J. Phys. Chem. 1988, 92, 1604-1612
1604
UV Laser-Induced Photochemistry of Fe(CO), on Single Crystal Surfaces in Ultrahigh Vacuum Francis G. Celii: Paul M. Whitmore,$ Arthur Amos Noyes Laboratory for Chemical Physics, California Institute of Technology, Pasadena, California 91 125
and Kenneth C. Janda* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: August 13, 1987)
We report the observation of dissociation resulting from irradiation of Fe(CO)5 deposited onto single crystal Alz03,Si( loo), and Ag( 110) surfaces in ultrahigh vacuum. Photolytic effects are observed without competition from laser-induced heating of the substrate. Decomposition is measured by mass spectrometric detection of the CO released from the surface. The postulated mechanism involves the direct absorption of 337-nm radiation by the physisorw Fe(CO)5 and subsequent fast dissociation. The Fe-containing photoproducts remain on the surface, although they are not identified. The dissociation yield is very similar for Fe(CO), on A1203 and Ag(ll0) surfaces, which indicates that the rate of photofragmentation is competitive with energy transfer to the surface for each of the three substrates. Also, an annealing process, which results in a short initial burst of Fe(CO)5 from the surface, is observed.
1. Introduction The photochemical modification of solid surfaces is a topic of much current and practical interest.’” The use of lasers in this application brings inherent submicrometer resolution to bear on the challenge of producing ever smaller semiconducting devices. One particular area that has met with much success is the light-induced deposition of conducting metallic structures by using volatile organometallic precursor^.^" There is a growing need, however, to examine these reactions under conditions that allow one to sort out contributions from various mechanisms such as photolysis, pyrolysis, electron-induced processes, and ablation. A potential problem for the stimulation of photochemical reactions a t an interface is the ability of the solid to quench the photoexcited adsorbate. The bulk solid of semiconductors and metals, in particular, possesses a variety of modes, active over many orders of magnitude in frequency, for accommodating excess energy. The fluorescence lifetime measurements of Harris and co-workers7-’’ experimentally document the quenching of excited-state species as a function of distance from various solid surfaces. No fluorescence, however, could be observed from molecules adsorbed directly on the Ni,’ Ag,8*11and GaAs’O surfaces. The line shapes of electron-energy loss (EELS) spectra of pyrazine monolayers on the Ag( 111) surface12 and for Nzon Al( 111) were attributed to lifetime broadening by nonradiative adsorbate-surface energy transfer with rates in excess of loL3s-I. Rates of many chemical reactions may be orders of magnitude too slow to effectively compete with quenching mechanisms near surfaces that deplete excited-state population on this time scale. The metal carbonyls are particularly attractive for the study of photochemical reactions at solid surfaces. They have potential practical importance as volatile sources of metal species for deposition, which can be efficiently decomposed with heat or light. Also, the photochemical reactivity of metal carbonyls in various media have been well-studied. The possibility that photoexcited metal carbonyls may undergo reaction at a solid surface is demonstrated by the fact that dissociation of the molecules effectively competes against further laser up-pumping in attempts at multiphoton ionization of many carbonyl^.'^-'^ We have investigated the photochemical decomposition of Fe(CO)5 at 337 nm on clean single crystals of sapphire, silicon, and silver. A wide range of substrate energy-transfer behavior is represented in the choice of insulator, semiconductor, and metal
0022-3654/88/2092-1604$01.50/0
substrates. The results obtained are discussed in terms of the metal carbonyl photochemistry observed in other media and energytransfer considerations for the reaction. Of special interest is the reported observation of a photochemical reaction at a clean and well-ordered metal surface.17 This complements the growing body of literature on the dynamics of photochemical dissociation processes at the surface of semiconductorsI8-*’ and insulator^.^^*^^
2. Background Spectroscopy and Photochemistry of Fe(C0)S The gas-phase absorption spectrum of Fe(CO), consists of an (1) Deutsch, T. F. In Laser Processing and Diagnostics; Bauerle, D., Ed.; Springer-Verlag: Berlin, 1984. (2) Osgood, R. M., Jr. Ann. Rev.Phys. Chem. 1983,34, 77. (3) BPuerle, D. In Laser Processing and Diagnostics; BLuerle, D., Ed.; Springer-Verlag: Berlin, 1984. (4) Ehrlich, D. J.; &good, R. M., Jr.; Deutsch, T . F. J . Vac. Sci. Technol. 1982, 20, 738. (5) Solanki, R.; Boyer, P. K.; Mahan, J. E.; Collins, G. J. Appl. Phys. Lett. 1981, 38, 572. (6) George, P. M.; Beauchamp, J. L. Thin Solid Films 1980, 67, L25. (7) Campion, A.; Gallo, A. R.; Harris, C. B.; Robota, H. J.; Whitmore, P. M. Chem. Phys. Lett. 1980, 73, 447. (8) Whitmore, P. M.; Robota, H. J.; Harris, C. B.; J . Chem. Phys. 1982, 76, 740. (9) Whitmore, P. M.; Robota, H. J.; Harris, C. B. J. Chem. Phys. 1982, 77, 1560. (10) Whitmore, P. M.; Alivisatos, A. P.; Harris, C. B. Phys. Rev. Lett. 1983.50, 1092. (1 1 ) Alivisatos, A. P.; Waldeck, D. H.; Harris, C. B. J. Chem. Phys. 1985, 82, 541. (12) Demuth, J. E.; Avouris, Ph. Phys. Rev. Lett. 1981, 47, 61. (1 3) Avouris, Ph.; Schmeisser, D.; Demuth, J. E. J . Chem. Phys. 1983, 79, 488. (14) Duncan, M. A.; Dietz, T. G.; Smalley, R. E. Chem. Phys. Lett. 1979, 44, 415. (15) Gerrity, D. P.; Rothberg, L. J.; Vaida, V. Chem. Phys. Lett. 1980, 74. 1 . (16) Fisanick, G. J.; Gedanken, A.; Eichelberger, T. S., IV; Kuebler, N. A.; Robin, M. R. J . Chem. Phys. 1981, 75, 5215. (17) Celii, F. G.; Whitmore, P. M.; Janda, K. C. Chem. Phys. Lett. 1987, 138. 257 . .., - .. (18) Hemminger, J. C.; Carr, R.; Somorjai, G. A. Chem. Phys. Leff. 1978, 57, 100.
