- Email: [email protected]
Charge exchange in low-energy D+ scattering from O-, CO-, and CsCl-adsorbed Pt(111) surfaces
Nuclear Instruments
and Methods in Physics Research B 100 (1995) 389-395
Beam Interactions with Materials 8 Atoms ELSEZVIER
Charge exchange in low-energy D+ scattering from 0-, CO-, and CsCl-adsorbed Pt( 111) surfaces R. Souda *, K. Yamamoto,
W. Hayami, T. Aizawa, Y. Ishizawa
National Institute for Research in Inorganic Materials, l-l Namiki, Tsukuba, Ibaraki 305, Japan
Abstract The capture and loss of valence electrons during low-energy (- 100 eV) D+ scattering from solid surfaces have been studied from a combination of experiments and molecular-orbital-energy calculations based on the discrete variational Xa method. The surface peak of D+ surviving neutralization is almost absent for 0 at the oxygenated Pt(ll1) surface whereas it is remarkable at the CO-adsorbed Pt(ll1) surface. The enhancement of the neutralization probability of D+ at the O/Pt(lll) surface is caused by the band effect of resonance neutralization mediated by the 0-Pt bond, but the band effect is not remarkable in scattering from molecularly-adsorbed CO. The other example of the band effect is presented concerning CsCl adsorption: Almost complete neutralization occurs when D + is scattered from CsCl dissociatively adsorbed on the Pt(lll) surface, whereas neutralization probability is suppressed to some extent if D ’ is scattered from CsCl adsorbed ionically on the H-terminated Si(OO1) surface.
1. Introduction It is well established that an ion captures a surface electron via resonant neutralization (RN) and/or Auger neutralization (AN). The relative role of these processes is thought to be dependent upon the energy position of the ion vacant level relative to that of the target valence band. So far, a considerable research effort has been devoted to alkali-metal ions and noble-gas ions, but a few investigations had been made of the other ions which might be classified as “reactive” ions. Among them, hydrogen is of particular interest since it is the simplest projectile and its neutralization behavior is known to be unique [l]. In general, ions scattered from the outermost surface layer are more likely to survive neutralization than those scattered from the deeper layers, and form surface peaks in their energy distributions. This is the case for the noble-gas ions such as He+. However, the surface peak in HC or Df scattering is almost completely absent for metal and semiconductor surfaces and appears only at highly ionic-compound surfaces such as alkali halides [2]. In order to gain better insight into the charge exchange phenomena, D+ scattering experiments have been made at a variety of chemisorption systems on metal and semiconductor surfaces [3]. in this paper, attention is mainly focused on 0-, CO- and CsCl-adsorbed Pt(ll1) surfaces. The mechanism
* Corresponding author, tel. +81 52 7449, E-mail: [email protected].
298 51 3351, fax +81
298
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSLII 0168-583X(94)00821-3
of neutralization and electronic excitation will be discussed from a combination of experiments and molecular-orbital (MO) energy calculations.
2. Experiment The experiments were performed in an ultrahigh vacuum (UHV) chamber equipped with facilities for standard surface characterization. The Dt ion was generated in a discharge-type ion source and was mass analyzed by using a Wien filter. A D+ beam was incident upon a surface with an angle of 80” measured from the surface, and the D+ ions scattered specularly through a laboratory scattering angle of 160” were detected by means of a hemispherical electrostatic energy analyzer operating with a constant energy resolution of 1 eV. The Pt(ll1) surface was prepared with a standard oxygen treatment and exhibited an excellent 1 X 1 pattern in low-energy electron diffraction. CsCl was evaporated in UHV from the tantalum basket and was deposited on the clean Pt(lll) surface and the hydrogen-terminated Si(OO1) surface, which were kept at room temperature.
