368
Langmuir 1986, 2, 368-372
Registry No. cis-la, 645-49-8; trans-la, 103-30-0; cis-lb, 6624-53-9; trans-lb, 1694-20-8; cis-lc, 46925-32-0; trans-lc, 1149-18-4;cis-ld, 1657-49-4;trans-ld, 1657-50-7;cis-le, 1657-45-0; trans-le, 1860-17-9;cis-lf, 1657-53-0;trans-lf, 1694-19-5;cis-lg, 14301-11-2;trans-lg, 838-95-9;CdS, 1306-23-6;CdSe, 1306-24-7;
W03, 1314-35-8;PbO, 1317-36-8;Bi203,130476-3; ZnSe, 1315-09-9;
CdO, 1306-19-0;FezO3,1309-37-1;V20,,11099-11-9;CCh, 56-23-5; CH2C12,75-09-2;CH3COCH3,67-64-1; CH,OH, 67-56-1;CH3CN, 75-05-8 n-pentane, 109-66-0;1,2,4-trimethoxybenzene,135-77-3; anisole, 100-66-3.
Dissociative and Molecular Bromine Adsorption on an Fe( 100) Surface P. A. Dowben" and M. Grunzef Fritz-Haber-Institut der Max-Planck-Gesellschaft, 1000 Berlin 33, FRG Received August 21, 1985. I n Final Form: December 16, 1985 We have studied bromine adsorption on an Fe(100) single-crystal surface between 110 and 550 K by X-ray and ultraviolet photoelectron spectroscopy. For the whole temperature range, the initial adsorption was found to be dissociative and lead to formation of a chemisorbed overlayer. At T 5 140 K, we observed the formation of solid bromine growing in a layer by layer mechanism. No evidence for iron halide formation was found under our experimental conditions.
Introduction In this paper we describe the adsorption behavior of bromine on an Fe(100) surface between 110 and 550 K. The results presented here complement our previous reports on X-ray photoemission experiments for bromine and iodine adsorption on an iron(100) surface.' Initial adsorption of bromine and iodine at T > 110 K is always dissociative and is followed by molecular halogen adsorption at substrate temperatures of T I 140 K for Br, and T I 230 K for I,, respectively. Angle-resolved photoemission studies have identified two molecular bromine adsorption states on Ni(100), and Fe(110)2adsorbed onto the chemisorbed atomic bromine overlayers at low temperatures. The spectral features of the molecularly adsorbed bromine have been interpreted as arising from Br, molecules having different orientation of their molecular axis with respect to the surface normal. Similar behavior has been observed with molecular I, adsorption on Ni( Fe(l10),4 and iron i ~ d i d e .In ~ a recent paper, Benndorf e t ala5studied bromine adsorption on an Ag(ll0) surface. The formation of AgBr following the initial dissociative chemisorption of bromine was indicated for T 1 130 K. Only at 130 K upon the AgBr corrosion layer was a molecular bromine state observed. As detailed in this paper, we found no evidence for iron bromide formation on Fe(100) under the low-pressure conditions applied in this study. The results for halogen adsorption on solid surfaces in general have been summarized and reviewed6r7 and a comprehensive survey of the literature may be found there. Experimental Section The photoemission experiments were performed in a commercial stainless steel UHV chamber (Leybold-Heraeus),described previously.s The chamber was equipped with a hemispherical electron energy analyzer, an A1 K a X-ray source (Ekln= 1486.6 eV), a differentiallypumped rare-gas discharge lamp, a quadrupole *Address correspondence to this author at Department of Physics, Syracuse University, Syracuse, NY 13244-1130. Present address: Department of Physics and Laboratory for Surface Science and Technology, University of Maine, Orono, ME 04469.