(19) Foord, J. S.; Jackman, R. B. Chem. Phys. Lett. 1984, 112, 190. (20) Creighton, J. R. J . Vac. Sci. Techno/. A. 1986, A4, 669. (21) Bartosch, C. E.; Gluck, N. S.;Ho, W.; Ying, Z. Phys. Rev.Lett. 1986, 57, 1425. (22) Bourdon, E. B. D.; Cowin, J. P.; Harrison, I.; Polyany, J. C.; Segner, J.; Stanners, C. D.; Young, P. A. J . Phys. Chem. 1984, 88, 6100. (23) Higashi, G. S.;Rothberg, L. J. J . Vac. Sei. Techno/. 8 1985, 83, 1460.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1605
UV Laser-Induced Photochemistry of Fe(CO)5
upper limits for the excited-state lifetime and estimated that loss intense charge-transfer band (cabs = 15 700 M-' cm-I) with a of C O occurs within 2.0 ps (300-310-nm excitation) or 0.6 ps maximum near 200 nm, a secondary maximum at 280 nm (cabs (275-280-nm excitation). The lifetime limits were derived from = 3900 M-l cm-I) assigned to a metal-ligand-charge-transfer band estimates of the up-pumping rate for second (ionization) photon (MLCT), plus a weaker absorption feature near 330 nm (cabs 500 M-' cm-'), which is thought to be a ligand field t r a n s i t i ~ n . ~ - ~ ~absorption obtained by using the laser fluence and absorption intensity and measurements of the branching ratio to several ion The N 2 laser (337 nm) used in our experiments excites the fragment products. molecule via the low-frequency tail of the ligand field transition. The bands are broad and unresolved, indicating that the electronic 3. Experimental Section states are likely to be mixed on a very short time scale. Elec3.1. UHV Chamber. The ultrahigh vacuum (UHV) chamber tron-impact spectra give some indication for triplet-state character constructed for this study consists of a main chamber mounted in analogous bands in Cr(CO),, while preliminary spectra for on a pumping stack, both constructed by MDC (Mountain View, Fe(CO)5 were not conclusive about this point.26 The weak CA). The primary system pump is a 4-in. diffusion pump (Ed60 M-I cm-I) employed in our exmolecular absorption (e wards E04) backed by a two-stage direct-drive rotary pump periment serves to indicate the range of experimental sensitivity (Alcatel Z-2008-A). The diffusion pump system is extensively that is available for further studies. interlocked to prevent pump fluids from reaching the main With absorption of light at 337 nm (85 kcal/mol), an Fe(CO), chamber, especially the rotary pump, which can be automatically molecule contains enough energy to lose a t least 2 CO moleisolated by a pneumatic valve with position indicator (MDC c u l e ~ . ~Trapping ~ - ~ ~ experiments using PF3have determined that AV-103-P). Above the diffusion pump is a right-angle liquidexcitation with light in the 352-192-nm range produces all possible nitrogen cold trap (Vacuum Generators CCT-100V), which also fragments-Fe(CO),, x = 2,3,4- with branching ratios that includes an integral gate valve to isolate the pump and trap, and suggest nonstatistical distribution of internal energy in the Fe(CO)4 to throttle the pumping speed for sputtering or dosing applications. While some fragment, following prompt initial loss of C0.27*28 The trap has a 16-h hold time and, more importantly, all surfaces doubt may be cast upon the branching ratios due to the ability exposed to the vacuum chamber remain cold until the trap becomes of PF3 to exchange with CO ligands of excited Fe(CO), fragments, completely empty. The combination of diffusion pump and trap the extent of decomposition a t 248 nm has been verified by a is quoted to have a pumping speed of 200 L d . Rough pumping gas-phase study in which Fe(CO),, x = 2,3,4, species have been from atmospheric pressure is accomplished with a pair of cryoidentified with IR absorpti~n.~',~* Recently, Waller et al. reported sorption pumps isolated by a bakeable metal seal valve (Granthat gas-phase phase photolysis of Fe(CO)5 at 193-nm proceeds ville-Philips Model 204). The cryopumps evacuate the chamber with a completely statistical distribution of energy into the C O to lo4 Torr at which point the diffusion pump can be used to product degrees of freedom.33 bring the pressure into the 10-7-10-8-Torr range. After bakeout Extensive study of the UV and IR photochemistry of metal at 200 OC, the pressure drops into the 10-9-Torr range at which carbonyls in inert matrices has been conducted.3e38 Irradiated point a water-cooled titanium sublimation pump efficiently getters Fe(CO)5 in a matrix loses 1 CO molecule per UV photon to yield Torr is achieved. At this and the base pressure of (1-2) X either Fe(C0)4 or a stable Fe(CO).,-X