Langmuir 1991, 7, 2881-2886
2881
Photoproduction of Gold Colloids and Films R. Krasnansky, S. Yamamura,t and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
R. Dellaguardia IBM, NSLS IBM- V6, Brookhaven National Laboratory, Upton, New York 11973 Received October 22, 1990. In Final Form: January 2,1991 The photochemical and the photothermal induced decomposition events of dimethylgold(II1) hexafluoroacetylacetonate (DMG-HF) have been monitored in real time on the microsecond time scale in solution and in condensed films. Photochemical decomposition of DMG-HF occurs via the triplet excited state of DMG-HF while photothermal decomposition proceeds via “seed” zones, zones which are initially produced and act as thermal conversion sites. Both photolytic and photothermal decomposition of DMG-HF produce intermediates that yield colloid gold and gold films.
Introduction Metal colloids, which are some of the earliest known colloids,’ have been studied in some detail.2g3 These colloids are of major interest for many catalytic processes as their colloidal forms can be used to coat surfaces, thus creating catalytic materials. The exact nature of the colloid has been worked out in some detail for gold2p3and for ~ i l v e r .Most ~ methods of production involve a chemical reduction of a metal salt in aqueous solution. However, ref 4 indicates the use of high-energy radiation to reduce silver salts to metallic silver. There is some interest to produce colloids by photochemical methods; they then can be produced directly on surfaces or in a variety of solvents by photolysis of suitable salt to produce a colloid in a known form. It is also desirable to produce these metals in a layered form on surfaces, as the products have useful properties in the microelectronics i n d ~ s t r y .In ~ the present studies we have investigated the decomposition of dimethylgold hexafluoroacetylacetone DMG-HF (1). DMG-HF is volatile and films of this material can be cast on surfaces. The concept of the present work is to use laser flash photolysis and steady-state methods to discern the individual processes that take place in the formation of colloids and films of gold following photochemical and photothermal decomposition of DMG-HF. Experimental Section Steady-State Fluorescence. Steady-state fluorescence measurements were made on a SLM/Aminco SPF-500 spectrofluorometer equipped with a LX300 UV illuminator, a 1200grooves/ mm grating, and a Hamamatsu R-928P photomultiplier tube. + On leave from Government Industrial Research Institute, Osaka, Midorigaoka 1, Ikeda, Osaka 563, Japan. (1) Faraday, Michael Philos. Trans. R. SOC.London 1857, 147, 145. (2) Turkevich,J.; Garton, G.; Stevenson,P. C. J. Colloid Sci., Suppl.
I1954, 26 (35), 26. (3) Mie, G. Ann. Phys. 1908, 25, 377. (4) Henglien, A. Pure Appl. Chem. 1984,56, (9), 1215. (5) Tyndall, G. W.; Jackson, R. L. J. Chem. Phys. 1988,89 (3), 1364. Jones, C. R.; Houle, F. A.; Kovac, C. A.; Baum, T. H. Appl. Phys. Lett. 1985,46 (I), 97. Baum, T. H.; Jones, C. R. Appl. Phys. Lett. 1985,47 (5), 538. Kodas, T. T.; Baum, T. H.; Comita, P. B. J.Appl. Phys. 1987,62 (l),281. Baum, T. H.; Marinero, E. E.; Jones, C. R. Appl. Phys. Lett. 1986,49 (18),1213. Baum,T. H.; Jones, C. R. J.Vac. Sci. Technol. 1986, E4 (5), Sept/Oct, 1187. Leyendecker, G.; Bauerle, D.; Geittner, P.; Lydtin, H. Appl. Phys. Lett. 1981,39 (ll),921. Randall, J. N.; Ehrlich, D. J.; Tsao, J. Y. J. Vac. Sci. Technol. 1985, E3 (l),Jan/Feb, 262. Houle, F. A.; Wilson, R. J.; Baum, T. H. J. Vac. Sci. Technol. 1986, A4 (6), 2452. Baum, T. H. J . Electrochem. SOC.1987, 134 (lo), 2616.