mass spectrometer, an electron gun, and an Ar+ ion gun for cleaning the specimen. The angle of incidence of the probing photon and electron sources was 40' off normal and the emitted electrons were collected normal to the surface. The spectra were recorded by direct pulse counting into a Nicolett instrument computer to improve the signal to noise ratio and then plotted on an n-y recorder. Binding energies in this paper refer to energies below EF of the clean iron substrate. Adsorption curves were determined from integrated XPS core level intensities or the XPS core level intensity maximum. The methods of cleaning the Fe(100) surface and estimating the concentration of the residual contaminents have been described elsewhere.' The oxygen residue on the surface in the experiments was estimated to 3% of a monolayer and slight, if any, increase was observed after an experimental run. The work function change measurements were obtained from the cutoff of the secondary electron photoemission background in He1 photoemission experiments. Earlier retarding potential diode work function change measurementsg were also repeated. Coverages throughout this paper will be denoted in terms of atoms per surface unit cell of the clean Fe(100) surface (1.214 X 1019atoms m-2),denoted by r, or, alternatively, with respect to the saturation coverage of the bromine chemisorbed overlayer at Exposures given here room temperature and denoted as 6'/6',. are not corrected for the ionization gauge sensitivity of bromine unless otherwise stated. Gaseous bromine was admitted to the chamber via a standard leak valve. The base pressure was generally 2 x mbar, and some bromine adsorption on the surface was found to occur during photoelectron spectra acquisition on the previously clean surface. This bromine adsorption was not found to alter our results significantly and remained less than 3% overall.
Results Bromine adsorption was followed as a function of ex(1) Dowben, P. A.; Grunze, M.; Tomanek,D. Phys. Scr. 1983, T4,106. (2) Dowben, P. A.; Mueller, D.; Rhodin, T. N.; Sakisaka, Y. Surf. Sci., in press. (3) McConville, C. F.; Woodruff, D. P. Surf. Sci. 1985, 152/153,434. (4) Mueller, D.; Rhodii, T. N.; Dowben, P. A. Surf. Sci. 1985,164,271. (5) Benndorf, C.; Kruger, B. Surf. Sci. 1985, 151, 271. (6) Grunze, M.;Dowben, P. A. Appl. Surf. Sci. 1982, 10, 209. (7) Farrell, H. H. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis. King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1984, Vol. 3b. (8) Grunze, M. Surf. Sci. 1979, 81, 603. (9) Dowben, P. A.; Jones, R. G. Surf. Sci. 1979, 88, 348.
0743-7463/86/2402-0368$01.50/063 1986 American Chemical Society
Langmuir, Vol. 2, No. 3, 1986 369
Bromine Adsorpion on an Fe(lO0) Surface
Bromine Exposure x 5
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Figure 1. X-ray photoemission spectra of bromine adsorption on Fe(100) at 300 K. The spectra of the bromine 2pIp and 2p3/,
Figure 2. Adsorption curve for dissociative Br2adsorption on Fe(100) at 300 K. The coverage (Ole, = 8.86 X lo-**atom m-2) is plotted as a function of exposure. The coverage has been determined by (0)the X-ray photoemission intensity of the bromine (3p,,,) core level, (0) retarding the potential diode work function change, and (+) the bromine 102-eVAuger electron peak height. Exposure is shown, both assuming no correction for ionization gauge efficiency and an ionization gauge efficiency for = 1.4. Br2 of SBr2/SN2