complex in which X is a pressure H20, CO, H2, and C 0 2 are the species present. During matrix atom or m o l e c ~ l e , ~in~contrast -~~ to the extensive deoperation, the helium closed-cycle refrigerator (see below) acts composition in the gas phase. Irradiation of matrices with high as a cryopump for condensables. Fe(CO)5 concentration can produce the Fe2(CO)9and Fe3(C0)12 The main chamber is a variation of the standard Varian-type condensation species.36 Fe2(C0)9 can undergo further photodesign (12-in. diameter), to which was added a second level, to chemistry, giving Fe2(C0)8,38while Fe3(C0)12appears not to be accomodate the helium refrigerator, and ports for optical access. photoactive, although it possesses intense absorption bands.40 The analytical tools in the chamber are LEED/Auger screens Of central importance in this study is the time scale of the initial (Varian), residual gas analyzer (VG SX-200 mass spectrometer), photochemical decarbonylation step. Since the rate of energy Ar+-ion sputter gun (Perkin Elmer), and nude ionization gauge transfer from an excited adsorbate to a substrate may be fast, (Granville Phillips Model 271). Two gas inlets were utilized, one especially for metal surfaces, photodissociation must also be fast for dosing from the background, the other for directed doses to be competitive. This criterion is fulfilled for Fe(CO), in which through a multichannel array. For laser-surface interaction the initial decarbonylation step occurs on the order of picoseconds. studies, optical entrance and exit ports at optimal angles for both From a multiphoton ionization study, Whetten et ale4'determined metal (0 = 88O in the IR, 65' in the UV) and semiconductor (0 = 75O) surfaces are provided. Differentially pumped uncoated (24) Dartiguenave, M.; Dartiguenave, Y . ;Gray, H. B. Bull. SOC.Chim. Fr., 1969, 4223. BaF2 flats (6-mm thickness) mounted on the 75O ports were used (25) Geoffroy, G. L.; Wrighton, M. Organometallic Photochemistry; as N2-laser entrance and exit windows throughout the course of Academic: New York, 1979. the work reported here. (26) Koerting, C . F.; Walzl, K. N.; Kupperman, A. J. Chem. Phys. 1987, The low-energy electron diffraction (LEED) screens and 86, 6646. (27) Nathanson, G.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J . Chem. electron gun are used for observation of surface order. The same Phvs. 1981. 74. 361. apparatus is employed for retarding-field Auger electron spec128) Yardley, J. T.; Gitlin, B.; Nathanson, G.; Rosan, A. M. J . Chem. troscopy (AES) to monitor surface cleanliness. Home-built Phys. 1981, 74, 370. electronics for the electron pass energy ramp, amplifier, and screen (29) Engleking, P. C.; Lineberger, W. C.; J. A m . Chem. SOC.1979, 101, 5569 __-_. modulation voltage, plus a high-voltage supply (Kepco OPS(30) Lewis, K. E.; Golden, D. M.; Smith, G. P.; J . A m . Chem. SOC.1984, 2000B) augment the Varian LEED screen and electron gun control 106, 3905. modules. The mass spectrometer acts as a residual gas analyzer, (31) Oudekirk, A. J.; Weitz, E. J . Chem. Phys. 1983, 79, 1089. thermal desorption spectrometer, and product detector for the UV (32) Ouderkirk, A. J.; Wermer, P.; Schultz, N. L.; Weitz, E. J . A m . Chem. SOC.1983, 105, 3354. photochemistry experiments. A channeltron electron multiplier (33) Waller, I. M.; Davis, H. F.; Hepbum, J. W. J. Phys. Chem. 1987, 91, provides high gain to amplify the ion current generated by an 506. electron-impact ionizatiofl source. A Faraday cup is also available (34) Burdett, J. K. Coord. Chem. Rev. 1978, 27, 1 . was fit around for gain calibration. A glass integrating nose (35) Poliakoff, M. Chem. SOC.Rev. 1978, 6, 527. the ionization region to restrict the pumping speed, increase the (36) Poliakoff, M.; Turner, J. J. J. Chem. SOC.,Dalton Trans. 1974, 210. (37) Poliakoff, M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1974, 2276. dwell time of product molecules, and thus increase the sensitivity ( 3 8 ) Poliakoff, M.; Turner, J. J. J . Chem. SOC.A 1971, 2403. of the mass spectrometer at the expense of time resolution. The (39) Graff, J. L.; Sanner, R. D.; Wrighton, M. S. J . A m . Chem. Soc. 1979, mass range of the quadrupole (0-200 amu) is sufficient to resolve -101. - - , -213. .- . the parent and fragment peaks of Fe(CO)5 (196 amu), but not (40) Tyler, D. R.;Levenson, R. A,; Gray, H. B. J. Am. Chem. SOC.1978,
-
N
N
100, 7888. (41) Whetten, R. L.; Fu, K.-J.; Grant, E. R. J. Chem. Phys. 1983, 79,
4899.
(42) Feulner, P.; Menzel, D. J . Vac. Sci. Technol. 1980, 17, 662.