3. Results and discussion Fig. 1 shows energy spectra of E. = 100 eV D+ ions scattered from CsCl (- 0.4 monolayer) deposited on (a) the H-terminated Si(OO1) surface and (b) the Pt(ll1) sur-
R. Souda et al. /Nucl. Instr. and Meth. in Phys. Res. B 100 (I 995) 389-395
390
face. The arrows on the abscissa indicate the positions of the elastic binary-collision energy for individual target atoms. The D+ ions scattered from the substrates themselves are almost completely neutralized and the ion yield is increased with the amount of deposited CsCl. In Fig. la, the surface peaks (peak A) for Cs and Cl appear accompanied by the energy-loss peak labeled B. One may think that peak B comes from multiple scattering rather than the electronic excitation. But, this assumption should be discarded by considering the fact that the energy loss value is almost identical even when the incident energy is changed. The energy spectrum shown in Fig. la is characteristic of the intrinsic CsCl surface. Peak B can be ascribed to the
(a 1 CsCIIH-S
(b)
CsCl
electron-hole-pair excitation because of the good correlation between the energy-loss value of peak B and the band-gap energy of CsCl [2,3]. By contrast, the D+ spectrum in Fig. lb exhibits no marked surface peaks for Cs as well as Cl, and the spectrum is composed mainly of a broad background indicated by the broken line. The background stems from D+ ejected from the surface after penetration into the Pt substrate. The Pt(lll) surface is also exposed to oxygen (200 L; 1 L = 1 langmuir = 1.3 X 10m4 Pa s) and carbon monoxide (10 L) at room temperature, and the results of D+ scattering (E, = 100 eV) are displayed in Figs. 2a and 2b, respectively. The intensities are normalized relative to each
i(OO1)
I Pt(lll1
Cl
1
20
I
I
I
I
I
60
40
Energy
t
80
‘#
‘JS Pt
I
.
IO0
(eV)
Fig. 1. Energy spectra of E, = 100 eV D+ scattered from CsCl (0.4 ML) deposited on (a) the H-terminated 8001) surface and (b) the clean Pt(ll1) surface. The energies for the elastic binary collision are shown by arrows with chemical symbols on the abscissa.
R. Souda et al. / Nucl. Instr. and Meth. in Phys. Res. B 100 (1995) 389-395
the absence of the surface peak in the D+ spectra is correlated closely to the ionicity of the surface atomic bond. Indeed, Fig. la is typical of the D+ spectra for the highly ionic-compound surfaces. On the other hand, the absence of the marked surface peaks in Fig. lb is thought to be caused by the breakdown of the ionic Cs-Cl bond, that is, the occurrence of the dissociative adsorption and the covalent orbital hybridization with the Pt atom [2,3]. As regards neutralization of the D+ ions, it is believed
other through beam current. The oxygenated Pt(ll1) surface shows a very small 0 surface peak relative to the background, whereas the 0 peak is remarkable at the CO/Pt(lll) surface. It should be noted that the backgrounds in Figs. lb and 2 are quite similar in shape irrespective of the species of the adatoms and have a cutoff energy at around 90 eV. In the literature [2,3], it has been revealed from a large number of D+ scattering experiments that the presence or
1
500 400
(a)
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1
0 / Pt(lll) .I * I
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.*,’ /
/ 4*
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-
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relative to each other.
, #’
I
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I
40
Fig. 2. Energy spectra of E, = 100 eV D+ scattered from the Pt(ll1) normalized
, II
I
I
80
100
(eV)
surface exposed to (a) 200 L O,, and (b) 10 L CO. The intensities are
392
R. Souda et al. / Nucl. Instr. and Meth. in Phys. Res. B 100 (I 995) 389-395
that the AN process makes important contribution, but AN cannot explain the observed remarkable chemical effect on the neutralization probability. On the basis of the experiments, we have suggested that RN plays an important role in neutralization of Df [2,3]. This assumption has recently been confirmed from the MO energy calculations for hydrogen interacting with various surface clusters [4]. In what follows, the demonstration is made for the 0- and CO-adsorbed Pt(ll1) surfaces since their structures are well established. The MO energy level is numerically calculated using the self-consistent-charge discrete variational Xa (SCC-
DV-Xa) method; the strategy of the calculation has been shown elsewhere [5]. CO is molecularly adsorbed at the on-top site of Pt(ll1) with the C end towards the Pt atom. Although a small 50 donation/2n back-donation type interaction exists between the MOs of CO via the valence band of the Pt surface [6], the calculation is made for (C-O-D)+ linear chain by changing the O-D separations. It might be possible that the ion-beam bombardment causes the dissociation of CO or the change of the adsorption site. However, the beam intensity is too small to induce such a strnctural change. On the other hand, oxygen is dissociatively adsorbed at the fee three-fold hollow site of the
50
40
7-A-T ‘Pt(lll),
30
2cI-
10
0 -
-10 l-
-20 b-
-30
-40
0
1.0
2.0
3.0
4.0
d (a.u.1 surface. Fig. 3. Energy-level diagram for the (COD)+ cluster simulating D+ scattering from 0 of CO adsorbed molecularly on the Ptilll) The D Is component is shown as a function of the internuclear distance. The calculated molecular-orbital energies for CO are displayed on the right-hand side.
