9862
J. Phys. Chem. 1995, 99, 9862-9868
Heavy-Ion Radiolysis of Cyclooctane Jay A. Laverne" and Laszlo Wojnarovitst Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: February 7, 1995; In Final Form: April IO, 1995@
The major products produced in radiolysis of cyclooctane with 1-15 MeV protons, 10-30 MeV carbon ions, and 20-30 MeV oxygen ions were investigated in the absence and presence of iodine radical scavenger and compared with the y and 5-20 MeV helium ion radiolysis published in a previous work (J.Phys. Chem. 1994, 98, 8014). While almost 70% of the cyclooctyl radicals survive until a few microseconds in y-radiolysis, the yields with carbon ions and oxygen ions are about an order of magnitude lower. It appears that all of the hydrogen atoms produced in y-radiolysis react by hydrogen atom abstraction from the medium molecules, whereas in the heavy-ion tracks enhanced intratrack reactions eliminate most of the hydrogen atoms before such reactions can occur to any significant extent. The yield of products derived from the excited singlet state steadily decreases with the linear energy transfer (LET), and in carbon and oxygen ion radiolysis its yield is less than 20% of that found in y-radiolysis. Product formation through radical mechanisms first slightly increases with particle LET until about 50 eV/nm, and then the total yields of all products decrease to half of that found in y-radiolysis. Much of the change in product yields with increasing LET can be explained by a radical mechanism, but it also appears that there is a change in initial radical formation due to a shift toward an increased probability of triplet excited-state formatiofi over that of singlet excited states.
Introduction Many of the experimental studies on the heavy-ion radiolysis of hydrocarbons have been performed with cyclic compounds.'-3 The relatively few products formed in these systems helps one to elucidate the effects of particle track structure on the radiation chemistry. The additional use of an efficient radical scavenger such as iodine allows one to determine the role of the major radicals produced. Most of the attention has focused on the radiolyses of cyclopentane and of cyclohexane and only recently have results on the heavy-ion radiolysis of cyclooctane been p ~ b l i s h e d . ~In - ~a previous work6 the basic radiation chemistry of cyclooctane as determined with y-rays and 5-20 MeV helium ions was presented. That study has been expanded to include the radiolysis of cyclooctane with protons of 1- 15 MeV, carbon ions of 10-30 MeV, and oxygen ions of 20-30 MeV, and the results are presented here. The use of such a wide range of particles allows one to better understand the relationship between particle track structure and the observed radiation chemistry. The y-radiolysis of cyclooctane can be characterized by a high yield of C-H decomposition (G(H2) = 5.9 molecules/ 100 eV7) and by a low yield of C-C scission (G(C-C) 0.45): In the C-H decomposition cis-cyclooctene, cis-bicyclo[3.3.0]octane (pentalane), cis-bicyclo[5.1.O]octane, and bicyclooctyl form with yields of 3.22, 0.65, -0.06, and 1.92.6 The total yield of these products agrees well with that of the complementary molecular hydrogen yield. This agreement is a very important difference from the more studied cyclopentane and cyclohexane radiolysis where in both cases the hydrogen deficit8 balance has ~ 1 0 % Product formation in cyclooctane radiolysis from C-H decomposition processes takes place by three different mechanisms.8 One of them involves a dissociation from the singlet excited state giving an H2 molecule (G = 2.45) and a carbene type intermediate. No radical intermediates have been detected from this mechanism, and it appears that the carbene rearranges
'
Permanent address: Institute of Isotopes of the Hungarian Academy of Sciences P.O. Box 77, Budapest, H-1525, Hungary. *Abstract published in Advance ACS Abstracts, May 15, 1995.
