J . Phys. Chem. 1993,97, 11249-1 1252
11249
Photodissociation of Copper Hexafluoroacetylacetonate in the Charge-Transfer Absorption Region Jeffrey A. Bartz,’ Douglas B. Galloway,$ L. Gregory Huey,! Thomas Glenewinkel-Meyer, and F. Fleming Crim’ Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received: May 25, 1993; In Final Form: August 18, 1993e
The combination of ultraviolet laser photolysis and vacuum ultraviolet photoionization with time-of-flight mass spectrometry experimentally identifies the primary products of 222- and 252-nm photodissociation of copper hexafluoroacetylacetonate, Cu(hfac)2, in a molecular beam. The photolysis produces extensive ligand fragmentation and results in a number of different copper-containing species. One of the fragments, Cu(hfac), appears over a period of about a microsecond after the photolysis pulse. None of the fragments has a large translational energy. For example, about 2% of the available energy appears as kinetic energy of the Cu(hfac) fragment. The extensive ligand loss and slow decay are consistent with ligand fragmentation causing large contaminations in photolytically deposited Cu films from Cu(hfac)z and caged recombination inhibiting photoreduction of Cu(hfac)2 in solution.
I. Introduction We have applied a combination of ultraviolet (UV) laser photolysis and vacuum ultraviolet (VUV) photoionization with time-of-flight mass spectrometry to identify the primary photodissociation products of the compound bis( 1,l ,1,5,5,5-hexafluoropentane-2,4-dionato)copper [copper hexafluoroacetylacetonate, C u ( h f a ~ ) ~1,] ,in a molecular beam. Cu(hfac)? is a member of a class of volatile organometallic compounds formed by chelating a metal to the conjugate base of the enol form of a p-diketone, also called an acetylacetonate (acac). The acac complexes are fairly stable and are good sources of volatile organometalliccomplexes for chemicalvapor deposition or laserassisted chemical vapor deposition. These compounds are especially important for metals, such as Cu, that form few other volatile complexes. Chemical substitution on the alkyl groups of the acac ligand directly influences the physical and chemical properties of acac complexes. For example, fluorination of the methyl groups, to form an hfac ligand, greatly increases the volatility of the organometallic complex relative to the unsubstituted acac compound.’ Fluorine substitution also influences the efficiency of the photoreduction of Cu(I1) acac complexes. Gafney and Lintvedt2 found that the quantum efficiency for Cu(11) reduction doubles in Cu(hfac)2comparedto theunfluorinated Cu(acac)z complex. F3c\ 0
-0
/ cF3
4
1
The results of Houle et al. motivated our investigation.’ They studied the composition of photodeposited copper films from 1 Present address: Department of Chemistry, Washington University, St. Louis, MO 63130-4899. thesent address: UOP Research Center, Des Plaines, IL 60017. 8 Present address: Cooperative Institute for Research in Environmental Science, University of Colorado/NOAA, Boulder, CO 80309-0216. Abtract published in Advance ACS Abstructs, October 1, 1993. t
as a function of wavelength and photolysis conditions. Excimer laser (248-nm) deposited copper films had contaminationin molar ratios of 1 Cu:68 C:l6 0:15 F. By comparison, the ratios in the precursor compound 1 are 1 Cu:lO C:4 0:12 F. The extensive contamination in the copper films, in mole ratios greater than those of the atoms in the complex, indicates extensive fragmentation of the ligands at or near the surface. Others have studied the laser photochemistry of metal acac compounds but have concentrated on the multiphoton dissociation of the compounds and detected the products by laser-induced fluorescence5-’ or multiphoton ionization and mass spectrometry.’J-lO A particularly relevant example is the work of Mariner0 and Jones in which they report timedependent evolution of Cu atoms in the 248-nm photodissociation of 1.5 They found that the laser-induced fluorescence signal from Cu rises as the delay between pump and probe lasers increases. The signal reaches a maximum level at 25 ns delay and remains constant for delays up to 600 ns. Here we study the photolysis of C ~ ( h f a c ) ~ 1,, in a molecular beam using photoionization time-of-flight mass spectrometry to identify some of the primary photofragments. Our approach combines ultraviolet photolysis, vacuum ultraviolet ionization, and time-of-flight mass spectrometry to