J. Phys. Chem. 1996, 100, 18757-18763
18757
Fluorescence in the Heavy Ion Radiolysis of Benzene Jay A. LaVerne Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: June 14, 1996; In Final Form: September 10, 1996X
Fluorescence emission has been measured in the radiolysis of neat liquid benzene with 1-15 MeV protons, 5-20 MeV helium ions, 5-30 MeV lithium ions, and 10-30 MeV carbon ions using single photon counting techniques. Companion studies were performed with 90Sr-90Y β-particles with an average energy of about 1.66 MeV. Within 30-50 ns following the passage of the heavy particles the fluorescence intensity decreases rapidly and then levels off to a much slower decay rate which is very nearly the same for β-particles and high-energy protons and helium ions. The lifetimes of both the fast and slow components of the fluorescence decrease with increasing linear energy transfer (LET), and the magnitude of the fast component increases with respect to the slower one. Quenching of the fluorescence by radicals or other transient species produced in the particle track are thought to be responsible for the observations. At low LET the relative yield of the fluorescence decreases with increasing LET, presumably due to the quenching. However, at higher LET the fluorescence yield increases with increasing LET. Self-annihilation reactions of the triplet states to give singlet states may be the source of these observations. These experiments are the first in which the temporal variation of fluorescence emission has been determined in a neat hydrocarbon liquid irradiated with heavy ions.
Introduction The production and decay of excited states play important roles in the radiation chemistry of hydrocarbon liquids.1,2 Both molecular products and radicals are known to be descendent from the excited states of certain medium molecules. For example, the yields of molecular hydrogen and cycloalkenes have been determined in the decomposition of the singlet excited states induced in irradiated cycloalkanes.3 It is further well established that the yields of final products are usually dependent on the type of irradiating particle.4 Energy deposited by the passage of ionizing radiation is nonhomogeneously distributed in the medium. The spatial distribution of the energy and the resultant reactive species define the particle track structure, which is largely responsible for the observed yields of final products. High linear energy transfer (LET ) stopping power, -dE/dx) particles such as protons, helium ions, or other heavy ions can have relatively high concentrations of reactive species in their tracks. The result is an increase in the probability of second-order reactions involving excited states that would not otherwise occur or would proceed only to a small extent with more conventional radiation such as fast electrons or γ-rays. It would be extremely helpful for elucidating the relationship between track structure and final product formation by directly measuring the yields and the decay of transient excited states in the track. This work represents the first successful attempt to measure the time-resolved emission from a neat liquid hydrocarbon irradiated with heavy ions. There is very little direct evidence on the effect of particle track structure on the chemistry of excited states produced in heavy particle tracks. The observation of emission from aromatic compounds added to hydrocarbons is a well-established technique for examining radiolytic products.5,6 Emission from the solute represents both ionic and excited state processes of the medium. A few investigations have used this technique in the heavy ion radiolysis of hydrocarbons.6-14 Although it is difficult to interpret detailed track structure from these experiments because one is observing both ionic and excited state X
Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01766-2 CCC: $12.00
transients, they are important for an overview of the processes in heavy ion tracks. All of the studies find a decrease in emission with increasing LET. The observations are generally ascribed to an increase in second-order reactions with increasing LET. Many details, especially the relationship between excited state and final product formation, have yet to be explained satisfactorily. Benzene is the prototypical aromatic system used in radiation chemical studies. It is a fairly stable compound toward radiation damage, and the major lower molecular weight products have been determined using β- and γ-radiolysis.15 The only product examined substantially with higher LET particles is molecular hydrogen, the yield of which increases by more than an order of magnitude from γ-radiolysis to carbon ion radiolysis.16 One of the mechanisms proposed for the excess formation of hydrogen is the combination of molecular excited states in the particle track.17 However, the direct link between the two species has not been proved. Burns and co-workers examined the effects of track structure on the production of triplet states in benzene using the isomerization of cis-but-2-ene as a probe.18 They concluded that that there was substantial self-annihilation of the triplet states in proton radiolysis to give the singlet state. However, no previous study has examined the emission from the excited singlet states produced in the radiolysis of neat benzene with high LET particles. The observed fluorescence lifetime of benzene is about 27 ns with UV radiation19,20 and with fast electrons.21 The excited singlet state yield is estimated to be about 1.5 molecules/100 eV,22,23 so it should play an important role in the radiolysis of neat benzene. Direct examination of the yield and decay of the excited singlet state in benzene will eventually be of great importance in understanding the radiation chemistry of this compound with heavy ions. These experiments are especially important because they represent one of the very few techniques suitable for measuring the temporal dependence of the radiation chemical processes induced by heavy ions. Here the results of measurements on the fluorescence in neat benzene irradiated with protons of 1-15 MeV (average LET24 of 43-9 eV/nm), helium ions of 5-20 MeV (average LET of © 1996 American Chemical Society
18758 J. Phys. Chem., Vol. 100, No. 48, 1996
LaVerne
Figure 1. Heavy ion window assembly showing the beam direction, the collimator, the magnetic suppression region, and the exit window. The single photon counting apparatus is in a light-tight box containing sample cell, quartz light guide, photomultipliers (PMT1 and PMT2), and the iris.
