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Nov 14, 1996 - Chartered Semiconductor Manufacturing Pte. Ltd., 60 Woodlands Industrial Park D, Street 2, Singapore 738406. J. Phys. Chem. , 1996, 100...
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J. Phys. Chem. 1996, 100, 18138-18144

Thermal and Photoinduced Desorption and Decomposition of Fe(CO)5 on Clean and Oxygen-Modified Ru(001) H. H. Huang,* C. S. Sreekanth, C. S. Seet, and G. Q. Xu* Department of Chemistry, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260

L. Chan Chartered Semiconductor Manufacturing Pte. Ltd., 60 Woodlands Industrial Park D, Street 2, Singapore 738406 ReceiVed: April 24, 1996; In Final Form: August 13, 1996X

The thermal and photoinduced desorption and decomposition of Fe(CO)5 on clean and O-covered Ru(001) surfaces were studied. Adsorption of Fe(CO)5 on Ru(001) is associated with a partial decomposition, resulting in the formation of CO and Fe(CO)x fragments. In the thermal desorption spectrum for mass 28, the surfacestabilized decomposition product gives rise to a peak at 270 K, whereas the first and second monolayers of molecular Fe(CO)5 desorb at 190 and 160 K, respectively. The photochemical studies of Fe(CO)5 at monoand multilayered molecular coverages on Ru(001) were carried out by UV irradiation at various wavelengths (290-450 nm). Irradiation at wavelengths > 370 nm resulted in photodesorption, while photodecomposition showed significant contribution at shorter wavelengths. The total cross sections for the photochemical process closely follow the UV absorption spectrum of Fe(CO)5 in the gaseous phase, suggesting that the photoreaction is mainly due to the direct absorption of UV photons by the adsorbed Fe(CO)5 molecule. The photodecomposition yields reactive intermediates that subsequently form Fex(CO)y clusters. These species thermally decompose, desorbing the CO moieties and depositing Fe atoms on the surface. Dissociative adsorption of Fe(CO)5 has also been observed on O-covered Ru(001). O adatoms create inhomogeneity in the adsorption sites for Fe(CO)5, as seen from the broadened desorption peak of the first molecularly adsorbed layer of Fe(CO)5. In contrast to the clean surface, irradiation of Fe(CO)5 adsorbed on O/Ru(001) at 290 nm produced relatively lower yields of photodecomposition and a higher extent of photodesorption attributable to the more effective quenching of electronically excited Fe(CO)5 by the O-covered surface.

1. Introduction Surface photochemistry has been developing rapidly due to its fundamental importance and technological relevance.1-4 Selective metal deposition on surfaces has important applications in the areas of heterogeneous catalysis and thin film technology. In this context metal carbonyls have drawn considerable interest as convenient precursors due to their high vapor pressures and the ease with which they can be decomposed. The decomposition can be effected thermally by heating the substrate, photochemically by UV irradiation, and also by energetic electrons. The processes involved in thin film deposition by thermal and photodecomposition of metal carbonyls on metal and semiconductor surfaces have been extensively studied by many workers.5-16 The use of metal carbonyls as precursors in heterogeneous catalysis has also been probed by their interactions with metals and metal oxides.17-20 The metal carbonyls are particularly photosensitive with respect to dissociative loss of CO. This observed photoactivity has been attributed to the presence of unbound or weakly bound excited states at energies corresponding to UV radiation.21 Though removal of three to five CO moieties from the Fe(CO)5 in the vapor phase with ultraviolet light is energetically possible, the actual pattern of fragmentation has been proposed to proceed by the formation of Fe(CO)4 as the primary photoproduct followed by a rapid sequential loss of the remaining CO moieties.22,23 In contrast, irradiated Fe(CO)5 in inert matrices loses one CO per photon, yielding Fe(CO)4 or a stable complex X

Abstract published in AdVance ACS Abstracts, October 15, 1996.