to cyclooctene or bicycl~alkanes.~ In the other two mechanisms cyclooctyl radicals are produced with concomitant formation of scavengeable hydrogen atoms (G = 2.3), or without formation of scavengeable hydrogen atoms (G = 1.0).* In y-radiolysis the hydrogen atoms almost exclusively abstract another hydrogen atom from the bulk medium to give more cyclooctyl radicals and molecular hydrogen. Disproportionation and combination reactions of the cyclooctyl radicals (kdlk, = 0.736) lead to the formation of cyclooctene and bicyclooctyl. Since all of the bicyclooctyl is believed to be formed by cyclooctyl radicals it is estimated that the total yield of the latter is about 6.6 radicals/ 100 eV. The energy deposited in cyclooctane, or any other liquid, by y-radiolysis is distributed nonhomogeneously throughout the medium in regions called spurs. Each spur can be characterized by a number and spatial distribution of cyclooctyl radicals and other cyclooctane decomposition products. The lifetime of spurs is less than 1 ps as determined by the relaxation of the nonhomogeneous spatial distributions to nearly homogeneous. About 70% of the cyclooctyl radicals will survive the spur and react homogeneously in the bulk medium.6 Such a large escape yield means that most of the cyclooctyl radicals can be scavenged by a small concentration of a radical scavenger such as iodine. With an increase in the incident particle linear energy transfer (LET = -dE/&, stopping power) radical concentrations increase which enhances radical-radical reactions as compared to radical diffusion into the bulk. At a given scavenger concentration far fewer radicals will be scavenged in high-LET heavy-ion radiolysis than found in y-radiolysis. The track effects in hydrocarbons are further complicated by the competition of hydrogen atom-atom reactions in competition with hydrogen atom abstraction reactions that lead to cyclooctyl radical production. Nevertheless, observation of the yields of the major products in the radiolysis of cyclooctane with various particle types gives a significant amount of information on particle track structure. An important question in radiation chemistry is whether the primary decomposition yields are independent of the type of
0022-3654/95/2099-9862$09.Q0/00 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 24, 1995 9863
Heavy-Ion Radiolysis of Cyclooctane
................
-
........ '.'''I' 'H ...... .........I . . . . . . . . . . . . . . 4He I
I
-
.....
-
0
-
. . . . . . . . . . . . . . . . . .-.
--
.... ........ ...... .....
'*C
+
0
10
20
30
0
Particle Energy, E, (MeV) Figure 1. Production of cyclooctene (Go&, molecules/100 particles) from neat cyclooctane as a function of initial particle energy, E,, for protons (a),carbon ions (A),and oxygen ions (+), this work; helium ions (0);6 and 210Po-aparticles (O).I5 The dashed line shows the y-radiolysis limit.6The solid lines were obtained by a best fit of the data to eqs 1 and 2. ionizing particle. Bums and Barker3 concluded in early studies that it was not necessary to postulate an LET dependence of the radical yields produced by the decomposition of the singlet excited state in liquid cyclohexane. Later however hinted that there might be a change in the primary decomposition yields with changing particle type and LET. The radiolysis of cyclooctane offers an answer since pentalane is formed entirely in a unimolecular decomposition of the singlet excited state.2s'o The lifetime of this state is very short, and it is not sensitive to secondary reactions. Previous results6 on product yields with y-rays and helium ions are combined here with the proton, carbon, and oxygen ion radiolysis data in order to evaluate the effect of LET on initial product yields.
Experimental Section Heavy-ion irradiations were performed using the 9-MV Tandem Van de Graaff accelerator of the Notre Dame Nuclear Structure Laboratory. The details of the accelerator facility, window assembly, and radiolysis procedure have been described elsewhere.I2,l3 Particle energy incident to the accelerator exit window was determined by magnetic analysis to within 0.1%. All irradiations were performed with completely stripped ions and energy loss of the particle in passing through the various windows was determined from standard stopping power tables.I4 Energy deposited into the sample was typically about 5 x lOI9 eV in 20 mL of sample (total dose of -50 krad). Cyclooctane was purified by distillation as described elsewhere.6 Product analysis of the irradiated samples was performed within a few days of the radiolysis using a gas chromatograph-mass spectrometer operated in a selected ion monitoring mode. Details of the analysis technique were described in former I
Results and Discussion All of the particles used in these experiments were completely stopped in the sample. Their ranges in the cyclooctane varied from 2.7 mm for 15 MeV protons to 21 p m for 20 MeV oxygen ions. Therefore, it is the integral or track average radiation chemical yields (Go) which have been measured. Figure 1 shows the track averaged production of cyclooctene (GO&, molecules/100 incident particles) in neat cyclooctane as a function of incident energy (Eo) for the various particles. Also shown in this figure is a datum obtained using low-energy *loPo
10
20
30
Particle Energy, E, (MeV) Figure 2. Radiation chemical yields of cyclooctene from neat cyclooctane as a function of initial particle energy. The symbols are for the track-averaged yields with the same representation as in Figure 1. The solid lines are for the track averaged yield and the dotted lines are for the track segment yields as obtained from the appropriate manipulation of eqs 1 and 2 with the same parameters used in Figure 1. The dashed line shows the y-radiolysis limit.6 TABLE 1. Radiation Chemical Yields (Go,moleculedl00 eV) in the Radiolvsis of Neat Cvclooctane