Time-Resolved Transient Absorption. Excitation was achieved with either a Lambda Physik Model 100 excimer laser or a Candela SLM-500Mflash lamp pumped dye laser. A XeCl/ He gas mixture yielded a 308-nm, 150-mJ,10-nsfwhmlaser pulse. A 8 X 10-5 M LD-490 laser dye solution in 1/1methanol/water yielded a 480-501 nm range, ,A, = 490 nm, 500-ns fwhm laser pulse. The analysislight was a 450-W OrielXe arc lamp powered by a PRA Model 302 power supply and pulsed with a PRA Model m-305 pulser; wavelength selectivity was achieved with a monochromator and various cutoff and band-pass filters. The signal was detected by a Hamamatsu R-928 photomultiplier tube. Transient signals were captured with either a Tektronix Model 7912AD or a 7912HB programmable digitizer (response time = 0.4 ns), a Tektronix 7B10 timebase, a Tektronix Model 7A13 differential comparator (response time = 4.7 ns), and an appropriate computer system. The intensity of transmitted light was captured on a Tektronix 7D12 A/D converter equipped with a M2 Sample & Hold module. Transient absorption signals for translucent samples were obtained with the analysis light collinear with the sample cell and detector; laser excitation of the sample occurred at 90°, < 4 5 O for films, to the optic line. An electronic synchronizer dictates shutter opening, for the analysis light, pulsing of the Xe lamp, the determination of I,,, and fiially the firing of the excitation laser. The real time resolved trace was obtained and converted into optical density vs time. Steady-State Absorption. Steady-state absorption spectra were obtained on a Perkin-Elmer Model 552 spectrophotometer equipped with a DZA tungsten projector lamp, a 055-0505 deuterium lamp, and a Hamamatsu R-928photomultiplier tube. Photoreactor. Continuous irradiation was achieved with a Rayonet Model RPR-100 photoreactor equipped with Rayonet RYR-3000%,lamps. The photoreactor provided the 1-cm2sample [quanta] s-l. cell with (8.38 f 0.69) X Dimethylgold Herafluoroacetylacetonate. Film samples were prepared by either sandwiching neat DMG-HF between two quartz slides or coating one faceof an evacuated 1-cm2quartz cell with neat DMG-HF. Liquid samples were deoxygenated by bubbling with prepurified nitrogen gas saturated with the respective solvent for 30 min. Materials. DMG-HF was used as received from Cyanamid, Aldrich tetrahydrofuran was distilled prior to use, 96 % 1,1,1,5,5,5hexafluoro-2,4-pentanedione(HFACAC),spectral grade meth0 1991 American Chemical Society
Krasnansky et al.
2882 Langmuir, Vol. 7, No. 12, 1991 [quanta] / '
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Figure 1. Absorption spectra of DMG-HF (a) 6.18 X 10" M in M in M in acetonitrile, (c) 6.06 X methanol, (b) 6.05 X M in M THF, and (e) 6.22 X chloroform, (d) 5.99 X cyclohexane. 20
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Figure 2. Absorption spectra, 1.1 X M DMG-HF in cyclohexane continuously irradiated with 300.0 nm light, for (A) 0 min, (B) 1 min, (C)2 min, (D)3 min, (E) 5 min, (F) 10 min, (G) 15 min, and (H) 30 min. ylcyclohexane, HPLC grade cyclohexane and benzene, 99 % spectrophotometricgrade carbon tetrachloride,and spectralgrade acetonitrile were used as received from Aldrich. SPECTRAR grade chloroform, acetone, and anhydrous methanol were used as received from Mallinckrodt. Perylene (99+ %) was purchased from Aldrich and recrystallized from ethanol. Oxygen and prepurified nitrogen were used as received from the Mittler Co. Results a n d Discussions Photochemistry of DMG-HF. PhotophysicalCharacterization. The absorption spectra of DMG-HF in several solvents, Figure 1, indicate that DMG-HF absorbs strongly a t both 300 and 308 nm. No appreciable DMGH F absorption occurs a t 490 nm. Figure 2 shows the absorption spectra (350-750 nm) of DMG-HF cyclohexane solutions following irradiation in a photoreactor a t 300.0 nm (termed low intensity steadystate irradiation). The resulting solutions are clear, scatter light, and have a pink tinge typical of gold colloids. Turkevich et a1.2have previously shown that, in accordance with the theoretical predictions of Mie,3 gold colloids exhibit spectra that depend on the colloid diameter; the absorption maximum systematically shifts toward the red as the colloid diameter increases from 200 A. On irradiation, the DMG-HF absorption decreases while a decomposition product with an absorption maximum a t 258 nm appears. In step with the work of Baum et a1.: the (6) Klassen, R. B.; Baum, T.H.Organometallics 1987,8, 2477.