R. Souda et al. /Nucl. Instr. and Meth. in Phys. Res. B 100 (1995) 389-395
Pt(ll1) surface with a height of 0.85 A [7]. The simulation is made by taking the simplest (Pts-O-D)+ cluster. Figs. 3 and 4 show the u-orbital energies for the (COD) + cluster and the q-orbital energies for the (Pt,OD)+ cluster, respectively. The results are plotted relative to the valence-band-top (or the highest occupied molecular orbital; HOMO) positions so that the variations of orbitals with the D 1s character can be surveyed as a function of the separation, d, of the deuteron from the 0 atom. The distance of closest approach for E, = 100 eV Dt on 0 is 0.65 au. (1 a.u. = 0.53 A>. The population of the D 1s atomic orbital in each MO is indicated by the length of the horizontal bar. Also displayed on the right-
393
hand side of the diagram are the electronic structures calculated by using the (CO) and (Pt,O) clusters. In Fig. 3, the D 1s character appears mainly in the 5u and 6a orbitals at a large separation. With decrease of the separation, the 6a orbital is promoted and, finally, the D 1s character is concentrated on the 7u orbital. The position of the Fermi level at the Pt(lll) surface is also indicated in the figure by considering the experimental fact that 4u orbital is located at about 12 eV below the Fermi level position [8]. The ur orbital energies shown in Fig. 4 have basically the same behavior as in the (COD)+ system. Note that, at a large separation, the D Is character appears in the MOs
171
-
50
p*
T d
40
30
( Pt 30)
20
10
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-10
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-
+02p
-20
-30
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-
25a1
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-40 0
I
I
I
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2.0
3.0
I
tc II
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d (a.u.1 Fig. 4. Energy-level diagram for the @t,OD)+ cluster simulating D+ scattering from 0 dissociatively adsorbed at the fee 3-fold hollow site of the Pt(ll1)
surface.
394
R. Souda et al. / Nucl. Instr. and Meth. in Phys. Res. B 100 (I 995) 389-395
around the HOMO position, which in this case coincides with the Fermi-level position. The distribution of the D 1s character in 27a,, 28~2, and 29at orbitals at moderate separations indicates that D Is does satisfy the resonance condition in energy with the valence band states. Before interaction with D+, the 29a, orbital has the Pt 5d, 6s and 0 2p characters, whereas the 28a, and 27a, orbitals are composed mostly of the Pt Sd atomic orbital. The 30n, orbital originates from D Is and is hybridized with the Pt Sd, 6s and 0 2p orbitals. With the decrease of the separation, the orbital character changes and the D 1s component is, in turn, transferred to the MOs with higher energies. Such an orbital promotion is caused by the antibonding interaction with the 0 2s, 2p orbitals as evidenced by the downward shift of the correlated bonding orbitals. It is notable that the D 1s character disappears almost completely in the valence-band orbitals for d < 2.0 a.u. This state is the so-called “surface molecule” [9]. The above calculation clearly shows that D+ captures a valence electron via RN. However, the calculated MO energy diagrams are not sufficient to elucidate the experimental results shown in Fig. 2. In reality, ion neutralization is a highly dynamical process and the neutralization probability is thought to be determined at least by two competitive factors. One is the duration of the ion-surface interactions, T = lo-l5 s, and the other is the lifetime of the D 1s hole. The occurrence of complete neutralization at metal [Pt(lll)] and semiconductor [Si(OOl), H-terminated Si(OOl>] surfaces can be ascribed to the band effect of RN, that is, the D 1s hole cannot be localized in the MOs formed by D+ and the target atom, and diffuses irreversibly into the valence-band states of the surrounding atoms within the collision time T. On the other hand, the presence of the 0 peak for the CO/Pt(lll) surface shows that the hole is rather localized in the molecular orbitals of the (COD)+ complex and cannot diffuse into the band of the Pt(ll1) substrate. The situation is similar to the wellknown quasi-resonant neutralization of the He+ ions [lo131, where the electron (or the hole) is localized in the MOs formed by He 1s and the resonating semi-core orbital of the target. In these cases, the average neutralization probability is shown to be