136-68 eV/nm), lithium ions of 5-30 MeV (average LET of 265-141 eV/nm), and carbon ions of 10-30 MeV energy (average LET of 686-549 eV/nm) are presented. Comparative studies were performed using 90Sr-90Y β-particles having an average energy of 1.66 MeV (LET of 0.15 eV/nm). Both the decay rates and the total fluorescence intensities were determined for each ion relative to that for the β-particles using single photon counting techniques. The results of these experiments are presented, and their significance with respect to the radiation chemistry of liquid benzene is discussed. Experimental Section Irradiations were performed with 1H, 4He, 7Li, and 12C ions using the 10-MV FN Tandem Van de Graaff accelerator of the Notre Dame Nuclear Structure Laboratory.25 The window assembly and emission detection are shown in Figure 1. A collimator of 4 mm diameter was used to define the beam profile and to attenuate the particle current. Typical beam currents into the sample were (2-3) × 104 particles/s. The collimator was followed by a magnetically suppressed drift region to sweep away scattered electrons. The ion beam passed successively through a titanium exit window (3.6 mg/cm2), a nitrogen drift region (0.8 mg/cm2), and a mica window attached to the sample cell (4-6 mg/cm2). 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.26 Irradiations with β-particles were performed using a 1 mCi 90Sr-90Y source with maximum energies of 0.54 and 2.25 MeV, respectively.27 The β-particles were collimated to give a particle flux similar to that of the heavy ions and attenuated in energy with a 243 mg/cm2 aluminum absorber. Average β-particle energy was determined by the addition of sufficient aluminum absorber until no emission could be detected. Stopping power tables28 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 a given discriminator setting (see below). An average β-particle
energy was obtained from the appropriate integration of the energy spectrum given by Slack and Way.27 For the 243 mg/ cm2 aluminum absorber used in these experiments, the average β-particle energy was estimated to be 1.66 MeV. Fluorescence was detected using a time-resolved single photon counting technique (cf. Figure 1).29 The optical detection system was housed in a light-tight 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 4-6 mg/cm2) for the particle beam. The fluorescence that passed through a rear Supracil window was collected with a light guide made from a 2.54 cm diameter quartz rod 40 cm long. The light guide allowed for greater efficiency in light collection. 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. Additional attenuation of the number of photons incident to PM2 was obtained by a remotely adjustable iris. Both photomultipliers were RCA 8850s. 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 counter, and the start of a time to amplitude converter, TAC (Ortec Model 567), set for a range of 500 ns. 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
Fluorescence in Benzene Radiolysis
J. Phys. Chem., Vol. 100, No. 48, 1996 18759 1.5 ns. The contribution of Cerenkov radiation to the observed β-particle fluorescence was subtracted directly. All time profiles were obtained with the TAC time window at 500 ns, and the average counts in channels 3000-4000 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 benzene that was further passed through a column containing activated silica gel. Repeated filtering had no observable effect on the fluorescence. All samples consisted of neat liquid benzene carefully deaerated and at room temperature (25 °C).
