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Nuclear stimulated desorption of isolated cadmium atoms from structured surfaces
Surface Science 442 (1999) L1001–L1005 www.elsevier.nl/locate/susc
Surface Science Letter
Nuclear stimulated desorption of isolated cadmium atoms from structured surfaces Y. Ashkenazy a, I. Kelson a, *, H.H. Bertschat b, K. Potzger b, A. Weber b, W.-D. Zeitz b, ISOLDE-Collaboration at CERN c a School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel b Bereich Festko¨rperphysik, Hahn–Meitner-Institut Berlin GmbH, D 14109 Berlin, Germany c 1211 Gene`ve 22, Switzerland Received 26 July 1999; accepted for publication 8 September 1999
Abstract Measurements of nuclear stimulated desorption (NSD) of 107Cd from a (111) nickel single crystal surface covered with a fraction of a monolayer of palladium were performed. The total desorption probability and the polar angular distribution were determined. An anisotropic distribution with a minimum in the upward direction was found. This distribution is compared with molecular dynamics calculations and is shown to be consistent with desorption from step sites. It is also qualitatively consistent with independent results on similar systems obtained by the perturbed angular correlation method using 111mCd. The total desorption probability agrees with the predicted value. Future work required to turn NSD into a useful analytic tool is discussed. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Atomistic dynamics; Cadmium; Ion–solid interactions; Molecular dynamics; Palladium; Silver
Methods involving isolated radioactive probe atoms have many possible applications in the study of surfaces, interfaces and bulk matter. One particular method is nuclear stimulated desorption (NSD) [1], in which the recoil obtained during the decay of the probe atom may cause it to desorb from a surface onto which it had been deposited. This process can be regarded as a scattering event of a stationary atom originating in an equilibrium site on the surface. The underlying premise is that * Corresponding author. Tel: +972-364-086-70; fax: +972-364-293-06. E-mail address: [email protected] (I. Kelson)
the probability of desorption and its angular distribution contain information both about the characteristics of this site and about the adsorbate– substrate interaction. This is particularly true for isolated, highly diluted atoms, which can hardly be detected by other methods (e.g. by scanning tunneling microscopy). The basic NSD phenomenology was demonstrated experimentally for a few isotopes, such as 99Mo and 47Ca [2,3]. However, the surfaces involved in those experiments were not properly prepared or characterized and thus the obtained results could not be critically evaluated. In this letter we report results of the first NSD
0039-6028/99/$ - see front matter © 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 09 7 1 -1
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experiment involving a well prepared and properly characterized surface. Similar surfaces had also been studied independently by another radioactive probe atom method (PAC, perturbed angular correlation), providing a check on the interpretation of the observations [4–6 ]. Moreover, the physical system had been simulated by realistic molecular dynamics calculations [7], thus allowing direct comparison between experiment and theory. The probe which was used in this experiment was an isotope of cadmium, [8]. 107Cd (6.5 h halflife) decays by electron capture to an isomeric state 107mAg (55 s half-life). In the course of the decay, in which a monoenergetic neutrino is emitted, the daughter nucleus receives 8.7 eV of recoil energy. This is sufficient, in principle, to cause it to desorb from the surface onto which it is originally adsorbed if the recoil is in an appropriate direction. The silver atom, initially formed in an ionized state is neutralized within a few fs, as is indirectly corroborated by our experimental findings. The recoiling daughter nucleus itself, 107mAg, decays to the ground state, 107Ag, by emitting characteristic radiations: 93 keV gammas in 4.6% of the decays and monoenergetic conversion electrons in the rest. In the experiment the recoiling species are simply captured by geometrically welldefined collectors and the subsequent radiation is measured. The objectives of this first experiment were primarily to demonstrate, in a specific example, the feasibility of the use of as a practical NSD probe in a controlled environment, to study the methodological aspects and to determine the relevant quantitative parameters. The entire experiment was carried out at ASPIC (apparatus for surface physics and interfaces at CERN ) [9] under strict ultrahigh vacuum conditions ( low 10−9 Pa range). The experiment was performed on a (111) nickel single crystal, sputtered and annealed for several times [9] at a 1000°K. About 1/3 monolayer (ML) of isoelectric palladium was subsequently evaporated on it in order to enhance the number of steps. The choice of this particular substrate was also motivated by the role it played in PAC experiments on the magnetic properties of thin Pd layers, using another isotope of cadmium, 111mCd.
