- Email: [email protected]
Observation of two reaction pathways for water formation from hydroxyl radical covered Pt(111)
1 May 1998
Chemical Physics Letters 287 Ž1998. 475–479
Observation of two reaction pathways for water formation from hydroxyl radical covered Ptž 111/ Kyle M. Backstrand, Michael A. Weibel, Toby D. Hain, Thomas J. Curtiss
)
UniÕersity of Utah, Salt Lake City, UT 84112, USA Received 20 November 1997; in final form 20 November 1997
Abstract Intense molecular beams of hydroxyl radicals produced in a supersonic corona discharge source were focused and state-selected by an electrostatic hexapole. Thermal desorption spectra from a PtŽ111. surface dosed with the resulting pure beam of OD radicals showed features corresponding to D 2 O and O 2 desorption. Two distinct types of adsorbed OD were observed depending on the dosing temperature. Dosing at 215 K generated a low temperature Ž215 K. D 2 O desorption feature. Dosing at 275 K generated a high temperature Ž400 K. D 2 O desorption feature. Possible origins of these two features are discussed. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Surface reactions catalyzed by transition metals typically proceed through a series of elementary reaction steps occurring between radicals adsorbed on the surface. Ideally one measures the rate constant for a particular elementary step by monitoring product yield as a function of reactant concentration. Measuring rate constants as a function of surface temperature allows Arrhenius activation energies and pre-exponential factors to be determined leading to greater insights into the reaction mechanism. Investigations of such catalytic systems have concentrated on elementary steps involving stable gas phase reactants or the desorption behavior of the products because these are the most easily studied. Consider, for example, the water reaction on PtŽ111., one of the most well-studied prototypical catalysis systems )
Corresponding author: E-mail: [email protected]
w1–5x. Experimentally it is easy to control the flux of O 2 , H 2 , or H 2 O striking the PtŽ111. surface as well as the surface temperature. Under conventional low pressure, UHV conditions molecular hydrogen and molecular oxygen dissociatively adsorb on the surface. A sequential addition of two hydrogen atoms to an oxygen atom produces a water molecule that quickly desorbs. It is also possible for two hydroxyl radicals to combine to produce a water molecule that quickly desorbs and an adsorbed oxygen atom. The following elementary steps are believed to play an important role in the waterrPtŽ111. reaction system: H Ž2g . ° 2H Ža. Ž 1. O Ž2g . ° 2O Ža. H Ža. q O Ža. ° OH Ža. OH Ža. q H Ža. ™ H 2 O Ža. 2OH Ža. ° H 2 O Ža. q O Ža.
Ž 2. Ž 3. Ž 4. Ž 5. Ž 6.
H 2 O Ža. ™ H 2 O Ž g . Rate constants for reaction steps involving adsorbed
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 1 8 3 - 3
476
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
OH radicals Že.g. steps 4 and 5. are difficult to measure directly because it is experimentally difficult to independently control the OH surface coverage. Estimates for these rate constants have been obtained by maintaining the O 2 and H 2 flux and surface temperature at conditions believed to assure the dominance of the reaction of interest. For example, Anton and Cadogan recently reported rate constants for reaction Ž5. measured using modulated beam relaxation spectroscopy ŽMBRS. w6,7x. They dosed the surface with D 2 while scattering a modulated beam of O 2 from the surface and analyzed the scattered D 2 O waveform to extract rate constants. By maintaining a low total coverage with the hydrogen coverage significantly exceeding that of oxygen, Anton and Cadogan were able to operate under conditions where reaction Ž5. was thought to dominate reaction Ž4. in water production. A much more direct approach is to control the OH coverage directly by dosing the surface with OH radicals. Hain et al. recently reported the development of a pure source of hydroxyl radicals suitable for this purpose w8,9x. Here we report the first results from experiments employing this unique source to dose a PtŽ111. surface with OD radicals.