Figure 2. Fluorescence intensity from neat liquid benzene irradiated with β-particles as a function of time and the prompt or instrument response function due to the Cerenkov radiation from the passage of β-particles in water.
impinging on the sample while the time window of the TAC was still open were detected by the pileup gate, which 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 and the multichannel analyzer were controlled by a personal computer. Except where explicitly stated, the total start counts were 5 × 107 for all particles. The pile-up effect due to the observation of only the first of multiple stop photons was kept small by maintaining the count rate of stop pulses to 2-3% of the start rate. The main source of background is the chance arrival of a second particle within 500 ns of that which gave the start signal. The time profiles of the fluorescence from benzene were obtained with the various energy heavy ions and from β-particles using identical conditions. Cerenkov radiation produced by β-particles in water was used to obtain the prompt or instrument response function. The width of the prompt peak was about
Results and Discussion Fluorescence Profiles. Figure 2 shows the observed fluorescence intensity as a function of time for β-particles. The profile was obtained with a total of 5 × 107 β-particles. Also shown in this figure is the prompt response due to Cerenkov radiation. Several peaks are observed in the prompt intensity profile following the main peak. These satellite peaks are probably due to ringing in the photomultiplier and the light guide. They are also apparent in the responses due to β-particles and the heavy ions, but they cause no particular problem since they are relatively easy to deconvolute out (see below). The fluorescence intensity in β-particle radiolysis shows an initial sharp peak due to the Cerenkov that is followed by a slower, exponential decay due to the emission of the benzene excited singlet states. The observed fluorescent intensities for the protons, helium ions, lithium ions, and carbon ions are shown in Figure 3 as a function of time. The same number of particles (5 × 107) was used to produce each of the fluorescence profiles in Figure 3. As observed with β-particles, there is a rapid increase in fluorescence intensity associated with the passage of the particle that is followed by a slower decrease at longer times. However,
Figure 3. Fluorescence intensity from neat liquid benzene irradiated with 1-15 MeV protons, 5-20 MeV helium ions, 5-30 MeV lithium ions, and 10-30 MeV carbon ions as a function of time. The same total number of particles (5 × 107) was used in the accumulation of the fluorescence profiles for each of the ions.
18760 J. Phys. Chem., Vol. 100, No. 48, 1996 the overall shapes of the fluorescence intensity profiles are very dependent on particle type and energy. With 15 MeV protons one observes a fluorescence decay that is almost identical to that found with β-particles except for the Cerenkov component in the latter. There is no Cerenkov radiation with any of the heavy ions because of their relatively low velocities. Experiments performed with 15 MeV protons in solutions of aromatic scintillators in ethanol showed no fluorescence due to Cerenkov radiation.14 As the proton energy decreases, and LET increases, a rapid decrease in intensity is observed immediately following the passage of the particle. At longer times the fluorescence decay rate slows and appears to approach that found with β-particles. The same general trend is observed with all of the ions. The main variation in the fluorescence decay with increasing particle LET is that the relative magnitude of the initial rapid decay increases with respect to the later slow decay. Something in the particle track appears to be affecting the normal fluorescence of the benzene at short times following the passage of the heavy particle. In a series of papers West and Miller8-11 observed fluorescence from dilute solutions of benzene in cyclohexane irradiated with UV photons and low-energy protons and helium ions. They also observed a fast decay rate at short times followed by decay rates that were common for all of the particles. The magnitude of the fast component relative to the slower one also was found to increase with increasing LET. They ascribed their findings to quenching of the benzene excited state by radicals produced from the decomposition of the cyclohexane. A similar quenching mechanism was previously reported by Berlman and coworkers.7 With increasing LET the density of radicals and other transient species in the track increases. Since the lifetime of the benzene excited state is long, about 27 ns,19,20 there is a high probability that it will survive long enough to undergo second-order reactions in the particle track. The result is an apparent quenching of the fluorescence. However, the overall chemistry and the nature of the excited state of neat benzene are very different from that found in dilute solutions. Benzene Excited States. The analysis of the excited state chemistry in neat liquid benzene is complicated by the formation of an excimer from the coupling of an excited singlet state with a ground state molecule.30 The overall reaction scheme for the initial decomposition of benzene and the formation of excited states is as follows.
