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Simultaneous mass and kinetic energy analysis of Ar+ and Ar2+ ions photodesorbed from condensed argon
Nuclear Instruments and Methods in Physics Research B 101 (1995) 200.. 202
Beam Interactions with Materials & Atoms ELSEVIER
Simultaneous mass and kinetic energy analysis of Ar ’ and Ari ions photodesorbed from condensed argon L. Philippe”,*, G. DujardirFb,
M.J. Besnard Ramageaqb, L. Hellnera-b, T. Hirayamab*c, G. Comteta, M. Roseb
aLaboratoire pour 1‘Utilisation da Rayonnement Electromagndtique (LURE) B2t. 2090. lJGcersit6 Paris-&d, 91405 0rsa.v Cede.*, France b Laboratoire de Photophysique Molt;culaire. Centre National de la Recherche Scientijiyue. Bit. 213. Unioersiti Paris-Sad, 91405 Orsal Cede.u, France ‘Department of Phvsics, Gakushuin University. l-5-1 Mejiro, Toshimaku. Tokio 171. Japan
Abstract Mass and kinetic analysis of photodesorbed ions from argon multilayers shows that the kinetic energy distributions depend on both the mass of the ions and the photon excitation energy. Measured kinetic energies are in good agreement with previously proposed models for photodesorption of ions at hv = 40 eV and hv = 100 eV.
1. Introduction A basic issue of ion photodesorption involves the sequence of processes which follow the primary electronic excitation. A direct desorption mechanism in the primary electronic excited state is usually not likely to occur since a variety of relaxation processes may compete with the direct bond breaking. Possible relaxation processes include recombination with electrons, emisson of photons, excitation of phonons In order to get a detailed insight into these complex mechanisms it is essential to analyse the translational energy of photodesorbed ions. Such kinetic energy measurements are rather scarce in the literature and have been previously performed without mass selection of the desorbed ions [ 141. They were then limited to special situations where only one ionic species is desorbed. Here we report results from a new experiment where translational energy measurements of photodesorbed ions can be achieved with simultaneous kinetic energy and mass analysis. Condensed rare gases, in particular condensed argon. offer a number of advantages for ion desorption studies because of simplicity and of their well known physical and electronic properties. However, until recently it was believed that ion photodesorption could not exist from condensed rare gases. Ion desorption has been mentioned for the first time in 1990 both for valence [S] and core [6] photoexcitation. Several primary electronic processes including formation of ionic satellite states [S], *Corresponding
author.
Elsevier Science B.V. SSDI 0168-583X(95)00289-8
direct double and triple ionization [S], 2p core excitation [6] and formation of exciton pairs [7.8] have been shown to produce ion desorption from condensed argon. In this paper, we consider ion photodesorption from condensed argon as produced by formation of ionic satellite states and by direct double photoionization.
2. Experimental Synchrotron radiation from Super-AC0 in Orsay, disOpersed by a grazing incidence monochromator with a I A bandpass, is used as a photon source of variable energy in the 20-100 eV range. Experiments are performed in an ultrahigh vacuum chamber (basse pressure of 7 x lo- l1 mbar) equipped with a liquid-helium flow cryostat. Pure argon is condensed at low temperature (15 K) on a polycrystalline substrate which can be cleaned by resistive heating. The thickness of thin argon layers is measured as described in Ref. [4]. Ions are detected along the sample normal. The ion spectrometer includes a cylindrical mirror analyzer (CMA) with a 0.25 eV energy resolution followed by a mass quadrupole for simultaneous kinetic energy and mass analysis of desorbed ions. Reported ion kinetic energies are relative to the vacuum level of the sample.