energy LET0 (MeV) (eV/nm) cyclooctene bicyclooctyl pentalane 3.22 1.92 0.65 6oCo-y 1.25 0.16 0.3 1 2.73 2.03 'H 1 46.5 2.03 0.36 2 32.7 2.74 2.09 0.38 19.2 2.73 5 2.16 0.46 2.82 10 12.3 0.52 2.92 2.20 15 9.43 1.56 0.20 146.6 2.40 4He 5 1.65 0.23 106.2 2.40 10 0.29 2.52 1.67 15 85.4 1.70 0.27 72.4 2.54 20 0.67 0.12 1.42 '2C 10 740.8 0.67 0.14 696.5 1.41 15 0.82 0.16 654.3 1.64 20 0.98 0.16 1.82 25 616.6 1.08 0.18 583.0 1.79 30 0.89 0.13 1.50 I60 13.9 1047 0.63 0.13 994.5 1.42 22.5 0.14 1.61 0.79 32.3 929.0 a-parti~1es.I~ The latter point is slightly lower than the present data would suggest, but since it was obtained at a much larger dose and uncertainties in energy determination are greater at low energies the agreement is good. At a given energy there is a progressive decrease in cyclooctene production with increasing particle nuclear charge. This decrease can be observed in the curvature of the plots of Figure 1 or from the integral Go values that are given in Figure 2 and Table 1. For a given ion the yield of cyclooctene is nearly linear with particle energy and the yield decreases with increasing particle nuclear charge. Figures 3 and 4 show the integral Go values for bicyclooctyl and pentalane, respectively, as a function of initial particle energy. It is readily observed from Figures 2-4 that the yields all generally increase with increasing particle energy although the rate of change may be different for the different products. At a given energy product yields for the different types of particles are not always less than the limiting value found with y-rays. It is usually more instructive to investigate the causes for the variation in product yields by plotting the data of Figures 2-4 as a function of particle LET. The correct method of presenting the data is to determine the differential or track segment yield (G, = d(G&o)/dEo) as a function of the track ion
Laverne and Wojnarovits
9864 J. Phys. Chem., Vol. 99, No. 24, 1995 3 r
4i-
t
'H
h
a,
C a, c
u
3 .
0
-00 u
w
2 ,
:0
0
I
0
1
10
20
30
Particle Energy, E, (MeV)
l[
0 100
IO'
102
1o3
LET, or LET (eV/nm)
Figure 3. Radiation chemical yields of bicyclooctyl from neat cyclooctane as a function of initial particle energy. The symbols are for the track averaged yields with the same representation as in Figure 1. The solid lines are from a linear fit of the data. The dashed line shows the y-radiolysis limit.6
Figure 5. Radiation chemical yields of cyclooctene from neat cyclooctane as a function of particle LET. The symbols are for the track averaged yields as a function of track average LET0 with the same representation as in Figure 1. The solid lines are for the track segment yield as given in Figure 2 as a function of track segment LET. The dashed line shows the y-radiolysis limit.6
using many different particles have shown that the equations 6oco-y
._________----
0.6 h
al C
([I -
([I 0.4 +
C
a,
Q
w
uo 0.2 0.0
'
0
10
I
I
20
30
Particle Energy, E, (MeV) Figure 4. Radiation chemical yields of pentalane in neat cyclooctane as a function of initial particle energy. The symbols are for the track averaged yields with the same representation as in Figure 1. The solid lines are from a linear fit of the data. The dashed line shows the y-radiolysis limit.6 segment LET. Track segment yields for each ion can be determined from the tangents to the curves of the data in Figure 1. However, not all products are generally measured over a wide range of energy because of the limited time constraints on heavy-ion accelerator facilities. It is much more common to present integral GOvalues as a function of the track average LET0 as given by
One must remember that the integral Go value is similarly related to the track segment G; value by
Go = ( l / E o ) ~ E cdE Gi If the track segment yield is proportional to the track segment LET, then the integral GOvalue is similarly proportional to the integral LETo. It is usually not possible to directly obtain differential yields from data as given in Figure 1 because of the large amount of scatter that would be introduced. It is much more common to fit the data to an equation and take the derivative of that equation. Previous experimentsL6with the Fricke dosimeter
well represent the heavy ion data over a wide range of energies. In these equations G, is the limiting y-ray value, GBis the yield at the energy of the Bragg peak (EB),and a and m are parameters to be determined for each ion. It is straightforward to obtain both the integral and differential yields from these equations. The solid lines in Figure 1 show the results of fitting these equations to the data for cyclooctene production in neat cyclooctane irradiated with the various heavy ions. The integral and differential yields derived from eqs 1 and 2 are shown as solid and dotted lines, respectively, in Figure 2 . It can be observed that at a given energy there can be a large difference between integral and differential yields. At a given energy the track segment Gi values are generally larger than or equal to the integral Go values. It is easy to show that the differential track yield should only equal the integral yield when the initial energy of the particle approaches zero or when the integral yield becomes independent of the incident particle energy.I7 It should be noted that differential yields can always be obtained from plots of the data as given in Figure 1, but the above equations may not always be the best choice for fitting the data. Circumstances in which the products are both created and destroyed with changing LET can lead to much different behavior than predicted by the above equations. The results observed in Figure 3 and discussed below suggests that the yield for bicyclooctyl represents such a case. Figure 5 shows the integral Go values for the production of cyclooctene in neat cyclooctane as a function of the integral LET0 for the different particles. The solid lines on the same figure show the results for the differential yields as a function of the track segment LET. It can be seen that over the range of energies used here the agreement between the two methods is generally good. At low particle energies it is expected that for a particular ion the differential yields are somewhat linear with the track segment LET. Obviously, at very high energies the particle track will break up into isolated spurs and the yields will exhibit a different dependence on particle LET. The highenergy (low LET) proton data may be showing some of the effects of this breakup.