1606 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
high enough to observe the parent ions of larger Fe,(CO), (x 3 2, y 3 4) cluster products that may be formed. The crystal sample to be studied was mounted on a standard manipulator (Huntington PM-600-XYZ-RC) configured for a 2.5-in. sample offset from the rotation axis. The coaxial motion of the manipulator assembly is levered to allow tilt of the sample in addition to the x, y , z, and rotary motions. The crystal mounting assembly, consisting of 0.001-in. tantalum foil, which was spotwelded to tungsten rods, was electrically isolated and provided resistive heating to higher than 1100 "C. The T a foil also held a chromel-alumel thermocouple pair in contact with the crystal. The crystal sample was cooled by a helium closed-cycle refrigerator. The refrigerator provides two cold stations of 15 K (2 W of power for sample cooling) and -77 K (25 W for heat shrouds and crystal support precooling). The refrigerator was retrofitted onto a Conflat flange for UHV compatibility (Cryosystems, Inc.). A bakeout control system43was constructed to allow unattended baking of the UHV chamber to 200 OC without adverse effects to the refrigerator cold head, which must be kept below 90 "C. A flexible braid of 99.999% copper couples the refrigerator cold head (second stage) and the crystal assembly.44 Sapphire spacers are employed in the cooling path to both provide electrical isolation and act as a thermal switch-good thermal conductivity at low temperature for sample cooling, poor thermal conductivity above room temperature-to isolate the copper braid during sample heating. This is especially important for studies using Si and AI2O3crystals, which for cleaning purposes must be heated above the melting point of Cu. The W support rods and Cu electrical leads are also precooled (-200 K) by using the first stage of the refrigerator. The crystal mount is described in more detail elsewhere.45 Two sample inlets were used during this study. For initial experiments, vapor was dosed from background through a leak valve (Vacuum Generators MD-7R) and 1/8-in. stainless steel tubing; this method had the disadvantage of admitting a large load of gases such as Fe(CO), into the chamber. To avoid this problem, a calibrated dosing line was used, which was equipped with a multichannel array doser, to direct a significant fraction (-0.3) of the gas sample onto the crystal face.46 An 80-pmdiameter capillary tube limited the flow from a calibrated volume, so the flux of molecules emitting from the multichannel array could be determined by monitoring the pressure drop in the sample volume. With a pressure of 1.O Torr in the calibrated volume, 1.1 X lOI4 molecules/s are emitted from the doser. Thermal desorption spectra were used to compare doses obtained from the doser to those obtained by filling the entire chamber with sample gas. 3.2. Sample Preparation. The sapphire and silicon crystal samples were obtained as polished wafers and were not polished further. They were cleaned by heating under vacuum: A1203 (obtained from Union Carbide cleaved - 3 O O to the c axis) to 1050 0C;47.48Si( 100) p-doped (from California Technical Services) to 900 0C.49,50 The cleanliness of the Si surface was verified by AES, but only faint LEED spots were observed. Charging effects caused difficulties in obtaining LEED and Auger spectra of A1203. Since photochemistry can be enhanced by a roughened metal surface, effort was expended to polish, clean, and characterize the silver crystal to eliminate the effects of surface roughness. A 2-mm-thick disk of '/*-in. diameter was spark cut and mechanically polished to 0.25-pm flatness by using a series of abrasive papers and diamond paste. The orientation was within of
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-
(43) Celii, F. G.; Whitmore, P. M.; Janda, K. C. J . Vat. Sci. Techno/.A 1986, A4, 1939. (44) Samson, S . ; Goldish, E.; Dick, C. J. J . Appl. Crystallogr. 1980, 13, 425. (45) Celii, F. G.; Ph.D. Thesis, California Institute of Technology, 1986. (46) Campbell, C. T.; Valone, S. M. J . Vuc. Sci. Techno/. 1985, A3, 408. (47) Williams, R. S., personal communication. (48) Wei, P. S . P.; Smith, A. W. J . Vac. Sci. Technol. 1972, 9 , 1209. (49) Meyer, F.; Bootsma, G. A. Surf. Sci. 1969, 16, 221. (50) Chung, Y . W.; Siekhaus, W.; Somorjai, G. A. Surf. Sci. 1976, 58, 341.
Celii et a]. Fe*
i
CO'
I
FeCO'
CIx1
0
x1 00-
IO p
1
I
25
I
I
50
75
100
1
125
I
I
'50
I75
2bo
ION MASS h k )
Figure 1. Mass spectrum of Fe(CO), observed during a gas dose. Intensity scale changes are shown on the spectrum. The most intense fragment ions, CO' and Fe', were monitored during UV photochemistry experiments. Features can also be seen for fragments containing the 54Fe isotope, which has -6% natural abundance.