core levels were taken following cleaning of the surface (A) and (B) 2 X lo4, (C) 4 X lo4, (D) 6 X lo4, (E) 8 X lo4, and (F) 12 X lo4 mbar s Br, exposure, uncorrected for ion gauge sensitivity. posure by using XPS, as shown in Figure 1. With initial bromine adsorption a t 300 K, the (3s)(3plI2),( 3 ~ , / ~and ), (3d) core levels were observed with binding energies of 256.2 f 0.4, 189.1 f 0.3, 182.7 f 0.2, and 69.0 f 0.2 eV, respectively, and half-widths of 3.7, 3.4, 2.9, and 2.7 eV, respectively. A small shift of -0.3 eV in electron binding energy is found as a function of coverage. The bromine core levels a t saturation coverage for 300 K were observed at 255.9, 188.8, 182.4, and 68.8 eV for the (3s), (3p, 2), (3p3/2), and (3d) levels, respectively, with no observa le changes in half-width. The intensity of the bromine (3p3/2 and 3p1/2) orbitals relative to the Fe(3p) XPS core level has been used to estimate the bromine saturation coverage a t room temperature on Fe(100), using the procedure described elsewhere.1° Corrections for the attenuation of the Fe(3p) signal through the bromine overlayer using an electron mean free path of 20 as well as core level ionization cross sections for A1 K a radiation,14 analyzer efficiency, and geometry were taken into account in calculating the coverage from the XPS intensities. The saturated bromine surface overlayer at 300 K has thus been estimated to have a coverage of r = 0.8 f 0.2 (9.7 X lola bromine atoms m-2). Both retarding potential diode work function change (rpd) and the work function change determined from the secondary electron cutoff in He I photoelectron spectra increase proportionally with the bromine core level signal and the 102-eV bromine Auger electron peak height, with adsorption at 300 K. The overall increase in work function with Br2 adsorption a t room temperature was found to be 1.16 f 0.05 eV by the rpd method and 1.4 f 0.1 eV from
b
(IO) Fadley, C. S. In Electron Spectroscopy Theory, Techniques, and Applications. Brundle, C . R., Baker, A. D., Eds.; Academic Press: London, 1978; Vol. 2. (11) Penn, D. R. J. Electron Spectrosc. Rel. Phenom. 1979, 9, 29. (12) Powell, C. J. Surf. Sci. 1974, 44, 29. (13) Dowben, P. A.; Grunze, M. J . Electron. Spectrosc. Rel. Phenom. 1983, 28, 249. (14) Jorgensen, C. K.; Berthou, H. Faraday Discuss. Chem. Soc. 1972, 54, 269. (15) Somerton, C.; McConville, C. F.; Woodruff, D. P.; Jones, R. G. Surf. Sci. 1984, 136, 23. (16)Holland, L.; Steckelmacher, W.; Yarwood, J. Vacuum Manual Spon, E., Spon, F. N., Eds.; Academic Press: London; 1974.
t
I
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C
a
4 Y
s.
.-
c
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Q C e C
-
Binding Energy [eV] Figure 3. Ultraviolet helium I photoemission spectra of bromine adsorption on Fe(100) at 110 K. Spectra were taken for the clean surface (A) and following (B) 4 X lo4, (C) 8 X lo4, (D)16 X lo4, (E) 28 X lo”, (F) 52 X lo4, and (G)175 X lo4 mbar s Brz exposure.
the cutoff of secondary electrons in He I ultraviolet photoemission data. The discrepancy between the two techniques cannot be explained, but we note that in the retarding potential diode work function measurements bromine adsorption on the diode might lead to erroneous results. In Figure 2 we show the correlation between the potential diode work function measurements, the integrated band intensity of the bromine 3p3/2core level, and the 102-eV bromine Auger peak height. Adsorption of bromine a t 110 K leads initially to the formation of dissociated bromine overlayer. The He I1 ultraviolet photoemission data in Figure 3 show, up to an exposure of 16 X lo4 mbar s (spectrum D), only one adsorbate induced emission band a t 5.3 eV below E p This feature arises from the 4p orbitals of atomic bromine. For
Dowben and Grunze
370 Langmuir, Vol. 2, No. 3, 1986
Bromine Exposure x I d 6 mbar sec ( S e r 2 / S ~ ,= I 4)
--
< ?2
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PO
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’
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compared below to the UPS spectrum of the multilayered molecular Br, structure adsorbed on Fe(100): (A) helium I spectrum of FeBr,; (B) helium I gas-phase spectrum of Br,; (C)helium I UPS spectrum of Br2adsorbed on Fe(100) at 110 K following 300 X lo4 mbar s Br, exposure.