2. Experimental Hydroxyl radicals were produced from He saturated with water vapor in a supersonic corona discharge beam source w10x. An electrostatic hexapole lens focused and state-selected w11,12x the hydroxyl radical beam. A beam-stop located at the midway point of the hexapole blocked from the beam any hydrogen or oxygen atoms produced in the discharge as well as undissociated water. The measured focusing spectrum was similar to those reported previously w8x with the peak of the < J V M J : s < 32 32 32 : rotational state appearing at a hexapole voltage of V0 s 8.7 kV. With V0 fixed at 8.7 kV the beam exiting the hexapole had a OD flux density of J s 8 = 10 11 OD radicals cmy2 sy1 with greater than 99% < 32 32 32 : state purity. The beam passed through a field free region of 12.4 cm before striking a single crystal PtŽ111. target. Typically, the M J rotational quantum number is randomized in the absence of an orienting field w13x, so that the rotational state distri-
bution of radicals reaching the Pt target contained equal numbers of M J s " 32 , " 12 . The Pt crystal was purchased from Monocrystals Company, polished and cut to within 18 of the Ž111. plane as determined by Laue X-ray diffraction. The crystal was spot-welded to Pt posts that were attached to a liquid nitrogen cold finger mounted in an UHV chamber with a base pressure of PUH V s 1 = 10y9 Torr. Temperatures were measured with a PtrPty 10% Rh thermocouple spot welded to the sample. The OD beam impinged on the sample with an angle of incidence of 708. Repeated cycles of exposure to O 2 Ž P UHV s 1 = 10y7 Torr. at T s 1125 K followed by Ar ion sputtering and annealing at T s 1375 K were used to clean the PtŽ111. surface. This procedure has been shown previously to produce a clean, well-ordered PtŽ111. surface w14x. No surface diagnostic tools were available to confirm the surface quality. However, O 2 thermal desorption spectra were comparable to those seen previously w15,16x and have been shown to be sensitive to the presence of common surface contaminants. In this Letter we report the results from two thermal desorption experiments. These were measured with a differentially pumped quadrupole mass spectrometer ŽQMS.. The axis of the mass spectrometer ionizer was oriented perpendicular to the OD molecular beam axis and approximately 208 from the surface normal. An elliptical spot approximately 1.5 = 4.4 mm was exposed to the OD beam for 5–20 min corresponding to a dose of approximately 0.5–2 L Ž1 L ' 4.8 = 10 14 radicals cmy2 .. The sample was then translated to a position locating the exposed spot at approximately 2 mm from a 3-mm-diameter aperture separating the UHV chamber from the QMS chamber. The sample was then resistively heated and the desorption flux was monitored with the QMS.
3. Results Fig. 1 shows a thermal desorption spectrum of the D 2 Oq signal from the QMS taken after the sample was exposed to approximately 1.5 L of OD radicals at a surface temperature of T s 275 K. The temperature ramp rate was high, approximately b s 25 K sy1 . We believe the water desorbing from the surface under these conditions was generated by the
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
Fig. 1. Thermal desorption spectrum of D 2 O from PtŽ111. after a 1–2 L OD radical dose at 275 K.
disproportionation of OD, i.e. from reaction Ž5. above. This assumes that the OD does not dissociatively adsorb, but remains intact on the surface. Although we have no direct evidence supporting this assumption, Anton and Cadogan, based on waveform analysis in their MBRS experiments, maintain that OD does not dissociatively adsorb at temperatures below T s 550 K w7x. In general, careful analysis of thermal desorption spectra can yield quantitative estimates of three kinetic parameters of interest: the desorption order in coverage, the pre-exponential factor Ž nd ., and the activation energy Ž Ed . w17,18x. This requires analysis of a series of spectra at varying coverages andror temperature ramp rates. We are currently pursuing such studies. Reaction Ž5. is expected to be second order in OD coverage. Typically, second-order desorption spectra are fairly symmetric about the desorption maximum, in contrast to the asymmetric spectrum seen in Fig. 1. However, with high ramp rates and low pumping speeds asymmetric spectra with high temperature tails are observed w19x. Although the data in Fig. 1 contain insufficient information to extract quantitative estimates of nd and Ed , we can perform simulations of desorption spectra using estimates available from the literature w7,20x. Such simulations can be made to
477
agree with the data in Fig. 1 by varying the initial OD coverage, an unknown quantity for these experiments. This agreement is somewhat arbitrary, but does provide a crude consistency check. Fig. 2a shows a desorption spectrum of the D 2 Oq signal under conditions comparable to those for Fig. 1 except the dosing temperature was 215 K rather than 275 K. Here we see behavior distinctly different than that observed in Fig. 1 with the primary D 2 O desorption feature appearing at Žor below. the dosing temperature of T s 215 K. We saw no substantial D 2 O desorption at temperatures higher than T s 300 K. Molecular oxygen was found to desorb from the surface at high temperatures Žsee Fig. 2b. with a spectrum similar to that observed previously by many groups studying the desorption kinetics of O 2 w15,16x. A similar O 2 desorption spectrum was observed under the dosing conditions corresponding to Fig. 1 as well.