3. Results and discussion Experiments are performed with 20 monolayers (ML) of condensed argon. This thickness is choosen in order to
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L. Philippe et al. / Nucl. Insir. and Meth. in Phys. Res. B 101 (1995) 200-202
avoid any interaction, by Auger neutralization or by image charge interaction, with the metallic substrate [4]. Ion photodesorption is studied at two photon excitation energies. 40 and 100 eV. As previously discussed [5]. the primary electronic photoexcitation producing ion desorption at hv = 40 eV is the formation of ionic satellite states (Art)* whose binding energies relative to the vacuum level lie in the 34-41 eV range [9] and which correspond to the ejection of a 3p electron and the simultaneous excitation of a 3p electron to an unoccupied valence band. At hv = 100 eV, the primary electronic excitation producing ion desorption is considered to be the formation of Arf + doubly charged ions [S] corresponding to the removal of two correlated 3p electrons and whose the binding energy is expected to be around 40 eV [lo]. Preliminary measurements previously reported [4,1 l] have shown that the kinetic energy of photodesorbed ions at hv = 100 eV and hv = 40 eV is much smaller that what is expected from a direct desorption process. It was concluded that relaxation processes, mainly by radiative electronic transitions [4], play an important role in the desorption process. However, it was not possible to determine if the kinetic energy of desorbed ions depends on the photon energy. These previous kinetic energy measurements [4,1 I] were indeed performed without mass selection of the desorbed ions. As noted in Ref. [4], the kinetic energy distribution (KED) of desorbed ions at hv = 40 eV was narrower and had a maximum at a lower energy than the distribution at hv = 100eV. However, this effect could not be unambiguously ascribed to the Arf ion desorption since other impurity ions were also desorbed at hv = 40 eV. Furthermore. the kinetic energy distribution was assumed to be that of the dominant Ar+ desorbed species and the KED of minority species like Ar: could not be obtained. By using the new spectrometer described in paragraph II, we perform simultaneous mass and kinetic energy analysis of desorbed ions from argon multilayers. KED of desorbed Ar+ and Ar: ions for hv = 40 eV and hv = 100 eV are shown in Fig. 1 for 20 monolayers of condensed argon. Some of these KED curves are quite compatible with similar results previously obtained without mass selection of the desorbed ions [4]. For example, at hu = 100 eV the KED of Arf in Fig. I has a maximum at 1.1 eV and a width of about 1.2 eV. In Ref. [4], the KED of Ar+ at hv = 100 eV for the same argon thickness (20 ML) has the same shape and a maximum at 0.8 eV and a similar width of 1.2 eV. The energy difference of about 0.3 eV between the curve maxima in Fig. 1 and in Ref. [4] results from the uncertainties in calibrating the various spectrometers. However, this does not affect the comparison between various KED curves (like in Fig. 1) recorded with the same spectrometer. The energy of the maximum (M) and the width (W) of each KED curve of Fig. 1 are shown in Table 1. We note that the KED
ffinencEnergy (eV)
(e;)
Kmuc E&y
Fig. 1. Kinetic energy distributions of desorbed Ar+ and Ar2+ ions at hv = 40 eV (left) and hv = 100 eV (right).
