather other losses of ions such as detachment of protons.

ð8Þ ð9Þ

CCl4 •þ f CCl3 þ þ Cl•

ð10Þ

CCl3 þ þ Cl
f ½CCl3 þ 3 3 3 Cl


ð11Þ

Radiolytic reaction in pure CCl4 has been widely investigated for decades, and the characteristic absorption peak at ca. 470 nm has attracted the attention of many researchers. There are many reports on the assignment of this absorption, such as CCl4
,21 CCl4•+,22 ion pair (CCl4•+ 3 3 3 Cl
),23,24 and a complex occasionally separated by solvent such as methylcyclohexane (CCl3 + 3 3 3 Cl
).12,25 From the inspection of the previous reports in sequential order and examination on the optical absorption by TD-DFT (Table S1 and Figure S3 in the Supporting Information), the complex (CCl3 + 3 3 3 Cl
) is the most probable species having the 470 nm absorption.12,25 See Table S1 in the Supporting Information also for the TD-DFT results of other intermediates. In the case of concentrated CCl4 solutions, direct ionization of CCl4 giving rise to CCl4•+ becomes predominant rather than the ionization of RH2 (eq 1), which might monotonically decrease the yield of RH2•+ according to the fractional density of RH2 and CCl4. However, as seen in Figure 4a, the absorption peak remained almost constant, slightly increasing up to 9 mol dm
3. The slight increase is probably due to the increase of absorbed energy arisen from the density change from n-dodecane (0.75 g cm
3) to CCl4 (1.594 g cm
3). The almost flat tendency of the peaks is rationalized by the hole transfer from CCl4•+ to RH2 given by eq 9. This is supported by the fact that the ionization potential (Ip) of CCl4 (11.47 eV) is higher than those of alkanes.18,26 However, once CCl4•+ decomposes to CCl3+ and Cl•, CCl3+ could not withdraw an electron from RH2, since the Ip of CCl3 is small (8.78 eV).25 This scheme is consistent with the present experiments, where no delayed formation of RH2•+ was observed and the peak intensity of RH2•+ suddenly fell off at high [CCl4]. This drop is explained quantitatively by an immediate hole transfer from CCl4•+ to adjacent RH2 molecule just after the ionization of CCl4, which is referred to as the adjacent effect27 or static quenching.28,29 The solid line in Figure 4a was calculated by a sphere of action model:28 a[1
exp(
cNAV)], where a, c, V, and NA are a scaling factor, concentration of n-dodecane, 4πRs3/ 3, and the Avogadro number, respectively. The hole transfer was presumed to occur if the reactants were in the sphere of radius Rs. The best fit indicated Rs = 1.1 nm, which is about 3 times larger than the radius of CCl4. The Rs is, however, overestimated originating from the random stringlike structure of n-dodecane molecule of which the most extended trans-conformational length is ca. 1.6 nm.27 Thus the obtained Rs is understood as the sum of typical reaction radius (0.5 nm) and slight diffusion of CCl4•+ within its lifetime. Let us return to the dependence of half-lifetime on CCl4 concentration. At less than 0.03 mol dm
3, the lifetime simply increased with CCl4 as shown in Figure 4b. This can be linked to the decrease of the diffusion constant of negative charge from eth
to Cl
and the resultant slowing of geminate recombination expressed by eq 5. However, the decrease of lifetime seen at >0.3 mol dm
3 cannot be predicted by only the decrease of the 10169

