4096
J . Phys. Chem. 1984, 88,4096-4099
Solvent Ionization in the 248-nm Photolysis of Anthracene in Cyclohexane with High- Intensity Laser Pulses’ Myran C. Sauer, Jr.,* Alexander D. Trifunac, Daniel B. McDonald, and Ronald Cooper? Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: September 9, 1983)
The photolysis of 2 X 10” M anthracene in liquid cyclohexane containing 0.05 M SF6 with high-intensity pulses of 248-nm light causes an unexpectedly high conductivity signal decaying on a 1-ps time scale. Positive ion scavengers accelerate the decay of this signal, strongly supporting the conclusion that the high-mobilityc-C6HI2+ion is a major product of the photolysis. The nature of the photolytic process is elucidated by other observations reported here: conductivity dependence on light intensity and excited-state quenchers, hydrogen production, and consumption of anthracene.
Introduction We have reported preliminary results on the dc conductivity signals observed when anthracene dissolved in cyclohexane and other solvents is photolyzed with intense laser pulses from an excimer laser.’ In SF6-saturated solutions the signals at times longer than about 5 pus are quantitatively comparable if one takes into account the different mobilities of the molecular ions in various solvents. The same should hold true for all times more than about 20 ns after the laser pulse, but samples dissolved in cyclohexane show anomalously high conductivity at times shorter than 1 p s . This unexpected result cannot be explained in terms of simultaneous or consecutive two-photon ionization of anthracene followed by the scavenging of electrons by SF6. The high-conductivity signal unique to the cyclohexane samples can be explained, however, if cyclohexane positive ions, which have an anomalously high mobility in liquid cyclohexane, are produced in the course of the phot~lysis.~In the following we present experimental results which confirm this suggestion and probe the nature of the photolytic processes. Experimental Section A diagram of the experimental arrangement has been given earlier.2 The light from a Lambda Physik excimer laser (Model EMG, 102, about 200 mJ per pulse, 20-11s fwhm) operating at 248 nm was focused and collimated (1.1 cm X 0.35 cm) so as to pass entirely between the electrodes (4 cm long, 1.2 cm wide, spacing 0.7 cm) of the conductivity cell. The light diverged about 50% in traversing the length of the electrodes. The maximum intensity entering the sample cell was typically about 2 X 10’’ photons cm-2 per pulse over an area of about 0.4 cm2 at the cell entrance window. The light beam passed no closer than about 1 mm to the electrodes. The dc-conductivity of the solution was measured with a circuit similar to ones described earlier$5 with 900 V across the electrodes, a load resistance of 92 ohms, and a capacitance of 55 pF (see Figure 1). In the case of the measurements described here, the ions are essentially too short-lived to reach the electrodes; the circuit senses them by a “displacement current” effect. This displacement current is measured via the voltage produced across the load resistance. A Tektronix 7633 oscilloscope (7A13 amplifier) was used to amplify the signal before putting it into a Biomation 8100 transient recorder. The 10-90% rise time of the measurement system is approximately 19 ns. The molar absorbtivity of anthracene at 248 nm is 1.2 X lo5 M-’ cm-’; therefore, a low concentration was used in order that the distribution of ions would be reasonably uniform throughout the path of the light between the electrodes. The uniformity was greatly enhanced by the fact that the ground state of anthracene was bleached because of the high photon intensity and the low M) concentration of anthracene. ((2-4) X Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052,Australia.
0022-3654/84/2088-4096$01.50/0
The solvents (Burdick and Jackson) were purified by passage through a column of activated alumina. The anthracene (Aldrich, gold label) was used as received. A previously described syringe technique6 was used for handling solutions, which were degassed by bubbling with helium or argon. For measurement of hydrogen production, the photolyzed sample was withdrawn from the cell with a syringe and introduced to a Van Slyke apparatus combined with a gas chromatograph.6 This allowed extraction, separation, and measurement of the dissolved gas. C 0 2 was used as an electron scavenger in these experiments, because it interfered less with the chromatography. Changes in optical absorbance due to the photolysis were measured with a Varian Model 2300 spectrophotometer.