0
Time I (min.)
Figure 3. Plot of colloidal gold diameter in 300.0-nm photoM DMG-HF in cyclohexane reactor decomposition of 1.1X vs irradiation time. decomposition product absorbing a t 258 nm is seen as the detached HFACAC ligand. The colloidal solution obtained from the low intensity steady-state irradiation is different to that obtained with high-intensity laser irradiation. The absorption spectra of the products from low intensity continuous irradiation closely resemble the absorption spectra of gold colloids presented by Turkevich. The product of the photoinduced decomposition of DMG-HF is a gold colloid, the diameter of which increases as a function of irradiation time of DMG-HF, Figure 3. The diameter of the gold colloid was found to be stable on discontinuing the irradiation, indicating that stable gold colloids of specific diameter can be reproducibly made by this technique. Colloids produced by pulsed 308-nm laser light are black and are partially annihilated by further intense irradiation a t 308 nm. Such annihilation was not seen with the colloids produced by low-intensity steady-state irradiation. It is suggested that photolysis initially produces a gold cluster or a gold/carbon colloid. This "initial" material can absorb light leading to further decomposition, the process continuing until a gold colloid of high purity is developed. DMG-HF does not fluoresce and excitation leads to radicals and/or triplet excited states. The role of the excited state of DMG-HF as a precursor for the photodecomposition or as an alternative pathway competing with the photodecomposition has been addressed through studies of bulk photochemical decomposition in the presence of a quencher. It is reasoned that if the excited state of DMG-HF is directly involved in the photodecomposition pathway, then quenching of the excited state should have an effect on the quantum yield of photodecomposition, a. In cyclohexane oxygen quenches the excited state of DMG-HF with a bimolecular quenching rate constant of (2.25 f 0.05) X lo9 M-' s-l. The @ values of deoxygenated and oxygenated DMGHF/cyclohexane solutions were monitored by absorption spectroscopy. The a values of DMG-HF are 0.0340 f 0.0010 and 0.0092 f 0.0014 for the deoxygenated ([OZ] 10-6M) and the oxygenated ([02] = 1.15 X M) systems, respectively. The net photodecomposition of DMG-HF is greatly enhanced in the absence of oxygen. The decrease in of DMG-HF with increasing oxygen concentration
-
Langmuir, Val. 7, No. 12,1991 2883
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Figure 4. Transient absorption spectra, A,, = 308 nm, 4.60 X M DMG-HF (A) 0.2 pa, (B)0.5 ps, (C)1.0 ps, (D) 2.0 pa, and (E) 4.0 pa after laser pulse.
demonstrates that the excited state of DMG-HF, the identification of which will be presented shortly, plays a direct role in the decomposition. The CP of DMG-HF decreases dramatically in polar media, e.g., @ = 0.0063 f 0.0013 in deaerated acetonitrile. Hydrogen abstraction from the solvent appears to have little effect on the quantum yield of DMG-HF photodecomposition, @cyclohexane = 0.0340 & 0.0010 vs @benzene = 0.0350 f 0.0010. These observations suggest that the intermediate involved in the decomposition is uncharged and not a radical. These data mediate against a ligand to metal transfer transition causing decomposition and support a ligandcentered transition scheme. Pulsed Laser Studies. Flash photolysis of solutions or films of DMG-HF gives rise to short-lived intermediates as well as permanent products. The transient absorption spectra following 308-nm laser excitation of DMG-HF in THF, Figure 4, possess broad features with major transitions centered a t 399 and 430 nm; the transients collapse over a few microseconds leaving behind a broad nonstructured absorption spectrum. The spectra of DMG-HF are similar to those of HFACAC with (1)a loss of resolution, (2) a 31-nm bathochromic shift attributable to complex formation, and (3) the presence of the long-lived nonstructured absorption spectrum. The transient absorption signals observed with DMGH F under 308-nm excitation did not decay to the baseline. The observed transient absorption signals are a composite of (1)a simple single exponential decaying to the baseline and (2) a signal that grows attributable to formation of a species with a broad absorption, Figure 5. The yield of the long-lived species is sensitive to analyzing light intensity; increasing the analyzing light intensity, i.e. by pulsing the light source, dramatically reduces the amount of permanent product. Flash irradiation of a DMG-HF solution produces progressively a dark colloidal material that is annihilated on further intense pulsed illumination. The transient absorption signals of DMGHF in several solvents have been simulated with a computer using a single exponential decay with a permanent component, Table I.