the (110) plane as determined by Laue X-ray backscattering. The surface that resulted was slightly, but visibly scratched, presumably on the 1.0-0.25-pm scale. Chemical polishing (100 mL of saturated K2Cr207,2 mL of saturated NaCI, 10 mL of H2S04diluted to twice the volume) gave a highly reflective surface and removed some of the mechanical damage, as could be seen by eye and in the clarity of the Laue spots. The crystal was rinsed with ethnaol after mounting and just prior to admission into the chamber. Cleaning under vacuum was accomplished by using standard procedure^.^'-^^ Combinations of mild heating (220 "C) in 10" Torr O2to remove surface carbon, room temperature dosing with CO to "clean off" atomic oxygen, and cycles of Ar+-ion sputtering followed by annealing at 300 "C resulted in a clean silver surface as determined by Auger spectro~copy.~'The resolution was high enough to differentiate between the silver Auger transition at 262 eV and the carbon peak at 272 eV. The ratio of the three Ag peaks was -7:2:1 at 365, 302, and 262 eV, respectively. The HeNe alignment laser reflected from the surface was not sharp, indicating some roughness on the 0.6-pm scale. However, the LEED pattern obtained from the clean and annealed crystal indicates an ordered (1 10) surface on the 50-100-A dimensiomS6 No evidence of facetting was detected.57 Fe(CO)S (Alfa Chemical, 99.5%) was subjected to numerous freeze-pump-thaw cycles to remove excess CO before each fill of the dosing volume. The observed cracking pattern of Fe(C0)5 during a dose, shown in Figure 1, is in reasonable agreement with published mass ~ p e c t r a .The ~ ~purity ~ ~ ~ of the Fe(CO)Sdelivered to the chamber was constantly monitored by observing the intensity ratio between m / e 28 (CO') and m / e 56 (Fe+) peaks. Experiments were conducted with doses that exhibited approximately a 5:l CO+:Fe+ ratio, the lowest value that was observed. Doses were administered to the crystal face in a configuration with no line-of-sight between the crystal face and any hot filaments. Filaments were switched off during longer (>1 min) doses. 3.3. Thermal Desorption. Thermal desorption spectra of Fe(CO), were obtained on the A1203and silicon crystal samples. A particular ion mass signal detected at the mass spectrometer is monitored as the temperature of the crystal is ramped. Because the entire crystal mount in the vicinity of the crystal and heater (51) Campbell, C. T.; Paffett, M. T. Surf. Sci. 1984, 143, 517. (52) Engelhardt, H. A.; Menzel, D. Surf. Sci. 1976, 57, 591. (53) Bradshaw, A. M.; Engelhardt, A,; Menzel, D. Ber. Bumen-Ges. Phys. Chem. 1972, 76, 500. (54) Bowker, M.; Barteau, M. A.; Madix, R. J. Surf. Sci. 1980, 92, 528. (55) Albers, H.; van der Waal, W. J. J.; Gijzeman, 0. L. J.; Bootsma, G. A,; Surf. Sci. 1978, 77, 1. (56) Ertl, G . ;Kiippers, J. Low Energy Electrons and Surface Chemisrry: Verlag Chemic Weinheim, West Germany, 1974. (57) Steiger, R. F.; Morabito, J. M., Jr.; Somorjai, G. A,; Muller, R . H. Surf. Sci. 1969, 14, 279. (58) Winters, R. E.; Kiser, R. W. Inorg. Chem. 1964, 3, 699. (59) Foster, M. S.; Beauchamp, J. L. J . A m . Chem. SOC.1975, 97, 4808.
UV Laser-Induced Photochemistry of Fe(CO)5
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1607
/
/"
m
f\
Figure 2. Schematic diagram for the UV photochemistry experiment. The output of the pulsed N2+laser (10 Hz,400 kW)is spacially apertured so that only the crystal face is irradiated. The fluence at the crystal is 20 pJ/cm2 when the laser is unfocused The components are as follows: (a) N2 laser, (b) HeNe laser, (c) beam shaping aperatures, (d) neutral density filter, (e) power meter, (f) electronic shutter, (g) focusing lens, (h) LEED, Auger optics, (1) sample surface, (j)refrigerator, (k) mass spectrometer, (1) x , y recorder, (m) signal averager, (n) IBM-PC
are at the same low temperature during a dose, adsorption by other surfaces can interfere with the thermal desorption spectrum. However, desorption from the crystal sample was isolated by using a glass integrating nose cone.42 The sample was positioned to within 1 mm of the nose-cone aperature. Because of the lower local pumping speed due to the nose cone, a slow heating rate (-2 K/s, manually controlled) was employed, compared to a more common rate of 10 K/s. 3.4. Photochemistry. A schematic of the UV laser-induced photochemistry experiment is depicted in Figure 2. A Molectron UV-400 N 2 laser produced 400 kW of 337.1-nm light in a IO-ns pulse, but the output beam was too large for irradiation of the crystal samples. Care was taken to reduce and shape the spot size with iris diaphragms and aperatures so that only the crystal surface was irradiated. Even with these precautions, background desorption signals of m / e 28 (CO') and m / e 44 (CO,') were observed upon laser irradiation. We assume this to be due to stray UV light, which induces photodesorption from metal surfaces in the chamber.60q61 The background CO' and COz' desorption signals, observed from the A1203or Si surfaces without Fe(C0)5 dosing, were approximately the same intensity. With the laser well-aligned, the laser-induced background CO+ intensity was never more than 5% of the photolysis signal and was usually much smaller. After the aperature, the UV laser beam was approximately 1 X 3 mm and spreads to 4 X 3 mm across the crystal face due to the angle of incidence ( 7 5 O ) . The energy density at the crystal face was 20 pJ.cm-z. A 3 1.4-cm focal length lens positioned at a distance of 29.5 cm from the crystal was sometimes employed and is estimated to have increased the energy density by a factor of -50-100, the uncertainty arising from the spot size measurement. A calibrated pyroelectric joulemeter (Cooper Lasersonics 53) with quartz condenser lens in combination with a boxcar integrator (Stanford Research Systems SR250) monitored the laser pulse energy, which was constant throughout these experiments. Quartz (60) McAllister, J. W.; White, J. M.; J . Chem. Phys. 1973, 58, 1496. (61) Lichtman, D.; Shapira, Y.CRC Crit. Rev. Solid State Mater. Sci. 1978, 8, 93.