determine the photodissociation pathways. The ionization method, VUV singlephoton ionization, is similar to that of other authorsll-l3 although we generateVUV radiationby resonant four-wave mixing. Singlephoton VUV ionization using 125-nm (9.9-eV) photons is a general, energy-selective ionization method for the detection of primary products from photodissociation of organometallic complexes. The energy of the VUV photon limits detection to those products with ionization potentials of 9.9 eV or less. Because this energy is near the ionizationthreshold of many species, there is little excess energy for secondary dissociations. Therefore, most products detected by the mass spectrometer result from primary photodissociation and single-photon ionization. Published ionization potentia1s1&l6 and other results from our laboratory17aguide our assignment of primary photodissociation fragments in these experiments. 11. Experimental Approach
Detailed information about the design of the molecular beam apparatus appears elsewhere.l* Figure 1 shows a portion of the apparatus, which has two differentially pumped chambers separated by a skimmer. The organometalliccompound resides in a sample reservoir, inside the molecular beam sourcechamber, approximately 5 cm behind the nozzle. Thermal coaxial cable
0022-3654/93/2097-11249$04.00/00 1993 American Chemical Society
Bartz et al.
11250 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993
ions
acceleration electrodes
t
\ W
vuv
4-2 -
backing plate
Figure 1. The experimental apparatus.
heats this holder, which has an inlet for the external carrier gas, He, and an outlet that connects to the nozzle. Typically, the sample reservoir operates at a temperature of 80 OC and the carrier gas pressure (Matheson Ultra-high Purity He, 99.999%) is about 150 Torr to create about a 1% seed ratio. The nozzle pulses at 20 Hz and produces 1-ms long burst of gas. The valve sits in an aluminum holder, wrapped in thermal coaxial cable. Typically, the nozzle operates at 90 OC, a higher temperature than the sample reservoir, to reduce condensation or sublimation of the organometallic compound in the nozzle assembly. The seeded carrier gas travels out of the nozzle and expands,producing a molecular beam that passes through a skimmer and travels into the interaction region, about 13 cm from the nozzle. In the interaction region, 250-nm UV light crosses the beam of organometallic molecules and photolyzes it. After a short time delay (0-1000 ns), 125-nm VUV light ionizes the resulting photofragments. An extraction field accelerates the ions into the field-free region of a time-of-flight mass spectrometer where they separate by mass. The apparatus uses two separate Nd:YAG-pumped dye laser systems. The first produces UV light for photolysis by frquencydoubling the dye laser light and then mixing that doubled light with the fundamental of the Nd:YAG laser in a KDP crystal. The secondgenerates VUV light by four-wave mixing in mercury. Light from the second dye laser (625 nm) passes through a KDP crystal, which doubles the frequency of a portion of the beam. The two wavelengths (625 nm 312.5 nm) travel collinearly through a lens and into a Hg heat pipe where the combination of two 312.5-nm UV photons and one 625-nm visible photon produces a single 125-nm (9.9 eV) VUV photon through resonant four-wave mixing.I7 Compound 1 is well-suited to our molecular beam technique, since it has a vapor pressure of about 1 Torr at 75 OC.I The synthesis of 1 is simple,19 but it is also commercially available from Strem Chemical Company. Although 1 is somewhat moisture-sensitive,19water contamination is easy to detect because the anhydrous complex is a dark blue and the hydrate is light green. The gas-phase absorption spectrum20of 1 has two main features. One, centered at 300 nm, is a ligand based x to x* transition, and the other is a ligand-to-metal charge-transfer transition with a maximum around 240 nm.21 The cross sections for the two transitions are similar, 6 X 10-17 cm2 for the x to x* transition and 2 X l&17 cm2 for the charge-transfer band.22 The combination of availability, volatility, reasonable cross sections, and ease of handling make Cu(hfac)2 a good organometallic complex to study.
200
500
400
Mas
Figure 2. The VUV-only (125nm, 9.9 eV) photoionizationmass spectrum of 1. The signal near mass 400 is from background in the chamber. The subtraction of the VUV only signal in the data analysis removcs it.