Table 1 Energy of the maximum (M) and width (W) of the KED of Ar+ and Ar: photodesorbed ions at hv = 40 eV and hv = 100 eV. Uncertainties of the M values of the order of 0.3 eV are mainly related to the uncertainty in calibrating the spectrometer Excitation Ion satellite hv = 40 eV
states
Doubly charged ho = 100 eV
ions
Arf
Ar;
M = 0.8 eV W = 1.1 eV
M = 0.3 eV W = 0.6 eV
M = 1.1 eV W = 1.2eV
M = 0.8 eV W = 0.8 eV
depends on the photon energy both for Ar+ and Ar: ions and that for a given photon energy the kinetic energy of Ari ions is reduced as compared to that of Ar+ions. This reduction is even more pronounced at 40 eV than at 1OOeV since the ratio M(Ar:)/M(Ar+) of the energy maxima is 0.73 at hv = 100 eV whereas it is only 0.37 at hv = 40 eV. These ratios can be understood within the model which was proposed in Ref. [4]. According to this model the Ar+ + ions produced at hv = 100 eV interact with neighbour Ar atoms and relax by fluorescence towards the repulsive Ar+ + Ar+ potential energy curve. Considering the Ar+ + + Ar and Ar+ + Ar+ potential energy curves calculated by Cachoncinlle et al. [ 123, the total kinetic energy release in the Ar+ - Ar’ repulsion can be estimated by two different methods. In Ref. [4], a kinetic energy release of 5.7 eV was obtained by substracting the fluorescence energy (6.8 eV) plus the energy of the Ar+ + Ar+ limit in the solid (27.6 eV) from the double ionization energy in the solid (39.8 eV). Since this latter energy is not well known, we choose here to estimate the kinetic energy release by a different way. After fluorescence relaxation from the equilibrium geometry of the Ar’ + + Ar potential energy curve (see Ref. [4] ), one reaches the Ar+ + Ar’ curve at a potential energy which can be estimated from the calculations of Cachoncinlle et al. [12] to be 4.7 eV above the Ar+ + Ar+ dissociation energy. This 4.7 eV value is then a more precise estimation of the total kinetic energy release during the Ar’ - Ar’ separation. In order to obtain the kinetic
V. FROZEN
GASES
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L. Philippe et al. ! Nucl. Insrr. and M&h. it? Phvs. Res. B 101 1199.51 200-202
energy of the desorbed Ar+ ions, one needs to divide the total kinetic energy into two parts (each Ar’ ion takes half of the total energy) and to subtract the barrier energy (2 eV) which prevents ions to desorb from the argon solid. Subtracting the barrier energy after dividing into two parts the total kinetic energy release, like it was done in Ref. 141, is equivalent to consider that the Coulomb repulsion occurs in the solid and that one of the expelled Arf ions leaves the solid afterwards. Here we occurs at the consider that the Ar+ - Ar’ separation surface of the solid argon and consequently we first substract the 2 eV barrier energy and then divide into two in order to obtain the kinetic energy of desorbed Ar’ ions. This leads to a predicted value of 1.35 eV for the kinetic energy of desorbed Ar + ions. Similarly, the kinetic energy of desorbed Ar: may be estimated from Ar; ’ + Ar and Ar: + Ar+ potential energy curves. From recent calculations of Langhoff 1131, the total kinetic energy release in the Ar; + Arf repulsion is exactly equal to that of the Ar+ + Ar+ system. Assuming that the Ar; ion takes f of this 2.7 eV energy the At-Z+ desorbed ion is expected to get a 0.9 eV kinetic energy. These predicted values of kinetic energies of Ar’ ions (1.35 eV) and Ar: ions (0.9 eV) are in good agreement with experimental values. Similar estimations at hv = 40 eV can be done only qualitatively since the (Ar+)* + Ar and (AT;)* + Ar potential energy curves are not known. (Ar+)* + Ar ((Ari)* + Ar) is expected to relax by fluorescence towards Ar+ + Ar (Art + Ar). Since the bound state of Ar: + Ar is much less bound (z 0.2 eV) than that of Ar+ + Ar (z 1.2 eV), the Ar: + Ar repulsive state is thought to be much less repulsive than the Ar+ + Ar one. It follows that the kinetic energy of desorbed Arf ions is expected to be smaller than $ of that Ar+ ion. This is also in qualitative agreement with experimental findings. We emphasize that within this model the interaction of desorbing ions with other argon atoms in the solid has been
neglected. The main effect of these surrounding atoms is expected to be a screening of the various Ar+ + - Ar. Ar+ - Ar’, interactions. This screening effect should be taken into account in a complete picture of the desorption mechanism and would most probably weaken the above mentioned inter-atomic Ar+ - Ar+. Arf ~ Ar+ repulsive interactions and then decrease the kinetic energy of desorbed ions.
References [II T.R. Pian, M.M. Traum.