I90
I85
Figure 6. Adsorption curve for Br2adsorption on Fe(100) at 110 K. The coverage has been determined from the XPS intensity of the bromine (3p3/2)core level. The coverage O/O, = 1.0 is the saturation coverage for bromine adsorption at 300 K (8.86 X 10l8atoms m-, or 4.4 X 1Ol8 molecules m-,). The data (+) has been corrected for self-attenuationof the bromine signal in the assumed to vary by I = Io exp(-d/Xe)where d is the overlayer ~ ~ ~ ~ 1 thickness of the overlayer and Xe is the electron mean free path.I5
Where significant differences between the coverages corrected for photoemission signal attenuation and the uncorrected coverage exist, the original measurements are shown as 0.
Figure 4. Gas-phase UPS spectra for FeBrzZoand BrZz1are
195
1
M)
mbar sec ( S ~ , * / S N= ~I )
I80
Binding Energy [eV]
Figure 5. XPS spectra of the bromine (3Pl/z) and (3p3z) core levels are shown following adsorption of Brz at 110 K on de(l00). Spectra were taken following (A) 1 X lo”, (B) 10 X lo”, (C)30 x IO”, and (D) 150 X 10” mbar s Br, exposure.
a detailed discussion of the assignment of these bromine orbitals see ref 1,2, and 17. Spectra C and D in Figure 3 closely resemble the ultraviolet photoemission data for a room temperature bromine-saturated surface.l Only a t higher exposures a broadening of the features and the evolution of photoemission bands identified as arising from molecular adsorbed or condensed bromine are 0bserved.l The correlation between the He I ultraviolet photoemission spectrum of gaseous bromine and the condensed bromine layer is shown in Figure 4. For the later discussion we also included the He I ultraviolet spectrum for gaseous FeBr,. Some representative X-ray photoelectron spectroscopic data (Br 3p1j2and 3p3/2) for adsorption at 110 K are shown in Figure 5. The transition between atomic and molecular adsorption of bromine occurs be(17) Dowben, P.A.; Grunm, M.;Varma, S.Solid State Commun. 1986, 57, 631.
tween spectra B and C. There is an energy binding shift to lower energies between the saturated atomic overlayer and molecular adsorption. This shift, within the error bars, was observed for all bromine core levels to be 0.2 eV or less. The adsorption curve for bromine a t 110 K was measured by degrading the resolution of our spectrometer to 2 eV and following the increase in Br 3p3/, intensity as a function of exposure. In Figure 6 the bromine coverage as calculated from the XPS data is plotted vs. exposure. The coverage e/e, = 1.0 is the saturation coverage for bromine adsorption a t 300 K (8.86 X 10l8atoms or 4.4 X 10l8molecules m-2). The data (+) have been corrected for self-attenuation of the bromine signal in the overlayer, assumed to vary by I = Io exp(-d/Ae) where d is the thickness of the overlayer and A, is the electron mean free path.l’ Where significant differences between the coverages corrected for photoemission signal attenuation and the uncorrected coverage exist, the original measurements are shown as 0. The saturation exposure for bromine adsorption at 300 K, determined by the AES, XPS, and rpd work function is 12 X lo4 mbar s (f0.2 X IO4 mbar s), assuming that the ionization gauge cross section of Br2 is equal to that of N2 By use of the saturation coverage of bromine atoms on Fe(100) a t 300 K estimated from LEEDg of r = 0.73 (or 8.86 X 10l8bromine atom m-2), the initial dissociative sticking coefficient is So = 0.31, uncorrected for ionization gauge sensitivity. The ionization gauge cross section is roughly proportional to the number of electrons in the molecule.16 Thus, we estimate the ion gauge cross section of molecular bromine relative to molecular nitrogen at SBr2/SN21.4 from the relative ionization gauge sensitivity of molecular chlorine to molecular nitrogen SClZ/SNz = 0.68.16 An estimate of the absolute initial sticking coefficient from the XPS, AES, and work function data for dissociative Br2 adsorption a t 300 K is therefore So 2 0.43. Adsorption curves for bromine on iron (100) were recorded also a t higher temperatures by the intensity of the Br 3p3/, core level intensity as shown in Figure 7. The saturation coverage obtained a t the higher temperature corresponded well to those reported previously obtained during desorption experiment^.^ The initial sticking coefficient, shown in Figure 7, decreases with increasing temperature. Included in Figure 7 are also XPS data on the initial sticking coefficient for atomic bromine adsorption a t 110 K, showing that So also decreases a t T < 300 K. The change in work function with adsorption at 110
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Bromine Adsorpion on an Fe(100) Surface
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Langmuir, Vol. 2, No. 3, 1986 371
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Figure 7. Initial dissociative adsorption sticking coefficient plotted as a function of temperature. The sticking coefficient has been corrected for ionization gauge sensitivities and coverage (see text).