Results and Discussion Figures 1 and 2 show the nature of the conductivity signals observed when cyclohexane and n-hexane, each containing 2 X 10” M anthracene, are photoionized with pulses of 248-nm light. With no SF6, the signal at early times is large because of the high mobility of the electron, which is a major product of the photoionization process. The electron decay occurs by its reaction with anthracene and by its homogeneous “free-ion” recombination with positive ions. (Geminate recombination of the electron is essentially complete by 20 ns.) It can be shown on the basis of known rate constants that the “crossover” seen in Figures 1 and 2 occurs because the SF6scavenges the free electrons (to yield long-lived, low-mobility free ions) before they can react with the positive ions to form nonconducting neutral species. If after a few nanoseconds the only ions in solution were A+ and SF6-, one would expect cyclohexane and n-hexane solutions (with SFs) to have qualitatively the same ratio of early signal to signal at few microseconds. A+ is not expected, on energetic grounds, to transfer charge to either cyclohexane or n-hexane. If, however, solvent ionization occurs (by some process to be discussed later), the order of magnitude higher mobility of the cyclohexane positive ion (compared with that of A+, SF6-, and C6HI4+)would explain the difference between cyclohexane and n-hexane, as has been previously s~ggested.~ Thus, the signal described by the filled circles (Figure 1) can be qualitatively explained if the high-mobility cyclohexane ion is converted to a “normal” mobility ion with a time constant of 200-300 ns. (The dashed lines in Figures 1 and 2 indicate the extrapolated initial free-ion signals based on the decay in the 5-100-ps region.) If the cyclohexane ion is the reason (1) Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US-DOE under Contract No. W-3 1-109-ENG-38. (2) Sauer, M. C., Jr.; Trifunac, A. D.; Cooper, R.; Meisel, D. Chem. Phys. Let. 1982, 92, 178. (3) Warman, J. M. Chem. Phys. Lett. 1982, 92, 181. (4) Beck, G.Int. J. Radiat. Phys. Chem. 1969, I , 361. (5) Beck, G.;Thomas, J. K. J . Chem. Phys. 1972, 57, 3649. (6) Hart, E.J.; Anbar, M. “The Hydrated Electron”; Interscience: New York, 1970;Chapter IX.
0 1984 American Chemical Society
Photolysis of Anthracene in Cyclohexane
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4097
0.03
0.1
1
100
10
Time bsec) 0.1
lilll8I
0.03
0.1
I
,
I881’1I
1
1
‘ 4 ! 1 8 U
1
1
10
1
l ” I 1 ’
100
Time bsec)
Figure 1. Observed conductivity signals in 2 X 10” M anthracene in cyclohexane: open circles, no SF,; closed circles, 0.05 M SF,.
Figure 3. Effect of triethylamine (TEA) and trans-Decalin on the conductivity signal from 2 X 10” M anthracene/0.05 M SF6 in cyclohexane. The data points are not shown; the quality of the data is similar to Figures 1 and 2. The concentrations of TEA going from the top curve to the lowest one were 0, 5 X 1X and 2 X 10” M. The dashed M trans-Decalin. curve is for 1.25 X 10
-
8-
-C,H;,
-C,Hi, + e- (C,H,, -C,Hi, + A- (C,H,, A*
6-
5-
+ e- (gas)
-A’
+ e- (gas) + e- (C,H,,
liquid) liquid)
liquid)
4-
5 2-
0-
-A-
0.03
0.1
1
10
100
Time bsec)
-2
(gas)
-
Figure 2. Observed conductivity signals in 2 X 10” M anthracene in n-hexane: circles, no SF,; squares, 0.09 M SF6.