S0022-3654(96)01174-4 CCC: $12.00

of Fe(CO)4 with the matrix atom or molecule.24,25 Irradiation of higher concentrations of Fe(CO)5 produced clusters such as Fe2(CO)9 and Fe3(CO)12.26 Photolysis of silica-adsorbed Fe(CO)5 and Fe(CO)5 sorbed in polymer films also resulted in a similar cluster formation.27,28 Thermal and photochemical decomposition of Fe(CO)5 has also been studied on several single-crystal surfaces. At liquid nitrogen temperature, Fe(CO)5 adsorbs molecularly in both mono- and multilayer regimes on Si(111)-7×7, Ag(111), Ag(110), Si(100), and Al2O3.8,9,11,14,29 Dissociative adsorption has been observed on Pt(111) and Ni(100).5,6,30,31 The photochemical studies for Fe(CO)5 on Ag(110), Si(100), and Al2O3 using a 337 nm laser14 suggested that the photoreaction involves a direct absorption of UV light by the adsorbed Fe(CO)5 molecule. Henderson et al.29 proposed that irradiation at 256 and 365 nm converted Fe(CO)5 into Fex(CO)y clusters on the Ag(111) surface. Irradiation of Fe(CO)5 adsorbed on Si(111) and Si(100) at 257 nm also forms intermediates such as Fe(CO)x; however, cluster formation was not noted in this case.9,11 This work is concerned with the thermal and photoinduced desorption and decomposition of Fe(CO)5, adsorbed on the clean as well as the O-adatom-covered Ru(001) surfaces. The previous photochemical studies of Fe(CO)5 adsorbed on various substrate surfaces were carried out at several discrete wavelengths. The objective of the present work is to study the dependence of the photochemistry of adsorbed Fe(CO)5 on the UV wavelength. This is achieved by the use of a continuous light source in the range 290-450 nm. From our studies on O-adatom covered Ru(001), we also hope to understand the © 1996 American Chemical Society

Desorption and Decomposition of Fe(CO)5 on Ru(001)

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influence of the chemical modification of the surface on the decomposition process. 2. Experimental Section The experiments were carried out in a stainless steel ultra high-vacuum (UHV) system pumped by a combination of ion and titanium sublimation pumps. The ultimate vacuum obtained after bakeout was around 1.0 × 10-10 Torr. The system was equipped with low-energy electron diffraction (LEED) and quadrupole mass spectrometer (QMS) probes. The mass spectrometer was covered by a glass shield with a 3 mm opening in the front to reduce any possible contributions from the filaments used for sample heating in TDS (thermal desorption spectrometry) measurements. The Ru(001) crystal was mounted on a X-Y-Z rotary manipulator by two 0.5 mm diameter tungsten rods that were spot-welded to the side edges of the crystal. The temperature of the crystal was measured by a 0.003 in. W5%Re/ W26%Re thermocouple spot-welded to the upper edge of the crystal. All gases used in the experiments were from Matheson and were of 99.99% purity. Background dosing was used for the adsorption of CO and O2. The crystal was cleaned by Ar ion sputtering and repeated cycles of oxygen adsorption and desorption. The cleanliness of the surface was verified by the thermal desorption features of CO32 and confirmed by a sharp (1×1) LEED pattern. All the thermal desorptions were conducted by resistively heating the crystal with 15 A of current. The oxygen-covered surface was prepared by first exposing the crystal to 10 langmuirs of the gas at 90 K and subsequently annealing the crystal to 600 K. Iron pentacarbonyl with a purity of 99% was obtained from Merck. Fe(CO)5 was dosed directly onto the surface by means of a microcapillary array doser. In our experiments, efforts were made to ensure the purity of Fe(CO)5. The glass storage for Fe(CO)5 connected to the leak valve was covered with aluminum foil, preventing photoinduced decomposition. Before dosing, Fe(CO)5 in the storage was purified by repeated cycles of freeze-pump-thaw to remove the residual CO content. The thermal decomposition of Fe(CO)5 on the stainless steel wall of the doser was difficult to completely eliminate. We used oxygen every morning to passivate the doser, which effectively reduced the extent of decomposition in the doser.29b During the dosing process, the crystal was maintained at 90 K and the background pressure increased by about (1.0-2.5) × 10-10 Torr. The Fe(CO)5 exposure was calibrated by TDS using the background dosing. A monolayer coverage of molecular Fe(CO)5 was defined at the completion of the first Fe(CO)5 desorption peak at 190 K. It was obtained by first adsorbing Fe(CO)5 in excess, followed by annealing the crystal up to 158 K. To avoid or minimize the dissociation caused by the stray electrons from the ionizer of the mass spectrometer, the sample was separated from the ionizer by about 15 cm and the MS was covered with a glass piece that has a 5 cm long and 3 mm diameter tube in the front. In addition, the MS filaments were kept off during the dosing process, minimizing the possible exposure to the stray electrons. After taking these precautions, we carried out a detailed study on Mo(CO)6 to examine if the stray electrons have any appreciable effects on TDS. On COsaturated Ru(001), Mo(CO)6 adsorbs molecularly. The COdesorption feature resulting from the decomposition of surface fragments Mo(CO)x (x < 6) was not observed upon the adsorption of Mo(CO)6, but was readily detected after UV irradiation. This result clearly suggests that stray electrons did not cause significant dissociation in our configuration. The photon source employed consisted of a 1000 W Xe arc lamp (Oriel Optics) in combination with a grating monochro-

