Geminate recombination kinetics of solute radical ions

Radiat. Phys. Chem Vol 19, No 4, pp. 277-282, 1982

0146-57241821040277-06503.0010

Printed m Great Brtttun

Pergamon Press Ltd

GEMINATE RECOMBINATION KINETICS OF SOLUTE RADICAL IONS S I N G L E T E X C I T E D S T A T E F O R M A T I O N IN C Y C L O H E X A N E SOLUTIONS OF BIPHENYL S. TAGAWA~,M. WASHIO, Y. TABATA and H. KOBAYASHI Nuclear Engineering Research Laboratory, Faculty of Engineering, University of Tokyo, Tokaimura, Ibaraki, Japan 319-11

(Received 23 August 1981) Abstract--Transient absorption spectra of the solute anion, cation and triplet state and the solute fluorescence in the pulse radiolysis of 0.1 mole l-I biphenyl in cyclohexane were observed on a nanosecond timescale longer than Ins after a 20 ps pulse. The formation of the solute excited singlet state is mainly due to the geminate ion recombination reaction even in the high concentrated solutions. The decay of the solute ions obeys the reciprocal square root dependence on time longer than 10 ns from the end of a 10 ps pulse. The slope of this reciprocal square root plots agrees with the literature value on a longer timescale obtained by microwave absorption. The yield of free ions obtained from the intercept of the slope agrees also with the literature values obtained by the field clearing method. Ratio of the formation rate of the solute excited triplet state to the decay rate of the solute anion changes in a time range between 5 and 20 ns. It is very well correlated with a theoretical calculation of spin correlation decay of the germinate ion pairs by Brocklehurst, although the formation of the solute triplet state was observed even on a timescale shorter than 5 ns from the end of a 20 ps pulse, where loss of spin correlation is negligibly small.

1. I N T R O D U C T I O N RECENTLY we have been studying the formation of solute excited singlet states in saturated, ~1-4~ aromatic °) hydrocarbon and other ~6" solvents by using a picosecond pulse radiolysis system for emission spectroscopy with a response down to 20ps. Two kinds of the formation processes of excited singlet states of solute molecules have been observed in a picosecond time region in these solvents. ¢t4~ The ratios of two components are significantly depending on the solute concentrations and kinds of solvents."-" In cyclohexane, a fairly large slower formation component has been observed, contrasting with other saturated hydrocarbon solvents. In diluted cyclohexane solutions, the slower formation component of solute excited singlet states is mainly due to the solute geminate ion recombination and partly due to the energy transfer from the excited singlet states of solvent molecules. ¢4~

tResearch Center for Nuclear Science and Engineering, University of Tokyo, Tokai-mura, Ibaraki, Japan 319-11. RPC Vol 19. No 4----R

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The faster component has not been made clear, but could be due to (~erenkov light. ~4"7) Nanosecond pulse radiolysis data show that certain amount of solute excited triplet states in cyciohexane are produced within the limit of the detection system, while the remaining component is generated over about 100 nsfl ) The development of the excited triplet states over 100 ns is replaced by decay of the solute anions,c9"1°~ Geminate recombination decay kinetics of solute ions in pulse irradiated solutions of biphenyl in cyclohexane have been extensively investigated in a time rage from 50ns to 5 p.s by using both microwave and optical absorption techniques. "1~ Now, in our pulse radiolysis system, absorption spectroscopy with a response down to 80 ps °2) is available as well as emission spectroscopy."-" The present paper describes mainly the behavior of solute anion, cation, and excited states including both singlet and triplet excited states in 0.1 tool i-1 biphenyl cyclohexane solutions in a time range from 1 to 50 ns. The geminate ion decay on the shorter timescale is discussed in comparison with works reported by other authors, tH) and the mechanism of the slower formation of solute excited singlet state in concentrated cyciohexane solutions is explained.