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130 x ld‘ mbarsec (Intensity. 2 I Ln
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and different saturation exposures. The LEED studies for bromine chemisorbed on Fe( 100) have been interpreted in terms of two types of overlayer structures with a continuously variable overlayer net size for a wide ranges caused by long-range bromine-bromine repulsive forces. As the overlayer compresses the heat of adsorption decreases as a result of increased repulsion between bromine atoms, being brought closer together? The repulsive forces between halogen atoms has been employed to explain the core level binding energy shifts as a function of coverage for bromine on Fe(100)’ and for chemisorbed iodine on Ni(100).16 The overlayer structures that have been employed to interpret the LEED patterns for Br/Fe(100) place all the bromine atoms on the surface in approximately equivalent chemical states for any given c ~ v e r a g e .This ~ explains the core level XPS half-width independence with chemisorption coverage as described previously.’ At 110 K, initial adsorption is also dissociative, as concluded from the valence band photoemission data. The first indications for adsorbed molecular bromine species occur after an exposure of 16 X lo4 mbar s or at a coverage of 6/19,, 0.8 with respect to the room temperature saturated surface (7.1 X 1018bromine atom/m2). With the start of molecular adsorption, we also find an expected attenuation of the iron d-band in He I and He I1 photoemission experiments and iron core level intensities. This subsequent molecular Br2 adsorption occurs on top of a dissociatively chemisorbed bromine ~verlayer.’*~J~ Thus, the initial XPS core level binding energy shifts are a result of repulsive ‘nteractions between chemisorbed bromine atoms, as we Jbserved for bromine adsorption at 300 K. The small diff zence in core level binding energies between the saturated chemisorbed bromine overlayer observed at 300 K an. molecularly adsorbed Br2 observed with large exposure:- .sn Fe(100) at 100 K has been explained in detail else here, using a Born-Haber cycle and the “equivalent core approximation.lJ8 Molecular adsorption of bromine a t 110 K leads to the formation of bromine multilayers, which is expected from the high heat of sublimation (AHsub = 46.5 kJ/mol). In the adsorption curve of Figure 6 the steps a t 16 and 55 X lo4 mbar s exposure indicate a layer by layer growth mechanism of the first two bromine molecular overlayers. After an exposure of 940 X lo4 mbar s we estimated the thickness of the bromine overlayer to be -7 f 1 monolayers from both the attenuation of the substrate iron signal and the bromine X P S intensities. For these bromine multilayers, ultraviolet photoemission data show features that can be correlated to the molecular bromine orbitals as indicated in Figure 4. However, the intensity distribution between the emission bands is different than in the gas-phase spectra of randomly oriented molecules. The photoemission intensities of the oriented adsorbed molecular bromine orbitals depend upon the incident light and electron collection geometry. We thus postulate that the condensed bromine molecules are not randomly oriented but have a preferential orientation of their molecular axis to the surface normal resulting in a different intensity distribution in our photoemission data taken a t the normal emission angle. The angular dependence in photoemission from bromine multilayers or. Fe(110) has been described previously2and an orientation of Br2 with
I
1
I
I
250
300
350
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Temperature ( K )
Figure 8. Thermal desorption spectra of bromine adsorbed on Fe(100) at 110 K using the (3p312)bromine XPS signal. The heating rate is 10 deg/s for both spectra; 130 X 10” (a) and 940 x 10” mbar s (b).