Figure 1. Mass 28 TD spectra: (a) 18 langmuir CO exposure and b-e obtained after various Fe(CO)5 exposures on a clean Ru(001) surface. The corresponding Fe(CO)5 exposures are (b) 0.24, (c) 0.32, (d) 0.95, and (e) 1.26. (f) The mass 196 TD spectrum after an exposure of 1.7 langmuir Fe(CO)5 onto clean Ru(001).

mator. The incident power density of the radiation was measured by using the Melles Griot 13 PEM 001 power/energy meter, and the bandwidth of radiation was kept at 20 nm. The temperature rise of the crystal during irradiation was negligible, which rules out the possibility of thermal effects. 3. Results and Discussion 3.1. Fe(CO)5 on Clean Ru(001). The thermal desorption of Fe(CO)5 was monitored for masses corresponding to Fe(CO)5 and various possible fragments, such as CO, Fe, and Fe(CO)x (x ) 1-4). Figure 1a shows the TDS of CO from Ru(001), at the saturation exposure (18 langmuirs) which is consistent with published data.32,33 Figure 1b-e shows the thermal desorption spectra monitored for mass 28 at different coverages of Fe(CO)5 on the Ru(001) surface. At a low exposure of 0.24 langmuir of Fe(CO)5 the TDS shows one peak, g, in the lowtemperature region at 270 K and two peaks at 405 and 460 K (Figure 1b). The two peaks at 405 and 460 K, by comparison with Figure 1a, can be attributed to the desorption of CO, a product of Fe(CO)5 decomposition, from the Ru(001) surface. The possible contribution from the coadsorption of CO formed in the doser cannot be ruled out. The CO desorption features reach saturation at an exposure of 0.32 langmuir of Fe(CO)5 (Figure 1c). However, the saturation coverage of CO from Figure 1c is 0.45 ML (monolayer), which is less than a value of 0.68 ML for the saturation coverage of CO on the clean Ru(001) surface.33 Also to be noted at this Fe(CO)5 exposure are the saturation of the γ peak at 270 K and the appearance of a new peak, β, at 190 K. The β peak grows with the increasing exposure of Fe(CO)5. However, the peak temperature remains independent of exposure, suggesting a first-order desorption kinetics. At an exposure of 1.26 langmuirs, the β peak saturates

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Figure 2. TD spectra for masses (a) 56, (b) 84, and (c) 168 at 1 ML coverage of molecular Fe(CO)5 on a clean Ru(001) surface.

and an additional peak, R, appears at 160 K. We assign the R and β peaks to the second and first molecularly adsorbed layers of Fe(CO)5, respectively. The first molecular layer, due to its stronger interaction with the surface, desorbs at a higher temperature than the second layer. The TD spectra of Fe(CO)5 for mass 196 (Figure 1f) also showed only two peaks at 160 and 190 K, confirming that molecular desorption occurs at these temperatures. We have estimated the desorption energies, assuming first-order desorption kinetics and a pre-exponential factor of 1013 s-1.34 The desorption energies thus obtained (for a heating rate of about 3 K/s) are 48.1 and 40.3 kJ/mol for the 190 and 160 K peaks, respectively. These values are comparable to the heat of sublimation (37.6 kJ/mol) of Fe(CO)5.8 The second molecular layer experiences predominantly the intermolecular forces between Fe(CO)5 molecules. Therefore, its desorption energy is closer to the heat of sublimation. The desorption temperature of 190 K, observed in this work for the first molecular layer of Fe(CO)5, is higher than the corresponding values of 181 and 162 K for Fe(CO)5 adsorbed on Ag(111)29 and Si(111),9 respectively. This suggests a stronger interaction of Fe(CO)5 with the Ru(001) surface. To understand the nature of the 270 K peak we have recorded the TD spectra for mass numbers 56, 84, and 168 (Figure 2) at 1 ML coverage of molecular Fe(CO)5 on Ru(001). These masses correspond to the cracking fragments Fe+, Fe(CO)+, and Fe(CO)4+, respectively. The spectra show two low-temperature peaks at 190 and 270 K. The peak at 190 K also appears in the TD spectrum of mass 196, corresponding to Fe(CO)5+ (Figure 1f), where it was assigned to the desorption of the first molecularly adsorbed layer of Fe(CO)5 on Ru(001). Hence, this peak (190 K) in Figure 2a-c can be attributed to the cracking of the Fe(CO)5 in the ionization region of the mass spectrometer. However, the 270 K peak in Figure 1b-e could be detected for masses up to a highest value of 168, corresponding to Fe(CO)4+. The TDS for mass 196 does not yield any detectable