at most 0.5 [9,13]. The considerably small 0 peak at the O/Pt(lll) surface suggests that the D 1s hole is diffusive. The MO calculation shown in Fig. 4 reveals that RN, though suppressed in the close encounters due to the formation of the surface molecule, occurs at moderate separations (d > 2.0 au.) as a consequence of the hybridization of D 1s with the 27-29a, orbitals of the (P&O) cluster. Among them, the 29a, orbital formed with 0 2p and Pt 5d, 6s states is of most importance due to the strong hybridization with the D 1s orbital. In this case, RN occurs such that the hole of D + is initially transferred to the 29a, orbital and subsequently diffuses into the valence-band states of the Pt surface. It is thus concluded that the absence of the 0 peak at the O/ Pt(ll1) surface is caused by the band effect of the
Pt(ll1) surface mediated by the 0-Pt bond. The same mechanism holds for neutralization of D+ scattered from dissociatively-adsorbed Cs and Cl on the Pt(ll1) surface. On the other hand, the ionicity of the Cs-Cl bond causes the bottleneck of hole diffusion [2-41, which results in the appearance of the surface peaks as is typically seen in Fig. la. The spectral backgrounds shown in Figs. lb and 2 are assignable not to survival of D+ but to re-ionization of neutralized DC (D+--+ Do + DC). Re-ionization occurs in the collision with the adatoms just before leaving the surface due to the electron promotion mechanism [4]. As is seen in Figs. 3 and 4, indeed, the D Is electron can be promoted along antibonding MO in the close encounter (d < 2.0 au.). This is followed by ionization (resonance ionization; RI) due to electron diffusion into the open conduction-band states of the Pt(ll1) substrate. If the resulting ions survive RN on the receding path from the surface, they form the background in the D+ energy spectra. Re-ionization results in excitation of the D 1s electron to the conduction-band state, so that the leading edge of the background is located by several eV below the elastic collision energy for Pt. It is remarkable that the intensity of the background at the CO/Pt(lll) surface is considerably large compared to that at the O/Pt(lll) surface, although the coverage of the adatoms is not known. It is likely that the background intensity depends on the microscopic surface atomic structures; oxygen of the molecularly-adsorbed CO is located farther from the Pt(ll1) surface than the dissociatively-adsorbed 0 atom. Since the orbitals of the empty conduction-band states are spatially extended toward the vacuum side compared to the occupied valence-band state, the antibonding 6a and 7a orbitals in the (COD)+ complex may interact preferentially with the former rather than the latter, leading to the enhancement of RI relative to RN in probability.
4. Conclusion The mechanism of charge exchange in low-energy Dt scattering from the 0-, CO-, and CsCl-adsorbed Pt(ll1) surfaces has been discussed within the framework of the electron promotion mechanism and the band effect of resonant tunneling. It is revealed that neutralization of D+ scattered from molecularly-adsorbed CO on the Pt(ll1) surface is suppressed in probability because of the localized nature of the D 1s hole in the (D-O-C) complex. The same is essentially true for D+ scattering from ionically adsorbed CsCl on the H-terminated Si(OO1) surface. On the other hand, the atomically adsorbed 0, Cs, and Cl on Pt(ll1) have significant orbital hybridization with the Pt Sd, 6s states through which the D Is hole can diffuse irreversibly into the valence-band states of Pt(lll), resulting in almost complete neutralization of D+. (Re)ionization of Do occurs if the 1s electron is promoted along the
R. Souda et al./Nucl.
Instr. and Meth. in Phys. Res. B 100 (1995) 389-395
antibonding molecular orbitals and diffuses into the conduction-band state. The spectral background in D+ scattering becomes remarkable if the surface is covered with the adatoms on which Do can be ionized during collision. It is suggested that the background intensity is enhanced if the adatom is situated farther from the surface since the D 1s orbital interacts with the empty conduction-band state rather than the occupied valence-band state.
Acknowledgement We are indebted to Professor H. Adachi of Kyoto University for use of the DV-Xcx calculation program.
References [I] W. Eckstein,
in: Inelastic Particle-Surface Collisions, Springer Series of Chemical Physics vol. 17, eds E. Taglauer and W. Heiland (Springer, New York, 1981) p. 157.
