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J. Phys. Chem. 1996, 100, 1682-1688
Magnetic Field Effects on the Solute Luminescence of Alkane Solutions Irradiated with Heavy Ions Jay A. LaVerne* and Brian Brocklehurst*,† Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: August 14, 1995; In Final Form: October 23, 1995X
The effects of track structure on the luminescence decays in cyclohexane and in 2,2,4-trimethylpentane (isooctane) solutions of 2,5-diphenyloxazole (PPO) have been determined in the presence and in the absence of an external magnetic field. Irradiations were performed with protons of 1-15 MeV and with helium ions of 2-20 MeV energy. Companion studies were performed with 90Sr-90Y β-radiolysis. The magnetic field effect is due to the hyperfine interaction of nuclear spins in the geminate pair of solvent radical ions produced. In both solvents, the effect of the magnetic field on luminescence decreases with increasing linear energy transfer (LET) from about 40% for β-particles to only a few percent with helium ions. Magnetic field effects with protons decrease in time whereas they are constant with β-particles. This result is attributed to the overlap of initially isolated spurs during the evolution of the proton track; the probability of nongeminate recombination increases with the number of neighboring ion pairs. The total luminescence intensity per incident particle remains constant with proton energy but increases slightly with increasing helium ion energy. At a given particle energy, the intensity is greater in cyclohexane than in isooctane. The pulse shapes of the luminescence decays reflect the distributions in ion recombination times, and very little variation in luminescence decay rates is observed with increasing LET.
Introduction Energy deposited by the passage of ionizing radiation in hydrocarbons leads to a series of clusters of ions and excited states (spurs) stochastically placed along the particle path.1 The spatial distribution of these species and the radicals that they form make up the particle track structure, and it is largely responsible for the observed yields of final products. Increasing the linear energy transfer (LET ) stopping power, -dE/dx) as in the case of proton, helium ion, or other heavy ion radiation increases the concentration of reactive species in the particle track, and intratrack radical-radical reactions can be preferred over radical diffusion into the bulk medium. For instance, in the fast electron radiolysis of cyclohexane almost 80% of the cyclohexyl radicals produced react in the bulk medium whereas less than 10% escape the tracks of low-energy carbon ions.2 Almost all of the studies on the chemical effects of track structure have been made from observations of the final product formation.3 Only a few studies have attempted to observe the formation of ions and excited states in the heavy ion radiolysis of hydrocarbons,4-11 but it is well-known that scintillation yields decrease markedly with increasing LET.12 Luminescence from aromatic compounds added to hydrocarbons has long been used to examine the radiolytic production of ions and excited states.5,13 The initial ionization and direct excitation of the solvent and the subsequent reactions can be represented by
RH -Df RH+ + e- or RH*
(R1)
RH+ + e- f RH*
(R2)
RH* + S f RH + S*
(R3)
RH+ + S f RH + S+
(R4)
† Permanent address: Chemistry Department, University of Sheffield, Sheffield S3 7HF, United Kingdom. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-1682$12.00/0
e- + S f S-
(R5)
S+ + S- f S + S*
(R6)
S* f S + hν
(R7)
where RH is the solvent and S is the solute. When moderate concentrations of solute are used, the reactions of S+ with eand RH+ with S- can be neglected. The luminescence from the solute represents both ionic and excited state processes of the medium. Although it is difficult to interpret detailed track structure from these studies, they are important for an overview of the processes in heavy ion tracks. These experiments are especially important because they offer a technique for determining the temporal dependences of the radiation chemical processes induced by heavy ions. The time resolution is a few hundred picoseconds, and if large aromatics are added to scavenge electrons, the range of ion recombination times can extend to hundreds of nanoseconds. However, excitation transfer (R3) must be complete within a few nanoseconds. In contrast to the case in aromatic solvents, luminescence in alkane solutions is largely due to the recombination of solute ions. Energy transfer from the solvent to the solute and triplettriplet annihilation are minor processes.14-16 Geminate recombination of radical ions gives rise to