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2. E X P E R I M E N T A L Details of the picosecond pulse radiolysis system for emission spectroscopy with a response time of 20ps composed of a specially-designed linear accelerator (t3-tS) and very fast response optical detection systems (t-" have been reported previously. The detection system of the pulse radiolysis system for absorption spectroscopy with a response time of 80 ps(n) is mainly composed of a very fast response photodiode (R1328U, Hamamatsu, T. V.), a sampling oscilloscope (4s sampling head, Tektronix), a computer (PDPll/34), and a display unit. A transient digitizer (R7912, Tektronix) was used in the present experiment instead of the sampling oscilloscope. The time resolution of the present system was about 1 ns, determined by the rise time of the transient digitizer. Solutions were irradiated by 10ps single electron pulse. The radiation dose given to the sample was calculated from the absorption of solvated electrons in water at 720nm. The extinction coefficient of 1.85x 104mole-t I cm-t at 720 mm(re and the yield of 4.1/100 eV at 1 ns(t" were used in this calculation. In addition, a correction was made for the difference between electron densities of the sample and of water. The radiation dose per 10 ps pulse was in a range from 400 to 1200rad. Cyclohexane (Tokyo Kasei Co. Ltd., spectrograde) was passed through a column charged with fresh activated alumina before use, and was checked by a spectrometer for all experiments, Biphenyl (Tokyo Kasei Co. Ltd., zone-refiner) was used as received. All of the solutions used were degassed on a vacuum line. Other experimental details were already reported elsewhere. "-7) 3. R E S U L T S Figure 1 shows transient spectra for a solution of 0.1 mole I-t biphenyl in cyclohexane. The spectrum at 1 ns after a 10 ps pulse has two absorption peaks at 380 and 410nm, and a small shoulder around 360nm. These absorption peaks and shoulder are due to cation (at 380 nm), anion (at 410nm), and triplet state (around 360nm) of biphenyl.(tSJg) The spectrum at 50 ns after a I0 ps pulse has an absorption peak at about 370 nm and a shoulder at 410 nm. The absorption peak and the shoulder are due to triplet state and anion of biphenyl, respectively. (9,t°) Figure 2 shows typical oscilloscope traces of the absorption of solute anion monitored at 410nm, cation at 380 nm, and triplet state at 360 nm in the pulse radiolysis of 0.1 moll -t biphenyl in cyclohexane. The data of the anion and the triplet state of biphenyl display the typical initial rapid decay of geminate ions and the rapid formation of the triplet state followed by the slow formation. It is very interesting to note that decay of the anion and formation of the triplet state were observed immediately after the rise time of the present system (about 1 ns). Ratio of the formation rate of the solute triplet to decay rate of the solute anion is constant on nanosecond time scale longer than

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FIG 1. Transient spectra of solute anion, cation, and triplet absorptions obtained in the pulse radiolysis of 0.1 mole !-t biphenyl in cyclohexane: ©, at 1 ns, and 0, at 50 ns after 10 ps pulse.

20 ns and becomes smaller on a timescale shorter than 20 ns. Decay of the biphenyl cation monitored at 380 nm is observed in several nanoseconds and then the decay is masked by formation of the triplet state on a longer time scale. Figure 2 shows also typical oscilloscope traces of the solute fluorescence in pulse radiolysis of 0.1 mol l-t biphenyl in cyclohexane solution observed by a photodiode and a transient digitizer. Most of the solute fluorescence is formed within the limit of the time resolution of the present system and the remaining component is produced over about 3 ns. Figure 3 shows G values of the solute anion (number/100eV) formed in 0 . 1 m o l l -1 biphenyl cyclohexane solutions as a function of reciprocal square root of time from the end of pulse. The G value of biphenyl anion was determined based on the molecular extinction coefficient at 410nm of 4 x l 0 4 m o l - l l c m - ~ . (2°) Thomas et al. reported that absorption at 410 nm is mainly due to biphenyl anion, based on electron scavenger experiments (N,O, SF6, 02). (gb) The data points can be well represented by a straight line in Fig. 3 on a timescale longer than 10 ns. The value of the intercept is 0.15 and agrees with the G value of free ions in cyclohexane. (2~)

279

Geminate recombination kinetics of solute radical ions (d)

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FIG. 3. G value of the solute anion following pulse irradiation of cyclohexane containing 0.1 mole I-' biphenyl, plotted against the reciprocal sq'u~re root of the time from the end of the pulse,~

(a) Transient absorption spectra Absorption spectra of the excited triplet state and anion of biphenyl in nanosecond time region, (9''°) and of the ions of biphenyl at 200 ps after a pulse (') have been reported for cyclohexane solutions of biphenyl. The absorption spectrum at 1 ns after a pulse in Fig. 1 agrees fairly well with the previous picosecond data (') rather than the previous nanosecond data, (gJ°) although the absorption peaks are at 380 and at 410 nm in the present work and at 390 and at 410 nm in the previous workJ =) Absorption spectra are composed of those due to cation (at 380 nm), anion (at 410 nm) and triplet (around 360 nm) at times within I ns after 10 ps pulse. Absorption of the biphenyl cation was not

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observed in nanosecond time region in the previous work. ~9"~°>It was made clear in the present work that the absorption peak of the biphenyl cation is only within several nanoseconds, and then the absorption is masked by the troplet state, as shown in Fig. 2(d). The absorption spectra change very fast on a several nanosecond timescale, as understood by comparing with results reported before. ~9''°'22~ (b) Mechanism of formation process of solute

excited singlet states in concentrated cyclohexane solutions Two kinds of the formation processes of excited singlet states of solute molecules in cyclohexane solution on a picosecond timescale after 10ps pulse were previously reported by the present authors.
there are discrepancies in diluted solutions among different authors. °~'7'23-25). (c) Geminate recombination kinetics of solute ions The geminate recombination kinetics of solute ions measured by conductivity method have been extensively studied on a nanosecond timescale longer than 50 ns following pulse irradiation of cyclohexane solutions. is given by (1)