neutral density filters were used to variably attenuate the laser power. A He/Ne laser was employed for initial alignment of the laser beam, then for reproducible placement of the crystal sample back to the irradiation position in front of the mass spectrometer. The mass spectrometer ion pulses were observed as a current pulse from the channeltron multiplier, which was amplified by a current-to-voltage converter (Keithly Model 427, l@-lOIO V/A, 0.01-300 ms rise time). With a 300-ms time constant being used to average the effect of individual laser pulses, the output of the amplifier was fed into a chart recorder for yield measurements. A 100-MHz amplifier (LeCroy TTB 1000) and transient digitizer (Biomation 805) monitored the ion signal wave form. The digitizer and optical shutter were interfaced to an IBM-PC through a TecMar Industries Labmaster board and a home-built interface. 4. Results 4.1. Thermal Desorption. An example of thermal desorption traces obtained during this study are shown in Figure 3 for the case of Fe(C0)5 on sapphire. The m / e 56 signal is displayed as a function of the crystal temperature, which is monitored by an attached chromel-alumel thermocouple. The slow heating rate, necessary with use of the glass nose cone, was controlled manually, leading to some variation in the peak shapes and position. Also, the thermal couple was mounted on the support, well away from the center of the sapphire face, so the measured temperature is higher than the temperature where desorption is monitored by the mass spectrometer. The thermal desorption data is thus used only to qualitatively characterize the surface layer and cannot be used to obtain bond energies from a Redhead analysis. Doses are reported in units of langmuirs (1 langmuir = 10" Torrs), as determined by using the uncorrected ion gauge pressure. This quantity was measured directly in the case of doses from background and by extrapolation, using integrated thermal desorption curves, for doses from the multichannel array system. The desorption profiles shown in Figure 3 are interpreted in terms of two binding states of Fe(CO)5on A1203. At low coverage, Fe(CO)5 bonds directly to the surface and gives rise to the thermal desorption peaks (cy) between 260 and 280 K. At higher coverage, multilayers of Fe(CO)5 can be condensed above the first layer and these produce the thermal desorption feature ( p ) at -225
Celii et al.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
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Fe(CO)5/A1203 2K/S&
(with integrding nosecone)
detect, mk 56
(Fe')
( d ) 35L
( c ) 23L
i b ) 11L
0
0 150
200
250
303
350
400
integrating nose cone in place (Figure 4b), the time response of the signal is dominated by the pumping time constant within the nose cone volume, 10 ms. Without the nose cone (Figure 4c), the desorption signal becomes a direct measure of the laser-induced pressure change as a function of time. We were not able to measure any distinctive, short time signals, which could be used to estimate the translational energy distribution for the product C O molecules. Figure 5 shows data obtained with focused UV irradiation of Fe(C0)5 overlayers on the Si(100) surface. In contrast to the unfocused laser experiments, the CO+ signal generated by the focused UV light decreases with time due to the depletion of molecules from the surface. In addition, laser-induced Fe+ signals are observed. At coverages of less than a monolayer, an Fe+ signal is only observed during the initial few laser pulses. At coverages of 2 or more monolayers, laser irradiation causes a constant flux of Fe-containing compounds to be desorbed in addition to a large initial burst. Furthermore, an Fe(CO), burst is observed if the shutter is closed to stop laser irradiation and then reopened without additional gas dosing. At coverage of less than a monolayer, the continual laser-induced signal at CO+ ( m / e 28) and the absence of a similar Fe+ ( m / e 56) signal indicates that molecular C O is the source of the m / e 28 signal. Thus, a dissociation process must be occurring on the Si(100) surface, which results in the desorption of CO. The Fe(CO), fragment remains on the surface. The Fe+ signal observed at low coverage arises from a process that is distinct from the main photodecomposition process. We have observed similar signals when exposing adsorbate-covered GaAs surfaces to IR in which no dissociation process is energetically possible. We therefore assign this feature to an "annealing" process, in which Fe(CO)5 molecules are desorbed from unstable binding sites. Both processes, the photodecomposition,which yields desorbed CO, and the annealing phenomenon, which results in a short burst of Fe(CO), from the surface, also occur at coverages greater than a monolayer. The process that causes the continual desorption of Fe-containing species from high-coverage surfaces is much less efficient than the decomposition. We believe this process to be the pyrolytic desorption of Fe(CO)5 due to heating of the overlayers. Another possibility for the high-coverage process is the desorption of condensation products, such as Fe2(C0)9, following the reaction of Fe(C0)4 and Fe(C0)5. However, because of the mass coincidence between fragments of condensation products (e.g., Fez+) and unreacted Fe(CO)5 (e.g., Fe(CO),+), no information could be obtained on the importance of this pathway. No iron-containing compounds could be detected by AES, as desorption is induced by the incident electron ~ u r r e n t . 6 ~ 4.2.2. A1203. Results obtained on the AlZ0, surface are shown in Figures 6-8. Figure 6 shows both the UV dissociation yield, as monitored by the laser-induced CO+ signal, and the surface coverage, as determined by the area under the Fe+ thermal desorption curves, as a function of the gas dosage at 130 K. The laser-induced CO+ signal that is generated with the unfocused laser beam (-20 pJ.cm-z) increases with dosage but exhibits saturation after -2-3 monolayers. The Fe+ thermal desorption peak area data indicate that the adsorbate coverage increases with dosage, but with decreasing efficiency after 1 monolayer coverage. Assuming an initial sticking coefficient of 1, the sticking rate for the second and subsequent layers is determined to be 0.2, in agreement with the value obtained for condensed layers of Fe(CO)5 on Si(100).19 Since the laser desorption signal levels off at a dosage of 20 langmuirs, we conclude that the yield of C O is limited either by physical or chemical trapping when the irradiated Fe(C0)5 is not in the top layer. Figure 7 shows the time dependence of the detected CO+ signal when the Fe(CO)5 on A1203is irradiated with a focused laser, 1 mJ/cm2. While the signal appears to decrease exponentially at early times, a plateau is seen at long irradiation time. The initial Fe(CO)5 coverage for the displayed data was submonolayer (2-
450
5co
T('K)
Figure 3. Thermal desorption curves for several dosages of Fe(CO)5on
A1203. Desorption from the surface layer and from multilayers can be distinguished. Because Al2O3must be heated indirectly and the temperature was measured on the crystal mount, the actual temperature of the surface is lower than that measured.
