- Email: [email protected]
The influence of lateral interactions on the angular distribution in photodesorption
_..... :.:.:j:.:.:::.:.:~...~.~. .‘.‘.‘.:.:
.:::‘.;:i:::,+: :(:::,.
Surface Science North-Holland
287/288
Hirayama
..:.
‘, Arne de Meijere
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Received
28 August
1992; accepted
:,,:
‘kurface science
(1993) 160-164
The influence of lateral interactions in photodesorption Hiroyuki
. . . . ,:.:. :.,.:.:.::j.::,:::::i:~:.:::jj:i
......A.
.,.
for publication
on the angular distribution
and Eckart
Hasselbrink
Faradayweg 4-6, 1000 Berlin 33, Germany 24 November
1992
Photodesorption and photodissociation are induced when 0, adsorbed on Pdtlll) is irradiated with UV light. Both processes are non-thermal. In addition thermal desorption of molecular oxygen is observed as a consequence of a displacement process by the nascent atomic oxygen. Angle-resolved time-of-flight spectra were recorded. For the adsorbed molecular superoxo species these measurements show a pronounced maximum in the translational energy at a desorption angle of 60”. Moreover, this peak is only observed at certain azimuthal angles indicating C,, symmetry.
1. Introduction Oxygen adsorbed on metal single crystal surfaces such as Pd(ll1) [l-4], Pt(ll1) [5,6] and Ag(ll0) [7] has served as a model system for the study of surface photochemistry. Time-of-flight (TOF) measurements are an avenue to shed light onto the dynamics of these processes involving desorption of molecules from the substrate. In this paper we will report angle-resolved measurements of the desorption flux and translational energy distribution of 0, photodesorbed from Pd(ll1). Molecular oxygen adsorbs on Pd(lll1 in three binding states, denoted (pi, LX*, and q, corresponding to three different adsorbate structures. These binding states appear in thermal desorption spectrometry (TDS) at desorption temperatures of 200, 155, and 120 K, respectively. Wolf et al. [8] have observed by high-resolution electron energy loss spectroscopy (HREELS) that upon irradiation with UV light, 0, dissociates to form surface oxygen. TDS ’ Permanent Research Japan.
address: NEC Corporation, Laboratories, 34 Miyukigaoka,
0039-6028/93/$06.00
0 1993 - Elsevier
Microelectronics Tsukuba 305,
Science
Publishers
measurements show that the total coverage of all oxygen species on the surface decreases at the same time [2]. Time-of-flight measurements show that molecular oxygen desorbs from the surface with a translational energy (E,,,,,)/2k corresponding to a temperature of 650 K, which is far above the actual sample temperature of 100 K. Hence, these measurements demonstrate the non-thermal character of these processes. Additionally a slow component is observed in the time-of-flight spectra, which has been interpreted as being due to a displacement process by the nascent atomic oxygen. The excitation mechanism is still under debate, but recent measurements of the excitation spectrum [4,9] favor an interpretation in terms of a transient attachment of a hot substrate electron to the 30; orbital of 0,. This additional charge transfer into an antibonding orbital is likely to cause the molecular bond to break since the molecular bond is, due to charge transfer, already weakened in the adsorbed state. The measurements are consistent with a position of the 3a, orbital 4 eV above the Fermi level in agreement with recent experiments on Pt(lll1 [lo]. Moreover, attachment of additional charge causes attraction by its image. Upon reneutraliza-
B.V. All rights reserved
H. Hirayama et al. / The injluence of lateral interactions on the angular distribution in photodesorption
tion the molecule finds itself displaced towards the substrate where repulsive interactions might dominate, eventually leading to desorption. Hence, depending on initial position and momenta, dissociation or desorption might result. The desorption direction is determined by the direction of steepest decent of the repulsive leg of the O,-Pd interaction potential. This direction does not necessarily have to be parallel to the surface normal. In this paper we will discuss dependencies of the time-of-flight spectra on the polar and azimuthal detection angles. The observations will be interpreted within the model of the desorption process outlined above. In particular we will show that the adsorption geometry of superoxo 0, is influenced by coadsorbed atomic oxygen. The binding geometry as well as a lateral force induced by the atomic oxygen results in rich structure in the angular distributions analogous to that found in electron stimulated desorption ion angular distributions (ESDIAD).