Cu (hfac)p-CFg
I
\ Cu( hf ac)CF3
1
Cu ( hf ac)
200
250
300
350
400
Mass Figure 3. The 252-nm photodissociation mass spectrum of 1.
Cu(hfac)CFg
+
III. Results A. Primary Photodissociation Products. A 9.9-eV photon has sufficient energy to ionizethe parent molecule 1sinceits ionization potential, measured by electron impact, is 9.86 f 0.05 eV.15 The VUV photoionization mass spectrum in our apparatus (Figure 2) shows only the parent molecule ion, mass 477. (Each copper-
300
1I
Cu( hf ac) I
200
250
300
350
400
Mass Figure 4. The 222-nm photodissociation mass spectrum of 1.
containing feature has two isotopes of copper, 63Cu and W u . For simplicity, we list only the ion containing the lighter isotope.) Other photoions, from secondary dissociations, most likely have appearance potentials above 9.9 eV. The appearance potential of one conceivable fragment ion, the parent molecule minus one CF3, is 11.6 i 0.1 eV.15 The mass spectra after photodissociation of 1 at wavelengths of 252 and 222 nm appear in Figures 3 and 4, respectively. All the features in Figure 3 appear to come from the absorption of only one 252-nm photon and correspond to the masses 408, CUO~C~H~F~(CU(~~~C)~-CF~, Cu(hfac)z "minusn CF3); 339, Cu02C6HF9 (Cu(hfac)CF3; 270, C U O ~ C S H(Cu(hfac)); F~ and 201, CuO~C4HF3(Cu(hfac)-CF3). (Our inference of single photon absorption comes from the measured variation of the yield with photolysis laser pulse energy.) The same masses appear in Figure 4 from the 222-nm photodissociation of 1. Again, all
The UV Photodissociation of Cu(hfac)z in a Molecular Beam The Journal of Physical Chemistry, Vol. 97,No. 43, 1993 11251
Cu (hfac)
Cu(hfac)*-CF,
\
I
0.0
0.2
0.4
0.6
0.8
1 .o
Laser Delay (ps) Figure 6. The integrated intensities of Cu(hfac) signal at various delays between 252-nm photolysis and V U V ionization lasers.
200
250
300
350
400
Mass Figure 5. The photodissociation mass spectra of 1 at varying delays ( a = 0, b = 50, c = 100, d = 200, e = 500, andf= lo00 ns) bctwccn 252-nm
photolysis and V U V ionization lasers.
depend on the absorption of one UV photon. At least one of the features in both mass spectra, Cu(hfac)CFp, appears to come from decay of a larger ion in the extraction region of the mass spectrometer. The arrival time distribution of this fragment has a shape that is characteristic of the decay of a metastable ion.23 B. Fragment Time Evolution. The photodecomposition of Cu(hfac)Z at 252 nm, in the ligand-to-metal charge-transfer band, is slow enough for us to observe significant changes in the signal as the delay between the two lasers increases from 0 to 1000 ns. Figure 5 compares the mass spectra for the 252-nm photolysis of 1 at laser delays of 0, 50, 100, 200, 500, and 1000 ns. The Cu(hfac) featuregrows with the delay between the lasers. Figure 6 is a plot of the integrated area of detected Cu(hfac) versus the delay time for delay times from 50 to 1000 ns. The line is a fit of an exponential rise to those points with a time constant of (2.2 f 0.7) X 106 s-1. Determining the rise time of this feature is difficult, since the molecular beam travels normal to the two laser beams. At longer times after the dissociation, the products have moved further from their formation point, and we adjust the VUV laser beam position to compensate for this. Thus, the asymptotic value of the signal is unknown in Figure 6. It is apparent though, that therise timeis on theorder of a microsecond from a number of separate determinations. C. Translational Energy. The width of each product’s arrival time distribution in a time-of-flight mass spectrometer operating under Wiley-McLaren space-focused conditions, which correct for the variations in the initial positions of the products, reflects the velocity distribution of those fragments from the photodissociation event and, thus, the amount of initial excitation partitioned into translational energy.24 This analysis is analogous to one-dimensional Doppler spectroscopy.25 Because the average energy of each Cu-0 bond in 1 is 1.70 f 0.11 eV (39.2 f 2.6 kcal/mo1)26 and the energy of a 252-nm photon is 4.92 eV, there
is about 1.52 eV of energy available following the removal of a complete hfac ligand. By fitting the Cu(hfac) arrival time distrjbution to a Gaussian function, we find the translational energy of this product is 0.03 f 0.01 eV, which is only 2 f 1% of the available energy.