time-dependent magnetic field effects due to the hyperfine interaction between unpaired electrons and nuclei.17,18 Only one study with heavy ions has looked at the effect of magnetic fields on luminescence, and that was in the proton and R-particle radiolysis of solid anthracene.11 Here the results of measurements of the effects of magnetic fields on the luminescence in cyclohexane and in 2,2,4-trimethylpentane (isooctane) solutions of 2,5-diphenyloxazole (PPO) irradiated with protons of 1-15 MeV and with helium ions of 2-20 MeV energy are presented. Comparative studies were performed using 90Sr-90Y β-particles. PPO was chosen for its high solubility, high fluorescence yield, and short lifetime of 1.53 ns.19 The observed luminescence decays largely reflect the dynamics of the precursors of the excited PPO © 1996 American Chemical Society
Magnetic Field Effects on Solute Luminescence
J. Phys. Chem., Vol. 100, No. 5, 1996 1683
Figure 1. Heavy ion window assembly showing the beam direction and the two slits (S1 and S2). The single-photon counting apparatus is in its light-tight box with the sample cell, quartz light guide, photomultipliers (PMT1 and PMT2), high and low pass filters (F1 and F2), and the iris (I).
molecule, not its rate of decay (R7). Preliminary results with helium ions have been published elsewhere.20 Experimental Section Irradiations with 1H and 4He ions were performed using the 9-MV FN Tandem Van de Graaff accelerator of the Notre Dame Nuclear Structure Laboratory.21 The window assembly is shown in Figure 1, and it is essentially the same as previously described 22 except for an additional collimator of 0.5 mm diameter to allow for the attenuation of the ion beams. Completely stripped ions were used, and energies were varied by changing machine parameters. Absolute energy was determined by magnetic analysis with an uncertainty in energy equal to (0.3 mg/cm2 of range. Energy loss by the ions in passing through windows was determined using standard stopping power tables.23 Irradiations with β-particles were performed using a 1 mCi 90Sr-90Y source with maximum energies of 0.54 and 2.25 MeV, respectively.24 The β-particles were collimated to give a particle flux similar to that of the heavy ions and, in some cases, attenuated in energy with aluminum absorbers. The range of β-particle energies used in the experiments was determined by the addition of sufficient aluminum absorber until no luminescence could be detected. Stopping power tables25 were used to determine the energy loss to the aluminum absorber by the maximum energy β-particle. This energy loss is equivalent to the energy range of the β-particles sampled with given discriminator settings, see below. An average β-particle energy was obtained from the appropriate integration of the energy spectrum given by Slack and Way.24 Luminescence was detected using a time-resolved singlephoton counting technique.26 The optical arrangement was similar to that used previously for β-radiolysis,17 and it is shown in Figure 1. The optical detection system was housed in a lighttight box through which passed the window assembly. The box could be moved away from the accelerator beam line and the window assembly replaced with a device holding the β-particle source and its collimator. A quartz sample cell was made from 2.54 cm diameter tubing 2.54 cm long. One end of the cell had a mica window (typical thickness of 6-8 mg/cm2) for the particle beam. The luminescence which passed through a rear Supracil window was collected with a light guide made from a 2.54 cm diameter quartz rod 40 cm long. A motor drive allowed the placement of a permanent magnet of 0.15 T around the sample cell. This field is much larger than the hyperfine
coupling of the radical cations with electrons, so that the field effect is saturated.27 Residual effects of the magnetic field on the photomultipliers were negligible because of the light guide. Nitrogen was passed through a prebubbler and then through the sample cell to remove residual oxygen. A large fraction of the photons produced by the incident particle was collected by the light guide and observed by the start photomultiplier (PM1). The light guide contained a notch for scattering a small portion of the photons into the stop photomultiplier (PM2). An iris and a combination of long and short-wavelength filters were placed in front of PM2. Only photons between 300 and 400 nm reached PM2. Both photomultipliers were RCA 8850s which could distinguish between one or more incident photons. The signal from PM2 was passed through a variable nanosecond delay, and both signals were amplified with fast timing amplifiers (Ortec Model 574) and passed out of the target area to constant