Fig. 2. Thermal desorption spectra of Ža. D 2 O and Žb. O 2 from PtŽ111. after a 1–2 L OD radical dose at 215 K.
478
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
4. Discussion The thermal desorption spectra in Figs. 1 and 2a demonstrate that there are at least two distinct surface ‘phases’ for adsorbed OD radicals on PtŽ111.. We speculate that at low dosing temperatures OD radicals may form islands or multilayers. In the early 1980s, using ultraviolet and X-ray photoelectron spectroscopies, Fisher and coworkers identified adsorbed hydroxyl radicals after adsorbing water molecules on a PtŽ111. surface that had been precovered with oxygen at T s 100 K w3,21x. Subsequent work by White and coworkers measured the kinetics of water formation from this waterroxygen phase Žtermed intermediate ‘I’. using static SIMS w5,22x. They established that there was a 4:3 hydrogenroxygen ratio in ‘I’ and suggested a structure composed of rows of hydroxyl radicals bridged by water molecules. The thermal desorption spectra observed from ‘I’ contain a distinctive H 2 O desorption feature peaked at 210 K, just slightly less than our dosing temperature in Fig. 2a. We believe that the overlayers we prepared by dosing PtŽ111. with OD radicals at low temperatures may be similar to those prepared by co-adsorbing water and oxygen. Decomposition of this overlayer occurs near 210 K resulting in water formation. At higher dosing temperatures the intermediate phase of adsorbed OD is thermodynamically unstable. Under these conditions hydroxyl radicals may be adsorbed in a more highly dispersed configuration leading to higher activation energies for the ‘decomposition’ of this overlayer and the higher temperature desorption feature observed in Fig. 1.
5. Summary Here we have reported the first use of molecular beams of hydroxyl radicals to prepare overlayers of the HO–Pt intermediate. The molecular beam methods we have developed provide the means to independently control the surface coverage of adsorbed intermediates thereby facilitating the study of their surface chemistry. For example, studies currently underway in our laboratory are exploring the coverage dependence of the OD disproportionation kinetics using thermal desorption techniques. Also, by
modulating the hydroxyl radical beam and simultaneously dosing the surface with hydrogen, the kinetics of reaction Ž4. can be studied in detail with MBRS methods. And finally, given the reactive nature of radicals, we might expect a direct, Eley– Rideal reaction mechanism to be operative as has been observed previously in atomrsurface scattering studies w23–25x. Our beam techniques are general and have been applied to produce well-characterized beams of CF and CF3 radicals as well w26x. Other radicals with first order Stark effects that are amenable to our beam focusing methods include CH, SH, CH 3 –O, SiF, SiH, SiF3 , and SiH 3 . We are confident that our beam techniques will provide an important new tool for exploring the rich and complicated chemistry of surface reactions.