El
395
R. Souda, W. Hayami, T. Aizawa, S. Otani and Y. Ishizawa, Phys. Rev. B 42 (1990) 7761; B 43 (1991) 10062; B 45 (1992) 14 358; B 50 (1994) 1934; 4733. [31 R. Souda, W. Hayami, T. Aizawa, S. Otani and Y. Ishizawa, Phys. Rev. Lett. 69 (1992) 192; Phys. Rev. B 48 (1993) 17255; Surf. Sci. 290 (1993) 245. 141 R. Souda, K. Yamamoto, W. Hayami, B. Tilley, T. Aizawa and Y. Ishizawa, Phys. Rev. B, in press. [51 H. Adachi, M. Tsukada and C. Satoko, J. Phys. Sot. Jpn. 45 (1978) 875. 161 H.P. Bonzel, Surf. Sci. Rep. 8 (1987) 43. [71 K. Mortensen, C. KIink, F. Jensen, F. Besenbacher and I. Stensgaard, Surf. Sci. 220 (1989) L701. G. Pirug and H.P. Bonzel, Surf. Sci. 133 Bl M. K&nova, (1983) 321. 191 S. Tsuneyuki, N. Shima and M. Tsukada, Surf. Sci. 186 (1987) 26. [lOI R.L. Erickson and D.P. Smith, Phys. Rev. L&t. 34 (1975) 297. ml N.H. Talk, J.C. Tully, .I. Kraus, C.W. White and S.H. Neff, Phys. Rev. L&t. 36 (1976) 747. ml A. Zartner, E. Taglauer and W. Heiland, Phys. Rev. Lett. 40 (1978) 1259. t131 J.C. Tully, Phys. Rev. B 16 (1977) 4324.
and Methods in Physics Research B 100 (1995) 389-395
Beam Interactions with Materials 8 Atoms ELSEZVIER
Charge exchange in low-energy D+ scattering from 0-, CO-, and CsCl-adsorbed Pt( 111) surfaces R. Souda *, K. Yamamoto,
W. Hayami, T. Aizawa, Y. Ishizawa
National Institute for Research in Inorganic Materials, l-l Namiki, Tsukuba, Ibaraki 305, Japan
Abstract The capture and loss of valence electrons during low-energy (- 100 eV) D+ scattering from solid surfaces have been studied from a combination of experiments and molecular-orbital-energy calculations based on the discrete variational Xa method. The surface peak of D+ surviving neutralization is almost absent for 0 at the oxygenated Pt(ll1) surface whereas it is remarkable at the CO-adsorbed Pt(ll1) surface. The enhancement of the neutralization probability of D+ at the O/Pt(lll) surface is caused by the band effect of resonance neutralization mediated by the 0-Pt bond, but the band effect is not remarkable in scattering from molecularly-adsorbed CO. The other example of the band effect is presented concerning CsCl adsorption: Almost complete neutralization occurs when D + is scattered from CsCl dissociatively adsorbed on the Pt(lll) surface, whereas neutralization probability is suppressed to some extent if D ’ is scattered from CsCl adsorbed ionically on the H-terminated Si(OO1) surface.
1. Introduction It is well established that an ion captures a surface electron via resonant neutralization (RN) and/or Auger neutralization (AN). The relative role of these processes is thought to be dependent upon the energy position of the ion vacant level relative to that of the target valence band. So far, a considerable research effort has been devoted to alkali-metal ions and noble-gas ions, but a few investigations had been made of the other ions which might be classified as “reactive” ions. Among them, hydrogen is of particular interest since it is the simplest projectile and its neutralization behavior is known to be unique [l]. In general, ions scattered from the outermost surface layer are more likely to survive neutralization than those scattered from the deeper layers, and form surface peaks in their energy distributions. This is the case for the noble-gas ions such as He+. However, the surface peak in HC or Df scattering is almost completely absent for metal and semiconductor surfaces and appears only at highly ionic-compound surfaces such as alkali halides [2]. In order to gain better insight into the charge exchange phenomena, D+ scattering experiments have been made at a variety of chemisorption systems on metal and semiconductor surfaces [3]. in this paper, attention is mainly focused on 0-, CO- and CsCl-adsorbed Pt(ll1) surfaces. The mechanism
* Corresponding author, tel. +81 52 7449, E-mail: [email protected].
298 51 3351, fax +81
298
0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSLII 0168-583X(94)00821-3
of neutralization and electronic excitation will be discussed from a combination of experiments and molecular-orbital (MO) energy calculations.
2. Experiment The experiments were performed in an ultrahigh vacuum (UHV) chamber equipped with facilities for standard surface characterization. The Dt ion was generated in a discharge-type ion source and was mass analyzed by using a Wien filter. A D+ beam was incident upon a surface with an angle of 80” measured from the surface, and the D+ ions scattered specularly through a laboratory scattering angle of 160” were detected by means of a hemispherical electrostatic energy analyzer operating with a constant energy resolution of 1 eV. The Pt(ll1) surface was prepared with a standard oxygen treatment and exhibited an excellent 1 X 1 pattern in low-energy electron diffraction. CsCl was evaporated in UHV from the tantalum basket and was deposited on the clean Pt(lll) surface and the hydrogen-terminated Si(OO1) surface, which were kept at room temperature.