IV. Discussion Products from the photodissociation of 1in the charge-transfer bond 252 and 222 nm, apparently come from the absorption of one photon. Moreover, thedetectedamount ofoneof theproducts, Cu(hfac), depends strongly on the delay time between the photolysis and ionization lasers. In contrast, the products from the photodissociation of 1 in the ligand A to A* transition come from multiphoton dissociation, and the amount of each varies little with the delay between the two lasers.*’ Because of these differences, we concentrate on the charge-transfer band, where the excitation wavelength is closest to that used by Houle et aIs3 in the 248-nm Cu photodeposition. The extensive contamination of laser-deposited Cu films3may partially arise from ligand fragments. Neither complete ligands nor ligand fragments, however, appear in our mass spectra. The most likely reason is that the ionization potentials of these species are greater than the energy available in our VUV photons.16 Nonetheless, the presence of Cu-containing fragments suggests the formation of ligand fragments as well. Also missing from the data is the observation of Cu atom production in the 252-nm photolysis. Low laser power at 252 nm (less than 0.25 mJ/pulse) preventsour producing detectableamountsof Cu. It is reasonable that a larger photon flux, more typical of film deposition conditions, would generate Cu atoms by multiphoton dissociation. Our time-dependent results provide a consistent explanation of the published photolysis behavior of 1. With our method of photodissociation and detection we observe an increase in the size of the Cu(hfac) feature with the delay between photolysis and ionization lasers. Scheme I provides a possible explanation of the time evolution that we observe. The Cu(hfac)2 (1) absorbs a photon in the ligand-to-metal charge-transfer band, which breaks the Cu-0 bond and reduces the metal from Cu(I1) to Cu(1) to form 2 in a homolytic dissociation. The hfac ligand, however, remains attached to the metal through a two-electron coordinate covalent bond. The unimolecular decay of 2 by cleaving the
11252 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 SCHEME I
Bartz et al.
Acknowledgment. J.A.B. wishes to thank the US.Department of Education for fellowship support at the University of WisconsinMadison, and T.G.M. gratefully acknowledges the Alexander von Humboldt Foundation for a Feodor-Lynen Fellowship. The Army Research Office supports this work. F3C’
CF3
1
1
hv
+ hfac. 3
remaining Cu-0 bond proceeds on the slower time scale of statistical decomposition. Characteristically, this statistical processes places little of the excess energy in relative translation. This time-dependence may explain the solution photolysis results of Gafney and Lintvedt,*who obtain a quantum efficiency less than 0.04 for photolytic decomposition of l.z Our gas-phase results, which show that compound 1 loses completeligands slowly, are consistent with the solution phase behavior. In the photolysis of 1in ethanol solution, the solvent cage may force the ligand and metal radicals to recombine, reducing the quantum efficiency of photodecomposition. Furthermore, the solvent cage may inhibit the other photolysis pathways we observe in the gas phase and further decrease the quantum efficiency observed in solution. The slow dissociation of intact ligands probably also hinders the photodeposition of Cu from 1. An ideal metal source for metal vapor deposition would decompose rapidly to generate metal atoms and unreactive gaseous byproducts. Our data show that 1 loses complete ligands in about a microsecond. This contrasts with the results of Marinero and Jones, who found a faster rise time, 25 ns, for the production of Cu atoms as detected by laserinduced fluorescence.5 The origin of this difference is likely to be their production of Cu atoms by multiphoton dissociation compared to our one-photon decomposition. The extensive fragmentation and time-dependent behavior in the photodissociation of 1 in the charge-transfer band support the suggestion that the poor film quality observed by Houle et al. in the photodecomposition of 1 comes from incorporation of ligand fragments in the film.