fraction discriminator/ single channel analyzers (CFD1 and CFD2) (Ortec Model 583). The discriminators were set so that only signals produced by multiple photons incident to PM1 were passed through CFD1 while only signals produced by single photons incident to PM2 were passed through CFD2. The output from CFD1 was sent to a ratemeter, a pileup gate, and the start of a time to amplitude converter (TAC) (Ortec Model 567). A stop signal for the TAC was obtained from CFD2. Valid start signals were obtained from the TAC and counted while the time-converted signal was fed to a 4096-channel multichannel analyzer (Ortec Model 916). Coincidental events produced by multiple particles impinging on the sample while the time window of the TAC was still open were detected by the pileup gate, which voided the TAC conversion cycle and incremented a counter. The true number of incident particles was taken to be the difference between the valid start counts from the TAC and the coincidence counts. The counter of the valid starts, the multichannel analyzer, and the magnet motor drive were controlled by a personal computer. Successive collection of the time profile of the luminescence was made with and without a magnetic field using small time intervals (about 10 min) to compensate for any drift in the electronics. Total start counts ranged from 5 × 107 to 1 × 108, depending on particle energy. A correction was applied to the data for ‘pileup’ in the detection system: of two photons produced from the same start signal only one can be detected. The pileup effect was kept small by maintaining the count rate of stop pulses to 2-3% of the start rate. The main source of
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Figure 2. Relative luminescence decay for 10 mM PPO solutions in cyclohexane with (O) β-particles, (9) 1 MeV and (×) 15 MeV protons, and ([) 2 MeV and (+) 20 MeV helium ions. The peaks have been normalized at their maxima.
background is the chance arrival of a second particle within 1 ms of that which gave the start signal. Therefore, incident particle rates were kept below 2 × 104 particles/s. The time profiles of the luminescence with and without a magnetic field from cyclohexane and isooctane solutions of PPO were obtained with the various energy heavy ions. Identical conditions were used to obtain the luminescence produced by β-particles. Cerenkov radiation produced in water was used to obtain the prompt or instrument response function. The contribution of Cerenkov radiation to the observed β-particle luminescence was obtained from ethanol solutions of PPO; the correction (8% for cyclohexane, 12% for isooctane) may be slightly too large because of a small amount of direct PPO excitation in ethanol solutions. The width of the prompt peak was about 1.5 ns. All time profiles were obtained with the TAC time window at 1 µs, and the average counts in channels 30004000 of the multichannel analyzer were used to obtain the background counts. Integration of the background-corrected time profile was used to give the total intensity. Solutions were made from Aldrich HPLC grade cyclohexane and isooctane. The solutes were passed through activated silica gel. Repeated filtering had no observable effect on the luminescence. Gas chromatographic/mass spectrometric analysis of the cyclohexane showed that the cyclohexene impurity was much less than 1 µM. Solutions were 10 mM PPO, which was used as received. Results and Discussion Luminescence Decays. Luminescence decays for solutions of 10 mM PPO in cyclohexane are shown in Figure 2 for β-particles, 1 and 15 MeV protons, and 2 and 20 MeV helium ions. Similar results were observed in isooctane and therefore are not shown. The curves in Figure 2 have been normalized at the peak maxima to aid in comparison of the pulse profiles. For the most part, the shape of the luminescence peak is the same for β-particles, high-energy protons, and high-energy helium ions. The two lowest energy ions (1 MeV protons and 2 MeV helium ions) appear to have a higher luminescence in the long-time limit. This result appears to be real, but such data must be treated with caution because of the poor statistics for these low-energy ions. There are a number of techniques for fitting luminescence decays in order to extract decay rates.26 Normally these methods use a sum of exponential functions to describe the decay process.
Figure 3. Luminescence intensity in the region of interest relative to the intensity at the peak maximum for 10 mM PPO solutions in cyclohexane and isooctane as a function of the track average LET with β-particles, protons, and helium ions. The dashed lines are the average values with β-particles at 3-10, 30-60, and 200-400 ns.