3. Results and discussion Fig. 1 shows energy spectra of E. = 100 eV D+ ions scattered from CsCl (- 0.4 monolayer) deposited on (a) the H-terminated Si(OO1) surface and (b) the Pt(ll1) sur-
R. Souda et al. /Nucl. Instr. and Meth. in Phys. Res. B 100 (I 995) 389-395
390
face. The arrows on the abscissa indicate the positions of the elastic binary-collision energy for individual target atoms. The D+ ions scattered from the substrates themselves are almost completely neutralized and the ion yield is increased with the amount of deposited CsCl. In Fig. la, the surface peaks (peak A) for Cs and Cl appear accompanied by the energy-loss peak labeled B. One may think that peak B comes from multiple scattering rather than the electronic excitation. But, this assumption should be discarded by considering the fact that the energy loss value is almost identical even when the incident energy is changed. The energy spectrum shown in Fig. la is characteristic of the intrinsic CsCl surface. Peak B can be ascribed to the
(a 1 CsCIIH-S
(b)
CsCl
electron-hole-pair excitation because of the good correlation between the energy-loss value of peak B and the band-gap energy of CsCl [2,3]. By contrast, the D+ spectrum in Fig. lb exhibits no marked surface peaks for Cs as well as Cl, and the spectrum is composed mainly of a broad background indicated by the broken line. The background stems from D+ ejected from the surface after penetration into the Pt substrate. The Pt(lll) surface is also exposed to oxygen (200 L; 1 L = 1 langmuir = 1.3 X 10m4 Pa s) and carbon monoxide (10 L) at room temperature, and the results of D+ scattering (E, = 100 eV) are displayed in Figs. 2a and 2b, respectively. The intensities are normalized relative to each
i(OO1)
I Pt(lll1
Cl
1
20
I
I
I
I
I
60
40
Energy
t
80
‘#
‘JS Pt
I
.
IO0
(eV)
Fig. 1. Energy spectra of E, = 100 eV D+ scattered from CsCl (0.4 ML) deposited on (a) the H-terminated 8001) surface and (b) the clean Pt(ll1) surface. The energies for the elastic binary collision are shown by arrows with chemical symbols on the abscissa.
R. Souda et al. / Nucl. Instr. and Meth. in Phys. Res. B 100 (1995) 389-395
the absence of the surface peak in the D+ spectra is correlated closely to the ionicity of the surface atomic bond. Indeed, Fig. la is typical of the D+ spectra for the highly ionic-compound surfaces. On the other hand, the absence of the marked surface peaks in Fig. lb is thought to be caused by the breakdown of the ionic Cs-Cl bond, that is, the occurrence of the dissociative adsorption and the covalent orbital hybridization with the Pt atom [2,3]. As regards neutralization of the D+ ions, it is believed
other through beam current. The oxygenated Pt(ll1) surface shows a very small 0 surface peak relative to the background, whereas the 0 peak is remarkable at the CO/Pt(lll) surface. It should be noted that the backgrounds in Figs. lb and 2 are quite similar in shape irrespective of the species of the adatoms and have a cutoff energy at around 90 eV. In the literature [2,3], it has been revealed from a large number of D+ scattering experiments that the presence or
1
500 400
(a)
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I
I
1
1
0 / Pt(lll) .I * I
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I
-
l
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0 I
60 Energy
relative to each other.