K appears to level off at a value of 2.1 f 0.2 eV, about 700 meV higher than the maximum change observed with adsorption a t 300 K. Continuing exposure a t 110 K substrate temperature leads to the formation of bromine multilayers as evident from the increase in the bromine XPS band intensity and the decrease in substrate emission. Desorption of these multilayers occurs at 140 K as shown in Figure 8 by the decrease of the Br 3p3/, emission. Around 350 K the onset of bromine desorption from the dissociative layer takes place as described in detail el~ewhere.~
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Discussion Bromine adsorption on Fe( 100) a t room temperature and above is always dissociative as determined from LEEDQand photoemission experiments.‘ The LEED data have been interpreted as a sequence of a commensurate and variable incommensurate overlayers with a maximum coverage at room temperature of r = 0.73. From our XPS experiments, we estimate the maximum coverage to be r = 0.8 f 0.2, in good agreement with the LEED data. From the proportionality of the core level intensity with the low-energy bromine Auger data and the change in work function we conclude that the adsorbate phase is best described by a chemisorption layer and does not involve iron bromide formation on the surface. If three-dimensional compound formation would occur, we expect the coverages measured by the three techniques having different surface sensitivity to give different saturation values
(18) Tomaek, D.; Dowben,P. A,; Grunze, M. Surf. Sci. 1983,126,112. (19) Haefa, R.A.Kryouakuumteknik Springer Verlag: Berlin, 1981. (20) Berkowitz, J.; Sheets, D. G.;Garritz, A. J.Phys. Chem. 1979, 70, 1305. (21) Potts, A. W.;Price, W. C. Trans. Faraday SOC.1971, 67, 1242.
Dowben and Grunze
372 Langmuir, Vol. 2, No. 3, 1986 the molecular axis parallel with the surface normal was found. Figure 4 also shows that our photoemission results from bromine layers do not resemble the features expected for FeBrz. Thus we have no evidence for the formation of a corrosion layer as described for the Ag/Brz system a t low substrate temperature^.^ In Figure 6 we also indicated our experimental sticking coefficientsderived with the corrected ion gauge sensitivity. The experimental data points have been approximated by a straight line although in all our adsorption curves at 110 K an S-shape behavior is found up to Old,, 0.8. We suspect, however, that this shape is caused by instabilities in the bromine pressure and the pressure gradients between sample and ion gauge. The initial sticking coefficient at 110 K is So = 0.26 and decreases to SI = 0.094 and S, = 0.015 with subsequent molecular multilayer formation. The experimental conditions under which multilayer formation was observed were such that the dosing pressure (p = lo-' mbar) exceeded the equilibrium pressure of solid bromine a t 110 K (-2 X mbar) by orders of magnitude.,, With our calibration of the sticking coefficient, the condensation coefficient of molecules bromine into the second molecular layer at a gas temperature of 300 K on a bromine substrate at T, = 110 K is ac = 0.015, which is an extremely low value and cannot be rationalized by an inspection of the gas temperature dependence of condensation coefficients of other gases.Ig Typically, the condensation coefficient has a value of around unity and decreases only by 2040% with an increase of gas temperature by several hundred degrees. We cannot rule out that our results are obscured by artifacts, e.g., X-ray induced photodesorption leading to higher desorption rates than expected under thermodynamic equilibrium conditions. Also, bromine has a very low thermal conductivity (about 1% of that of typical metals), making it possible that the temperature of the outer molecular bromine layer is higher than the measured temperature of our iron substrate due to the thermal and X-ray radiation and the exothermal heat of condensation of bromine. Since at -130 K the equilibrium pressure of solid bromine and the adsorbed thin film of molecular bromine on iron becomes equal to or exceeds the dosing mbar, condensation terminates at this pressure of 1 X temperature. We suspect that one or a combination of the possible effects mentioned above might be the cause of the extremely low condensation coefficient derived from our experiments. Our adsorption curves at 110 K resemble those reported by Benndorf et for bromine adsorption on an Ag(ll0) surface at 130 K. He also observed distinct breaks in the adsorption curve but interpreted his results as indicative for a layer by layer growth of AgBr. The formation of AgBr was also concluded from desorption data, which show the desorption of AgBr molecules around 650 K. This contrasts with the behavior of the Br/Fe(100) system, where a continuous desorption of bromine from the chemisorption layer with increasing temperature was found.Q Another indication of a difference in the lowtemperature behavior of the Br,/Ag(llO) and Brz/Fe(lOO) system is derived from the work function changes a t low temperatures (A$ = -2.1 f 0.2 eV for Fe(ll0); A$ = -1.44 f 0.05 eV for Ag(ll0)). The constant saturation value found by Benndorf et al.5 between 120 and 130 K corresponds well with our He I cutoff measurements for dissociative bromine adsorption a t 300 K (1.4 f 1 eV), whereas a further increase by -0.7 eV at low temperatures
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(22) Fischer, J.; Bingle, J. J Am. Chem. SOC.1955, 77, 6,511.