However, in the systems studied here a considerable amount of luminescence is due to the combination reaction of ionic pairs to give excited solute molecules (R6). This reaction is diffusion controlled and it is not strictly described by exponential functions. Nevertheless, preliminary work on the deconvolution of the luminescence decay peaks has found that two or three exponentials can well represent the observed decays. For example, the lowest energies excepted, the proton data for cyclohexane can be fitted with ‘lifetimes’ of 1.7-1.8, 10-12, and 65-75 ns; corresponding values for isooctane are 2.52.7, 12-14, and 75-80 ns. In both media, the contributions of these terms change smoothly with proton energy. However, exponential functions probably have no physical significance in these systems, and no further use of this description will be attempted here. The relative decays of the luminescence peaks for the different incident particles can be scrutinized by comparing the luminescence intensity in selected regions of interest to that at the peak. Figure 3 shows the ratios of the luminescence intensities in the ranges 3-10, 30-60, and 200-400 ns after the maximum to that at the peak (0.5 ns before the maximum to 2 ns after it, in this case) as a function of the track average LET28 of the incident particle. In the ranges 3-10, 30-60, and 200-400 ns following the peak, the observed luminescence intensity in cyclohexane is about 45, 7.5, and 2%, respectively, of that at the peak. The corresponding luminescence intensities in isooctane are 70, 18, and 6%, respectively. The dashed lines in
Magnetic Field Effects on Solute Luminescence Figure 3 are the average values obtained from different β-particle experiments in the various regions of interest. It can be seen that the luminescence intensities with heavy ions are almost independent of the track average LET, and they are very nearly the same as found with β-particles. The results at very low ion energies are again anomalous (cf. Figure 2). At the longer times, there may be a slight decrease in luminescence intensity with increasing LET of the heavy ion. However, there is considerably more scatter in the data at long times because of the low number of total counts, and within experimental error the luminescence pulse shapes are approximately the same. It must also be remembered that the initial nonhomogeneous spatial distribution of reactive species is almost completely relaxed within a few hundred nanoseconds. There is not much residual information on the track structure for the long-time experiments to sample. Previous experiments by Ito et al.10 found that the luminescence decays of 10 mM PPO in cyclohexane had essentially the same shape for 241Am R-particles (4 MeV) and 24 MeV helium ions, in agreement with the present results. However, Ito et al. also found that the short-time (e10 ns) decay with R-particles was faster than with 60Co γ-rays while they were both similar at longer times. From the data in Figures 2 and 3 it appears that the luminescence decays with β-particles and with high-energy heavy ions are very nearly the same shape at all times. However, if the decay after a few nanoseconds is due solely to ionic recombination, then the luminescence with β-particles represents more ions because of the loss of spin correlation, see below. Berlman et al.4 compared scintillators with different fluorescence lifetimes and demonstrated the role of the quenching of solute excited states by transient species such as radicals, electrons, or radical ions. A similar mechanism was suggested by the work of Miller and West.6-8 Excited state lifetimes have also been shown to decrease with increasing photon LET.29,30 One might therefore expect to observe a large LET effect on luminescence decay rates at short times, but such a result is not found. Presumably, the excited state of PPO decays sufficiently rapidly that it does not significantly interact with the transient species produced in cyclohexane or in isooctane at these LETs. Luminescence Yields. The integrated luminescence decay curves can be used to obtain the relative formation of excited PPO molecules in the tracks of the various particles. Figure 4 shows the relative number of photons per incident particle for β-particles, protons, and helium ions in cyclohexane and isooctane. The track average radiation chemical yield of excited PPO for any particle at a given energy is proportional to the slope of a line through that data point and the origin. In both solvents, the radiation chemical yields appear to be independent of particle energy for protons. The results with helium ions seem to suggest that the yields increase slightly with increasing particle energy, especially in cyclohexane. Furthermore, at a given particle energy the radiation chemical yields of PPO luminescence decrease on changing from β-particles to protons to helium ions. The ratio of luminescence yields for 5.8 MeV R-particles to that of β-particles was found to be 0.063,4 which agrees well with the present value of 0.076 for 5 MeV helium ions. The relative yield of PPO luminescence is less in isooctane than in cyclohexane for all particles studied here. It is expected that for a given particle the total ionization and consequently the PPO luminescence yield would be similar for these two hydrocarbons. The singlet excited state in cyclohexane has a lifetime of ∼1 ns,16 and it can transfer energy to solutes whereas the singlet excited state in isooctane has not been observed.31
J. Phys. Chem., Vol. 100, No. 5, 1996 1685
Figure 4. Relative total luminescence intensity per incident particle as a function of energy for β-particles, protons, and helium ions in 10 mM PPO solutions of cyclohexane and isooctane.