Identification of the Triplet Nature of the ShortLived Species. The transient excited state of DMG-HF is deactivated by molecular oxygen. The quenching ~
(7) Weast, R. C. CRC Handbook of Chemistry and Physics;CRC Press, Inc.: Boca Raton, FL, 1979; p E-56. Meites, C. Handbook of Analytical Chemistry; McGraw-Hill Book Company, Inc.: New York; 1963; p 1-47.
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process leads to the formation of singlet oxygen, O2('Ag), observed by IR detection. The detection of 02('A,) produced via energy transfer from 3DMG-HFadds weight to the suggestion that the precursor of the decomposition of DMG-HF is triplet in nature. In the present system the DMG-HF plays the role of a sensitizer. Transients could not be detected in any solvent under 490-nm excitation. However, photochemistry was observed in filmsunder these conditions and will be discussed subsequently. No room temperature luminescence of either DMG-HF or HFACAC has been detected. Excited triplet states transfer excess energy to a solute acceptor of lower triplet energy via a spin-allowed exchange mechanism,899 which is not the case with free radicals. Perylene, which possesses a relatively low energy triplet (11000 cm-l) and a strong transient triplet-triplet with a maximum absorption a t 485 nm,lo is used to deactivate the suspected DMG-HF triplet in the present study. Figure 6 shows the transient absorption decays observed a t both 400 and 485 nm for perylene, DMG-HF, and DMGHF/perylene solutions. A t 485 nm, perylene alone exhibits a long-lived species typical of the cation of perylene produced via two-photon ionization, DMG-HF alone exhibits its typical transient decay, and DMG-HF/ perylene solutions exhibit a transitory species which cannot be assigned to a composite of the two previous decays. At 400 nm, perylene alone shows practically no transient absorption, DMG-HF alone exhibits species reported earlier which appear instantaneously, and DMG-HF/ perylene mixtures show a new transient absorption decay which appears rapidly (but, not instantaneously) and decays more slowly than that of DMG-HF. This species is assigned to the perylene triplet and was confirmed by using similar studies with the donor / acceptor pair acetone/ perylene. Figure 7 shows that acetone exhibits no transient (8) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph 181; American Chemical Society: Washington, DC; 1984; p 35. Tilly, M.; Pappas, B.; Pappas, S. P.; Schnabel, Y. Y.; Thomas, R. K. J. Imaging S a . 1989, 33 (2), Mar-Apr, 62. (9) Birks, J. B.; Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970; pp 391-394. (10)Koiki, K.; Kanemoto, A.; Kurabayashi, Y. The Lowest Triplet Leuels of Organic Molecules; Theoretical Chemistry Laboratory: Sendai, Japan, 1987.
2884 Langmuir, Vol. 7, No. 12, 1991
Krasnansky et al.
Table I. Computer Simulation Parameters, DMG-HF's Transient Absorption Decay Profiles in Various Solvents solvent system [DMG-HF] M/105 ko (s-9/10-5 &, ns perm OD/103 (c/co)tc O C 1.18f 0.02 8470 6.22 2.01 f 0.20 2.02320 cyclohexane
cc4
2.10
1.78 f 0.06 0.842 f 0.059 8.29 f 0.12 1.16 f 0.10 5.59f 0.06 4.59f 0.54 4.07f 0.44 3.45f 0.05 4.31 f 0.09 4.01 f 0.08
6.50 benzene 4.85 THF, dry 6.06 chloroform 3.09 methanol 6.50 0.10 M CH30H in CH3CN 6.50 acetonitrile 6.50 0.10 M H2O in CH3CN 6.50 0.54 M HzO in CH3CN 6.50 1.04 M H20 in CH3CN water a a Insoluble in given solvent. * Not applicable. Reference 7.