References w1x E. Fridell, A. Elg, A. Rosen, B. Kasemo, J. Chem. Phys. 102 Ž1995. 5827. w2x B. Hellsing, B. Kasemo, V.P. Zhdanov, J. Catal. 132 Ž1991. 210. w3x G.B. Fisher, J.L. Gland, Surf. Sci. 94 Ž1980. 446. w4x L.K. Verheij, M.K. Hugenschmidt, B. Poelsema, G. Comsa, Surf. Sci. 233 Ž1990. 209. w5x G.E. Mitchell, S. Akhter, J.M. White, Surf. Sci 166 Ž1986. 283. w6x A.B. Anton, D.C. Cadogan, Surf. Sci. Lett. 239 Ž1990. L548. w7x A.B. Anton, D.C. Cadogan, J. Vac. Sci. Technol. A 9 Ž1991. 1890. w8x T.D. Hain, M.A. Weibel, K.M. Backstrand, P.E. Pope, T.J. Curtiss, Chem. Phys. Lett. 262 Ž1996. 723. w9x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w10x P.C. Engelking, Rev. Sci. Instrum. 57 Ž1986. 2274. w11x F. Harren, D.H. Parker, S. Stolte, Comments At. Mol. Phys. 26 Ž1991. 109. w12x J. Bulthuis, J.J. van Leuken, S. Stolte, J. Chem. Soc. Farad. Trans. 91 Ž1995. 205. w13x S.R. Gandhi, T.J. Curtiss, R.B. Bernstein, Phys. Rev. Lett. 59 Ž1987. 2951. w14x R.G. Musket, W. Mclean, C.A. Colmenares, D.M. Makowiecki, W.J. Siekhaus, Appl. Surf. Sci. 10 Ž1982. 143. w15x C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, Surf. Sci. 107 Ž1981. 220. w16x J.L. Gland, Surf. Sci. 93 Ž1980. 487. w17x P.A. Redhead, Vacuum 12 Ž1962. 203. w18x D.A. King, Surf. Sci. 47 Ž1975. 384. w19x M. Wilf, P.T. Dawson, Surf. Sci. 60 Ž1976. 561. w20x E.M. Patrito, P.P. Olivera, H. Sellers, Surf. Sci. 306 Ž1994. 447.
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479 w21x w22x w23x w24x
G.B. Fisher, B.A. Sexton, Phys. Rev. Lett. 44 Ž1980. 683. J.R. Creighton, J.M. White, Surf. Sci. 122 Ž1982. L648. C.T. Rettner, J. Chem. Phys. 101 Ž1994. 1529. C.T. Rettner, Phys. Rev. Lett. 69 Ž1992. 383.
479
w25x B. Jackson, M. Persson, B.D. Kay, J. Chem. Phys. 100 Ž1994. 7687. w26x M.A. Weibel, T.D. Hain, T.J. Curtiss, J. Vac. Sci. Technol. A 15 Ž1997. 2238.
Chemical Physics Letters 287 Ž1998. 475–479
Observation of two reaction pathways for water formation from hydroxyl radical covered Ptž 111/ Kyle M. Backstrand, Michael A. Weibel, Toby D. Hain, Thomas J. Curtiss
)
UniÕersity of Utah, Salt Lake City, UT 84112, USA Received 20 November 1997; in final form 20 November 1997
Abstract Intense molecular beams of hydroxyl radicals produced in a supersonic corona discharge source were focused and state-selected by an electrostatic hexapole. Thermal desorption spectra from a PtŽ111. surface dosed with the resulting pure beam of OD radicals showed features corresponding to D 2 O and O 2 desorption. Two distinct types of adsorbed OD were observed depending on the dosing temperature. Dosing at 215 K generated a low temperature Ž215 K. D 2 O desorption feature. Dosing at 275 K generated a high temperature Ž400 K. D 2 O desorption feature. Possible origins of these two features are discussed. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Surface reactions catalyzed by transition metals typically proceed through a series of elementary reaction steps occurring between radicals adsorbed on the surface. Ideally one measures the rate constant for a particular elementary step by monitoring product yield as a function of reactant concentration. Measuring rate constants as a function of surface temperature allows Arrhenius activation energies and pre-exponential factors to be determined leading to greater insights into the reaction mechanism. Investigations of such catalytic systems have concentrated on elementary steps involving stable gas phase reactants or the desorption behavior of the products because these are the most easily studied. Consider, for example, the water reaction on PtŽ111., one of the most well-studied prototypical catalysis systems )
Corresponding author: E-mail: [email protected]
w1–5x. Experimentally it is easy to control the flux of O 2 , H 2 , or H 2 O striking the PtŽ111. surface as well as the surface temperature. Under conventional low pressure, UHV conditions molecular hydrogen and molecular oxygen dissociatively adsorb on the surface. A sequential addition of two hydrogen atoms to an oxygen atom produces a water molecule that quickly desorbs. It is also possible for two hydroxyl radicals to combine to produce a water molecule that quickly desorbs and an adsorbed oxygen atom. The following elementary steps are believed to play an important role in the waterrPtŽ111. reaction system: H Ž2g . ° 2H Ža. Ž 1. O Ž2g . ° 2O Ža. H Ža. q O Ža. ° OH Ža. OH Ža. q H Ža. ™ H 2 O Ža. 2OH Ža. ° H 2 O Ža. q O Ža.