It is probable that the singlet excited state in isooctane decays before energy transfer to PPO can occur. Therefore, the difference in the PPO luminescence yields for two particles at the same energy is roughly equivalent to the yield of the singlet excited state of cyclohexane. Of course, the results would be for the yield of the excited singlet state at the time corresponding to the scavenging capacity of the PPO. From the data in Figure 4 it appears that the yield of the singlet excited state of cyclohexane decreases with decreasing helium ion energy, but it remains constant for protons. Luminescence yields in specific regions of interest for β-particles, 10 MeV protons, and 10 MeV helium ions are shown in Table 1. Absolute radiation chemical yields (G-values)32 were obtained by comparison with the measurements of Choi et al.33 on PPO solutions in cyclohexane excited with β-particles. The accuracy of such a comparison may be somewhat uncertain, but relative values should be precise. At long times, the isooctane yield decays much more slowly than that for cyclohexane. Such a result is expected, since the thermalization distance for electrons is greater in isooctane.34 The difference between the two media is the greatest at short times, probably the result of a contribution from reaction R3 in cyclohexane. As discussed above the relative decay rates of the luminescence are the same in both media, so that additional reaction of the excited PPO molecules with transient species is minimal. The reason for the overall decrease in luminescence yields with increasing LET must be found elsewhere. Magnetic Field Effects. A given fluorescence pulse shape is the product of the distribution of times of recombination of
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LaVerne and Brocklehurst
TABLE 1: G-Values for Luminescence at Various Regions of Interest following the Peak Maximum cyclohexane region of interest (ns) total -3 to 3 3-10 10-30 30-60 60-100 100-200 200-500
isooctane
β-particle
proton (10 MeV)
helium ion (10 MeV)
β-particle
proton (10 MeV)
helium ion (10 MeV)
0.65 0.398 0.130 0.0645 0.0246 0.0131 0.0119 0.00827
0.331 0.203 0.0661 0.0336 0.0122 0.00629 0.00582 0.00409
0.0562 0.0357 0.0107 0.00563 0.00195 0.000869 0.000743 0.000587
0.393 0.179 0.0887 0.0600 0.0264 0.0144 0.0140 0.0103
0.135 0.0635 0.0293 0.0194 0.00918 0.00535 0.00514 0.00350
0.0361 0.0181 0.00820 0.00500 0.00206 0.000922 0.000919 0.000849
Figure 5. Temporal dependence of the ratio of the luminescence with a magnetic field to that without a magnetic field for 10 mM PPO solutions of cyclohexane with (0) β-particles; (b) 15 MeV, (2) 10 MeV, and (1) 5 MeV protons; and ([) 20 MeV helium ions.