Figure 4. Energy levels involved in the photolysis of anthracene in cyclohexane with 248-nm light.
for the large early conductivity, then the addition of positive ion scavengers should cause the conversion to a normal-mobility ion to become faster. The effects of a series of concentrations of triethylamine and one concentration of trans-Decalin are shown in Figure 3. A concentration as high as 0.5 M trans-Decalin has no further effect. Another positive ion scavenger, NH,, has the same effect. Rate constants for reaction with trans-Decalin, triethylamine, and ammonia in the range of (2.5 f 0.5) X 10” M-’ s-I were obtained from analysis of these results, in agreement with previously measured rate constants for c-C6HI2+reaction^.^ It is clear from these results that the initial signal behaves qualitatively as would be expected if it were due to the high-mobility c-C6H12+ion. (Conductivity cannot give information on the structure of the high-mobility species; for convenience we will simply designate it as c-CqHlz+.) The evidence quite strongly supports the formation of c-C6H12+ in the case of the cyclohexane solutions. In the following, we will discuss observations relevant to the mechanism whereby it is formed. In addition to the results presented already, other experimental observations have a bearing on the mechanism in question: (1) the number of photons absorbed per anthracene molecule in the path of the light beam, (2) the effect of light intensity on the conductivity signal, (3) the production of hydrogen and the disappearance of anthracene, and (4) the effects of singlet and triplet quenchers.
A diagram depicting the energy levels involved is given in Figure 4. The process of ionization is initiated by absorption of a 248-nm photon by anthracene (A), for which c = 1.2 X los M-’ cm-’. The lowest excited singlet, AI, of anthracene is formed by internal conversion (IC) and has a lifetime of 4.9 ns, with about 60-70% of the decay being by intersystem crossing (ISC) to the lowest triplet state of anthracene (A3) and 30-40% by fluorescence to the ground state. The absorption of a 248-nm photon by A’ can easily ionize A and in principle produces a state energetic enough to ionize c-C6H12(liquid), if the state were to live long enough to transfer the energy. It is also possible that A+ is formed via A3, but this route cannot lead to c-C6H12+unless a third photon is involved. Power measurements made on single full-intensity laser pulses showed that the number of incident photons was 20 times the number of anthracene molecules in the path traversed (at 2 X lo4 M) and that the ratio of photons absorbed to the number of anthracene molecules in the light path was 10. (The absorption by the solvent is small in comparison.) Obviously, absorption of light by species produced by the “initial” photons must be involved in accounting for this. A value of E = 2 X lo4 M-I cm-’ for A3 at 248 nm (suggested by low-temperature studies6) is sufficient to account for the requisite number of absorbed photons, and a mechanism could be devised in which ionization of A’ or AS is followed by geminate recombination to “recycle” A’ or A3. This mechanism, however, can be ruled out by the magnitude of the concentration of free ions, produced per pulse. The observed 1X M concentration is approximately what would be expected
(7) Warman, J. M. In “The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis”; Baxendale, J. H., Busi, F., Eds.; D. Reidel: Boston, 1982; pp 433-533.
(8) Meyer, Y . H.; Astier, R.; Leclercq, J. M. J. Chem. Phys. 1972, 56, 801.
4098
Sauer et al.
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 I
0.03
I
0.1
1 Time (t~sec)
1
roo!G-
10
Figure 5. Effect of light intensity on the ratio of the “initial” signal to the “initial” free-ion level for 2 X lo-, M anthracene/0.05 M SF6 in cyclohexane. The free-ion “initial” concentrations (corresponding to the dashed line on graph) were determined from data (not shown) a t I = 10-200 gs. The different symbols correspond to different intensities per light pulse: open circles, maximum (100%) intensity (1 pulse); crosses, 26% (5 pulses); filled circles, 8.4% (15 pulses); triangles, 3.2% (101 pulses). The relative free ion yields are 100, 24, 5.2, and 0.88, respectively.