Heavy-Ion Radiolysis of Cyclooctane
J. Phys. Chem., Vol. 99, No. 24, 1995 9865
Track Average LET, (eV/nm) Figure 6. Integral Go values for the production of cyclooctene,
bicyclooctyl, and pentalane in neat cyclooctane as a function of track average LETo. Variation of the integral cyclooctene, bicyclooctyl, and pentalane yields as functions of the integral LET0 are shown in Figure 6. It is immediately seen that all three of these products decrease with increasing particle LETo. In the following discussion it will be assumed that product yields are dependent on LET0 regardless of the particle type. Such an assumption is not true as can be seen in some of the data in Figure 6. However, the difference is not very great in most cases. Since all of the bicyclooctyl and much of the cyclooctene are formed from reactions of cyclooctyl radicals, it may initially be surprising that the yields of these products decrease with increasing particle LET. An increase in LET is expected to increase radical-radical reactions in the track. However, a considerable fraction of the cyclooctyl radicals is formed by hydrogen abstraction reactiom6 The following reaction scheme can reasonably represent the chemistry in the particle track:
C,H,, C,H,,*
H
--
-
C8HI6**
pentalane
+ C,H,, 2H
-
H,
-
+ H,
(R2) (R3)
+ C,H,,
H,
In this mechanism CsHi6* represents a low-energy singlet excited molecule that mostly decays with the elimination of molecular hydrogen. The species CgH16** represents a triplet excited molecule that decays to radicals, and it also represents some ionic intermediate that is produced by ion-molecule
TABLE 2. Radiation Chemical Yields (GO, moleculedl00 eV) in the Radiolysis of Cyclooctane with 1 mM Iodine energy cyclooctyl ion (MeV) cyclooctene bicyclooctyl pentalane iodide 6oCo-y 1.25 2.20 0.49 0.62 4.60 1.08 'H 1 2.30 0.32 2.72 2 2.10 0.34 3.00 0.87 0.83 0.36 3.88 5 2.00 10 2.10 0.70 0.45 4.38 0.70 15 2.15 0.48 4.50 1.10 4He 5 1.80 0.22 1.42 10 1.90 1.05 0.23 1.60 15 1.80 0.27 2.02 0.95 0.95 20 1.85 0.28 2.25 '2C 10 1.54 0.61 0.13 0.66 0.65 15 1.61 0.14 0.59 20 1.87 0.79 0.16 0.86 0.73 0.16 0.77 25 1.90 30 2.12 0.91 0.18 0.98 0.93 '60 13.9 1.53 0.14 0.61 22.5 1.51 0.62 0.12 0.62 0.64 32.3 1.56 0.14 0.85 reactions and decays to molecular hydrogen and cyclooctyl radicals. With increasing LET the hydrogen atom abstraction reaction R7 is not able to effectively compete with reactions R10-Rl2 resulting in a net decrease in cyclooctyl radical production. Both the cyclooctene and bicyclooctyl yields decrease to approximately half their values from y-rays to oxygen ions. Measurements of the cyclooctyl radical yields at different LETS is one technique for estimating the effective concentration of radicals in the particle track. However, it is usually assumed that the initial yields of reactions R1 and R2 are independent of LET. As discussed below, this assumption may not always be valid. Iodine is an efficient radical scavenger with a rate constant of 5.7 x lo9 M-' s-I in its reaction with cyclooctyl radicals.Is The product of this reaction is cyclooctyl iodide and iodine atoms, and the latter mostly recombine to iodine. At an iodine concentration of 1 mM the lifetime of cyclooctyl radicals with respect to the iodine is about 200 ns, and in the case of y-rays the spherical spur is virtually relaxed to a homogeneous distribution. One is therefore sampling cyclooctyl radicals that survive or escape the spur. Mathematically, there is no equivalence to an escape yield with a columnar track structure. However, very few heavy ion tracks are strictly columnar in structure over their entire lifetime. The previously reported6 cyclooctyl iodide yields for 1 mM iodine solutions of cyclooctane irradiated with y-rays and with helium ions are combined with