The crystallographic identification of the substrate and its relative orientation on the sample mount were determined by low energy electron diffraction (LEED) at the beginning of the experiment. Auger spectroscopy was used to verify the cleanliness of the substrate following the annealing and sputtering stage. The magnitude of the palladium Auger peak was used to estimate the amount of Pd deposited on the nickel substrate. An amount of ca. 2×10−4 ML of 107Cd, produced by the mass separator ISOLDE, was deposited on the substrate using a two-stage evaporation process which was developed for and other probes and is described in detail elsewhere [9]. Most (over 90%) of the atoms were deposited in a 10 mm diameter circular area on the sample. Note for future experiments in combination with PAC, that this procedure could be applied for the concurrent deposition of both 107Cd and 111mCd, in arbitrary relative amounts. In this experiment the NSD phenomenology was determined only for the initial state obtained from the evaporation on the primary surface. It consisted of a series of essentially identical measurements, using the following procedure. The (radioactive) sample is placed at the central focal point (see Fig. 1) P. The decay products, atoms, which desorb from the substrate are collected on
Fig. 1. A schematic presentation of the basic observation geometry. The sample is placed at the focal point P, fixed on a rotation-translation manipulator, which enables it to rotate by an arbitrary angle h. The desorbed species are collected on the inner, vacuum side of the quartz tube. An external sodium iodide is used to measure the activity collected on the ‘collector’ area, with other parts of the quartz tube being shielded by lead.
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the inner surface of the quartz tube. The outer part of the tube is shielded by lead except for a small circular exposed area (the ‘collector’) placed directly above the central point. A sodium iodide detector is placed in proximity to this exposed area so that it can measure the 93 keV gamma-rays which traverse the quartz wall with minimal attenuation. In each cycle of the experiment the sample is kept at the central point in a certain predetermined inclination, the only geometric parameter allowed to vary, long enough (ca. 5 min) for 107Cd the 107mAg and populations on the sample and on the quartz tube to reach asymptotic equilibrium. Subsequently, the sample is removed and the population on the exposed area is determined. The entire procedure was carried out at room temperature. The special geometric setup described above allows a simple mathematical analysis. Let C be 0 the activity of the primary deposited on the sample. In the absence of desorption, this activity would be equal to C(s), the activity of the daughter 107mAg on the sample, in equilibrium with it. If the total desorption probability is P then one des clearly has C(s)=(1−P )C . The amount of des 0 desorbed activity 107mAg on the collector in equilibrium, C(c), is C (dP /dv)DV, where dP /dv is the 0 d d desorption probability density and DV is the solid angle subtended by the collector. The detector geometric efficiency is simply given by the solid angle subtended by it, so that the counting rate of the sample 107mAg gammas, R(s), is given by: R(s)=e c(1−P )C DV/4p, 0 des 0
est, dP /dv, is given by: d 1−P R(c) dP des d= . dv 4pe R(s)
L1003
(3)
Since the differential probability density depends, through the multiplicative constant (1−P ), on the total desorption probability, the des two have to be determined self-consistently. In the present experiment only the polar inclination angle h was varied. The sum (R(s)+R(c)) was measured directly with the sample kept at the central point with asymptotic equilibrium established with the collector. R(c) was extracted from the measurement of the collector activity with the sample removed, taking into account the exponential decay of the activity with the 107mAg lifetime. The measured values of the ratio g=
R(c) 4peR(s)
(4)
which is dP /dh expressed in units of (1−P ), d des are shown in Fig. 2 as a function of the inclination angle h. The graph is symmetric as expected and displays a pronounced minimum at h=0 (upward direction).
(1)
where c is the gamma branching ratio of the 107mAg decay and e is the intrinsic detector effi0 ciency for the 93 keV gammas. Similarly, for the collector, one has dP d DV. R(c)=e ecC 0 0 dv
(2)
where e is the geometric efficiency for counting the collector activity, which can be calculated with relatively high accuracy. Note, that in forming the ratio of the two counting rates, e , c, C and DV 0 0 cancel out. Thus, the experimental entity of inter-
Fig. 2. The measured values of g, defined by Eq. (4), as a function of h. The statistical uncertainty in the plotted values is very small, but it represents an average over an angular range of ca. 10°. The corresponding computed distributions from a substitutional step site and from a terrace site are given by the dashed curve and the dotted curve respectively. Note, that both experimental and theoretical curves do not involve any adjustable parameters.