those ions which can give excited states (radical ions) and the singlet fraction, Fs(t). In principle, the singlet fraction can be calculated from the magnetic field effect.27 It is given by
Fs(t) ) {(4Fs(0) - 1)Fs(t) + 1 - Fs(0)}/3
(1)
where Fs(t) is the singlet density for an initially pure singlet pair (Fs(0) ) 1). In a simple case, the time evolution of Fs can be calculated approximately from the hyperfine coupling constants of the ions. These constants are not known for PPO, but the constancy of the field effect after ∼50 ns for β-particles shows that limiting values are quickly reached. In a strong field, the value of Fs(∞) is simply 0.5, but at zero field it may be 0.25 or higher. Hyperfine interaction alone gives a limiting value of 1/3, but charge transfer from solvent to solute or between solute molecules decreases this value to an uncertain extent because there are two or more stages of spin evolution.27 The application of a magnetic field is expected to increase the photon intensity in the tail of the luminescence decay by making two of the triplet substates energetically inaccessible. Since only two states are involved, the probability that ionic recombination reactions involve singlet states is increased. Figure 5 shows the time dependence of the magnetic-fieldenhanced luminescence for β-particles, 15, 10, and 5 MeV protons, and 20 MeV helium ions in cyclohexane. The maximum relative field effect with β-particles is about 40%, which is slightly higher but comparable to the previous results of Brocklehurst.35 Similar results were observed in isooctane and not shown. Lower field effects are seen with protons, and the maximum effect decreases with decreasing proton energy. With 20 MeV helium ions the field effect has decreased to only a few percent, in agreement with the results of Klein and Voltz5 using R-particles. Such a low-field effect with helium ions
indicates that the proportion of isolated singlet ion pairs is considerably reduced. The magnitude of the field effect with β-particles appears to be very nearly constant over hundreds of nanoseconds after the initial rise, cf. Figure 5. These long times correspond to large initial cation-electron separations. The number of such recombination processes is much smaller than that occurring in the peak, but the temporal invariance of the field effect suggests that the spurs are isolated entities. The magnetic field effect with low-energy helium ions also shows very little temporal dependence, suggesting one single entity or track. However, protons give a field effect that passes through a maximum at ∼50 ns. This maximum is probably due to the overlap of initially isolated spurs as they expand by diffusion. The increased probability of reaction with neighboring ionic pairs decreases the amount of geminate recombination. A ‘typical’ spur produced by fast electrons in cyclohexane is expected to contain about 50 eV.36 If a similar energy is assumed for protons, then for the energy range of 1-15 MeV (LET 43.4-8.8 eV/nm) the average spur separation is expected to range from 1.2 to 5.7 nm. The initial track of a 1 MeV proton is expected to look like a series of spurs that initially overlap to a small extent whereas it will take some tens of nanoseconds for the spurs of a 15 MeV proton track to sufficiently expand so that they overlap. The magnitude and time dependence of the maxima in the magnetic field enhancements support such a model for proton tracks. Both media exhibit essentially the same type of field effects with protons, so differences in electron thermalization distances of the two media cannot play a large role in the overall track structure. The different electron thermalization distances will affect other aspects of the field enhancement. The relative magnetic field effect 50-75 ns following the peak is shown in Figure 6 as a function of the track average LET. Within the scatter of the data there is no noticeable difference between cyclohexane and isooctane solutions. Even the X-ray data for p-terphenyl in squalane37 blends in smoothly with the present data. There is a steady decrease in the magnetic field effect with increasing LET. The values range from about 40% with β-particles to virtually zero for low-energy helium ions. The apparent independence of the magnetic field effect on solute again suggests that the spatial distribution of ionization in alkanes is dominated by the properties of the incident particle. This result is probably just fortuitous because of the region of interest chosen. The time evolution of the magnetic field effect for β-particles and protons is shown in Figure 7, where the ratio of the luminescence with a field to that without a field is shown as a function of the track average LET for four regions of interest: 3-10, 10-30, 30-60, and 60-100 ns following the maximum. These results can be understood in terms of an increase in spur overlap as the LET increases. Early times correspond to small cation-electron separations, and so the chance of geminate