new results for protons, carbon ions, and oxygen ions in Table 2. Figure 7 shows the variation of cyclooctyl iodide, cyclooctene, bicyclooctyl, and pentalane yields in 1 mM iodine solutions as a function of track averaged LETo. There is a large decrease in cyclooctyl iodide yields with increasing LETo. The yields of cyclooctene and pentalane show a much smaller decrease with increasing LET0 while the bicyclooctyl yield initially increases and then decreases with increasing LETo. The same variation of the bicyclooctyl yield was observed for the bicyclohexyl yield in cyclohexane,' and it obviously reflects the complicated chemistry that has occurred in the track up to 200 ns. Pentalane yields show virtually no dependence on the presence of iodine as expected if it is only formed by the decomposition of the singlet excited state. Only at very high iodine concentrations could it interfere, if at all, with the formation or decay of the singlet excited state. A few experiments were performed at selected particle energies in which iodine concentrations were varied from 0.1 to 30 mM and the yields of cyclooctyl iodide measured. The
LaVeme and Wojnarovits
9866 J. Phys. Chem., Vol. 99, No. 24, 1995
'H
3
I 1o*
10'
1o3
Track Average LET, (eV/nm) Figure 7. Integral Go values for the production of cyclooctene, bicyclooctyl, pentalane, and cyclooctyl iodide in 1 mM iodine solutions of cyclooctane as a function of track average LETo. TABLE 3. Radiation Chemical Yields (GO, molecules/100 eV) in the Radiolysis of Cyclooctane with Various Concentrations of Iodine energy iodine cyclo- bicyclocyclooctyl ion (MeV) (mM) octene octyl pentalane iodide 6oCo-y 1.25 0.0 3.22 1.92 0.65 0.1 2.25 0.60 0.65 4.60 0.5 2.10 0.52 0.55 4.60 0.49 0.62 4.60 1.0 2.20 5.0 2.00 0.45 0.60 4.58 0.40 0.61 4.40 10.0 2.10 30.0 1.90 0.27 0.50 3.70 'H 10 0.0 2.82 2.16 0.46 0.95 0.49 2.70 0.1 2.35 0.2 2.30 0.89 0.50 3.10 0.75 0.44 4.10 0.5 2.15 1.0 2.10 0.70 0.45 4.38 0.54 0.45 4.40 5.0 2.13 10.0 2.15 0.53 0.47 4.60 0.46 0.45 5.00 30.0 1.90 4He 10 0.0 2.40 1.65 0.23 1.44 0.22 0.55 0.1 2.00 0.2 2.05 1.40 0.22 0.76 1.10 0.22 1.10 0.5 1.97 1.05 0.23 1.60 1.0 1.90 5.0 1.90 0.90 0.24 1.85 0.77 0.25 2.10 10.0 1.90 0.60 0.24 2.70 30.0 1.75 0.67 0.12 0.0 1.42 '2C 10 0.1 1.61 0.76 0.14 0.20 0.76 0.12 0.37 0.2 1.57 0.5 1.59 0.70 0.14 0.46 0.61 0.13 0.66 1.0 1.54 5.0 1.54 0.49 0.13 0.88 0.43 0.13 0.9 1 10.0 1.57 0.37 0.13 1.24 30.0 1.43 '
upper limit of iodine concentration is limited by its solubility while its lower limit is restricted to the lowest amount that will not be completely depleted for the given radiation dose. One particular advantage of the use of gas chromatographic-mass spectrometric techniques is that very low doses and thereby low iodine concentrations can be used. Previous results6 on the formation cyclooctyl iodide with y-rays and 10 MeV helium ions are given in Table 3 along with the results for 10 MeV protons and carbon ions. By varying the iodine concentration, one is effectively sampling the cyclooctyl radical yields at various times in the particle track. If those yields are not varying very quickly, then a simple temporal dependence of the cyclooctyl radical yields can be obtained from a plot of the cyclooctyl iodide yields as a function of the inverse of the