2. Experimental The experiments were carried out using a standard molecular beam apparatus 1111.Sample orientation and cleanliness were checked by low energy electron diffraction LEED. The crystal showed a sharp hexagonal pattern and a p(2 X 2) pattern after saturation with atomic oxygen. The azimuthal orientation of the sample was established by comparison of experimental and calculated LEED Z(V)-curves for the (01) and (11) spots. A KrF excimer laser was used for sample irradiation. This light source produces 10 ns pulses at a photon energy of hv = 5.0 eV. Typically pulse energies of less than 3 mJ/cm’ were employed in order to prevent noticeable substrate heating ( < 10 K>. A quadrupole mass spectrometer (QMS) mounted on a rotatable platform served for recording line-of-sight TD and TOF spectra. TOF spectra were fitted by a modified Maxwell-Boltzmann distribution, from which the desorption flux and the flux-weighted average translational energy (E,,,,, )/2k are derived. If
161
TOF spectra were bimodal, indicating a thermal desorption component, the latter was fitted by a Maxwell-Boltzmann distribution at the sample temperature and subtracted from the spectrum. Using a set of crystal holders, the azimuthal orientation of the sample could be varied in steps of 15”. The projection of the direction towards the detector onto the crystal defines the azimuthal direction of detection. Alignment of the crystallographic directions was checked by LEED with an accuracy of about 5”. The polar angle could be varied by rotating the QMS. The angle of incidence of the laser light was fixed at 45”. TOF spectra were recorded for every azimuth in polar angle steps of 12”. The volume of the ionization region of the QMS determines an angular resolution of 5”. Apart from preparing a saturated layer at 95 K sample temperature, it is possible to prepare oxygen layers in which preferentially one molecular species contributes to photodesorption. Lower binding energy species can easily be desorbed by raising the surface temperature. Additionally preadsorption of atomic oxygen suppresses population of the higher binding energy states [12]. We have used the following procedures: (1) The surface is saturated with 0, and irradiated at 153 K. At this temperature adsorption of (Ye- and cY,-O,(a) is suppressed. Hence, the desorption signal should originate solely from (piO,(a). (2) The sample was predosed with oxygen at 400 K resulting in about 0.2 ML coverage of O(a). After lowering the sample temperature to 130 K it was saturated with 0, and irradiated. This preparation results in predominant population of a,-O,(a). (3) The surface was predosed with oxygen at 400 K as above. Subsequently it was saturated with 0, at 95 K and irradiated with 8.4 X 10zl photons/cm2 to remove most of the cY,-O,(a) so that predominantly cr,-O,(a) is left on the sample. 3. Results For an O,(a) saturated sample at 95 K the photostimulated desorption flux exhibits cos@-de-
162
H. Hirayama et al. / The influence of lateral interactions on the angular distribution in photodesorption
pendence with /3 = 1.57 f 0.17. Hence, it is peaked towards the surface normal. Thermal desorption, which is observed concurrently due to displacement by newly formed atomic oxygen as already discussed, shows a cosp-dependence (p = 1.09 f 0.15) in agreement with the thermal character of this process. Measurements of the angular distribution of the desorption flux for the three preparations described above showed no marked deviation from the case of desorption from the saturated surface. The average translational energy is constant or decreasing for cri- and a,-O,(a) as the polar desorption angle is increased. In the [Tll] azimuthal direction we observe a decrease from 630 f 40 K in normal direction to 520 &-60 K at 60” for a,-O,(a) (fig. 1). The data for cu,-O,(a) show no variation of (E,,,,,)/2k. For cr,-O,(a) we observe a marked peak in the translational energy for observation in the [zll] azimuth. This peak occurs at a polar desorption angle of 60”. At increases to 1050 f 100 K this point (E,,,,,)/2k compared to 650 f 40 K for normal desorption. For larger angles it decreases again to 700 ? 100 K at 72”. This pronounced maximum of the translational energy was observed to have C,, symmetry upon azimuthal rotation of the crystal. The peak is not observed in the [12-i] direction, which
Fig. 2. Top view of the Pd(ll1) surface. The azimuthal directions at which TOF-spectra have been recorded are marked in the figure. The first, second, and third layers are shaded white, light and dark grey, respectively. The atomic oxygen pf2 X 2) structure is indicated by the small circles.
is 60” off the [zll] azimuth (fig. 2). Hence, a six-fold symmetry is ruled out. A check measurement on the [l?l] azimuth, which is 120” from [‘?lll, clearly showed the (E,,,,,)/2k peak, indicating three-fold symmetry. Summarizing our observations, we find that for LY,-and cY,-O,(a) the photodesorption flux peaked in the normal direction. The translational energy is rather constant, or may be slightly decreasing.