on the basis of free-ion yields in irradiated cyclohexane, if all of the anthracene (2 X IO” M) were photoionized by the first two photons absorbed per anthracene molecule. About 7-8 times greater concentration of free ions would be expected by the recycling mechanism outlined above. Recycling by conversion of A’ or A3 to the ground state cannot explain the large number of absorbed photons, because the quantum yield for fluorescence is low (-35%) and the triplet is long-lived on the time scale of the laser pulse. The photolytic mechanism we will propose to account for c-C6H12+formation will be seen to offer an explanation of this situation. Other processes of light absorption must also be considered. The absorption of additional photons by A+ and A- should not be significant despite the fact that molar extinction coefficients at 248 nm greater than lo4 M-’ cm-’ are likely.g$’OThe A+ and A- concentrations should be much smaller than the A concentration, because geminate recombination keeps [A+] low during the pulse and because most A- formation by e- scavenging occurs a t later times. We have observed that cyclohexane containing 0.05 M SF6but no anthracene results in a signal similar to that in Figure 1, but reduced by a factor of 5. This signal has an intensity dependence which indicates the occurrence of a simultaneous two-photon absorption by cyclohexane. However, this cannot be the cause of large number of photons absorbed per anthracene molecule. We propose a mechanism for anthracene-induced cyclohexane ionization which will also provide the necessary recycling of absorbing species. A process whereby A’ absorbs a photon producing excitation or ionization in a neighboring cyclohexane molecule is possible. Similar processes, which have been called “photoninduced inelastic collisions”, in photoionization with intense laser radiation in atomic systems have been observed experimentally’‘-I3 and treated the~retically,’~-’~ and the area has been recently reviewed.” Such a mechanism is consistent with other observations. The dependence of the observed conductivity signal on light intensity provides some information concerning the nature of the ionization process. Figure 5 shows the changes observed in the conductivity signal in the cyclohexane/SF6/anthracene system (9) Hoytink, G. J.; Velthorst, N. H.; Zandstra, P. J. Mol. Phys. 1960, 3, 533. (10) Rao, C. N. R.; Kalyanaraman, V.; George, M. V. In “Applied Spectroscopy Reviews”; Brame, E. G., Jr., Ed.; Marcel Dekker: New York, 1970; p 156. (11) Hellfeld, A. V.; Caddick, J.; Weiner, J. Phys. Rev. Len. 1978, 40, 1369. (12) Weiner, J.; Polak-Dingels, P. J . Chem. Phys. 1981, 74, 508. (13) BrCchignac, C.;Cahuzac, P.; DebarrC, A. J . Phys. B 1980, 13, L383. (14) Bellum, J. C.; George, T. F. J. Chem. Phys. 1978, 68, 134. (15) Bellum, J. C.; George, T. F. J. Chem. Phys. 1979, 70, 5059. (16) Weiner, J. J. Chem. Phys. 1980, 72, 2856. (17) George, T. F. J . Phys. Chem. 1982, 86, 10.
0.2
0.03
0.1
10
1
100
Time (psec)
Figure 6. Effect of xenon and O2on the conductivity signal; all samples contained 2 X lo-, M A in cyclohexane: circles, ca. 2 X lo-* M SF6; X, sample where 0.15 M xenon was present in addition to the SF,; crosses, 0.012 M 0 2 was present but no SF,. (Note: the load resistor was twice as large as for the other results given in other figures.)