product of the iodine scavenging rate constant and the iodine concentration. A plot of the cyclooctyl radical yields as a function of radical lifetime for the various heavy ions and y-rays are shown in Figure 8. An initial increase in cyclooctyl radical yields over a few tens of nanoseconds is observed with y-rays. This increase is due to the hydrogen abstraction reaction R7. The rate constant for reaction R7 is not known for cyclooctane, but it has been measured at 3 x lo7 M-' s-l for both cyclopentane and cyclohexane in water.I9 A simple viscosity scaling would suggest a rate of 1.2 x lo7 M-' s-l for this reaction in cyclooctane. Therefore, the lifetime of the hydrogen atom with respect to cyclooctane is about 11 ns in good agreement with the y-ray experiments. The results for helium ions and carbon ions with track averaged LETos of 106 and 740 eV/nm, respectively, show steady decreases in cyclooctyl radical yields with time. Intratrack reactions of the hydrogen atom appear to compete effectively with the hydrogen atom abstraction reaction and therefore must be over within tens of nanoseconds. The rate constant for the hydrogen atom-atom reaction R12 is given by 2k~12= 1.55 x lo9 M-' s-' in water20 and again a simple viscosity scaling leads to a value of 2kR12 = 0.61 x lo9 M-' s-l in cyclooctane. If one can assume that the only hydrogen atom reactions are reactions R7 and R12, then I ~ ]>> 0.15 for the heavy ions 2 k ~ 1 2 [ H>> ] ~~ R ~ [ H ] [ C ~orH[HI M. Cyclooctyl radical yields with protons decrease very slowly until about 100 ns and then followed by a rapid decrease at longer times. The slow decrease for protons could be due to a balance in the competing reactions for hydrogen atoms suggesting that for a proton at an LET0 of 12 eV/nm the average hydrogen atom concentration is about 0.1 M. Hydrogen atom concentrations are obviously much higher with the other particles. The long time limit yield of cyclooctyl radicals can be as much as an order of magnitude less than with y-rays where almost 70% of the cyclooctyl radicals survive until the microsecond time scale. It is a little difficult to estimate the initial concentration of cyclooctyl radicals because they are formed at short times by reactions R5 and R6 and at later times by reaction R7. As discussed above the relative amount of the contribution of reaction R7 to cyclooctyl radical formation depends on the LET. The initial spatial distributions of hydrogen atoms and cyclooctyl radicals produced by reactions R5 and R6 will be the same so the initial concentrations will be proportional to their initial yields, 2.3 and 4.3, respectively. For carbon ions one can essentially assume that no hydrogen atom abstraction reactions (R7) occur. The carbon ion data of Figure 8 shows that at times of -1 ns the cyclooctyl radical yield has dropped to half its initial value. At this point the competition between cyclooctyl radical-radical reaction (R8 R9, with a rate constant of 0.86 x lo9 M-' s-l 18) and radical scavenging is about the same if it further assumed that cross reactions (R10 and R11) are negligible. One can then estimate that the average cyclooctyl radical concentration is about 1 M. Clearly, these estimates of radical concentration give only the magnitude and further diffusion-kinetic calculations are necessary for detailed descriptions of particle tracks. Reactions R1 and R2 are very simplistic representations of the true mechanisms which are unknown. Presumably, both reactions take place for the most part by ionic intermediates. There is nothing to suggest that the initial production of ionic species is dependent on LET, but there are several things that suggest that the initial yields of products in reactions R3-R6 are dependent on particle LET. Pentalane is formed exclusively by the unimolecular decomposition of the excited singlet state of cyclooctane.2,10Figures 4 and 6 show that the yield of
+
Heavy-Ion Radiolysis of Cyclooctane
1
J. Phys. Chem., Vol. 99, No. 24, 1995 9867
6oco-y
IH
e 4 E!
” .-c (d
U L
3 1 bimolecular
0
Time (seconds) Figure 8. Temporal dependences of the integral Go values for the production of cyclooctyl radicals in the track of the various particles. Symbols are the same as in Figure 1. Helium ion and y-radiolysis data from ref 6.