1100 1000 z
900
6 t 5
800 700 600 500 0
20
40
60
0
20
40
60
Polar Angle Fig. 1. Translational energy, (I?,,,,, )/2k, of photodesorbed Oz for two azimuthal directions as a function of polar desorption angle. The crystal was prepared with preferentially the (Y, CO), a2 (o), or (us (A) states. They are indicated in the figure. The azimuthal directions are equivalent if only the C,, surface symmetry is considered.
H. Hirayama et al. / The influence of lateral interactions on the angular distribution in photodesorption
For a,-O,(a) we observe a marked maximum in (E,,,,,)/2k at 60” in the [zll] azimuthal direction and the equivalent directions in C,, symmetry. These are the directions which point from an atop site towards the neighboring fee three-fold hollow sites. The lack of a peak in the desorption flux from a3-0,(a) in these directions may well be due to the cu,-O,(a) contributions in the prepared layer. Since desorption of a,-O,(a) is peaked in normal *direction the superposition of both might appear to be cosine. And, in fact, deconvolution assuming that ayz- and ‘~~-0, contribute as expected from the respective TDS areas and a,-O,(a) with a rather constant (E,,,,,) shows that the measurements are consistent with an a,-O,(a) desorption flux that also has a maximum at 60”.
4. Discussion Palladium has a fee crystal structure. The top layer of the (111) surface has C, symmetry. Atomic oxygen forms a p(2 X 2) structure on the (111) surface. Oxygen atoms populate the threefold hollow sites. The p(2 X 2) structure can align with the (111) surface by either exclusively populating hcp or fee sites. Recent sophisticated experiments show that on transition metals, such as platinum and nickel, atomic oxygen adsorbs in fee sites [13,14]. Hence, we assume that oxygen atoms populate fee sites on Pd(ll1) as well, considering the chemical similarity of these metals. The population of the p(2 x 2) grid by atomic oxygen reduces the symmetry from six-fold to three-fold in the same manner as accounting for the abc-stacking of a fee-(111) surface does. The interaction between coadsorbed atomic and molecular oxygen is dominated by lateral repulsion. These effects are apparent in the experimentally observed suppression of molecular adsorption by preadsorbed oxygen atoms. Hence, it is not surprising that coadsorbed atomic oxygen influences the photodesorption pattern, whereas one would not expect the second metal layer to have a significant effect upon a weakly bound molecular species like 0,. Therefore, we conclude that the atomic
163
z
02-desorption
n
%
b
[% 11 direction
Fig. 3. Schematic sketch of the orientation of adsorbed Q~O,(a) induced by lateral repulsion with neighboring atomic oxygen.
oxygen which we used to prepare +0,(a) induces the C,, pattern of desorption directions. The vibrational mode of 1015 cm-’ categorizes cY,-O,(a) as a superoxo-species for which two adsorption geometries have been discussed [15]: Either an atop adsorption with a tilted molecular axis, resulting from a single bond, or a flat adsorption in a bridge site with a zig-zaggeometry. In the presence of the atomic oxygen p(2 x 2) structure, 0.25 ML of Pd surface atoms have no neighboring oxygen atoms. These are expected to be the preferred adsorption sites for cY,-O,(a). In the case of an incomplete p(2 x 2) structure, adsorption on one of the three Pd atoms around each site where an oxygen atom is missing minimizes lateral repulsion. If we first look at the possible adsorption geometry - tilted adsorption on an atop site, see fig. 3 - it is probable that the molecular axis is oriented away from the neighboring atoms. For all discussed binding sites this results in orientation towards one of the neighboring fee sites. It is in these directions that we observe the peak in the (E,,,,,) data. Hence, it might well be the orientation of the molecular axis induced by the atomic oxygen which gives rise to the desorption pattern. Alternatively one can also speculate that in the excited state, when temporarily an electron is attached,
164
H. Hirayama et al. / The influence of lateral interactions
strong repulsive lateral forces direct the desorption in an off-normal direction. Such forces would also be directed away from the nearest oxygen atom. Thus, in both alternatives the responsible interaction is the same, and both mechanisms might well contribute. The proposed interpretation of the binding geometry is similar to the interpretation of measurements by Shinn and Madey who saw a six-fold pattern of O+-desorption beams in ESDIAD of 0, adsorbed on Cr(ll0). In this case O,(a) is also characterized as a superoxo-species. Their observation of a six-fold symmetry is interpreted to resemble the surface symmetry adopted by the weakly bound and, therefore, easily rotatable and adjustable super0x0 0,. We conclude from our observations that for superoxo 0, the angular dependence of desorption flux and translational energy of photodesorbed neutral oxygen molecules are influenced by the binding geometry and lateral interactions with coadsorbed atomic oxygen. These effects are more easily observed in the (Et,,,,,) versus angle data than in the velocity integrated desorption yield, i.e. flux, since the different (E,,,,,) of the 0, species serve to discriminate between them. This clearly demonstrates the power of the TOF method. We would like to thank H. Over and W. Moritz for the LEED calculations, P.A. Thiel and K.W.