, #’
I
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I
40
Fig. 2. Energy spectra of E, = 100 eV D+ scattered from the Pt(ll1) normalized
, II
I
I
80
100
(eV)
surface exposed to (a) 200 L O,, and (b) 10 L CO. The intensities are
392
R. Souda et al. / Nucl. Instr. and Meth. in Phys. Res. B 100 (I 995) 389-395
that the AN process makes important contribution, but AN cannot explain the observed remarkable chemical effect on the neutralization probability. On the basis of the experiments, we have suggested that RN plays an important role in neutralization of Df [2,3]. This assumption has recently been confirmed from the MO energy calculations for hydrogen interacting with various surface clusters [4]. In what follows, the demonstration is made for the 0- and CO-adsorbed Pt(ll1) surfaces since their structures are well established. The MO energy level is numerically calculated using the self-consistent-charge discrete variational Xa (SCC-
DV-Xa) method; the strategy of the calculation has been shown elsewhere [5]. CO is molecularly adsorbed at the on-top site of Pt(ll1) with the C end towards the Pt atom. Although a small 50 donation/2n back-donation type interaction exists between the MOs of CO via the valence band of the Pt surface [6], the calculation is made for (C-O-D)+ linear chain by changing the O-D separations. It might be possible that the ion-beam bombardment causes the dissociation of CO or the change of the adsorption site. However, the beam intensity is too small to induce such a strnctural change. On the other hand, oxygen is dissociatively adsorbed at the fee three-fold hollow site of the
50
40
7-A-T ‘Pt(lll),
30
2cI-
10
0 -
-10 l-
-20 b-
-30
-40
0
1.0
2.0
3.0
4.0
d (a.u.1 surface. Fig. 3. Energy-level diagram for the (COD)+ cluster simulating D+ scattering from 0 of CO adsorbed molecularly on the Ptilll) The D Is component is shown as a function of the internuclear distance. The calculated molecular-orbital energies for CO are displayed on the right-hand side.
R. Souda et al. /Nucl. Instr. and Meth. in Phys. Res. B 100 (1995) 389-395
Pt(ll1) surface with a height of 0.85 A [7]. The simulation is made by taking the simplest (Pts-O-D)+ cluster. Figs. 3 and 4 show the u-orbital energies for the (COD) + cluster and the q-orbital energies for the (Pt,OD)+ cluster, respectively. The results are plotted relative to the valence-band-top (or the highest occupied molecular orbital; HOMO) positions so that the variations of orbitals with the D 1s character can be surveyed as a function of the separation, d, of the deuteron from the 0 atom. The distance of closest approach for E, = 100 eV Dt on 0 is 0.65 au. (1 a.u. = 0.53 A>. The population of the D 1s atomic orbital in each MO is indicated by the length of the horizontal bar. Also displayed on the right-
393
hand side of the diagram are the electronic structures calculated by using the (CO) and (Pt,O) clusters. In Fig. 3, the D 1s character appears mainly in the 5u and 6a orbitals at a large separation. With decrease of the separation, the 6a orbital is promoted and, finally, the D 1s character is concentrated on the 7u orbital. The position of the Fermi level at the Pt(lll) surface is also indicated in the figure by considering the experimental fact that 4u orbital is located at about 12 eV below the Fermi level position [8]. The ur orbital energies shown in Fig. 4 have basically the same behavior as in the (COD)+ system. Note that, at a large separation, the D Is character appears in the MOs
171
-
50
p*
T d
40
30
( Pt 30)
20
10
0 - 2&l _ 35e
-10
PtSd I
-
+02p
-20
-30
c
25a1
-
25a1
02s
-40 0
I
I
I
1.0
2.0
3.0
I
tc II
4.0
d (a.u.1 Fig. 4. Energy-level diagram for the @t,OD)+ cluster simulating D+ scattering from 0 dissociatively adsorbed at the fee 3-fold hollow site of the Pt(ll1)
surface.
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around the HOMO position, which in this case coincides with the Fermi-level position. The distribution of the D 1s character in 27a,, 28~2, and 29at orbitals at moderate separations indicates that D Is does satisfy the resonance condition in energy with the valence band states. Before interaction with D+, the 29a, orbital has the Pt 5d, 6s and 0 2p characters, whereas the 28a, and 27a, orbitals are composed mostly of the Pt Sd atomic orbital. The 30n, orbital originates from D Is and is hybridized with the Pt Sd, 6s and 0 2p orbitals. With the decrease of the separation, the orbital character changes and the D 1s component is, in turn, transferred to the MOs with higher energies. Such an orbital promotion is caused by the antibonding interaction with the 0 2s, 2p orbitals as evidenced by the downward shift of the correlated bonding orbitals. It is notable that the D 1s character disappears almost completely in the valence-band orbitals for d < 2.0 a.u. This state is the so-called “surface molecule” [9]. The above calculation clearly shows that D+ captures a valence electron via RN. However, the calculated MO energy diagrams are not sufficient to elucidate the experimental results shown in Fig. 2. In reality, ion neutralization is a highly dynamical process and the neutralization probability is thought to be determined at least by two competitive factors. One is the duration of the ion-surface interactions, T = lo-l5 s, and the other is the lifetime of the D 1s hole. The occurrence of complete neutralization at metal [Pt(lll)] and semiconductor [Si(OOl), H-terminated Si(OOl>] surfaces can be ascribed to the band effect of RN, that is, the D 1s hole cannot be localized in the MOs formed by D+ and the target atom, and diffuses irreversibly into the valence-band states of the surrounding atoms within the collision time T. On the other hand, the presence of the 0 peak for the CO/Pt(lll) surface shows that the hole is rather localized in the molecular orbitals of the (COD)+ complex and cannot diffuse into the band of the Pt(ll1) substrate. The situation is similar to the wellknown quasi-resonant neutralization of the He+ ions [lo131, where the electron (or the hole) is localized in the MOs formed by He 1s and the resonating semi-core orbital of the target. In these cases, the average neutralization probability is shown to be at most 0.5 [9,13]. The considerably small 0 peak at the O/Pt(lll) surface suggests that the D 1s hole is diffusive. The MO calculation shown in Fig. 4 reveals that RN, though suppressed in the close encounters due to the formation of the surface molecule, occurs at moderate separations (d > 2.0 au.) as a consequence of the hybridization of D 1s with the 27-29a, orbitals of the (P&O) cluster. Among them, the 29a, orbital formed with 0 2p and Pt 5d, 6s states is of most importance due to the strong hybridization with the D 1s orbital. In this case, RN occurs such that the hole of D + is initially transferred to the 29a, orbital and subsequently diffuses into the valence-band states of the Pt surface. It is thus concluded that the absence of the 0 peak at the O/ Pt(ll1) surface is caused by the band effect of the
Pt(ll1) surface mediated by the 0-Pt bond. The same mechanism holds for neutralization of D+ scattered from dissociatively-adsorbed Cs and Cl on the Pt(ll1) surface. On the other hand, the ionicity of the Cs-Cl bond causes the bottleneck of hole diffusion [2-41, which results in the appearance of the surface peaks as is typically seen in Fig. la. The spectral backgrounds shown in Figs. lb and 2 are assignable not to survival of D+ but to re-ionization of neutralized DC (D+--+ Do + DC). Re-ionization occurs in the collision with the adatoms just before leaving the surface due to the electron promotion mechanism [4]. As is seen in Figs. 3 and 4, indeed, the D Is electron can be promoted along antibonding MO in the close encounter (d < 2.0 au.). This is followed by ionization (resonance ionization; RI) due to electron diffusion into the open conduction-band states of the Pt(ll1) substrate. If the resulting ions survive RN on the receding path from the surface, they form the background in the D+ energy spectra. Re-ionization results in excitation of the D 1s electron to the conduction-band state, so that the leading edge of the background is located by several eV below the elastic collision energy for Pt. It is remarkable that the intensity of the background at the CO/Pt(lll) surface is considerably large compared to that at the O/Pt(lll) surface, although the coverage of the adatoms is not known. It is likely that the background intensity depends on the microscopic surface atomic structures; oxygen of the molecularly-adsorbed CO is located farther from the Pt(ll1) surface than the dissociatively-adsorbed 0 atom. Since the orbitals of the empty conduction-band states are spatially extended toward the vacuum side compared to the occupied valence-band state, the antibonding 6a and 7a orbitals in the (COD)+ complex may interact preferentially with the former rather than the latter, leading to the enhancement of RI relative to RN in probability.
4. Conclusion The mechanism of charge exchange in low-energy Dt scattering from the 0-, CO-, and CsCl-adsorbed Pt(ll1) surfaces has been discussed within the framework of the electron promotion mechanism and the band effect of resonant tunneling. It is revealed that neutralization of D+ scattered from molecularly-adsorbed CO on the Pt(ll1) surface is suppressed in probability because of the localized nature of the D 1s hole in the (D-O-C) complex. The same is essentially true for D+ scattering from ionically adsorbed CsCl on the H-terminated Si(OO1) surface. On the other hand, the atomically adsorbed 0, Cs, and Cl on Pt(ll1) have significant orbital hybridization with the Pt Sd, 6s states through which the D Is hole can diffuse irreversibly into the valence-band states of Pt(lll), resulting in almost complete neutralization of D+. (Re)ionization of Do occurs if the 1s electron is promoted along the
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antibonding molecular orbitals and diffuses into the conduction-band state. The spectral background in D+ scattering becomes remarkable if the surface is covered with the adatoms on which Do can be ionized during collision. It is suggested that the background intensity is enhanced if the adatom is situated farther from the surface since the D 1s orbital interacts with the empty conduction-band state rather than the occupied valence-band state.
Acknowledgement We are indebted to Professor H. Adachi of Kyoto University for use of the DV-Xcx calculation program.
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