K. The two states yield equal thermal desorption intensity a t a dose of -20 langmuir, suggesting that a dose of 10 langmuir produces a monolayer coverage. From these measurements, it is estimated that -30% of the flux from the array doser strikes the crystal face with unity sticking probability. Thermal desorption profiles obtained at m / e 28, identical with the displayed m / e 56 curves, indicate that Fe(CO), adsorbs molecularly on the sapphire surface. This is also the situation on the Si(100) and is expected to be for the weakly binding Ag( 110) s ~ r f a c e . ~We ~ ,did ~ ~not observe C O adsorption on AlZO3at 120 K, and C O does not adsorb on the Si(100) or A g ( l l 0 ) surfaces at this temperature. 4.2. UV Photochemistry on Surfaces. 4.2.1. Si(100). The results of a typical photochemistry experiment are depicted in Figure 4 for the case of Fe(CO)5 on Si(100). The specific procedure to obtain such data is as follows. After the surface is cleaned and cooled to 120 K, a measured gas dose of Fe(CO)5 is administered to the surface. The chamber is pumped down to 1 X lO-'O Torr and the crystal is rotated to the prealigned position facing the mass spectrometer. An optical shutter is toggled open to allow the laser irradiation to reach the crystal face. Desorbed species are ionized and mass filtered. To obtain Figure 4a, the signal was amplified with a 300-ms time constant, which, given the 100-Hz laser repetition rate, gives an effective dc signal. Thus, the photolysis signal is detected as a pressure increase that persists until the shutter is closed. With a laser fluence of 20 wJ.cm-2, the depletion of Fe(C0)5 adsorbates is slow, so the photolysis signal level is roughly constant. The data of parts b and c of Figure 4 are recorded with a 15-ps time constant by using a Biomation transient recorder. With the (62) Sexton, B. Surf. Sci. 1981, 102, 271. (63) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190.
-
(64) Celii, F. G.; Whitmore, P. M.; Janda, K. C., unpublished results. (65) Foord, J. S.; Jackman, R. B. Chem. Phys. Lert. 1984, 112, 190.
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1609
UV Laser-Induced Photochemistry of Fe(CO)5
b
0.0
10.0
0 .o
20.0
Time (msec)
2 .oo
4.00
Time (msec) (without integrating n o s e m )
(with integraiing nosecone)
Figure 4. CO signals observed due to UV photochemistry of Fe(CO), on Si(100). The laser is unfocused resulting in a fluence of 20 gJ/cm2. (a) signal recorded with a 300-ms time constant to yield a quasi-dc response; (b) signal-averaged trace of the time response with the mass spectrometer “nose cone” in place; (c) same as b except no nose cone.
-1
c Annealing
Annealing
0
0
1
I
IO
20
100
I IO
Dosage ( L )
$1 J
Phdolyt ic Decomposition
1
Figure 6. Comparison of the laser-induced C o t yield (solid circles) to the Fe(CO), coverage (solid squares) as a function of gas dosage. Fe(CO), coverage was determined as the area under the Fet mass peak in temperature-ramped desorption curves. The curvature in the Fe(CO), coverage plot suggests that multilayers start to form at a 10-langmuir dosage. This coverage is also where the Fet signal starts to persist beyond the first few laser shots. The laser photofragmentation yield levels off after 2-3 monolayer doses indicating that CO molecules produced below the top of the film become trapped.
Time (secr Low Coverage
High Coverage
(-2L)
(-
20L)
-
F I5. Mass spectrometer signals observed for irradiation of Fe(CO), on Si( 100) with a focused laser 1 mJ/cm2. (a) At low coverage a Fet signal is observed for the first few laser shots. This is attributed to laser-thermal desorption of Fe(CO),; (b) at the same coverage the C o t signal only slowly decreases with time; (c) for multilayer coverages, the Fet signal persists at a reduced level for longer times; (d) even for high coverage, the CO+ signal is slowly varying with time, indicating that it is dominated by photofragmentation. langmuir dosage = 0.2 monolayer), but similar curves were also obtained with dosages up to 20 langmuirs, similar to the results on the Si( 100) surface. For a single dissociation process we expect a CO+ signal decay described by
z/zo = axp(-@ut)
effective cross section (cm2). A plot of In (Z/Zo) vs t , rather than giving a single straight line whose slope represents Ow,displays a curvature, which indicates a secondary process is occurring at long irradiation times. From an estimated photon flux of 1.7 X 10l6cm-2-s-i the slope of the initial decrease in the CO+ signal (-0.025) gives an effective absorption cross section of 1.5 X cm2,in good agreement with the gas-phase-absorption cross section cm2)?-66 The dissociation for Fe(CO)5 at 337 nm ((1-1.5) X process at long times, with an effective cross section 15 times smaller than the short time process, could be due to further dissociation or reaction of Fe(CO), (x < 5) species, diffusion of Fe(CO)5 into the irradiated area, or dissociation from the lowintensity fringes of the laser spot on the surface. The fluence dependence of the CO+ yield (Figure 8) was determined by inserting neutral density filters in the path of the
-
(1)
in which 9 is the photon flux density (photonscm-2-s-’) and u the
(66) Bottka, N.; Walsh, P.J.; Dalbey, R. Z. J . Appl. Phys. 1983,54, 1104.