J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surf. Sci. 128 (1983) 13. c21 J.A. Kelber, R.R. Daniels, M. Turowski, G. Margaritondo, N.H. Talk and J.S. Kraus, Phys. Rev. 830 (1984) 4748. [31 J.A. Yarmoff and S.A. Joyce, Phys. Rev. B40 (1989) 3 143. M G. Dujardin, L. Hellner, L. Philippe, M.J. Besnard-Ramage and P. Cirkel, Phys. Rev. B48 (1993) 14 529. and R. ES1 G. Dujardin, L. Hellner, M.J. Besnard-Ramage Azria. Phys. Rev. Len. 64 (1990) 1289. C61G. Rocker, P. Feulner, R. Scheuerer, L. Zhu and D. Menzel, Phys. Ser. 41 (1990) 1014. 171 Y. Baba, G. Dujardin. P. Feulner and D. Menzel, Phys. Rev. Lett. 66 (1991) 3269. R. Scheuerer. E. Hudel and P. Feulner. CSI T. Schwabenthan, Solid State Commun. 80 (1991) 773. c91 G. Dujardin. L. Hellner, L. Philippe and M.J. BesnardRamage, in: Proc. 10th VUV Conf.,eds. F.J. Willeumier, Y. Petroff and 1. Nenner (World Scientific, Singapore. 1993) p. 378. DOI H.W. Biester. M.J. Besnard, G. Dujardin, L. Hellner and E.E. Koch, Phys. Rev. Lett. 59 (1987) 1277. [ill G. Dujardin, L. Hellner, L. Philippe. M.J. Besnard-Ramage, R. Azria, M. Rose and P. Cirkel, in: DIET V, eds. A.R. Burns, E.B. Steichel and D.R. Jennison. Springer series in Surf. Sci.. Vol 31 (1993) 337. Cl21 C. Cachoncinlle, J.M. Pouvesle, G. Durand and F. Spiegelmann, J. Chem. Phys. 96 (1992) 6085. Cl31 H. Langhoff. J. Phys. B 27 (1994) L709.
Beam Interactions with Materials & Atoms ELSEVIER
Simultaneous mass and kinetic energy analysis of Ar ’ and Ari ions photodesorbed from condensed argon L. Philippe”,*, G. DujardirFb,
M.J. Besnard Ramageaqb, L. Hellnera-b, T. Hirayamab*c, G. Comteta, M. Roseb
aLaboratoire pour 1‘Utilisation da Rayonnement Electromagndtique (LURE) B2t. 2090. lJGcersit6 Paris-&d, 91405 0rsa.v Cede.*, France b Laboratoire de Photophysique Molt;culaire. Centre National de la Recherche Scientijiyue. Bit. 213. Unioersiti Paris-Sad, 91405 Orsal Cede.u, France ‘Department of Phvsics, Gakushuin University. l-5-1 Mejiro, Toshimaku. Tokio 171. Japan
Abstract Mass and kinetic analysis of photodesorbed ions from argon multilayers shows that the kinetic energy distributions depend on both the mass of the ions and the photon excitation energy. Measured kinetic energies are in good agreement with previously proposed models for photodesorption of ions at hv = 40 eV and hv = 100 eV.
1. Introduction A basic issue of ion photodesorption involves the sequence of processes which follow the primary electronic excitation. A direct desorption mechanism in the primary electronic excited state is usually not likely to occur since a variety of relaxation processes may compete with the direct bond breaking. Possible relaxation processes include recombination with electrons, emisson of photons, excitation of phonons In order to get a detailed insight into these complex mechanisms it is essential to analyse the translational energy of photodesorbed ions. Such kinetic energy measurements are rather scarce in the literature and have been previously performed without mass selection of the desorbed ions [ 141. They were then limited to special situations where only one ionic species is desorbed. Here we report results from a new experiment where translational energy measurements of photodesorbed ions can be achieved with simultaneous kinetic energy and mass analysis. Condensed rare gases, in particular condensed argon. offer a number of advantages for ion desorption studies because of simplicity and of their well known physical and electronic properties. However, until recently it was believed that ion photodesorption could not exist from condensed rare gases. Ion desorption has been mentioned for the first time in 1990 both for valence [S] and core [6] photoexcitation. Several primary electronic processes including formation of ionic satellite states [S], *Corresponding
author.