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Figure 6. Triplet/triplet energy transfer from DMG-HF to perylene in cyclohexane observed by transient absorption A,. = 308 nm (I) &b = 400 nm and (11) &b = 485 nm of (A) 6.22X 10-5M DMG-HF, (B)1.85 X lo4 M perylene, and (C) a mixture of 6.22 X M DMG-HF and 1.85 X M perylene.
Figure 7. Triplet/triplet energy transfer from acetone to perylene in cyclohexane observed by transient absorption A.. = 308 nm (I) &,b = 400 nm and (11) &b = 485 nm of (A) 0.123 M M perylene, and (C) a mixture of 0.123 acetone, (B)1.85 X M acetone and 1.85 X M perylene.
absorption a t either 400 or 485 nm and that the acetone/ perylene mixture yields the same transitory species as the DMG-HF/ perylene mixture. The transient excited state of DMG-HF produced under 308-nm excitation is assigned to a triplet possessing an energy greater than 110oO cm-'. The outcome of the prior study is that excitation of DMG-HF leads, in less than 10 ns, to 3DMG-HF, which decomposes itself over microseconds to give a colloidal gold compound. Photolysis of Neat DMG-HF Films. Neat films of DMG-HF coated on one face of an evacuated 1-cm2quartz cell or sandwiched between two quartz plates have been studied. Excitation via 308 nm populates the electronic excited state of DMG-HF directly while no direct excitation occurs with 490-nm light. 308 nm Excitation. Excitation a t 308 nm into the absorption band of virgin DMG-HF films, either coated on a cell wall or sandwiched between quartz plates, immediately produces immediately transient species which resemble the transient absorption observed for DMG-HF
in solution. Neither a monoexponential decay model nor a monoexponential decay together with a permanent product model could satisfactorily simulate the transient absorption signals produced via radiation with defocused 308 nm; satisfactory simulation of the data was obtained with a second-order decay plus a permanent component. The second-order rate constant used was (2.94 f 0.86) X lo6 mol-' L s-1. Since the concentration of DMG-HF in the film is high; the population of 3DMG-HFproduced is much larger than that produced in solution. High local concentrations of the triplet excited state of DMG-HF give rise to triplet-triplet annihilation: and lower laser intensities yielded slower triplet decays. The triplettriplet annihilation accounts for the observed second-order nature of the transient absorption signal. Some of the permanent product produced is sputtered onto the walls of the cell, and in the case of the sandwiched sample, formation of a grainy black spot in the irradiation zone is observed. The chemical process occurring in the neat films on direct excitation is identical with that observed for
Photoproduction of Gold Colloids and Films Scheme I. Scheme of Events, 308 nm Induced Photolytic Decomposition of DMG-HF A
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Langmuir, Vol. 7, No. 12, 1991 2886
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Scheme 11. Scheme of Events, 490-nm Induced Photothermal Decomposition of DMG-HF
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Photothermal Decomposition o f DMG H i Around Heated S e e d Zones
DMG-HF in solution. Scheme I proposes the events that occur under 308-nm excitation of DMG-HF. 490 nm Excitation. Excitation a t 490 nm of virgin films of DMG-HF, either coated on a cell wall or sandwiched between quartz plates, yields no transients. Repeated 490-nm laser pulsing of the sample gradually gives rise to transitory species produced during the laser pulse, the intensities of which increase with the number of laser pulses used. I t is proposed that 490-nm irradiation gives rise to a low degree of decomposition, which produces “seed” particles in the DMG-HF film. Once formed, subsequent 490-nm laser light is absorbed by the “seed” particles causing a marked thermal increase in the immediate vicinity of the irradiation zone. The thermal zone leads to a further thermal decomposition of the DMG-HF complex. Multiple pulsing of the sample progressively increases the transient absorption signal of the species produced photothermally. The decay of the photothermally produced transient species, monitored a t 490 nm, is simulated well by a mo-
Figure 8. Transient absorption decays of DMG-HFcoated on one wall of an evacuated quartz cell, A,. = 490 nm, &,= 475 nm, after formation of artificially produced ‘seed zones” by (A) 10, (B)20, (C)30, (D)40, (E) 60, and (F)100 diffuse 308-nm laser pulses.