on the angular distribution
in photodesorption
Kolasinski for their helpful discussions, and G. Ertl for continuous support.
References [l] X. Guo, L. Hanley and J.T. Yates, Jr., J. Chem. Phys. 90 (1989) 5200. [2] L. Hanley, X. Guo and J.T. Yatq, Jr., J. Chem. Phys. 91 (1989) 7220. [3] M. Wolf, E. Hasselbrink, J.M. White and G. Ertl, J. Chem. Phys. 93 (1990) 5327. [4] E. Hasselbrink, H. Hirayama, A. de Meijere, F. Weik, M. Wolf and G. Ertl, Surf. Sci. 269/270 (1992) 235. 151 X.-Y. Zhu, S.R. Hatch, A. Campion and J.M. White, J. Chem. Phys. 91 (1989) 5011. 161 W.D. Mieher and W. Ho, J. Chem. Phys. 91 (1989) 2755. [7] S.R. Hatch, X.-Y. Zhu, J.M. White and A. Campion, J. Phys. Chem. 95 (1991) 1759. [8] M. Wolf, E. Hasselbrink, G. Ertl, X.-Y. Zhu and J.M. White, Surf. Sci. Lett. 248 (1991) L235. 191 F. Weik, A. de Meijere, H. Hirayama and E. Hasselbrink, J. Chem. Phys., submitted. 1101 W. Wurth, J. Stiihr, P. Feulner, X. Pan, K.R. Bauchspiess, Y. Baba, E. Hudel, G. Rocker and D. Menzel, Phys. Rev. Lett. 65 (1990) 2426. [ll] T. Engel, J. Chem. Phys. 69 (1978) 373. 1121 T. Matsushima, Surf. Sci. 157 (1985) 297. [13] K. Mortensen, C. Khnk, F. Jensen, F. Besenbacher and I. Stensgaard, Surf. Sci. 220 (1989) L701. 1141 J. Haase, B. Hillert, L. Becker and M. Pedio, Surf. Sci. 262 (1992) 8. [15] R. Imbihl and J.E. Demuth, Surf. Sci. 173 (1986) 395.
.:::‘.;:i:::,+: :(:::,.
Surface Science North-Holland
287/288
Hirayama
..:.
‘, Arne de Meijere
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Received
28 August
1992; accepted
:,,:
‘kurface science
(1993) 160-164
The influence of lateral interactions in photodesorption Hiroyuki
. . . . ,:.:. :.,.:.:.::j.::,:::::i:~:.:::jj:i
......A.
.,.
for publication
on the angular distribution
and Eckart
Hasselbrink
Faradayweg 4-6, 1000 Berlin 33, Germany 24 November
1992
Photodesorption and photodissociation are induced when 0, adsorbed on Pdtlll) is irradiated with UV light. Both processes are non-thermal. In addition thermal desorption of molecular oxygen is observed as a consequence of a displacement process by the nascent atomic oxygen. Angle-resolved time-of-flight spectra were recorded. For the adsorbed molecular superoxo species these measurements show a pronounced maximum in the translational energy at a desorption angle of 60”. Moreover, this peak is only observed at certain azimuthal angles indicating C,, symmetry.