Huang et al. signal at this temperature. We are unable to determine the exact nature of this adsorbed species. It, nevertheless, suggests the presence of a metal-containing decomposition product that may either desorb or decompose at this temperature, yielding Fe(CO)4 as a fragment. This is consistent with the observation (by IRAS) of iron carbonyl intermediates such as Fe(CO)x (x ) 4, 3) on Pt(111) and Ni(100) single-crystal surfaces, which also adsorb Fe(CO)5 dissociatively.30,31 On Pt(111) surface,30 detailed studies of adsorption and decomposition of Fe(CO)5 were carried out using reflection-absorption infrared spectroscopy (RAIRS). The results showed that iron pentacarbonyl adsorbs on Pt(111) both molecularly and dissociatively at 110 K. It was proposed that Fe(CO)5 changes its configuration from the bipyramid (stable in gaseous phase) to a square planar structure upon chemisorption. In addition, the sequential decomposition of Fe(CO)5 to form Fe(CO)4 and Fe(CO)3 surface intermediates was observed. Upon heating of the substrate these decomposed moieties recombine with coadsorbed CO to form Fe(CO)5, which then desorbs. However, this recombinative desorption of Fe(CO)5 was not observed on Ni(100).31 On Ni(100),31 the thermal decomposition of chemisorbed Fe(CO)5 was clearly demonstrated in their CO TPD and XPS studies. The CO TPD peaks at 180 and 450 K were related to the desorptions of molecular Fe(CO)5 and CO from Ni sites, respectively. A new feature of CO desorption at 290 K was attributed to the thermal decomposition of the Fe(CO)x (x < 5) surface intermediates. A similar CO TDS pattern was observed in our experiments on Fe(CO)5/Ru(001). In addition to the CO TDS peaks assigned to molecular desorption of Fe(CO)5 at 190 K and CO desorption from the Ru sites at 405 and 460 K, the 270 K peak, analogous to the 290 K peak in Fe(CO)5/Ni(100), was shown to originate from the surface intermediates Fex(CO)y formed after initial thermal decomposition of Fe(CO)5. A new CO-desorption feature at 330 K was also observed in the photoinduced and electron-induced dissociation of Fe(CO)5 on Ag(111), attributable to the formation of Fex(CO)y clusters upon electron or photon irradiation.29a The fact of observing the 270 K peak together with the CO desorption at 405 and 460 K from Ru sites in our experiments implies the partial decomposition of Fe(CO)5 on Ru(001). Furthermore, the absence of molecular desorption of Fe(CO)5 at exposures less than 0.24 langmuir suggests that the decomposition is the dominating channel at low coverages. A similar predominance of decomposition has also been reported for adsorbed Fe(CO)5 on Pt(111) and Ni(100) single-crystal surfaces at submonolayer coverages.30,31 The observation of dissociative adsorption of Fe(CO)5 on Ru(001) is interesting since a nondissociative adsorption of Fe(CO)5 has been noticed on Si(100), Ag(110), and also Al2O3.14,35 Similar nondissociative adsorption has also been reported for Mo(CO)6 on Si(111), Ag(111), and graphite basal plane.9,36,37 This has been explained by viewing the thermal decomposition on the surface as a competition between the metal atom and the substrate in CO bonding.38 Si(111), Ag(111), and the graphite basal plane are inert and do not adsorb CO even at liquid nitrogen temperature. The dissociation energy of Mo(CO)6 is 40.5 kcal/mol.39 Thus the dissociation of Mo(CO)6 on these surfaces tends to be thermally unfavorable. However, the Rh(100) surface, which interacts strongly with CO and desorbs CO at 500 K, adsorbs Mo(CO)6 dissociatively.10 Fe(CO)5 also has a similar dissociation energy (41.5 kcal/mol).39 The Ru(001) surface interacts strongly with CO, as evidenced by the high desorption temperatures (406 and 460 K). Consequently, the nondissociative adsorption of Fe(CO)5 on inert surfaces such as Si(100), Ag(110), and Al2O3, and a partial decomposition on Ru(001), appears to be reasonable. This is

Desorption and Decomposition of Fe(CO)5 on Ru(001)

J. Phys. Chem., Vol. 100, No. 46, 1996 18141

Figure 4. Plot of relative peak areas at 270 K (a) and 190 K (b), respectively (in Figure 3), normalized to photon intensity and the peak areas without irradiation, as a function of wavelength of the UV radiation.