Elsevier Science B.V. SSDI 0168-583X(95)00289-8
direct double and triple ionization [S], 2p core excitation [6] and formation of exciton pairs [7.8] have been shown to produce ion desorption from condensed argon. In this paper, we consider ion photodesorption from condensed argon as produced by formation of ionic satellite states and by direct double photoionization.
2. Experimental Synchrotron radiation from Super-AC0 in Orsay, disOpersed by a grazing incidence monochromator with a I A bandpass, is used as a photon source of variable energy in the 20-100 eV range. Experiments are performed in an ultrahigh vacuum chamber (basse pressure of 7 x lo- l1 mbar) equipped with a liquid-helium flow cryostat. Pure argon is condensed at low temperature (15 K) on a polycrystalline substrate which can be cleaned by resistive heating. The thickness of thin argon layers is measured as described in Ref. [4]. Ions are detected along the sample normal. The ion spectrometer includes a cylindrical mirror analyzer (CMA) with a 0.25 eV energy resolution followed by a mass quadrupole for simultaneous kinetic energy and mass analysis of desorbed ions. Reported ion kinetic energies are relative to the vacuum level of the sample.
3. Results and discussion Experiments are performed with 20 monolayers (ML) of condensed argon. This thickness is choosen in order to
201
L. Philippe et al. / Nucl. Insir. and Meth. in Phys. Res. B 101 (1995) 200-202
avoid any interaction, by Auger neutralization or by image charge interaction, with the metallic substrate [4]. Ion photodesorption is studied at two photon excitation energies. 40 and 100 eV. As previously discussed [5]. the primary electronic photoexcitation producing ion desorption at hv = 40 eV is the formation of ionic satellite states (Art)* whose binding energies relative to the vacuum level lie in the 34-41 eV range [9] and which correspond to the ejection of a 3p electron and the simultaneous excitation of a 3p electron to an unoccupied valence band. At hv = 100 eV, the primary electronic excitation producing ion desorption is considered to be the formation of Arf + doubly charged ions [S] corresponding to the removal of two correlated 3p electrons and whose the binding energy is expected to be around 40 eV [lo]. Preliminary measurements previously reported [4,1 l] have shown that the kinetic energy of photodesorbed ions at hv = 100 eV and hv = 40 eV is much smaller that what is expected from a direct desorption process. It was concluded that relaxation processes, mainly by radiative electronic transitions [4], play an important role in the desorption process. However, it was not possible to determine if the kinetic energy of desorbed ions depends on the photon energy. These previous kinetic energy measurements [4,1 I] were indeed performed without mass selection of the desorbed ions. As noted in Ref. [4], the kinetic energy distribution (KED) of desorbed ions at hv = 40 eV was narrower and had a maximum at a lower energy than the distribution at hv = 100eV. However, this effect could not be unambiguously ascribed to the Arf ion desorption since other impurity ions were also desorbed at hv = 40 eV. Furthermore. the kinetic energy distribution was assumed to be that of the dominant Ar+ desorbed species and the KED of minority species like Ar: could not be obtained. By using the new spectrometer described in paragraph II, we perform simultaneous mass and kinetic energy analysis of desorbed ions from argon multilayers. KED of desorbed Ar+ and Ar: ions for hv = 40 eV and hv = 100 eV are shown in Fig. 1 for 20 monolayers of condensed argon. Some of these KED curves are quite compatible with similar results previously obtained without mass selection of the desorbed ions [4]. For example, at hu = 100 eV the KED of Arf in Fig. I has a maximum at 1.1 eV and a width of about 1.2 eV. In Ref. [4], the KED of Ar+ at hv = 100 eV for the same argon thickness (20 ML) has the same shape and a maximum at 0.8 eV and a similar width of 1.2 eV. The energy difference of about 0.3 eV between the curve maxima in Fig. 1 and in Ref. [4] results from the uncertainties in calibrating the various spectrometers. However, this does not affect the comparison between various KED curves (like in Fig. 1) recorded with the same spectrometer. The energy of the maximum (M) and the width (W) of each KED curve of Fig. 1 are shown in Table 1. We note that the KED
ffinencEnergy (eV)
(e;)
Kmuc E&y
Fig. 1. Kinetic energy distributions of desorbed Ar+ and Ar2+ ions at hv = 40 eV (left) and hv = 100 eV (right).