was not observed in the Br,/Ag(llO) system. The observation that iron bromide formation does not occur in the Brz/Fe(lOO) system for T I550 K under high-vacuum conditions cannot be explained by the thermodynamics of the halide reaction but must be due to kinetic barriers. The heat of formation of FeBr, is exothermic by 59.7 kcal/mol. Dissociative chemisorption, however, will lead to surface halide formation on iron surfaces if defects are present at the surface as detailed elsewhere.23 The behavior of the initial sticking coefficient as a function of temperature as shown in Figure 7 can be qualitatively explained by an adsorption model involving a molecular precursor state for dissociative adsorption. Since we did not find any evidence for a molecular bromine state a t 110 K and t9/Om, 5 0.8 (r = 0.58 or 7.1 X 1018 atom/m2) after evacuation (PBr2 C low9mbar), we conclude that the bromine molecules impinging onto the bare surface either are reflected back into the gas phase or dissociate during their residence time on the surface. Molecular bromine adsorbed on clean iron {loo}must be unstable and tend toward dissociation, or the adsorption energy of molecular bromine must be so small that the molecular bromine does not adsorb irreversibly under these experimental conditions. From the absolute value of the dissociative sticking coefficient of So I0.23 at 110 K, it appears that some bromine molecules must have desorbed before dissociation, i.e., that the heat of Br, adsorption on clean iron is low. However, as with the abnormally low condensation coefficient, we cannot rule out that molecular bromine is photodesorbed with UV or X-ray radiation. From the increase in dissociative sticking coefficient with temperature (from 110 to 300 K) a small activation barrier for dissociation can be postulated. This activation barrier would be a t least 177 cal/mol. Such a small activation barrier for dissociation may be the reason for the observation that not all the molecules impinging on the surface dissociate. The decrease of the initial sticking coefficient at higher temperatures is then explained by the decrease in the equilibrium molecular bromine coverage during Br, exposure as a result of the low heat of adsorption. Since the rate of dissociation is proportional to the concentration of the molecular precursor the rate drops with increasing temperature.
Conclusion The initial adsorption of Br, of Fe(100) for 110 K 5 T I550 K is dissociative and leads to formation of a chemisorbed bromine overlayer. Subsequent Br, exposure to Fe(ll0) at 110 K leads to molecular Br, adsorption on the chemisorbed overlayer. We have observed layer by layer adsorption of bromine on Fe(100) at 110 K. A comparison of the initial sticking coefficients to dissociative adsorption suggests that there is an activation barrier from an adsorption precursor state to the dissociatively chemisorbed state of at least 7.5 X eV or 177 cal/mol. Acknowledgment. This work was supported by the Max Planck Gesellschaft and Deutsche Forschungsgemeinschaft (Sonderforschungs-bereicht6). Additional support was provided by the donors to the Petroleum Research Fund, administered by the American Chemical Society, and by Syracuse University. Registry No. Br,, 7726-95-6; Fe, 7439-89-6. (23) Mueller, D. R.; Rhodin, T.N.; Dowben, P. A. J. Vac. Sci. Technol., A 1986, 4.