Figure 3. Mass 28 TD spectra from 1 ML coverage of molecular Fe(CO)5 on Ru(001) after 10 min of irradiation at (a) 290 nm, 0.76 mW/ cm2; (b) 310 nm, 0.99 mW/cm2; (c) 330 nm, 1.17 mW/cm2; (d) 350 nm, 1.33 mW/cm2; (e) 370 nm, 1.31 mW/cm2; (f) 390 nm, 1.40 mW/ cm2; (g) 410 nm, 1.32 mW/cm2; and (h) 450 nm, 1.13 mW/cm2. (i) The mass 28 TD spectrum prior to irradiation.

also in agreement with the dissociative adsorption of Fe(CO)5 on the Ni(100) surface, which interacts strongly with CO and desorbs at 450 K.31 The photochemical studies of Fe(CO)5 on Ru(001) were carried out for one- and two-monolayer (ML) coverages of molecular Fe(CO)5. A monolayer (ML) is defined as the completion of the 190 K peak and was obtained by dosing the surface with 1.4 langmuirs of Fe(CO)5 and subsequently heating up to 158 K to remove the excess. This process ensures a fixed coverage before each UV irradiation. The crystal was then cooled to 90 K. The UV irradiation was carried out in the wavelength range 290-450 nm. The temperature of the substrate did not show any observable increase during the irradiation process, thereby excluding any contribution from the thermally induced decomposition. Figure 3 shows the TD spectra for mass 28 recorded after irradiating 1 ML of Fe(CO)5 on Ru(001) for a period of 10 min. Figure 3i shows the TD spectrum prior to UV irradiation. Irradiation in the wavelength range 450-370 nm results in some losses in the area of the 190 K peak, but no significant changes were noticed in the peak at 270 K attributed to the partially dissociated molecule. At wavelengths < 370 nm, adsorbed Fe(CO)5 undergoes extensive photochemical reaction as evidenced by (1) a sharp decrease in the area of the 190 K peak, (2) a significant increase in the area of the 270 K peak, and (3) appearance of a broad peak at 700 K. These effects were most pronounced at 290 nm radiation. The presence of the 270 K peak in the TDS results for Fe(CO)5 on clean Ru(001) was observed for mass 28 (Figure 1) and masses up to 168, Fe(CO)4+ (Figure 2). Irradiation at 290 nm resulted in a significant increase in the signal for mass 28 in the TD spectra, whereas the signal for mass 168 remained unchanged. It is possible that the photochemical reaction has

resulted in the formation of clusters Fex(CO)y in high yields. These clusters decompose thermally, depositing Fe atoms on the surface, yielding only CO as a detectable fragment in the TD spectra. Such a cluster formation has been suggested for photochemical studies of silica-adsorbed Fe(CO)527 and Fe(CO)5 in inert matrices.24,26 Formation of such clusters upon UV irradiation has also been invoked for Fe(CO)5 adsorbed on Ag(111).29 On this basis, we attribute the broad feature at 700 K in Figure 3 to the recombinative desorption of dissociatively adsorbed CO on Ru(001) partially covered by Fe. This is in agreement with the characteristics of CO desorption from Fecovered Ru(001).40 Figure 4 shows the increase in the 270 K peak area as well as the decrease in the 190 K peak area, normalized to photon intensity, as a function of wavelength. The 270 K peak seems to be unaffected at wavelengths > 370 nm. However, appreciable decrease in the intensity of the 190 K peak extends up to 450 nm. This clearly suggests that irradiation between 370-450 nm results only in photodesorption. However, at wavelengths < 370 nm, the intensity of the peak at 270 K rapidly increases with decreasing wavelength, which shows that the photodecomposition is significant in this wavelength region. We have also estimated the total cross sections, Q, for the photochemical process from the loss of the 190 K peak in Figure 3 using the relation

ln(I/I0) ) -nQt where I0 and I are obtained from the area of the 190 K peak before and after irradiation, and n and t represent the photon flux and the irradiation time. Figure 5 shows the wavelength dependence for the total photoreaction cross sections obtained from the above expression as a function of wavelength (the solid line). The gas phase UV-absorption data from refs 23 and 41 were also incorporated into the plot. Our cross section values of 2.0 × 10-18, 1.0 × 10-18, and 9.0 × 10-19 cm2 at 290, 310, and 330 nm, respectively, are within a factor of 2 compared with the gas phase values from ref 41. Our experimental results (solid line in Figure 5) closely follow the gas phase UVabsorption results of Fe(CO)5, suggesting that the desorption

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Huang et al.