as the light intensity was varied over a range of 30. The conductivity signal is plotted relative to a value of unity for the extrapolated initial free-ion signal at each intensity, which was determined from data (not shown) extending to 200 ps. The ratio of the signal at early times to the extrapolated initial free-ion signal decreases as intensity decreases. From results given in the legend of Figure 5, the free-ion level decreases in proportion to light intensity over the first factor of 3 but then falls off more rapidly than linearly as the intensity is lowered further. (In n-hexane, the free-ion yield behaves similarly.) These observations seem to require two ionization processes, one of which produces C-C6H12+ and is more sensitive to intensity than the other, which produces a positive ion with a “normal” mobility. We propose that these reactions are A’
+ C - C ~ H+I ~hU248
-
and A3 (and A’)
+ hY248
A-
-
+ C-C&12+
(1)
+ e-
(2)
A+
We believe reaction l occurs without the intermediate production of an electron, but part of the photolysis of A’ may result in production of e-, which is included in reaction 2. The electron produced in reaction 2 accounts for the large signal observed at early times when SF6 is not present (see Figure 1). The observed dependences of the initial conductivity signal and the extrapolated initial free-ion conductivity signal on laser intensity can be rationalized, qualitatively, on the basis of reactions 1 and 2. The more rapid decrease in the initial signal, due to c-C6HI2+, with decreasing intensity (see Figure 5) can be ascribed to the short lifetime of the A’ compared with that of A3. At low intensities more A‘ fluoresces or crosses to A3 before it can absorb a second photon. The fact that the total ionization also decreases more rapidly than linearly with intensity when the intensity is decreased by a factor of 10-30 can be ascribed to the fact that, at such low intensities, the anthracene absorption is no longer efficiently “saturated”, and therefore a greater fraction of the photons is absorbed by the ground state. Other experimental information related to the proposed reacM tions is that the presence of 0.15 M xenon (in a 2 X anthracene solution in cyclohexane containing SF6) causes a 30% decrease in the initial conductivity signal but does not affect the extrapolated initial free-ion yield (see Figure 6). This indicates ( 1 ) that the expected “heavy atom effect” promotion of the intersystem crossing of singlet to triplet anthracene occurs, (18) Head, A. D.; Singh, A,; Cook, M . G.; Quinn, M. J. Can. J . Chem. 1973, 51, 1624.
(19) Turro, N. J. “Modern Molecular Photochemistry”; Benjamin/Cummings: Menlo Park, CA, 1978; pp 191-193.
Photolysis of Anthracene in Cyclohexane
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4099
thereby decreasing the contribution of reaction 1, (2) that Xe does not rapidly quench A3, and (3) that the increase in free ions formed via reaction 2 compensates for the decrease in free ions from reaction 1. If the solution is saturated with O2 (0.012 M), both the initial and free-ion signal are decreased by about 30% (see Figure 6). The fact that the known rate constant for O2quenching of Ai is close to the diffusional rate (3 X 1O'O M-' s-l) and that quenching of A3 is only 10 times smallerZosupport the contention that both reactions 1 and 2 are affected due to 0, quenching. There are two reasons for reaction 1 producing A- rather than e-. First, the anthracene-induced photoionization of cyclohexane does not increase the yield of HZ,which is produced when e- and C6H]2+recombine.21 We have measured the hydrogen production in pure cyclohexane, cyclohexane with 0.07 M COZ,cyclohexane with 106-10-5 M anthracene, and cyclohexane with 0.07 M C 0 2 M anthracene. The lowest concentration of anand 2 X thracene causes a 10-20% decrease in the H, yield compared to the case of pure cyclohexane (quantum yield for H, of about 0.1); higher concentrations cause progressively larger decreases in the H i yield (65% decrease at M anthracene). If all the anthracene-induced ionization proceeds via reaction 1 rather than reaction 2, lowering the light intensity through a sample by the addition of anthracene should decrease the direct, biphotonic ionization of cyclohexane and hence the production of H2. Because the ionization of pure cyclohexane depends on the square of the light intensity, one might expect a somewhat larger concentration effect if no e-,c-C6H12+pairs were produced via anthracene absorption. The effect of anthracene concentration