Ž 2. Ž 3. Ž 4. Ž 5. Ž 6.
H 2 O Ža. ™ H 2 O Ž g . Rate constants for reaction steps involving adsorbed
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 1 8 3 - 3
476
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
OH radicals Že.g. steps 4 and 5. are difficult to measure directly because it is experimentally difficult to independently control the OH surface coverage. Estimates for these rate constants have been obtained by maintaining the O 2 and H 2 flux and surface temperature at conditions believed to assure the dominance of the reaction of interest. For example, Anton and Cadogan recently reported rate constants for reaction Ž5. measured using modulated beam relaxation spectroscopy ŽMBRS. w6,7x. They dosed the surface with D 2 while scattering a modulated beam of O 2 from the surface and analyzed the scattered D 2 O waveform to extract rate constants. By maintaining a low total coverage with the hydrogen coverage significantly exceeding that of oxygen, Anton and Cadogan were able to operate under conditions where reaction Ž5. was thought to dominate reaction Ž4. in water production. A much more direct approach is to control the OH coverage directly by dosing the surface with OH radicals. Hain et al. recently reported the development of a pure source of hydroxyl radicals suitable for this purpose w8,9x. Here we report the first results from experiments employing this unique source to dose a PtŽ111. surface with OD radicals.
2. Experimental Hydroxyl radicals were produced from He saturated with water vapor in a supersonic corona discharge beam source w10x. An electrostatic hexapole lens focused and state-selected w11,12x the hydroxyl radical beam. A beam-stop located at the midway point of the hexapole blocked from the beam any hydrogen or oxygen atoms produced in the discharge as well as undissociated water. The measured focusing spectrum was similar to those reported previously w8x with the peak of the < J V M J : s < 32 32 32 : rotational state appearing at a hexapole voltage of V0 s 8.7 kV. With V0 fixed at 8.7 kV the beam exiting the hexapole had a OD flux density of J s 8 = 10 11 OD radicals cmy2 sy1 with greater than 99% < 32 32 32 : state purity. The beam passed through a field free region of 12.4 cm before striking a single crystal PtŽ111. target. Typically, the M J rotational quantum number is randomized in the absence of an orienting field w13x, so that the rotational state distri-
bution of radicals reaching the Pt target contained equal numbers of M J s " 32 , " 12 . The Pt crystal was purchased from Monocrystals Company, polished and cut to within 18 of the Ž111. plane as determined by Laue X-ray diffraction. The crystal was spot-welded to Pt posts that were attached to a liquid nitrogen cold finger mounted in an UHV chamber with a base pressure of PUH V s 1 = 10y9 Torr. Temperatures were measured with a PtrPty 10% Rh thermocouple spot welded to the sample. The OD beam impinged on the sample with an angle of incidence of 708. Repeated cycles of exposure to O 2 Ž P UHV s 1 = 10y7 Torr. at T s 1125 K followed by Ar ion sputtering and annealing at T s 1375 K were used to clean the PtŽ111. surface. This procedure has been shown previously to produce a clean, well-ordered PtŽ111. surface w14x. No surface diagnostic tools were available to confirm the surface quality. However, O 2 thermal desorption spectra were comparable to those seen previously w15,16x and have been shown to be sensitive to the presence of common surface contaminants. In this Letter we report the results from two thermal desorption experiments. These were measured with a differentially pumped quadrupole mass spectrometer ŽQMS.. The axis of the mass spectrometer ionizer was oriented perpendicular to the OD molecular beam axis and approximately 208 from the surface normal. An elliptical spot approximately 1.5 = 4.4 mm was exposed to the OD beam for 5–20 min corresponding to a dose of approximately 0.5–2 L Ž1 L ' 4.8 = 10 14 radicals cmy2 .. The sample was then translated to a position locating the exposed spot at approximately 2 mm from a 3-mm-diameter aperture separating the UHV chamber from the QMS chamber. The sample was then resistively heated and the desorption flux was monitored with the QMS.