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Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
It is interesting to compare the observed distribution with predictions of model calculations. The theoretical results are obtained through molecular dynamics simulations, which treat the actual physical system as an ensemble of classically interacting atoms. The interaction between atoms is described by the semi-empirical potential derived from the second moment approximation (SMA) [10]. The total cohesive energy is written as the sum of an attractive term representing the tight-binding band energy due to the d-electrons, and a pairwise repulsive interaction term of the Born–Mayer type. Details of the integration algorithm and of the specific choice of parameters are described in Ref. [7]. Note that the palladium parameters were used to describe the interaction between the desorbing, neutral silver atom and the substrate. This approximation is motivated by the similarity of the interaction potential parameters of the different species [11]. The dashed curve in Fig. 2 gives the result of the calculation for a substitutional step site, taking into account the realistic conditions of the experiment. Namely, an average was taken over all contributing ledge directions and the finite solid angle subtended by the collector was folded into the distribution. Due to this averaging procedure there is little difference between the substitutional step site curve and the corresponding curves for step and kink sites. This common curve, however, is distinctly different from the one for a terrace site, also shown in Fig. 2 for demonstrative purposes. The terrace site distribution is essentially flat for h between 0 and 30°. We note, without presenting explicit quantitative results, that these conclusions are not very sensitive to the details of the employed theoretical model. It is instructive to point out that past PAC work [5] shows that under the conditions of the present experiment most Cd atoms which contribute to the PAC signal occupy substitutional step sites. Although a different substrate (a palladium crystal ) was used in that experiment, we expect the surface mobility of the cadmium atoms to be similar at room temperature for both cases. The fact that a sizable fraction of the surface is covered by palladium atoms in the present case makes this a more plausible assumption. Thus, the two experiments, utilizing totally
different physical processes, seem to be at least qualitatively consistent. Clearly, to draw more quantitative conclusions from such a comparison and to obtain detailed information about the actual site distribution, the two experiments must be performed concurrently on the same substrate. In order to extract the total desorption probability the observed differential curve was integrated and Eq. (3) solved. Using for the counter geometric efficiency the computed value e= 0.33±0.01, one gets P =0.22±0.01. This value des means that almost half of the atoms which recoil in the outward direction actually manage to desorb from the surface. The total desorption probability was stable throughout the experiment. This, incidentally, rules out the occurrence of diffusion of cadmium atoms into the bulk, which would have manifested itself as a reduction of that probability. The uncertainty in the ratio R(c)/R(s) is essentially determined by the statistical error in R(c). Since the total number of net counts from the collector was above 104 in the present case, this error was of the order of 1%. To check the consistency and the stability of the results we have repeated the measurement at h=0 periodically over a total time span of ca. 8 h. The sample activity C(s) remained unchanged within the statistical errors, except for the exponential time dependence of the decay. This simply implies that no 107Cd atoms were lost during this time. This was also the case for the ratio g. This demonstrates that the physical arrangement of the 107Cd atoms on the sample (i.e. the sites occupied by them) as well as the overall conditions of the surface were unchanged throughout the experiment. A further property of the desorbing species (107mAg atoms) which was investigated was their charge state. To this end the standard collection procedure, again for h=0, was followed, but this time with an electric bias applied between the sample and the collector. A bias of up to 50 V in both polarities had no measurable effect on the collected activity, implying that all collected species were electrically neutral. This, in fact, is to be explained by the fact that image charges in the metallic substrate would bind the 107mAg ions (if they are formed ) with more energy than available to them as a result of their recoil. This observation
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Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
is significant since the theoretical calculations used in the comparison with the experimental results actually assume the desorbed species to be neutral [7]. To conclude, we have performed for the first time an experiment on NSD from well-characterized surfaces. We have measured the total desorption probability and its polar angle dependence with high precision and have established the stability of the observed patterns over long times. The observations are consistent with realistic molecular dynamics simulations and with independent findings from PAC experiments. Although NSD measurements are inherently very accurate, the work presented here should only be regarded as a preliminary, semi-quantitative demonstration of principle. A number of experimental tests and improvements are required in order to turn this method into a useful tool. First, appropriately designed single crystals should be used. Nickel or palladium crystals, for example, vicinally cut at a small angle, would have steps facing predominantly in a given direction. Second, the desorption pattern should be monitored under varying conditions (e.g. temperature), which are known to modify the distribution of sites occupied by the active species. This would allow a more direct correlation between the site population and the NSD phenomenology, independently of theoretical modelling. Third and technically perhaps most important, a position sensitive device (e.g. a micro-channel plate) should be used to collect and measure the desorbing species. This would greatly improve the overall statistics and the two-dimensional angular resolution, allowing one to distinguish between sites with similar gross desorption characteristics [7]. Finally, we wish to express the
L1005
hope that the ensemble of improved NSD measurements might be used to study properties of the underlying interatomic interactions.