Magnetic Field Effects on Solute Luminescence
Figure 6. Relative magnetic field effect 50-75 ns after the peak as a function of track average LET for 10 mM PPO solutions of cyclohexane (closed symbols) and isooctane (open symbols) with β-particles ([), protons (9), and helium ions (b). Also shown are X-ray data (+) for p-terphenyl in squalane.37
J. Phys. Chem., Vol. 100, No. 5, 1996 1687 ization distance.34 Therefore, one observes a greater field effect at early times. With helium ions the individual energy loss events along the particle path are so close to each other that they coalesce to a continuous track. The observed field enhancement of 40% with β-particles corresponds to an initial singlet fraction, Fs(0), of about 0.7, cf. eq 1. However, the 2% field enhancement observed with low-energy helium ions corresponds to a value of only 0.27 for the initial singlet fraction. Since the lower limit for a completely homogeneous distribution of ionic pairs is 0.25, it can be assumed that such a condition very nearly exists in the helium ion track. With increasing LET, geminate recombination in the particle track will decrease in importance in relation to cross combination reactions due to the increased proximity of ionization processes. High-energy secondary electrons or δ rays should produce a larger proportion of geminate recombination with a resultant high-field effect. However, the results suggest that at track average LETs on the order of about 100 eV/nm there are very few high-energy δ rays in alkanes. Most of the energy loss outside of the track core must be deposited close to the particle track axis, and the ionization events produced from these processes are soon engulfed by the expanding track core. Conclusions The decrease in scintillation yield from β-particles to protons to helium ions is commonly ascribed to quenching of the excited states by transient species produced in the dense tracks.4,6-8,12 However, the present results suggest that these reactions are negligible with PPO solutions. The greater proportion of triplet state formation in high LET tracks has to be taken into account; one can estimate a reduction in the total luminescence by a factor of ∼2. The rest of the decrease in luminescence must be ascribed to intervention of radicals or other transient species with the precursors to excited PPO molecules. The results demonstrate the usefulness of time-resolved luminescence and magnetic field effects for studies of track structure of hydrocarbons. The conclusions are largely qualitative at present, and detailed modeling of the track structure and the diffusion-kinetic processes occurring within it is needed for further progress. An obvious next step is to correct the observed luminescence pulse shapes to distributions of those ion combination reactions which can lead to excited states (singlet and triplet). Such a process is feasible for isooctane, except that the course of spin evolution from ∼5 to 50 ns cannot be calculated accurately for PPO. In the case of cyclohexane there is the further complication of energy transfer from the solvent.
Figure 7. Ratio of the luminescence with a magnetic field to that without a magnetic field for 10 mM PPO solutions of cyclohexane and isooctane with β-particles and protons as a function of LET at (9) 3-10 ns, (b) 10-30 ns, (2) 30-60 ns, and ([) 60-100 ns following the peak maximum.
recombination is high, resulting in a low-field effect. At larger separations, longer times, the probability of recombination with neighbor ionic pairs (no spin correlation) is greater, which leads to a smaller field effect. At a given LET, spur overlap is more probable in isooctane because of the greater electron thermal-
Acknowledgment. One of us (B.B.) would like to thank Professor R. H. Schuler for support to visit Notre Dame and the Leverhulme Trust for an Emeritus Fellowship. We also thank Professor J. J. Kolata for his help in assembling the electronics for these experiments and for making the facilities of the Notre Dame Nuclear Structure Laboratory available. The latter is funded by the National Science Foundation. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3858 from the Notre Dame Radiation Laboratory. References and Notes (1) Burns, W. G.; Barker, R. In Aspects of Hydrocarbon Radiolysis; Gaumann, T., Hoigne, J., Eds.; Academic Press: New York, 1968; p 33. (2) LaVerne, J. A.; Schuler, R. H.; Foldiak, G. J. Phys. Chem. 1992, 96, 2588.