References and Notes (1) Severs, R. E.; Sadlowski, J. E. Science 1978, 201, 217-223. (2) Gafney, H. D.; Lintvedt, R. L. J. Am. Chem. Soe. 1971,93,16231628. (3) Houle, F. A.; Wilson, R. J.; Baum. T. H. J. Vac. Sci. Technol. A 1986, 4, 2452-2458. (4) Lintvedt, R. L. In Concepts of Inorganic Photochemistry; Adamson, A. W., Fleischauer, P. D., Ed.; Wiley: New York, 1975; pp 299-332. ( 5 ) Marinero, E. E.; Jones, C. R. J. Chem. Phys. 1985,82, 1608-1609. (6) Tyndall, G. W.; Jackson, R. L. J. Am. Chem. SOC.1987,109,582583. (7) Schendel, J.; Wehry, E. L. Anal. Chem. 1988,60, 1759-1762. ( 8 ) Mikami, N.; Ohki, R.; Kido, H. Chem. Phys. 1988,127, 161-171. (9) Mikami, N.; Ohki, R.; Kido, H. Chem. Phys. 1990,141,431440. (10) Morris, J. B.; Johnston, M. V. J. Phys. Chem. 1985,89,5399-5401. (1 1) Feldman, D.; Laukemper, J.; Welge, K.H. J. Chem. Phys. 1983,79, 278-282. (12) Ruhman,S.;Haas,Y.;Laukemper,J.;Preuss,M.;Stein,H.;Feldman, D.; Welge, K. H. J. Phys. Chem. 1984,88, 5162-5167. (13) (a) Schiihle, U.; Pallix, J. B.; Becker, C. H. J. Am. Chem. Soc. 1988, 110,2323-2324. (b) Boyle, J.; Pfefferle, L. J. Phys. Chem. 1990,94,33363340. (c) Van Bramer, S.E.; Johnston, M. V. Anal. Chem. 1990,62,26392643. (14) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R.
D.; Mallard, W. G.Gas-Phase Ion and Neutral Thermochemistry; Supplement 1 ed.; American Chemical Society and American Institute of Physics: New York, 1988; Vol. 17. (15) Reichert, C.; Bancroft, G. M.; Westmore, J. B. Can. J. Chem. 1970, 48, 1362-1370. (16) Westmore, J. B. Chem. Rev. 1976, 76, 695-715. (17) (a) Huey, L. G.Ph.D. Thesis, University of Wisconsin-Madison, 1992. (b) Hilbig, R.; Wallenstein, R. IEEE J. QE 1983, 19, 1759-70. (18) Hayden, C. C.; Penn, S. M.; Carlson-Miyskens, K. J.; Crim, F. F. Rev. Sci. Instrum. 1990, 61, 775-782. (19) Bertrand, J. A.; Kaplan, R. I. Inorg. Chem. 1966, 5, 489491. (20) Houle, F. A.; Baum, T. H.; Moylan, C. R. In Laser Chemical
Osgood, R. M.; Ed.; Cambridge Processing for Microelectronics;Ibbs, K. G.; University Press: Cambridge, 1989. (21) Fackler, J. P.; Cotton, F.A.; Barnum, D. W. Inorg. Chem. 1963,2, 97-101. (22) Johnson, P. R.; Thornton, D. A. J. Mol. Struct. 1975, 29, 97-103. (23) Proch, D.; Rider, D. M.; Zare, R. N. Chem. Phys. Lett. 1981, 81, 430-434. (24) Penn, S.M.; Hayden, C. C.; Carlson-Muyskens, K. J.; Crim, F. F. J . Chem. Phys. 1988,89, 2909. (25) Schmiedl, R.; Dugan, H.; Meier, W.; Welge, K. H. 2.Phys. A 1982, 304. 137-142. (26) Ribeiro da Silva, M. A. V.; Reis, A. M. M. V. Bull. Chem. Soc.Jpn. 1979. 52. 3080. (27) Bartz, J. A. Ph.D. Thesis, University of Wisconsin-Madison, 1992.