Acknowledgements This work was partially supported by the United States–Israel Binational Science Foundation and by the Israeli Ministry of Science. One of the authors (I.K ) acknowledges the hospitality of ISOLDE/CERN. The authors are indebted to H. Haas for many useful discussions.
References [1] I. Kelson, J. Phys. D 20 (1987) 1049. [2] H. Dumont, B. Perraillon, I. Kelson, D. Nir, A. Zidon, Phys. Lett. A 139 (1989) 406. [3] I. Kelson, D. Nir, G. Ringler, P.E. Haustein, J. Phys. D: Appl. Phys. 25 (1992) 1545. [4] T. Klas, R. Fink, G. Krausch, R. Platzer, J. Voigt, R. Wesche, G. Schatz, Surf. Sci. 216 (1989) 270. [5] E. Hunger, H. Haas, Surf. Sci. 234 (1990) 273. [6 ] H.H. Bertschat, H.-H. Blaschek, H. Granzer, K. Potzger, S. Seeger, W.-D. Zeitz, H. Niehus, A. Burchard, D. ForkelWirth, Phys. Rev. Lett. 80 (1998) 2721. [7] Y. Ashkenazy, I. Kelson, Model. Sim. Mat. Sci. Engng 7 (1999) 169. [8] R.B. Firestone, V.S. Shirley, S.Y. Frank Chu, Table of Isotopes, Wiley–Interscience, New York, 1996. CD-ROM; this is the source of all nuclear data in this paper. [9] K. Potzger, H.H. Bertschat, A. Burchard, D. Forkel-Wirth, H. Granzer, H. Niehus, S. Seeger, W.-D. Zeitz, Nucl. Instrum. Meth. B 146 (1998) 618. [10] M.W. Finnis, J.E. Sinclair, Phil. Mag. A 50 (1984) 45. [11] V. Rosato, M. Guillope, B. Legrand, Phil. Mag. A 59 (1989) 321.
Surface Science Letter
Nuclear stimulated desorption of isolated cadmium atoms from structured surfaces Y. Ashkenazy a, I. Kelson a, *, H.H. Bertschat b, K. Potzger b, A. Weber b, W.-D. Zeitz b, ISOLDE-Collaboration at CERN c a School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel b Bereich Festko¨rperphysik, Hahn–Meitner-Institut Berlin GmbH, D 14109 Berlin, Germany c 1211 Gene`ve 22, Switzerland Received 26 July 1999; accepted for publication 8 September 1999
Abstract Measurements of nuclear stimulated desorption (NSD) of 107Cd from a (111) nickel single crystal surface covered with a fraction of a monolayer of palladium were performed. The total desorption probability and the polar angular distribution were determined. An anisotropic distribution with a minimum in the upward direction was found. This distribution is compared with molecular dynamics calculations and is shown to be consistent with desorption from step sites. It is also qualitatively consistent with independent results on similar systems obtained by the perturbed angular correlation method using 111mCd. The total desorption probability agrees with the predicted value. Future work required to turn NSD into a useful analytic tool is discussed. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Atomistic dynamics; Cadmium; Ion–solid interactions; Molecular dynamics; Palladium; Silver
Methods involving isolated radioactive probe atoms have many possible applications in the study of surfaces, interfaces and bulk matter. One particular method is nuclear stimulated desorption (NSD) [1], in which the recoil obtained during the decay of the probe atom may cause it to desorb from a surface onto which it had been deposited. This process can be regarded as a scattering event of a stationary atom originating in an equilibrium site on the surface. The underlying premise is that * Corresponding author. Tel: +972-364-086-70; fax: +972-364-293-06. E-mail address: [email protected] (I. Kelson)
the probability of desorption and its angular distribution contain information both about the characteristics of this site and about the adsorbate– substrate interaction. This is particularly true for isolated, highly diluted atoms, which can hardly be detected by other methods (e.g. by scanning tunneling microscopy). The basic NSD phenomenology was demonstrated experimentally for a few isotopes, such as 99Mo and 47Ca [2,3]. However, the surfaces involved in those experiments were not properly prepared or characterized and thus the obtained results could not be critically evaluated. In this letter we report results of the first NSD
0039-6028/99/$ - see front matter © 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 09 7 1 -1
SU RF A LE CE TT S ER CIE S N CE
L1002
Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