3. Results Fig. 1 shows a thermal desorption spectrum of the D 2 Oq signal from the QMS taken after the sample was exposed to approximately 1.5 L of OD radicals at a surface temperature of T s 275 K. The temperature ramp rate was high, approximately b s 25 K sy1 . We believe the water desorbing from the surface under these conditions was generated by the
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
Fig. 1. Thermal desorption spectrum of D 2 O from PtŽ111. after a 1–2 L OD radical dose at 275 K.
disproportionation of OD, i.e. from reaction Ž5. above. This assumes that the OD does not dissociatively adsorb, but remains intact on the surface. Although we have no direct evidence supporting this assumption, Anton and Cadogan, based on waveform analysis in their MBRS experiments, maintain that OD does not dissociatively adsorb at temperatures below T s 550 K w7x. In general, careful analysis of thermal desorption spectra can yield quantitative estimates of three kinetic parameters of interest: the desorption order in coverage, the pre-exponential factor Ž nd ., and the activation energy Ž Ed . w17,18x. This requires analysis of a series of spectra at varying coverages andror temperature ramp rates. We are currently pursuing such studies. Reaction Ž5. is expected to be second order in OD coverage. Typically, second-order desorption spectra are fairly symmetric about the desorption maximum, in contrast to the asymmetric spectrum seen in Fig. 1. However, with high ramp rates and low pumping speeds asymmetric spectra with high temperature tails are observed w19x. Although the data in Fig. 1 contain insufficient information to extract quantitative estimates of nd and Ed , we can perform simulations of desorption spectra using estimates available from the literature w7,20x. Such simulations can be made to
477
agree with the data in Fig. 1 by varying the initial OD coverage, an unknown quantity for these experiments. This agreement is somewhat arbitrary, but does provide a crude consistency check. Fig. 2a shows a desorption spectrum of the D 2 Oq signal under conditions comparable to those for Fig. 1 except the dosing temperature was 215 K rather than 275 K. Here we see behavior distinctly different than that observed in Fig. 1 with the primary D 2 O desorption feature appearing at Žor below. the dosing temperature of T s 215 K. We saw no substantial D 2 O desorption at temperatures higher than T s 300 K. Molecular oxygen was found to desorb from the surface at high temperatures Žsee Fig. 2b. with a spectrum similar to that observed previously by many groups studying the desorption kinetics of O 2 w15,16x. A similar O 2 desorption spectrum was observed under the dosing conditions corresponding to Fig. 1 as well.
Fig. 2. Thermal desorption spectra of Ža. D 2 O and Žb. O 2 from PtŽ111. after a 1–2 L OD radical dose at 215 K.
478
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479
4. Discussion The thermal desorption spectra in Figs. 1 and 2a demonstrate that there are at least two distinct surface ‘phases’ for adsorbed OD radicals on PtŽ111.. We speculate that at low dosing temperatures OD radicals may form islands or multilayers. In the early 1980s, using ultraviolet and X-ray photoelectron spectroscopies, Fisher and coworkers identified adsorbed hydroxyl radicals after adsorbing water molecules on a PtŽ111. surface that had been precovered with oxygen at T s 100 K w3,21x. Subsequent work by White and coworkers measured the kinetics of water formation from this waterroxygen phase Žtermed intermediate ‘I’. using static SIMS w5,22x. They established that there was a 4:3 hydrogenroxygen ratio in ‘I’ and suggested a structure composed of rows of hydroxyl radicals bridged by water molecules. The thermal desorption spectra observed from ‘I’ contain a distinctive H 2 O desorption feature peaked at 210 K, just slightly less than our dosing temperature in Fig. 2a. We believe that the overlayers we prepared by dosing PtŽ111. with OD radicals at low temperatures may be similar to those prepared by co-adsorbing water and oxygen. Decomposition of this overlayer occurs near 210 K resulting in water formation. At higher dosing temperatures the intermediate phase of adsorbed OD is thermodynamically unstable. Under these conditions hydroxyl radicals may be adsorbed in a more highly dispersed configuration leading to higher activation energies for the ‘decomposition’ of this overlayer and the higher temperature desorption feature observed in Fig. 1.