1. Introduction Oxygen adsorbed on metal single crystal surfaces such as Pd(ll1) [l-4], Pt(ll1) [5,6] and Ag(ll0) [7] has served as a model system for the study of surface photochemistry. Time-of-flight (TOF) measurements are an avenue to shed light onto the dynamics of these processes involving desorption of molecules from the substrate. In this paper we will report angle-resolved measurements of the desorption flux and translational energy distribution of 0, photodesorbed from Pd(ll1). Molecular oxygen adsorbs on Pd(lll1 in three binding states, denoted (pi, LX*, and q, corresponding to three different adsorbate structures. These binding states appear in thermal desorption spectrometry (TDS) at desorption temperatures of 200, 155, and 120 K, respectively. Wolf et al. [8] have observed by high-resolution electron energy loss spectroscopy (HREELS) that upon irradiation with UV light, 0, dissociates to form surface oxygen. TDS ’ Permanent Research Japan.
address: NEC Corporation, Laboratories, 34 Miyukigaoka,
0039-6028/93/$06.00
0 1993 - Elsevier
Microelectronics Tsukuba 305,
Science
Publishers
measurements show that the total coverage of all oxygen species on the surface decreases at the same time [2]. Time-of-flight measurements show that molecular oxygen desorbs from the surface with a translational energy (E,,,,,)/2k corresponding to a temperature of 650 K, which is far above the actual sample temperature of 100 K. Hence, these measurements demonstrate the non-thermal character of these processes. Additionally a slow component is observed in the time-of-flight spectra, which has been interpreted as being due to a displacement process by the nascent atomic oxygen. The excitation mechanism is still under debate, but recent measurements of the excitation spectrum [4,9] favor an interpretation in terms of a transient attachment of a hot substrate electron to the 30; orbital of 0,. This additional charge transfer into an antibonding orbital is likely to cause the molecular bond to break since the molecular bond is, due to charge transfer, already weakened in the adsorbed state. The measurements are consistent with a position of the 3a, orbital 4 eV above the Fermi level in agreement with recent experiments on Pt(lll1 [lo]. Moreover, attachment of additional charge causes attraction by its image. Upon reneutraliza-
B.V. All rights reserved
H. Hirayama et al. / The injluence of lateral interactions on the angular distribution in photodesorption
tion the molecule finds itself displaced towards the substrate where repulsive interactions might dominate, eventually leading to desorption. Hence, depending on initial position and momenta, dissociation or desorption might result. The desorption direction is determined by the direction of steepest decent of the repulsive leg of the O,-Pd interaction potential. This direction does not necessarily have to be parallel to the surface normal. In this paper we will discuss dependencies of the time-of-flight spectra on the polar and azimuthal detection angles. The observations will be interpreted within the model of the desorption process outlined above. In particular we will show that the adsorption geometry of superoxo 0, is influenced by coadsorbed atomic oxygen. The binding geometry as well as a lateral force induced by the atomic oxygen results in rich structure in the angular distributions analogous to that found in electron stimulated desorption ion angular distributions (ESDIAD).