Table 1 Energy of the maximum (M) and width (W) of the KED of Ar+ and Ar: photodesorbed ions at hv = 40 eV and hv = 100 eV. Uncertainties of the M values of the order of 0.3 eV are mainly related to the uncertainty in calibrating the spectrometer Excitation Ion satellite hv = 40 eV
states
Doubly charged ho = 100 eV
ions
Arf
Ar;
M = 0.8 eV W = 1.1 eV
M = 0.3 eV W = 0.6 eV
M = 1.1 eV W = 1.2eV
M = 0.8 eV W = 0.8 eV
depends on the photon energy both for Ar+ and Ar: ions and that for a given photon energy the kinetic energy of Ari ions is reduced as compared to that of Ar+ions. This reduction is even more pronounced at 40 eV than at 1OOeV since the ratio M(Ar:)/M(Ar+) of the energy maxima is 0.73 at hv = 100 eV whereas it is only 0.37 at hv = 40 eV. These ratios can be understood within the model which was proposed in Ref. [4]. According to this model the Ar+ + ions produced at hv = 100 eV interact with neighbour Ar atoms and relax by fluorescence towards the repulsive Ar+ + Ar+ potential energy curve. Considering the Ar+ + + Ar and Ar+ + Ar+ potential energy curves calculated by Cachoncinlle et al. [ 123, the total kinetic energy release in the Ar+ - Ar’ repulsion can be estimated by two different methods. In Ref. [4], a kinetic energy release of 5.7 eV was obtained by substracting the fluorescence energy (6.8 eV) plus the energy of the Ar+ + Ar+ limit in the solid (27.6 eV) from the double ionization energy in the solid (39.8 eV). Since this latter energy is not well known, we choose here to estimate the kinetic energy release by a different way. After fluorescence relaxation from the equilibrium geometry of the Ar’ + + Ar potential energy curve (see Ref. [4] ), one reaches the Ar+ + Ar’ curve at a potential energy which can be estimated from the calculations of Cachoncinlle et al. [12] to be 4.7 eV above the Ar+ + Ar+ dissociation energy. This 4.7 eV value is then a more precise estimation of the total kinetic energy release during the Ar’ - Ar’ separation. In order to obtain the kinetic
V. FROZEN
GASES
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L. Philippe et al. ! Nucl. Insrr. and M&h. it? Phvs. Res. B 101 1199.51 200-202
energy of the desorbed Ar+ ions, one needs to divide the total kinetic energy into two parts (each Ar’ ion takes half of the total energy) and to subtract the barrier energy (2 eV) which prevents ions to desorb from the argon solid. Subtracting the barrier energy after dividing into two parts the total kinetic energy release, like it was done in Ref. 141, is equivalent to consider that the Coulomb repulsion occurs in the solid and that one of the expelled Arf ions leaves the solid afterwards. Here we occurs at the consider that the Ar+ - Ar’ separation surface of the solid argon and consequently we first substract the 2 eV barrier energy and then divide into two in order to obtain the kinetic energy of desorbed Ar’ ions. This leads to a predicted value of 1.35 eV for the kinetic energy of desorbed Ar + ions. Similarly, the kinetic