5. Summary Here we have reported the first use of molecular beams of hydroxyl radicals to prepare overlayers of the HO–Pt intermediate. The molecular beam methods we have developed provide the means to independently control the surface coverage of adsorbed intermediates thereby facilitating the study of their surface chemistry. For example, studies currently underway in our laboratory are exploring the coverage dependence of the OD disproportionation kinetics using thermal desorption techniques. Also, by
modulating the hydroxyl radical beam and simultaneously dosing the surface with hydrogen, the kinetics of reaction Ž4. can be studied in detail with MBRS methods. And finally, given the reactive nature of radicals, we might expect a direct, Eley– Rideal reaction mechanism to be operative as has been observed previously in atomrsurface scattering studies w23–25x. Our beam techniques are general and have been applied to produce well-characterized beams of CF and CF3 radicals as well w26x. Other radicals with first order Stark effects that are amenable to our beam focusing methods include CH, SH, CH 3 –O, SiF, SiH, SiF3 , and SiH 3 . We are confident that our beam techniques will provide an important new tool for exploring the rich and complicated chemistry of surface reactions.
References w1x E. Fridell, A. Elg, A. Rosen, B. Kasemo, J. Chem. Phys. 102 Ž1995. 5827. w2x B. Hellsing, B. Kasemo, V.P. Zhdanov, J. Catal. 132 Ž1991. 210. w3x G.B. Fisher, J.L. Gland, Surf. Sci. 94 Ž1980. 446. w4x L.K. Verheij, M.K. Hugenschmidt, B. Poelsema, G. Comsa, Surf. Sci. 233 Ž1990. 209. w5x G.E. Mitchell, S. Akhter, J.M. White, Surf. Sci 166 Ž1986. 283. w6x A.B. Anton, D.C. Cadogan, Surf. Sci. Lett. 239 Ž1990. L548. w7x A.B. Anton, D.C. Cadogan, J. Vac. Sci. Technol. A 9 Ž1991. 1890. w8x T.D. Hain, M.A. Weibel, K.M. Backstrand, P.E. Pope, T.J. Curtiss, Chem. Phys. Lett. 262 Ž1996. 723. w9x T.D. Hain, M.A. Weibel, K.M. Backstrand, T.J. Curtiss, J. Phys. Chem. A 101 Ž1997. 7674. w10x P.C. Engelking, Rev. Sci. Instrum. 57 Ž1986. 2274. w11x F. Harren, D.H. Parker, S. Stolte, Comments At. Mol. Phys. 26 Ž1991. 109. w12x J. Bulthuis, J.J. van Leuken, S. Stolte, J. Chem. Soc. Farad. Trans. 91 Ž1995. 205. w13x S.R. Gandhi, T.J. Curtiss, R.B. Bernstein, Phys. Rev. Lett. 59 Ž1987. 2951. w14x R.G. Musket, W. Mclean, C.A. Colmenares, D.M. Makowiecki, W.J. Siekhaus, Appl. Surf. Sci. 10 Ž1982. 143. w15x C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, Surf. Sci. 107 Ž1981. 220. w16x J.L. Gland, Surf. Sci. 93 Ž1980. 487. w17x P.A. Redhead, Vacuum 12 Ž1962. 203. w18x D.A. King, Surf. Sci. 47 Ž1975. 384. w19x M. Wilf, P.T. Dawson, Surf. Sci. 60 Ž1976. 561. w20x E.M. Patrito, P.P. Olivera, H. Sellers, Surf. Sci. 306 Ž1994. 447.
K.M. Backstrand et al.r Chemical Physics Letters 287 (1998) 475–479 w21x w22x w23x w24x
G.B. Fisher, B.A. Sexton, Phys. Rev. Lett. 44 Ž1980. 683. J.R. Creighton, J.M. White, Surf. Sci. 122 Ž1982. L648. C.T. Rettner, J. Chem. Phys. 101 Ž1994. 1529. C.T. Rettner, Phys. Rev. Lett. 69 Ž1992. 383.
479
w25x B. Jackson, M. Persson, B.D. Kay, J. Chem. Phys. 100 Ž1994. 7687. w26x M.A. Weibel, T.D. Hain, T.J. Curtiss, J. Vac. Sci. Technol. A 15 Ž1997. 2238.