2. Experimental The experiments were carried out using a standard molecular beam apparatus 1111.Sample orientation and cleanliness were checked by low energy electron diffraction LEED. The crystal showed a sharp hexagonal pattern and a p(2 X 2) pattern after saturation with atomic oxygen. The azimuthal orientation of the sample was established by comparison of experimental and calculated LEED Z(V)-curves for the (01) and (11) spots. A KrF excimer laser was used for sample irradiation. This light source produces 10 ns pulses at a photon energy of hv = 5.0 eV. Typically pulse energies of less than 3 mJ/cm’ were employed in order to prevent noticeable substrate heating ( < 10 K>. A quadrupole mass spectrometer (QMS) mounted on a rotatable platform served for recording line-of-sight TD and TOF spectra. TOF spectra were fitted by a modified Maxwell-Boltzmann distribution, from which the desorption flux and the flux-weighted average translational energy (E,,,,, )/2k are derived. If
161
TOF spectra were bimodal, indicating a thermal desorption component, the latter was fitted by a Maxwell-Boltzmann distribution at the sample temperature and subtracted from the spectrum. Using a set of crystal holders, the azimuthal orientation of the sample could be varied in steps of 15”. The projection of the direction towards the detector onto the crystal defines the azimuthal direction of detection. Alignment of the crystallographic directions was checked by LEED with an accuracy of about 5”. The polar angle could be varied by rotating the QMS. The angle of incidence of the laser light was fixed at 45”. TOF spectra were recorded for every azimuth in polar angle steps of 12”. The volume of the ionization region of the QMS determines an angular resolution of 5”. Apart from preparing a saturated layer at 95 K sample temperature, it is possible to prepare oxygen layers in which preferentially one molecular species contributes to photodesorption. Lower binding energy species can easily be desorbed by raising the surface temperature. Additionally preadsorption of atomic oxygen suppresses population of the higher binding energy states [12]. We have used the following procedures: (1) The surface is saturated with 0, and irradiated at 153 K. At this temperature adsorption of (Ye- and cY,-O,(a) is suppressed. Hence, the desorption signal should originate solely from (piO,(a). (2) The sample was predosed with oxygen at 400 K resulting in about 0.2 ML coverage of O(a). After lowering the sample temperature to 130 K it was saturated with 0, and irradiated. This preparation results in predominant population of a,-O,(a). (3) The surface was predosed with oxygen at 400 K as above. Subsequently it was saturated with 0, at 95 K and irradiated with 8.4 X 10zl photons/cm2 to remove most of the cY,-O,(a) so that predominantly cr,-O,(a) is left on the sample. 3. Results For an O,(a) saturated sample at 95 K the photostimulated desorption flux exhibits cos@-de-
162
H. Hirayama et al. / The influence of lateral interactions on the angular distribution in photodesorption
pendence with /3 = 1.57 f 0.17. Hence, it is peaked towards the surface normal. Thermal desorption, which is observed concurrently due to displacement by newly formed atomic oxygen as already discussed, shows a cosp-dependence (p = 1.09 f 0.15) in agreement with the thermal character of this process. Measurements of the angular distribution of the desorption flux for the three preparations described above showed no marked deviation from the case of desorption from the saturated surface. The average translational energy is constant or decreasing for cri- and a,-O,(a) as the polar desorption angle is increased. In the [Tll] azimuthal direction we observe a decrease from 630 f 40 K in normal direction to 520 &-60 K at 60” for a,-O,(a) (fig. 1). The data for cu,-O,(a) show no variation of (E,,,,,)/2k. For cr,-O,(a) we observe a marked peak in the translational energy for observation in the [zll] azimuth. This peak occurs at a polar desorption angle of 60”. At increases to 1050 f 100 K this point (E,,,,,)/2k compared to 650 f 40 K for normal desorption. For larger angles it decreases again to 700 ? 100 K at 72”. This pronounced maximum of the translational energy was observed to have C,, symmetry upon azimuthal rotation of the crystal. The peak is not observed in the [12-i] direction, which
Fig. 2. Top view of the Pd(ll1) surface. The azimuthal directions at which TOF-spectra have been recorded are marked in the figure. The first, second, and third layers are shaded white, light and dark grey, respectively. The atomic oxygen pf2 X 2) structure is indicated by the small circles.
is 60” off the [zll] azimuth (fig. 2). Hence, a six-fold symmetry is ruled out. A check measurement on the [l?l] azimuth, which is 120” from [‘?lll, clearly showed the (E,,,,,)/2k peak, indicating three-fold symmetry. Summarizing our observations, we find that for LY,-and cY,-O,(a) the photodesorption flux peaked in the normal direction. The translational energy is rather constant, or may be slightly decreasing.
1100 1000 z
900
6 t 5
800 700 600 500 0
20
40
60
0
20
40
60
Polar Angle Fig. 1. Translational energy, (I?,,,,, )/2k, of photodesorbed Oz for two azimuthal directions as a function of polar desorption angle. The crystal was prepared with preferentially the (Y, CO), a2 (o), or (us (A) states. They are indicated in the figure. The azimuthal directions are equivalent if only the C,, surface symmetry is considered.
H. Hirayama et al. / The influence of lateral interactions on the angular distribution in photodesorption
For a,-O,(a) we observe a marked maximum in (E,,,,,)/2k at 60” in the [zll] azimuthal direction and the equivalent directions in C,, symmetry. These are the directions which point from an atop site towards the neighboring fee three-fold hollow sites. The lack of a peak in the desorption flux from a3-0,(a) in these directions may well be due to the cu,-O,(a) contributions in the prepared layer. Since desorption of a,-O,(a) is peaked in normal *direction the superposition of both might appear to be cosine. And, in fact, deconvolution assuming that ayz- and ‘~~-0, contribute as expected from the respective TDS areas and a,-O,(a) with a rather constant (E,,,,,) shows that the measurements are consistent with an a,-O,(a) desorption flux that also has a maximum at 60”.