energy of desorbed Ar: may be estimated from Ar; ’ + Ar and Ar: + Ar+ potential energy curves. From recent calculations of Langhoff 1131, the total kinetic energy release in the Ar; + Arf repulsion is exactly equal to that of the Ar+ + Ar+ system. Assuming that the Ar; ion takes f of this 2.7 eV energy the At-Z+ desorbed ion is expected to get a 0.9 eV kinetic energy. These predicted values of kinetic energies of Ar’ ions (1.35 eV) and Ar: ions (0.9 eV) are in good agreement with experimental values. Similar estimations at hv = 40 eV can be done only qualitatively since the (Ar+)* + Ar and (AT;)* + Ar potential energy curves are not known. (Ar+)* + Ar ((Ari)* + Ar) is expected to relax by fluorescence towards Ar+ + Ar (Art + Ar). Since the bound state of Ar: + Ar is much less bound (z 0.2 eV) than that of Ar+ + Ar (z 1.2 eV), the Ar: + Ar repulsive state is thought to be much less repulsive than the Ar+ + Ar one. It follows that the kinetic energy of desorbed Arf ions is expected to be smaller than $ of that Ar+ ion. This is also in qualitative agreement with experimental findings. We emphasize that within this model the interaction of desorbing ions with other argon atoms in the solid has been
neglected. The main effect of these surrounding atoms is expected to be a screening of the various Ar+ + - Ar. Ar+ - Ar’, interactions. This screening effect should be taken into account in a complete picture of the desorption mechanism and would most probably weaken the above mentioned inter-atomic Ar+ - Ar+. Arf ~ Ar+ repulsive interactions and then decrease the kinetic energy of desorbed ions.
References [II T.R. Pian, M.M. Traum.
J.S. Kraus, N.H. Tolk, N.G. Stoffel and G. Margaritondo, Surf. Sci. 128 (1983) 13. c21 J.A. Kelber, R.R. Daniels, M. Turowski, G. Margaritondo, N.H. Talk and J.S. Kraus, Phys. Rev. 830 (1984) 4748. [31 J.A. Yarmoff and S.A. Joyce, Phys. Rev. B40 (1989) 3 143. M G. Dujardin, L. Hellner, L. Philippe, M.J. Besnard-Ramage and P. Cirkel, Phys. Rev. B48 (1993) 14 529. and R. ES1 G. Dujardin, L. Hellner, M.J. Besnard-Ramage Azria. Phys. Rev. Len. 64 (1990) 1289. C61G. Rocker, P. Feulner, R. Scheuerer, L. Zhu and D. Menzel, Phys. Ser. 41 (1990) 1014. 171 Y. Baba, G. Dujardin. P. Feulner and D. Menzel, Phys. Rev. Lett. 66 (1991) 3269. R. Scheuerer. E. Hudel and P. Feulner. CSI T. Schwabenthan, Solid State Commun. 80 (1991) 773. c91 G. Dujardin. L. Hellner, L. Philippe and M.J. BesnardRamage, in: Proc. 10th VUV Conf.,eds. F.J. Willeumier, Y. Petroff and 1. Nenner (World Scientific, Singapore. 1993) p. 378. DOI H.W. Biester. M.J. Besnard, G. Dujardin, L. Hellner and E.E. Koch, Phys. Rev. Lett. 59 (1987) 1277. [ill G. Dujardin, L. Hellner, L. Philippe. M.J. Besnard-Ramage, R. Azria, M. Rose and P. Cirkel, in: DIET V, eds. A.R. Burns, E.B. Steichel and D.R. Jennison. Springer series in Surf. Sci.. Vol 31 (1993) 337. Cl21 C. Cachoncinlle, J.M. Pouvesle, G. Durand and F. Spiegelmann, J. Chem. Phys. 96 (1992) 6085. Cl31 H. Langhoff. J. Phys. B 27 (1994) L709.