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rc is Onsager length, rc = e2/~KT, where ~ is dielectric constant of the solution, k is Boltzmann constant, and T is absolute temperature; D is sum of the diffusion coefficients of ions (anion and cation of biphenyl in the present work); ro is initial separation of ion pairs. Figure 3 shows that a time dependence of the form given by equation (1) describes the data on a timescale longer than 10 ns after a pulse. The equation (1) is re-written into the equation

(2) (2)

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G(t) is the yield of ion pairs still surviving at time t after a pulse per 100 eV; G~ is the yield of free ions per 100 eV. The intercept in Fig. 2 shows G~ and the value of the intercept is 0.15, which agrees very well with value of G~ previously reported in cyclohexane obtained by other methodfl 2> Equivalent lifetime, y, at which the yield of surviving geminate ions is equal to the free ion yield, y is equal to rc2DrDin equation (2). The sum of positive and negative ion mobilities of biphenyl (tx(~b2++/x(~b2-)) in cyclohexane is about 8 x l 0 - 4 c m 2 V - ' s - ' . <'''~ Using this value together with the Einstein relation (to obtain D), and taking rc = 279 ~ for cyclohexane at room temperature, the value of v(rc2/1rD) is 120 ns. This difference between the experimental and theoreti-

Geminate recombination kinetics of solute radical ions cal values cannot be explained so clear, although the most important possibility (the equation (2) is only correct on a longer timescale), of this discrepancy obtained by other method was discussed by other authors. "~ The experimental curve in Fig. 3 has a downward curvature on the several nanosecond timescale. It may provide the experimental data for the calculation of the initial distribution of ionpairs, tH.2~ The decay of the solute anion and the formation of the solute triplet were observed immediately after the rise time of the present system (about 1 ns), while no decay of the solute anion and no formation of the solute triplet state were reported on a several hundred picosecond timescale by other authors. <22) It is interesting and important that the formation of the solute triplet state was observed in the present work on timescale shorter than 5 n s after pulse in spite of no loss of spin correlation at these short times calculated theoretically by Brockelhurst. ~28) In the present work, the ratio of the formation rate of the solute triplet state to the decay rate of the solute anion is constant on the nanosecond timescale longer than 20 ns and decreases on the timescale shorter than 20ns. It is different from the previous data <9> where this ratio was constant just after the rise time of their system. The change of this ratio in the present work occurs in the time range between 5 and 20 ns. It is very well correlated with the time range of the loss of spin correlation calculated theoretically. ~28"29) More precise and higher time resolved measurements will be performed in the near future. 5. C O N C L U S I O N 1. Transient absorption spectra obtained in the pulse radiolysis of cyclohexane solution of biphenyl changes very drastically on a several nanosecond timescale after 10ps pulse. Two strong absorption peaks due to cation (at 380 nm) and anion (at 410 nm), of biphenyl were observed in the time range shorter than 1 ns after pulse. Weak absorption of the anion and strong absorption of the triplet state, which mask absorption of the cation, was observed on a nanosecond timescale longer than 10 ns after pulse. 2. In high concentrated cyclohexane solutions of biphenyl, the formation of the solute excited singlet state is mainly due to the geminate solute ion recombination reaction. 3. Even in a timescale shorter than 50 ns after pulse, the decay of the solute ion obeys reciprocal square root dependence on time within experi-

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mental error for times longer than 10 ns after pulse in 0.1mole1-1 biphenyl in cyclohexane. The G value of the free ions obtained here agrees quite well with the value obtained by other methods. <2t~ 4. Decay of the anion and formation of the triplet state were observed immediately after the rise time of the present system (about 1 ns) in spite of no loss of spin correlation at times shorter than 5 ns calculated theoretically, t2sJ The ratio of the formation rate of the solute triplet state to the decay rate of the anion becomes smaller on a timescale shorter than 20ns after 10ps pulse. It agrees very well with the theoretical calculation,t2s~

Acknowledgement--The authors are grateful to Mr. T Ueda and Mr. T. Kobayashi for running the picosecond pulse radiolysis system and for their maintenance of the linear accelerator. The authors would like to thank Mr. Y. Katsumura for his helpful discussions. REFERENCES 1. S. TAGAWA, Y. KATSUMURA and Y. TABATA, Chem.

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