4. Discussion Palladium has a fee crystal structure. The top layer of the (111) surface has C, symmetry. Atomic oxygen forms a p(2 X 2) structure on the (111) surface. Oxygen atoms populate the threefold hollow sites. The p(2 X 2) structure can align with the (111) surface by either exclusively populating hcp or fee sites. Recent sophisticated experiments show that on transition metals, such as platinum and nickel, atomic oxygen adsorbs in fee sites [13,14]. Hence, we assume that oxygen atoms populate fee sites on Pd(ll1) as well, considering the chemical similarity of these metals. The population of the p(2 x 2) grid by atomic oxygen reduces the symmetry from six-fold to three-fold in the same manner as accounting for the abc-stacking of a fee-(111) surface does. The interaction between coadsorbed atomic and molecular oxygen is dominated by lateral repulsion. These effects are apparent in the experimentally observed suppression of molecular adsorption by preadsorbed oxygen atoms. Hence, it is not surprising that coadsorbed atomic oxygen influences the photodesorption pattern, whereas one would not expect the second metal layer to have a significant effect upon a weakly bound molecular species like 0,. Therefore, we conclude that the atomic
163
z
02-desorption
n
%
b
[% 11 direction
Fig. 3. Schematic sketch of the orientation of adsorbed Q~O,(a) induced by lateral repulsion with neighboring atomic oxygen.
oxygen which we used to prepare +0,(a) induces the C,, pattern of desorption directions. The vibrational mode of 1015 cm-’ categorizes cY,-O,(a) as a superoxo-species for which two adsorption geometries have been discussed [15]: Either an atop adsorption with a tilted molecular axis, resulting from a single bond, or a flat adsorption in a bridge site with a zig-zaggeometry. In the presence of the atomic oxygen p(2 x 2) structure, 0.25 ML of Pd surface atoms have no neighboring oxygen atoms. These are expected to be the preferred adsorption sites for cY,-O,(a). In the case of an incomplete p(2 x 2) structure, adsorption on one of the three Pd atoms around each site where an oxygen atom is missing minimizes lateral repulsion. If we first look at the possible adsorption geometry - tilted adsorption on an atop site, see fig. 3 - it is probable that the molecular axis is oriented away from the neighboring atoms. For all discussed binding sites this results in orientation towards one of the neighboring fee sites. It is in these directions that we observe the peak in the (E,,,,,) data. Hence, it might well be the orientation of the molecular axis induced by the atomic oxygen which gives rise to the desorption pattern. Alternatively one can also speculate that in the excited state, when temporarily an electron is attached,
164
H. Hirayama et al. / The influence of lateral interactions
strong repulsive lateral forces direct the desorption in an off-normal direction. Such forces would also be directed away from the nearest oxygen atom. Thus, in both alternatives the responsible interaction is the same, and both mechanisms might well contribute. The proposed interpretation of the binding geometry is similar to the interpretation of measurements by Shinn and Madey who saw a six-fold pattern of O+-desorption beams in ESDIAD of 0, adsorbed on Cr(ll0). In this case O,(a) is also characterized as a superoxo-species. Their observation of a six-fold symmetry is interpreted to resemble the surface symmetry adopted by the weakly bound and, therefore, easily rotatable and adjustable super0x0 0,. We conclude from our observations that for superoxo 0, the angular dependence of desorption flux and translational energy of photodesorbed neutral oxygen molecules are influenced by the binding geometry and lateral interactions with coadsorbed atomic oxygen. These effects are more easily observed in the (Et,,,,,) versus angle data than in the velocity integrated desorption yield, i.e. flux, since the different (E,,,,,) of the 0, species serve to discriminate between them. This clearly demonstrates the power of the TOF method. We would like to thank H. Over and W. Moritz for the LEED calculations, P.A. Thiel and K.W.
on the angular distribution
in photodesorption
Kolasinski for their helpful discussions, and G. Ertl for continuous support.
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