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The Journal of Physical Chemistry A diffusion constant which just extends the lifetime of geminate decay. The lifetime should be constant at high enough [CCl4] to scavenge most of e
(eth
). We therefore thought that the initial distribution function is narrowed as a result of the high reactivity of CCl4 with e
, which might accelerate the geminate decay at high [CCl4]. The additional parameters are the initial distribution function of Cl
(r1 and r2 in eq 7) produced through eq 4 within the time resolution, and the diffusion constant of Cl
(DCl
). The measured C37 (50 mmol dm
3) was used to determine the fraction of eth
and Cl
at time zero. The eth
was changed to Cl
at the observed rate constant (k = 2.3  1011 mol
1 dm3 s
1), and the delayed geminate recombination was simulated by the Monte Carlo method. The half-lifetimes of total kinetic traces of RH2•+ were collected and shown as the colored solid lines in Figure 4b, where r2 was scanned from 2 to 9 nm with fixed DCl
= 5  10
5 cm2 s
1 and r1 = 2 nm. The calculated halflifetime shows a maximum at a certain [CCl4]; however, the peaks were always located at 0.01
0.02 mol dm
3. The diffusion constant of Cl
(or analogous species) is an unknown parameter and has been reported in a wide range from 10
6 to 10
4 cm2 s
1; for example, the sum of CCl3+ and Cl
in CCl4 is 9  10
5 cm2 s
1 (5.9  10
5 cm2 s
1 converted in n-dodecane using the viscosities of n-dodecane, 1.38 mPa s, and CCl4, 0.908 mPa s),11,18 Cl• in CCl4 is 1.1  10
5 cm2 s
1 (converted, 7.2  10
6 cm2 s
1),30 the sum of positive and negative charges in CCl4 is 7.6  10
4 cm2 s
1 (converted, 5  10
4 cm2 s
1),31 and Cl
in water is (1.0
2.0)  10
5 cm2 s
1 (converted, (0.7
1.3)  10
5 cm2 s
1).18,32 We also examined different diffusion constants of Cl
(1  10
5 and 1  10
4 cm2 s
1, shown in Figure S4 in the Supporting Information), but the maximum always stayed at 0.01
0.02 mol dm
3, although the lifetime of the low diffusion constant was longer than that of the high diffusion constant if they were compared at the same r2. In other words, the diffusion constant and initial separation distance are in a complementary relationship. However, a low DCl
(e.g., 1  10
5 cm2 s
1) forced r2 to be significantly reduced, e.g., 10.1 mol dm
3, which is followed by the sudden drop of initial yield of RH2•+. At such an extremely high [CCl4], the yield of RH2•+ is too small to analyze the kinetics with high accuracy. Regarding DCCl4•
, the value is controversial as is the case with DCl
. The diffusion constant of neutral CCl4 in acetone is as small as 3.29  10
5 cm2 s
1 (converted, 7.3  10
6 cm2 s
1).18 If this value, about 1 order of magnitude smaller than DCl
, is readily applied to the simulation, the lifetime is more elongated, resulting in further separation from the experiments. Therefore, DCCl4•
must be higher than DCl
. We tentatively used DCCl4•
of 2  10
4 cm2 s
1; however, it is unnatural to assume that CCl4•
is 4 times more mobile than Cl
from the viewpoints of their molecular weights and sizes. This might be because the geminate recombination between RH2•+ and Cl
expressed by eq 5 is not diffusion-controlled, because Cl
is a closed shell. In contrast, the recombination with CCl4•
given by eq 12 is expected to occur by diffusion control. These situations would account for the virtually higher DCCl4•
than DCl
. From the wide scanning of parameters, one parameter set that reproduced the experiment was DCl
= 5  10
5 cm2 s
1, τ(CCl4•
) = 2 ns, DCCl4•
= 2  10
4 cm2 s
1, and the narrowing of the initial distribution length r2 from 7 nm (0
0.2 mol dm
3) to 5.5 nm (5 mol dm
3) shown as the closed triangles in Figure 5a. The τ value was much longer than we expected from the reported lifetimes of CCl4•
in polar solvents as discussed above.37,39 Since solvation has been reported to promote the dissociation of CCl4•
,35 nonpolar solvent might suppress the dissociative path and extend the lifetime. In contrast, 1-butyl-3-methylimidazolium