on the hydrogen yield can be explained if only 5% of the anthracene-induced ionization leads to e-,c-C6H12+pairs. The second reason for postulating reaction 1 is that the relative magnitudes of the conductivity signals for e-, C6H12+, and the molecular free ions (see Figure 1) can be explained, if reactions 1 and 2 are of equal importance, with only a 10% contribution from reaction 3. This is in good agreement with our analysis of (3) the hydrogen yields. The calculated relative conductivities due to e- (absence of SF6), c-C6H12+(with SF6present), and.the free ions (from the second-order decay at 10-200 ps extrapolated to early times) were evaluated from several pieces of information: the mobilities of the ions, the total concentration of ions as a function of time, and the relative contribution of geminate ions and free ions to dc conductivity measurements. The ratio of the total ion concentration relative to the initial yield of free ions is given byZ2 [ions], [ions, freelo
5.38 X
where 1.1 is the sum of the ion mobilities (in cm2 V-' s-I ) of the ion pair, e, is the dielectric constant of the solvent, and t is the time in seconds. Theoretical and experimental works in these laboratories indicate that the geminate ions contribute approximately 0.4 as much to dc conductivity as do the same number of free ions. This efficiency is given by 1 - x , where x is the exponent in eq I.23$24 (20) Reference 19, pp 193, 589-591. (21) Dyne, P. J. Can.J . Chem. 1965, 43, 1080. (22) Van den Ende, C. A. M.; Warman, J. M.; Hummel, A. Radiat. Phys. Chem. 1984, 23, 55. (23) Schmidt, K. Chem. Phys. Lett. 1983, 103, 129. (24) McDonald, D.; Sauer, M. C., Jr., unpublished work.
WAVELENGTH (nm)
Figure 7. Spectral changes taking place in 4 X M anthracene cyclohexane, with 0.05 M SF6: sohd curve, before photolys~s,dashed curve, l o laser pulses; dotted 30 laser pulses
In the case of n-hexane, a similar analysis of the data indicates that less than 10% of the ionization goes via the path analogous to reaction 1. The end result of the photochemistry represented by reactions 1-3 is illustrated in Figure 7 which shows spectral changes taking place in the 230-390-nm region. The absorption which grows in at wavelengths somewhat above and below 250 nm as the anthracene is consumed is characteristic of partially hydrogenated (or fluorinated) anthracenes. The quantum efficiency of this process is about twice as great when SF6 is present, suggesting that recombination of e- with A+ does not efficiently lead to consumption of anthracene but that reaction 1 and the reactions of SF6- or F- with A+ do. Returning to the observation that about 10 photons are absorbed per anthracene molecule, it is possible that the production of Aand C6H12+ in reaction 1 provides the recycling mechanism. The mechanism suggested for reaction 1 suggests that the separation between A- and c-C6H12 is quite small because the c-C6H12 molecule initially ionized is likely to be a nearest neighbor to the anthracene. (This is in contrast with reaction 2 where the electron would move a significant distance away from the A'.) This small separation would lead to extremely efficient geminate recombination and a small probability that c-C6Hi2+would become free. A' would therefore be efficiently and rapidly reformed, and if the absorption coefficient for A' in reaction 1 were -5 X lo4 M-I cm,-' the absorption of 10 photons per anthracene molecule could be rationalized. Furthermore, this would help explain how a significant amount of free c-C6HI2+can be obtained from reaction 1 despite the small initial separation; the free-ion yield from reaction 1 would only need to be about 0.5%. Note that an alternate recycling mechanism based on reaction 3 automatically produces anthracene in its ground state. Since the yield of free ions from such a process should be similar to that from other photoionizations, the absorption of an anomalously large number of photons should also produce a much higher concentration of free ions (by about a factor of 20) and a similar increase in the yield of hydrogen. At this time, the details of reaction 1 are not clear. The mechanism, however, does provide a means whereby the observations concerning the generation of hydrogen, the large number of photons absorbed per anthracene molecule, and the unexpected (but now well estabilished) occurrence of solvent ionization can be accommodated. Registry No. A, 120-12-7; TEA, 121-44-8; c-C6HI2,110-82-7; cC&12+, 34473-67-1; SF,, 2551-62-4; NH3, 7664-41-7; n-hexane, 11054-3; trans-Decalin, 493-02-7.