Figure 5. Semilog plot of total cross sections for the photochemical process, estimated from the loss of the 190 K peak area (from Figure 3) as a function of wavelength (the solid line). The dashed and dotted lines show the UV-absorption cross section from refs 23 (cross symbol) and 41 (square symbol), respectively. The dotted line with square symbol was reproduced by interpolation of the plot shown in ref 41.

and decomposition of Fe(CO)5 on Ru(001) is a resonant process via direct electronic excitation of Fe(CO)5. The absorption at 330 nm has been attributed to the transitions within the ligand field of the metal atom, while the absorption at 290 nm is attributed to the metal to ligand charge-transfer transitions.3,14,21 The total cross section obtained from our experimental data of 9.0 × 10-19 cm2 at 330 nm is close to 1.5 × 10-18 cm2 for 337 nm radiation on Al2O3.14 Swanson et al.8 have reported a cross section of 1.2 × 10-16 cm2 on Si(111) at 249 nm, whereas Henderson et al.29 reported the photodissociation cross sections for 1 ML and 5 ML Fe(CO)5 as 9.2 × 10-18 and 9.4 × 10-18 cm2, respectively, on Ag(111) at 256 nm. Since these radiation wavelengths were not available in our experiments, a direct comparison cannot be made. Figure 6 shows the TDS of Fe(CO)5 on Ru(001) at a 2.8 ML molecular coverage, after irradiation at 290 nm for various lengths of time. Figure 6a shows the TDS prior to irradiation. Upon irradiation the intensities of both 160 and 190 K peaks decrease simultaneously. Concurrently a high-temperature shoulder on the 270 K peak appears. This shoulder grows into a peak at 300 K, which continues to increase in its intensity with irradiation time. It is noted that the peak at 270 K shows a decreasing trend at longer UV exposures. It may be possible that the photodecomposition of Fe(CO)5 in both molecular layers results in the formation of a three-dimensional complex on the Ru(001) surface, which decomposes at 300 K, giving out CO. Celli et al. studied the photochemistry of Fe(CO)5 adsorbed on Ag(110), Si(100), and Al2O3.14 On Ag(110), CO desorption was detected while irradiating Fe(CO)5/Ag(110) with 337nm laser light. The photofragments Fe(CO)x (x < 5) were assumed to be retained on the surface. It was found that the CO desorption yield was almost the same for all three different substrate surfaces (Ag(110), Si(100), and Al2O3). Their results are consistent with the direct UV absorption by the adsorbed Fe(CO)5 and further suggested that the dissociation process of excited Fe(CO)5(ads) is much faster than the surface quenching. Henderson et al.29 studied the photodissociation of Fe(CO)5 on Ag(111) at 90 K using 256 and 365 nm radiations. After irradiation, the intensity of the CO desorption at 330 K increased significantly, which resulted from the thermal decomposition of the Fex(CO)y clusters formed in the photodissociation of Fe-

Figure 6. Mass 28 TD spectra for 2.8 ML coverage of molecular Fe(CO)5 adsorbed on Ru(001) after irradiating at 290 nm for (a) 0, (b) 1, (c) 3, (d) 5, (e) 7, and (f) 10 min.