pentalane is very dependent on particle energy and LET. Its yield with oxygen ions is only 20% of that found with y-rays. Virtually no dependence of pentalane yield on iodine concentration is found in the data of Table 3. In the heavy ion track there is a high probability for the formation of two or more singlet molecules in each other’s proximity. In the relaxed singlet state two excited molecules may interact and send one to the ground state and the other to a higher excited or ionic state.2’ The net result is the loss of one singlet excited state. Such a reaction probably plays a role only in the track of very densely ionizing particles since the lifetime of the excited state of cyclooctane is very short, it is estimated to be ~ 0 . ns. 3 (The interaction of triplet molecules is much less probable because their lifetime is even shorter.22) The singlet excited molecules may also be depleted by radicals present in high concentrations in the track of highly ionizing particles. This scenario has been suggested by Holroyd et al.23based on their fluorescence decay studies with low-energy electrons in n-dodecane and also by West and Miller24on fluorescence decay studies of benzene in cyclohexane irradiated with protons and helium ions. Again, due to the short lifetime of the cyclooctane singlet excited state such reactions may have importance only at very high ionization densities. It is much more likely that with increasing particle LET fewer excited singlet states of the cyclooctane are being formed in reaction R1 and other methods not listed than it is that other reactions are interfering with reactions R3 and R4. The disproportionation to combination ratio for cyclooctyl radical reactions is 0.73, and it is usually assumed that all of the bicyclooctyl yield is due to the combination of cyclooctyl Therefore, in y-radiolysis 1.8 molecules/100 eV of the total cyclooctene yield of 3.22 come from the excited singlet state. The measured yield of pentalane is 0.65 so the cyclooctene produced from the singlet excited-state is approximately 2.8 times the observed pentalane yield. If one assumes that the branching ratio of the singlet excited state does not change with LET, only its yield does, then one may use the observed pentalane yield to estimate the total yield of the products due to singlet excited-state decomposition, the so-called unimolecular products. The total yield of products (the sum of the cyclooctene, bicyclooctyl, and pentalane yields) and the yield of unimolecular products are shown in Figure 9 as a function of track average LETo. Figure 9 shows that the total yield of the three major products produced in cyclooctane decreases as a function of track average LETo. Of these products those due to the unimolecular decay of the singlet excited state also show
100
101
1o2
103
Track Average LET, (eV/nm) Figure 9. Total product yields, bimolecular yields, unimolecular yields, and the ratio of bimolecular to unimolecular yields as a function of track average LETo. The dashed line shows the y-radiolysis limit.6
a large decrease with increasing LETo. The difference between the total yields and the unimolecular yields are due to the bimolecular processes (bimolecular because each product is due to the decomposition of two cyclooctane molecules). The bimolecular yields show an initial increase with increasing LET0 and then a decrease at LETos above =SO eV/nm. The initial increase in the bimolecular yield and the decrease in the unimolecular yield suggests that part of the precursor to the unimolecular product forming reaction is being replaced by the precursor to the bimolecular process. A major part of the bimolecular product formation is expected to take place through triplet excited molecule^.^^,^^ So it appears that the triplet excited state yield is increasing as the singlet excited-state yield decreases. In y-radiolysis the spurs produced in the energy deposition process contain only a few electron-cation pairs and geminate recombination dominates the decay of the charged species.27 With increasing LET the spurs are produced closer along the particle path and with the heavier ions they coalesce into columnar track. At sufficiently high LETS the recombination of the electron-cation pairs is completely random, and the relative probability of singlet to triplet excited-state formation is 1:3. The increased importance of cross combination reactions at the expense of geminate recombination reactions plays a decisive role in the relatively high yield of bimolecular product formation as compared to the unimolecular product formation in heavy-ion radiolysis. There is a lot of scatter in the data, but the observed ratio of the bimolecular to unimolecular product yields shown in Figure 9 increases from a value slightly greater than 1 to about 3 with increasing LET. This trend is exactly what one would predict from the above mechanism. The singlet-to-triplet replacement mechanism may not account for all of the apparent loss of products due to unimolecular processes. If it is assumed that all the recombination in y-radiolysis is geminate, and it is not, then the yield of unimolecular product formation should not decrease to less than 25% of its value with y-rays. However, it is observed that the unimolecular product yield with carbon and oxygen ions is about 20% of that found in y-radiolysis. The cause of the too low yields at high LET is not clear, but at least partly it may be due to experimental uncertainties and also to problems connected with the calculation of the unimolecular yield. However, since the decrease is rather high effects other than the change in charge recombination processes should also be considered, e.g., the reaction of two excited molecules or the quenching of excited
9868 J. Phys. Chem., Vol. 99, No. 24, 1995 molecules by radicals. Of course, it is possible that ionic precursors to the excited states may also be affected in some manner at very high LETs.