The pseudo-first-order rate for the association reaction to form the excimer, ka[C6H6], is about 1.2 × 1011 s-1, while the rate for the decomposition of the excimer back to the monomer and a ground state molecule, kd, is about 5.5 × 1010 s-1.31 For simplicity, the total decomposition rate of the monomer to products other than the excimer is given as km () kf + kic + kis) and the corresponding term for the excimer as ke () kf′ + kic′ + kis′). As will be seen below, the values of ka[C6H6] and kd are much greater than km and ke, respectively. Therefore, within a few hundred picoseconds following the absorption of energy an equilibrium between the monomer and the excimer
LaVerne is established. Only in very dilute solutions or at low temperatures can one slow the excimer formation rate so that only the chemistry of the monomer can be observed. The chemistry occurring in liquid benzene at 25 °C will always involve both the monomer and the excimer. Emissions from the monomer and excimer are broad and centered at about 280 and 320 nm, respectively.30 At wavelengths below 280 nm the absorption of the benzene becomes substantial and the photomultiplier low-wavelength cutoff is also at about the same value. Therefore, these experiments mainly monitor the emission from the excimer with some contribution from the low-energy tail of the monomer. Since the monomer and excimer can be considered to be in equilibrium on the time scale of these experiments, the actual state monitored is not important. However, it should be stressed that emission from only these two states can be observed. Emission from higher excited states cannot be detected in these experiments. The observed fluorescence decay is due to a combination of fluorescent decay rates, kf, internal conversion rates, kic, and intersystem crossing rates, kis, for both the monomer and excimer excited states. Each of these rates has been previously determined from a combination of flash photolysis studies on the neat liquid and dilute solutions. All of the reactions are first order, and at 25 °C the values for the monomer are 2.6 × 106, 2.5 × 107, and 8.7 × 106 s-1 for kf, kic, and kis, respectively, while those for the excimer are 0.6 × 106, 0.3 × 106, and 3.5 × 107 s-1 for kf′, kic′, and kis′, respectively.32,33 The mechanistic scheme has been written according to convention, but the actual product formed in each of the decomposition modes is not known. For example, the channel described as internal conversion may include formation of the ground state or molecular fragmentation. No further distinction is necessary at this point, but the details will be necessary to elucidate the overall radiation chemical mechanism. The decay kinetics for the excited state chemistry of benzene has been derived by several groups, but it is usually attributed to the initial work of Birks and co-workers.30,34 The rate equations for describing the kinetics of the monomer and the excimer can be found in these works, and they are not reproduced in their entirety here. Briefly, it is found that the fluorescence decay of each species can be described by a double exponential of the type [A exp(-λ1t) + B exp(-λ2t)], where A and B are different expressions for the monomer and excimer. The pseudorate λ1 is inversely proportional to the observed fluorescent decay of the system, while λ2 represents the formation of the equilibrium between the monomer and the excimer. Helman performed a complex temperature dependence on this system and determined that at 25 °C the values of km and ke were virtually identical.19 In dilute solutions of benzene one is mainly observing the chemistry of the monomer and λ1 ) km while λ2 ) km + kd. In liquid benzene at 25 °C ([C6H6] ) 11.2 M) the ratio of ka/kd has been determined to have a value of 0.2,31 and since km ) ke the pseudorates are given by λ1 ) km ) ke and λ2 ) km + kd + ka[C6H6]. Substitution of the measured rates readily shows that the lifetime for establishing equilibrium between the monomer and excimer, 1/λ2, is about 6 ps. This value is in general agreement with the measured rate of formation of the excimer.35 Therefore, in these experiments the observed fluorescence decay in neat liquid benzene is due to km or equivalently ke. Fluorescence Decay Rates. A deconvolution routine modeled after that given by O’Connor and Phillips29 was used to estimate the decay rates of the observed fluorescence profiles. It was found that a double-exponential decay curve was suitable to describe the observed decays for all of the particles. While
Fluorescence in Benzene Radiolysis
Figure 4. Lifetimes for the dual exponential deconvolution of the data of Figures 2 and 3 for protons (9), helium ions (b), lithium ions (2), and carbon ions ([) as a function of the track average LET. Solid symbols are for the slow component and open symbols are for the fast component. The dashed lines show the limiting values found with β-particles.
an exponential decay is expected for the longer decay, it should not strictly describe the second-order processes that are thought to be occurring at the shorter times. However, it was found that these short time processes were adequately described by an exponential function. The exponential function is probably robust enough to fit the observed decay. Alternatively, the mechanism for the short time decay is or appears to be first order. The lifetimes obtained from the deconvolution of the fluorescence decays are given in Figure 4 as a function of the track average LET of the incident particle. For β-particles the lifetime of the longer component is found to be 26.8 ns with a standard deviation of 0.1 ns, which is in excellent agreement with the 27.4 ns found with UV photons19 and the 26.3 ns found with pulsed electrons.21 The lifetimes of the longer component are essentially the same for all of the protons, but they decrease noticeably with increasing particle LET. At their lowest energies the values for lithium and carbon ions are 10.6 and 14.6 ns, respectively. These lifetimes have standard deviations of almost 1 ns because of the low fluorescence intensities. However, the lifetime for a 30 MeV carbon ion is 21.4 ns with a standard deviation of