(CO)5. Their results on the photodissociation of Fe(CO)5 multilayers separated from Ag(111) using an inert spacer layer (multilayers of n-decane) ruled out the possibility of a hotelectron tunneling process. The photodissociation of adsorbed Fe(CO)5 also has been studied on semiconductor surfaces, such as Si(100)11,45 and Si(111)-7×7.8,9 After UV irradiation on Fe(CO)5/Si(100) at 77 K, Jackman et al.11 found appreciable C and O retained on the surface up to 300 K, at which temperature molecular Fe(CO)5 would have completely desorbed. Since Fe(CO)5 does not thermally decompose on Si(100), the C and O AES signals detected at 300 K after UV irradiation were attributed to the photofragments Fe(CO)x (x < 5) retained by the surface. Annealing the surface above 300 K caused a reduction in C and O AES signals due to the thermal decomposition of the Fe(CO)x fragments. The existence of the photofragments Fe(CO)x was further confirmed by Bartosch et al.45 However, the results obtained on Si(111)-7×7 are rather inconsistent, especially regarding the existence of the photofragments, Fe(CO)x. After irradiating Fe(CO)5/Si(111)-7×7 with 193, 249, 308, and 337 nm, Fe(CO)x was not detected in the IR studies.8 Only CO desorption was detected during UV irradiation. A postradiation TPD study did not show any new CO desorption features attributable to the thermal decomposition of Fe(CO)x. A two-photon process was proposed to account for the complete dissociation of all CO ligands from Fe(CO)5, resulting in clean Fe deposited on the surface. In contrast, Gluck et al.9 observed Fe(CO)x fragments after irradiating Fe(CO)5/ Si(111) at 90 K with 257 nm laser light. We have measured the photoreaction cross sections of Fe(CO)5 on Ru(001) by continuously varying the wavelength from 290 to 450 nm. Since all the studies reported in the literature on the surface photochemistry of Fe(CO)5 were carried out on noble metals14,29 and semiconductors,8,9,11,45 our results confirmed the mechanism of direct UV absorption by adsorbed Fe(CO)5 occurring on a transition metal surface, Ru(001). The

Desorption and Decomposition of Fe(CO)5 on Ru(001)

Figure 7. Mass 28 TD spectra: (a) after an 18 langmuir exposure of CO; b-d after 0.13, 0.3, and 0.7 langmuir of Fe(CO)5 onto an ordered O-presaturated Ru(001) surface.

existence of the surface intermediates after irradiation was also detected. Our work interestingly shows the difference in the cutoff wavelengths for photodesorption and photodissociation. 3.2. Fe(CO)5 on O-Covered Ru(001). The oxygensaturated Ru(001) surface was prepared by exposing the Ru(001) surface to more than 10 langmuirs of oxygen at 100 K and subsequently annealing it up to 600 K to obtain an ordered overlayer. Oxygen adatom, due to its electronegativity and steric effects, destabilizes and also inhibits the CO adsorption. A TD spectrum of CO (mass 28) at saturation coverage from O-covered Ru(001) (shown in Figure 7a) is consistent with the published data.42 The peaks at 380 and 425 K in Figure 7a can be attributed to the linearly bonded CO, and the low-temperature features at 284 K and in the region 160-260 K are due to the desorption of bridge-adsorbed CO.42 The TD spectra of mass 28 after various Fe(CO)5 exposures to the O-saturated Ru(001) are presented in Figure 7b-d. Up to an exposure of 0.13 langmuir, the only feature observed is a doublet around 400 K, which, by comparison with Figure 7a, is assigned to the desorption of CO formed by decomposition of the metal carbonyl. This suggests that the decomposition of Fe(CO)5 is not significantly blocked by O adatoms. At an exposure of 0.3 langmuir, a broad peak appears at 190 K due to the desorption of the first molecularly adsorbed monolayer of Fe(CO)5, along with a broad feature in the temperature range 250-300 K. We assign this broad band at 250-300 K to the metal-containing decomposition fragment. At higher exposures (0.7 langmuir), the peak at 190 K saturates and another sharp feature appears at 160 K due to the second molecularly adsorbed monolayer. Compared with the desorption characteristics for Fe(CO)5 adsorbed on clean Ru(001) (Figure 1), the peak at 190 K is broadened considerably, suggesting inhomogeneity in the binding sites for Fe(CO)5 resulting from the presence of decomposed products as well as oxygen adatoms. In addition, the peak at 270 K, originally observed on the clean Ru(001) and attributed to the presence of a metal-containing decomposition fragment, develops into a band in the region 250-300 K

J. Phys. Chem., Vol. 100, No. 46, 1996 18143

Figure 8. Mass 28 TD spectra for 1 ML coverage of molecular Fe(CO)5 on O/Ru(001) after irradiating at 290 nm for (a) 0, (b) 1, (c) 3, (d) 5, (e) 7, and (f) 10 min.