cations in the ionic liquid were suggested to stabilize CCl4•
. Although the stabilities of radical anions of halogenated carbons and aromatics have been vigorously investigated,42 elusive aspects have still remained. For the absolute determination of the parameters, further discussion and experiment might be needed in the future. We should note again that it is not possible to fit the experiments over the whole CCl4 concentration unless CCl4•
is incorporated. Halocarbons and aryl halides are matters of great interest in synthetic carbon chemistry,43 for negative-type organic semiconductors such as fluorinated pentacene,44 and for functional materials such as resists for lithography.45 The electron acceptability and stability of these radical anions are some of the important characteristics in each field of chemistry. Especially in CAR of lithography, PAG effectively capture electrons, giving rise to counteranions and radicals immediately.46 An acid composed of a counteranion and proton plays a key role in the acid-catalytic reactions. The ability of electron capture, stability of the PAG radical anion, and spatial distribution of the generated counteranion would influence the accuracy of fine patterning in CAR systems. The present results are expected to produce a more detailed picture of chemical reaction between electrons and functional molecules in the fields of applications.

4. CONCLUSION The geminate charge recombination in an admixture of ndodecane and CCl4 was investigated by picosecond pulse radiolysis. The reactivity of CCl4 with eth
and e
was observed as the decay acceleration and decrease of the initial yield of infrared transient absorption, exhibiting a high rate constant of (2.3 ( 0.1)  1011 mol
1 dm3 s
1 and a small C37 of 50 ( 3 mmol, respectively. The absorption intensity of RH2•+ at 850 nm was almost constant and suddenly dropped at extremely high [CCl4], which was understood as hole transfer from CCl4•+ to RH2 via static quenching and immediate decomposition of CCl4•+ to CCl3+ that can no longer accept an electron from RH2. In contrast, the decay rate of RH2•+ showed a convex-type dependence on [CCl4]. The increase of half-lifetimes was analyzed by considering two reaction paths: (1) CCl4•
formed via eth
capture by CCl4 and (2) Cl
formed via DEA of e
by CCl4. DCl
smaller than DCCl4
can be understood as non-diffusion-controlled reaction of RH2•+ and Cl
. The decrease in lifetime of RH2•+ was rationalized by further narrowing the initial distribution of Cl
, due to the high energy loss efficiency and DEA of CCl4. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1, kinetic traces of Figure 1; S2, nanosecond transient absorption spectra of ndodecane with/without CCl4 or O2; S3, TD-DFT calculations of (CCl3+ 3 3 3 Cl
) complex in gas phase; S4, simulated half-lifetime using DCl
= 1  10
5 and 1  10
4 cm2 s
1; S5, initial distribution length (r2) that shows a good agreement with the experiments in the case of DCl
= 1  10
5 and 1  10
4 cm2 s
1; and S6, calculated viscosity, Onsager length, and density of the mixtures. Table S1, TD-DFT results of RH2•+, (RH2•+ 3 3 3 Cl
), RH•(
H+), (CCl3+ 3 3 3 Cl
), CCl3+, CCl3•, CCl4•
(Td symmetry, elongated, and compressed structures), and CCl4•+. This material is available free of charge via the Internet at http://pubs.acs.org. 10171

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (A.S.); kozawa@sanken. osaka-u.ac.jp (T.K.). Present Addresses †

Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.