1610 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
'
Celii et al.
10
--
04
. 0
C O yield &03
Fe(CO),/
337 nm irradiation
-
1
1
c
Fe (CO)5/Ag(110)
slope= - 2 5x10-2 ~
slope= -1 7 ~ 1 0 - ~
02
01
008 006
t 0
I
I
I
I
200
400 time (sec)
600
800
-
Figure 7. The dependence of the laser induced CO+ signal as a function of time for focused irradiation, 1 mJ/cm2. As discussed in the text, the initial slope is roughly consistent with the gas-phase absorption strength. The initial coverage was -0.4 monolayers (3.7-langmuir dose).
Fe(CO), dose
1 .o
.-0-
/'
tn + 0.6 0 0
.-
/:
0.4
+
-0
m
;0.2 0
#/! /
0
t
closed
open
closed
oper
t closed
28 (CO+)-
time
-
a) 8.3~ b) 8 = l O L c) 0 = 2 0 L Figure 9. Observed laser-induced CO+ and Fe+ signals for irradiation of Fe(CO), on Ag( 110) with 20 pJ/cm2. Only CO+ is observed at the
coverages studied (0.3-2 monolayers).
= 1.00
I
I
I
1
0.2
0.4
0.6
0.8
L a s e r Fluence
t
open
- m/e
/
0.5L 2.3L i0OL
0
A
0.8
A
I 1.0
+ 20pJ/cm2
Fgve 8. The dependence of the CO+ signal on laser fluence for Fe(CO)!, on A1203. The fluence and CO+ signal are each normalized to 1.O for each coverage at 20 pJ/cm2. Neutral density filters were used to adjust the laser power. All coverages tested show a similar fluence dependence. Three coverages are shown to indicate the scatter. We infer from the linearity of this plot that the photodecomposition is a 1-photon process.
unfocused N2 laser beam. The CO+ yield is normalized to that obtained with the unattenuated 20 pJ.cm-2 fluence. A straight line is obtained for all coverages, several of which are shown in the figure. The slope of 1.0 shows that the dissociation is a 1-photon process. The straight line is further evidence against a thermal mechanism, for which Arrhenius behavior (exponential dependence on laser fluence) would be expected. Although a linear dependence would be obtained in the limit of small temperature changes, this would not be sufficient to induce the observed dissociation of Fe(C0)5 at 120 K. The observed CO+ signal, in the limit of negligible fraction dissociated, can be written from eq 1 as zow = 1 - z/zo -Oat (2)
-
This approximation is seen to be valid given that the laser-induced CO+ signal is constant over the time scale of minutes (when using the determined value of u and a fluence of 20 pJ-cm-2, 10% dissociation is accomplished after 5 min of irradiation at 10 Hz). Also, the measurement technique for the fluence dependences of using a single dosage for the data obtained at a given coverage succeeds because the maximum fraction dissociated is 4% during the irradiation time used in that measurement. Equation 2 can be used to estimate the absolute mass spectrometer signal intensity. For the unfocused laser fluence (20 pJ.cm-2) and the determined cross section for dissociation, 3.9 X of the initial coverage is dissociated per pulse. Assuming one desorbed CO molecule per dissociated Fe(CO)5, a cos 0 de-
sorption flux (which implies a 10% collection efficiency into the nose cone for our geometry), a monolayer coverage of 1.2 x 1014 molecules/cm2 and an irradiated area of 1.2 cm2, 5.6 X lo8 molecules/pulse enter the nose cone. With the known local pumping time constant (- 15 ms) a pressure rise of 2.3 X lo-" Torr is calculated. This is the right order but smaller than the observed pressure rise of 6 X lo-" Torr and may indicate 2 or 3 C O S are produced from each Fe(CO)5. However, a more accurate calibration of the detection system is necessary before such a conclusion can be reached. 4.2.3. A g ( l l 0 ) . We have previously reported the observation of a fast photolytic decomposition involving Fe(CO)5 on the clean well-ordered surface of Ag( 1lo)." These results are shown in Figure 9. The laser-induced CO+yield from Fe(CO)5 on Ag( 110) is the same as from Fe(CO)5 on A1203,within experimental error, for the three indicated dosages. In addition, the experiments were repeated after subjecting the silver surface to -40 h of sputter-anneal cycles (sputtering, 500 V, 30 pA Ar+, 1 h; annealing 300 OC, 5-15 min), which should eliminate any effects due to surface roughness. No change in CO+ yield was observed following this procedure. We thus believe that Fe(CO)5 undergoes photodecomposition even on the Ag( 110) surface in spite of the fact that Ag is a very fast quencher of electronic excitation. 4.2.4. Summary of Results. Our observations of the photodissociation of Fe(CO)5 on the single crystal surfaces of A1203, Si( loo), and Ag( 1 10) are summarized as follows: UV irradiation of Fe(CO)5 on each surface yields an increase in the CO+ mass signal, which is attributed to the dissociation of Fe(CO)5 by the 337-nm radiation. The observed CO+ yield is the same for Fe(CO)5on Ag(ll0) and A1203at three coverages. (The yield appears slightly higher in the case of Fe(CO)5 on Si( loo), but a different channeltron electron multiplier, whose gain was not calibrated, was employed during the Si( 100) experiments.) At low Fe(CO)5 coverages (