Conclusions The heavy-ion radiolysis experiments demonstrate that in the radiolysis of cyclooctane the ionization density of the particle has a profound effect on the competition between radicalradical and radical-molecule reactions. This competition leads to a decrease in the yields of products with increasing particle LET. The sum of the yields of cyclooctene, bicyclooctyl, and pentalane with oxygen ions is about half of that found with y-rays. Iodine scavenging experiments have shown that the yield of cyclooctyl radicals with oxygen ions is about 5% of that found with y-rays. However, radical mechanisms cannot account for the total change in the chemistry observed. With increasing LET up to -50 eV/nm product yields due to bimolecular or radical mechanisms increase at the expense of product yields due to unimolecular decomposition processes. At the highest LET studied here (-1000 eV/nm), the data suggest that the yield of the excited state of cyclooctane is about 20% of its value with y-rays. The present results clearly indicate that the LET influences not only the reactions of radicals, but also the yields of primary decompositions. This influence most probably occurs through changes in the charge recombination mechanism at the LETs used here.
Acknowledgment. We thank Prof. J. J. Kolata for making the facilities of the Notre Dame Nuclear Structure Laboratory available to us. The Nuclear Structure Laboratory is supported by the U S . National Science Foundation. The research described herein was supported by the Office of Basic Energy Sciences of the U S . Department of Energy. This is contribution NDRL-3802 from the Notre Dame Radiation Laboratory. References and Notes (1) Laverne. J. A.; Schuler, R. H.; Ross, A. B.; Helman, W. P. Radiat. Phys. Chem. 1981, 17, 5.
LaVeme and Wojnarovits (2) Foldiak, G., Ed. Radiation Chemistp of Hydrocarbons; Elsevier: Amsterdam, 1981. (3) Bums, W. G.; Barker, R. In Aspects of Hydrocarbon Radiolysis, Gaumann, T., Hoigne, J., Eds.; Academic Press: London, 1968; p 33. (4)Foldiak, G.; Wojnarovits, L. Acta Chim. Acad. Sci. Hung. 1974, 82, 269. ( 5 ) Foldiak, G.; Roder, M.; Wojnarovits, L. Fifh Working Meeting on Radiation Interactions; Mai, H., Brede, O., Mehnert, R., Eds.; Zfi: Leipzig, 1991; p 21. (6) Wojnarovits, L.; Laverne, J. A. J . Phys. Chem., 1994, 98, 8014. ( 7 ) Radiation chemical yields, G values, are given in units of molecules of productl100 eV of absorbed energy. (8) Wojnarovits, L.; Laverne, J. A. J. Phys. Chem., in press. (9) Wojnarovits, L; Szondy, T.; Szekeres-Bursics, E.; Foldiak, G. J. Photochem. 1982, 18, 273. (10) Bums, W. G.; Reed, C. R. V. Trans. Faraday Soc. 1970,66,2159. (1 1) Laverne, J. A,; Schuler, R. H.; Foldiak, G. J. Phys. Chem. 1992, 96, 2588. (12) Laverne, J. A.; Schuler, R. H. J. Phys. Chem. 1984, 88, 1200. (13) Laverne, J. A.; Schuler, R. H. J. Phys. Chem. 1987, 91, 6560. (14) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range of Ions in Solids; Pergamon: New York, 1985. (15) Foldiak, G.; Roder, M.; Wojnarovits, L. J. Phys. Chem. 1994, 98. 5770. (16) Laverne, J. A,; Schuler, R. H. J. Phys. Chem. 1994. 98, 4043. (17) The relationship G, = d(GoEo)/dEo is equivalent to Go Eo dGd dEo. (18) Laverne, J. A.; Wojnarovits, L. J. Phys. Chem. 1994, 98, 12635. (19) Neta, P.; Fessenden, R. W.; Schuler, R. H. J. Phys. Chem. 1971, 75, 1654. (20) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref: Data 1988, 17, 513. (21) Brocklehurst, B. J . Chem. Soc., Faraday Trans. 1992, 88, 2823. (22) Wojnarovits, L. Fijth Working Meeting on Radiat. Interact.; Mai, H.; Brede, 0.;Mehnert, R., Eds.; Zfi: Leipzig, 1991; p 64. (23) Holroyd, R. A.; Preses, J. M.; Hanson, J. C. Radiat. Res. 1993, 135, 312. (24) West, M. L.; Miller, J. H. Chem. Phys. Lett. 1980, 71, 110. (25) Plotnikov, V. G.; Ovcinnikov, A. A. Usp. Khim. 1978, 47, 444. (26) Plotnikov, V. G. Radiar. Phys. Chem. 1985, 26, 519. (27) Hummel, A. Radiation Chemistry of Alkanes and Cycloalkanes. In The Chemistty ofAlkanes and Cycloalkanes; Patai, S . , Rappoport, Z., Eds; John Wiley and Sons: New York, 1992; p 743.
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