1688 J. Phys. Chem., Vol. 100, No. 5, 1996 (3) LaVerne, J. A.; Schuler, R. H.; Ross, A. B.; Helman, W. P. Radiat. Phys. Chem. 1981, 17, 5. (4) Berlman, I. B.; Grismore, R.; Oltman, B. G. Trans. Faraday Soc. 1963, 59, 2010. (5) Klein, J.; Voltz, R. In Radiation Research, Proceedings of the Sixth International Congress of Radiation Research; Okada, S., Imamura, M., Terashima, T., Yamaguchi, H., Eds.; Japanese Association for Radiation Research: Tokyo, 1979; p 257. (6) West, M. L. J. Phys. Chem. 1977, 81, 377. (7) Miller, J. H.; West, M. L. J. Chem. Phys. 1977, 67, 2793. (8) West, M. L.; Miller, J. H. J. Phys. Chem. 1979, 83, 1205. (9) West, M. L.; Miller, J. H. Chem. Phys. Lett. 1980, 71, 110. (10) Ito, Y.; Azuma, T.; Katsumura, Y.; Aoki, Y.; Tabata, Y.; Kimura, K. Radiat. Phys. Chem. 1987, 29, 31. (11) Fuchs, C.; Klein, J.; Voltz, R. Radiat. Phys. Chem. 1983, 21, 67. (12) Birks, J. B. The Theory and Practice of Scintillation Counting; Pergamon: Oxford, 1964. (13) Brocklehurst, B. Radiat. Res. ReV. 1970, 2, 149. (14) Ludwig, P. K.; Huque, M. M. J. Chem. Phys. 1968, 49, 805. (15) Ludwig, P. K. AdV. Radiat. Chem. 1972, 3, 1. (16) Sauer, M. C., Jr.; Jonah, C. D. Radiat. Phys. Chem. 1994, 44, 281. (17) Brocklehurst, B. Int. ReV. Phys. Chem. 1985, 4, 279. (18) Steiner, U. E.; Ulrich, T. Chem. ReV. 1989, 89, 51. (19) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1965. (20) LaVerne, J. A.; Brocklehurst, B. Radiat. Phys. Chem. 1996, 47, 71. (21) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1987, 91, 5770. (22) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1987, 91, 6560. (23) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range of Ions in Solids; Pergamon: New York, 1985.
LaVerne and Brocklehurst (24) Slack, L.; Way, K. Radiations from RadioactiVe Atoms in Frequent Use; United States Atomic Energy Commission: Washington, D.C., 1959. (25) Berger, M. J.; Seltzer, S. M. Stopping Powers and Ranges of Electrons and Positrons, 2nd ed.; United States Department of Commerce: Washington, D.C., 1983. (26) O’Connor, D. V.; Phillips, D. Time-correlated Single Photon Counting; Academic Press: London, 1984. (27) Brocklehurst, B. Int. ReV. Phys. Chem. 1985, 4, 279. (28) The track average LET is defined as the integral of the stopping power from the incident particle energy, E0, to zero; i.e., LET ) (1/Eo) ∫0E0(-dE/dx) dE. (29) Holroyd, R. H.; Preses, J. M.; Hanson, J. C. Radiat. Res. 1993, 135, 312. (30) Brocklehurst, B.; Munro, I. H. In Synchrotron Radiation and Dynamic Phenomena; Beswick, J., Ed.; American Institute of Physics: New York, 1991; p 465. (31) Rothman, W.; Hirayama, F.; Lipsky, S. J. Chem. Phys. 1973, 58, 1300. (32) The radiation chemical yield, G-value, is the number of species formed per 100 eV of energy absorbed. (33) Choi, H. T.; Hirayama, F.; Lipsky, S. J. Phys. Chem. 1984, 88, 4246. Note that this work relies on a value of 1.0 for the fluorescence efficiency of 9,10-diphenylanthracene. If this value is take to be 0.9 (Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.), then the quoted G-values for luminescence should be reduced by 10%. (34) Schmidt, K. H.; Allen, A. O. J. Chem. Phys. 1970, 52, 2345. (35) Brocklehurst, B. Faraday Discuss. Chem. Soc. 1977, 63, 96. (36) LaVerne, J. A.; Pimblott, S. M. J. Phys. Chem. 1995, 99, 10540. (37) Brocklehurst, B. Chem. Phys. Lett. 1993, 211, 31.
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