noexponential model yielding a unimolecular decay rate constant of (3.67 f 0.78) X lo4 s-l. This result is reminiscent of that obtained for simple solutions of DMGH F although the lifetime of the excited state is much longer in the film. I t is believed that this longer lived species is the thermally produced radical of the DMG-HF complex. Sputtering of black decomposition material is seen to occur within the l-cm quartz cell. “Seed” particles have been artificially produced by diffuse 308-nm irradiation of a DMG-HF film coated on one face of a quartz cell light followed by recoating of a DMG-HF film over the irradiation zone. The results of various extents of “seedling” the DMG-HF film in a closed system are seen in Figure 8. As seen previously, increasing the amount of “seed” particles increases the extent of formation of the transient absorbing species. In this closed system, however, one arrives a t a point where further pretreatment with 308-nm light leads to a decrease in the transient signal. This is due to the loss of DMG-HF material in formation of the “seed” particles and the larger extent of sputtering seen on seed particle formation. Scheme I1 proposes the events that occur under 490-nm excitation of DMG-HF films. Extraction of the soEd materials with T H F or cyclohexane yields a pale red solution typical of colloidal gold, and some residual material. The solid material produced on photolysis takes on one of three major forms: (1)a black material that can be degraded by intense light, (2) a material that is reddish brown in the dry form or a reddish colloidal material in solution which cannot be degraded with intense light, or (3) pure “yellow” gold films or solid gold chunks. The crystallinities of each of these materials were examined by use of X-ray diffraction. Each powder diffraction demonstrates a pattern typical of metallic gold.” Examination with a field microscope shows that even the black material contained traces of “yellow” metallic gold particles. Either thermal decomposition or continued photoreactor illumination of either the black or the reddish brown material increases the degree of metal gold present in the films. On heating, particles fuse to form a gold cluster that leads to a narrowing of the X-ray reflection. A definite conclusion about the black material cannot be made a t this time. The material is either predominantly amorphous or the particle size is small, thus giving rise to line (11) X-ray Powder Data File, #4-0784; American Society for Testing and Materials: Philadelphia, PA, 1960; p 571.
2886 Langmuir, Vol. 7, No. 12, 1991 broadening. Heating the black material to about 250 OC leads to its conversion to the reddish brown gold phase.
Conclusion Both 308- and 490-nm photoinduced decomposition of DMG-HF can be monitored. Our results suggest that the mechanism of the photoinduced decomposition of DMGH F is quite different under 308- and 490-nm excitation. The time-resolved photochemically and photothermally initiated DMG-HF transient absorption profiles do not completely decay to the baseline. The composite transient absorption signal of DMG-HF is composed of a transitory species with unimolecular decay and a permanent component that remains until bulk transport removes the permanent decomposition product from the irradiation zone. As triplet/triplet energy transfer occurs from DMGH F to perylene, the transient absorbing species of 308 nm excited DMG-HF has been assigned to a triplet state; triplet/triplet energy transfer from acetone to perylene confirmed the production of the perylene triplet. Photolytic decomposition with low-intensity steady-state 300nm illumination produces colloidal gold. The mean gold colloid diameter has been monitored spectroscopically and grows as a function of irradiation time. The gold colloid thus formed, however, was not annihilated with a focused 308-nm laser. Photolytic decomposition of DMG-HF with high-intensity laser illumination produces a black colloid
Krasnansky et al. that is annihilated through a thermal jump on further irradiation. It is proposed that this initial solid material produced is either a gold cluster or a Au/C colloid. Repeated annihilation and "reaggregation" or this material ultimately leads to pure gold colloids and films. Since no transients are detected with 490-nm excitation of DMG-HF solutions or initially with neat DMG-HF films, a small degree of two-photon-initiated decomposition of DMG-HF is involved in the mechanism of decomposition. In neat DMG-HF films, initial illumination produces microscopic seed zones via decomposition through a thermal processes through the temperature jump of the flash, and thermally produced DMG-HF radicals appear to be produced. Further illumination of the "seed" zones produces regions of high temperature which accelerates and heightens the thermal decomposition of nearby DMGHF. Sputtering of material has been seen in open cells coated with DMG-HF exemplifying the high energy impinged on the system. Transient absorption signals have been instantaneously produced by artificially creating "seed" zones by preirradiation of the sample with 308-nm light.
Acknowledgment. The authors thank the IBM Corporation and the NSF for financial support of this work. Registry No. 1, 63470-54-2; Au, 7440-57-5; 02,7782-44-7; perylene, 198-55-0.