on O-covered Ru(001). However, the features due to the desorption of bridge-bonded CO seen in Figure 7a were not observed in the decomposition of Fe(CO)5 on the O-saturated Ru(001). Figure 8 shows the TDS for mass 28 of 1 ML of molecular Fe(CO)5 on O/Ru(001) after irradiation at 290 nm. The 1 ML coverage of molecular Fe(CO)5 on O/Ru(001) was obtained in a similar way as described in the case of the clean surface. The striking features in Figure 8 are the decrease in the intensity of the 190 K peak, the increase in the intensity of the broad feature in the range 225-300 K, and the appearance of a new peak at 340 K with increasing exposure time. The total cross section for the photochemical process obtained from the decrease in the 190 K peak yields a value of 1.8 × 10-18 cm2. This compares well with the total cross section obtained for the clean surface. However, the increment in the broad band (225-300 K) is relatively small, indicating a higher photodesorption yield than that observed in the case of the clean surface. This has probably resulted from the more efficient quenching of photoexcited Fe(CO)5 by the O-covered metal surface. The peak at 340 K, which appears only at longer irradiation times (g3 min), is probably due to the formation of more than one type of photodecomposed fragments. Irradiation at 290 nm also influences the relative intensities of the peaks at 380 and 425 K. Prior to irradiation the 380 K peak is more prominent (Figure 8a). After the exposures to 290 nm for varying lengths of time, the 380 K peak decreases in area with increasing exposure time, whereas the 425 K peak appears to be unaffected by the irradiation. For exposure longer than 5 min, the 380 K peak becomes less intense than the 425 K peak. However, we are unable to interpret this change in the relative intensities of 380 and 425 K peaks with irradiation. Previous studies have shown that chemical modification of the substrate surfaces has significant effects on the photochemistry of adsorbed metal carbonyls. On CO-saturated Rh(100),

18144 J. Phys. Chem., Vol. 100, No. 46, 1996 molecular Mo(CO)6 underwent photodissociation more readily than Mo(CO)6 on a clean surface, which is probably due to the lower quenching efficiency of excited Mo(CO)6 on a COsaturated surface.43,44 On K-precovered Cu(111) and Si(111)7×7 surfaces, the observed photodissociation of Mo(CO)6 was mainly attributable to the photogenerated hot-electron tunneling from the substrate to the adsorbed Mo(CO)6 molecules.46 The adsorption of potassium decreased the work function of the surface, thus allowing the photogenerated hot-electrons to tunnel through the adsorbate-substrate barrier to adsorbed Mo(CO)6, forming negative ions, which might lead to dissociation. In our experiment, the preadsorption of oxygen on Ru(001) enhanced the quenching efficiency of the excited Fe(CO)5, resulting in a lower extent of photodissociation. 4. Conclusions We have carried out studies on the adsorption of Fe(CO)5 on clean as well as O-covered Ru(001) in mono- and multilayer regimes. On both surfaces, Fe(CO)5 adsorbs dissociatively as well as molecularly. In the TD spectra of Fe(CO)5 adsorbed on O/Ru(001), the desorption feature of the first monolayer of molecularly adsorbed Fe(CO)5 is broadened due to inhomogeneity in the adsorption sites. We have also studied the photochemistry of Fe(CO)5 on both the clean and O-covered Ru(001) surfaces using a continuous light source in the wavelength range 290-450 nm. Under the conditions of 1 ML coverage of molecular Fe(CO)5 on Ru(001), only photodesorption was observed at wavelengths > 370 nm of radiation. However, photodecomposition contributes significantly at wavelengths < 370 nm, yielding surface-stabilized intermediates, Fe(CO)x, which undergo clustering. The clusters thus formed decompose thermally to CO and metal atoms. The total cross sections for the photochemical process closely follow the UVabsorption results of gaseous Fe(CO)5, suggesting that the decomposition of the adsorbed Fe(CO)5 takes place by a direct absorption mechanism. Irradiation of Fe(CO)5 adsorbed in multilayers on Ru(001) at 290 nm resulted in the formation of a possible three-dimensional complex. The TD spectra after irradiation of Fe(CO)5 adsorbed on O-covered Ru(001) surface at 290 nm suggest relatively lower photodecomposition but higher photodesorption yields in comparison to the clean Ru(001). This may be attributed to the more effective quenching of excited electronic states of adsorbed Fe(CO)5 on O/Ru(001) than on the clean surface. Acknowledgment. This work was supported by the National University of Singapore under Grant No. RP910681. C.S.S. would like to thank NSTB of Singapore for the award of a fellowship. References and Notes (1) Zhu, X.-Y. Annu. ReV. Phys. Chem. 1994, 45, 113. (2) Ho, W. Surf. Sci. 1994, 299, 996.

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