’ ACKNOWLEDGMENT We greatly appreciate the experimental help of Dr. K. Okamoto, Mr. K. Kaseda, Mr. K. Takeya, and Mr. S. Suemine at ISIR, Osaka University. We would like to extend our gratitude to Prof. Y. Hatano at Japan Atomic Energy Agency for his advice on dissociative electron attachment. This work was supported in part by a grant-in-aid for scientific research from Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT) and from the Japan Society for the Promotion of Science (JSPS). ’ REFERENCES (1) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (b) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (2) (a) De, S.; Pascher, T.; Maiti, M.; Jespersen, K. G.; Kesti, T.; Zhang, F.; Ingan€as, O.; Yartsev, A.; Sundstr€om, V. J. Am. Chem. Soc. 2007, 129, 8466. (b) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 3030. (c) Nogueira, A. F.; Montanari, I.; Nelson, J.; Durrant, J. R.; Winder, C.; Sariciftci, N. S. J. Phys. Chem. B 2003, 107, 1567. (d) Meskers, S. C. J.; van Hal, P. A.; Spiering, A. J. H.; Hummelen, J. C.; van der Meer, A. F. G.; Janssen, R. A. J. Phys. Rev. B 2000, 61, 9917. (e) Offermans, T.; Meskers, S. C. J.; Janssen, R. A. J. Chem. Phys. 2005, 308, 125. (3) International Technology Roadmap for Semiconductor (ITRS), 2010 ed.; http://www.itrs.net/. (4) Kozawa, T.; Saeki, A.; Tagawa, S. J. Vac. Sci. Technol., B 2004, 22, 3489. (5) (a) Saeki, A.; Kozawa, T.; Tagawa, S.; Cao, H. B. Nanotechnology 2006, 17, 1543. (b) Saeki, A.; Kozawa, T.; Tagawa, S.; Cao, H. B.; Deng, H.; Leeson, M. J. Nanotechnology 2008, 19, 015705. (c) Saeki, A.; Kozawa, T.; Tagawa, S. Appl. Phys. Lett. 2009, 95, 103106. (6) (a) van den Ende, C. A. M.; Warman, J. M.; Hummel, A. Radiat. Phys. Chem. 1984, 23, 55. (b) Zhang, T.; Lee, Y. J.; Kee, T. W.; Barbara, P. F. Chem. Phys. Lett. 2005, 403, 257. (c) Healy, A. T.; Underwood, D. F.; Lipsky, S.; Blanka, D. A. J. Chem. Phys. 2005, 123, 051105. (d) Borovkov, V. I.; Anishchik, S. V.; Anisimov, O. A. Radiat. Phys. Chem. 2003, 67, 639. (e) Siebbeles, L. D. A.; Emmerichs, U.; Hummel, A.; Bakker, H. J. J. Chem. Phys. 1997, 107, 9339. (f) Long, R. H.; Lu, H.; Eisenthal, K. B. J. Phys. Chem. 1995, 99, 7436. (g) Hyde, M. G.; Beddard, G. S. Chem. Phys. 1991, 151, 239. (h) Trifunac, A. D.; Sauer, M. C.; Jonah, C. D. Chem. Phys. Lett. 1985, 113, 316. (i) Jonah, C. D. Radiat. Phys. Chem. 1983, 21, 53. (j) Sauer, M. C.; Jonah, C. D. J. Phys. Chem. 1980, 84, 2539. (k) Thomas, J. K.; Johnson, K.; Klipper, T.; Lowers, R. J. Chem. Phys. 1968, 48, 1608. (7) (a) Tagawa, S.; Washio, M.; Kobayashi, H.; Katsumura, Y.; Tabata, Y. Radiat. Phys. Chem. 1983, 21, 45. (b) Tagawa, S.; Hayashi, N.; Yoshida, Y.; Washio, M.; Tabata, Y. Radiat. Phys. Chem. 1989, 34, 503. (c) Yoshida, Y.; Tagawa, S.; Washio, M.; Kobayashi, H.; Tabata, Y. Radiat. Phys. Chem. 1989, 34, 493. (d) Yoshida, Y.; Tagawa, S.; Kobayashi, H.; Tabata, Y. Radiat. Phys. Chem. 1987, 30, 83. (e) Yoshida, Y.; Mizutani, Y.; Kozawa, T.; Saeki, A.; Seki, S.; Tagawa, S.; Ushida, K. Radiat. Phys. Chem. 2001, 60, 313.

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