experiment involving a well prepared and properly characterized surface. Similar surfaces had also been studied independently by another radioactive probe atom method (PAC, perturbed angular correlation), providing a check on the interpretation of the observations [4–6 ]. Moreover, the physical system had been simulated by realistic molecular dynamics calculations [7], thus allowing direct comparison between experiment and theory. The probe which was used in this experiment was an isotope of cadmium, [8]. 107Cd (6.5 h halflife) decays by electron capture to an isomeric state 107mAg (55 s half-life). In the course of the decay, in which a monoenergetic neutrino is emitted, the daughter nucleus receives 8.7 eV of recoil energy. This is sufficient, in principle, to cause it to desorb from the surface onto which it is originally adsorbed if the recoil is in an appropriate direction. The silver atom, initially formed in an ionized state is neutralized within a few fs, as is indirectly corroborated by our experimental findings. The recoiling daughter nucleus itself, 107mAg, decays to the ground state, 107Ag, by emitting characteristic radiations: 93 keV gammas in 4.6% of the decays and monoenergetic conversion electrons in the rest. In the experiment the recoiling species are simply captured by geometrically welldefined collectors and the subsequent radiation is measured. The objectives of this first experiment were primarily to demonstrate, in a specific example, the feasibility of the use of as a practical NSD probe in a controlled environment, to study the methodological aspects and to determine the relevant quantitative parameters. The entire experiment was carried out at ASPIC (apparatus for surface physics and interfaces at CERN ) [9] under strict ultrahigh vacuum conditions ( low 10−9 Pa range). The experiment was performed on a (111) nickel single crystal, sputtered and annealed for several times [9] at a 1000°K. About 1/3 monolayer (ML) of isoelectric palladium was subsequently evaporated on it in order to enhance the number of steps. The choice of this particular substrate was also motivated by the role it played in PAC experiments on the magnetic properties of thin Pd layers, using another isotope of cadmium, 111mCd.
The crystallographic identification of the substrate and its relative orientation on the sample mount were determined by low energy electron diffraction (LEED) at the beginning of the experiment. Auger spectroscopy was used to verify the cleanliness of the substrate following the annealing and sputtering stage. The magnitude of the palladium Auger peak was used to estimate the amount of Pd deposited on the nickel substrate. An amount of ca. 2×10−4 ML of 107Cd, produced by the mass separator ISOLDE, was deposited on the substrate using a two-stage evaporation process which was developed for and other probes and is described in detail elsewhere [9]. Most (over 90%) of the atoms were deposited in a 10 mm diameter circular area on the sample. Note for future experiments in combination with PAC, that this procedure could be applied for the concurrent deposition of both 107Cd and 111mCd, in arbitrary relative amounts. In this experiment the NSD phenomenology was determined only for the initial state obtained from the evaporation on the primary surface. It consisted of a series of essentially identical measurements, using the following procedure. The (radioactive) sample is placed at the central focal point (see Fig. 1) P. The decay products, atoms, which desorb from the substrate are collected on
Fig. 1. A schematic presentation of the basic observation geometry. The sample is placed at the focal point P, fixed on a rotation-translation manipulator, which enables it to rotate by an arbitrary angle h. The desorbed species are collected on the inner, vacuum side of the quartz tube. An external sodium iodide is used to measure the activity collected on the ‘collector’ area, with other parts of the quartz tube being shielded by lead.
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Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
the inner surface of the quartz tube. The outer part of the tube is shielded by lead except for a small circular exposed area (the ‘collector’) placed directly above the central point. A sodium iodide detector is placed in proximity to this exposed area so that it can measure the 93 keV gamma-rays which traverse the quartz wall with minimal attenuation. In each cycle of the experiment the sample is kept at the central point in a certain predetermined inclination, the only geometric parameter allowed to vary, long enough (ca. 5 min) for 107Cd the 107mAg and populations on the sample and on the quartz tube to reach asymptotic equilibrium. Subsequently, the sample is removed and the population on the exposed area is determined. The entire procedure was carried out at room temperature. The special geometric setup described above allows a simple mathematical analysis. Let C be 0 the activity of the primary deposited on the sample. In the absence of desorption, this activity would be equal to C(s), the activity of the daughter 107mAg on the sample, in equilibrium with it. If the total desorption probability is P then one des clearly has C(s)=(1−P )C . The amount of des 0 desorbed activity 107mAg on the collector in equilibrium, C(c), is C (dP /dv)DV, where dP /dv is the 0 d d desorption probability density and DV is the solid angle subtended by the collector. The detector geometric efficiency is simply given by the solid angle subtended by it, so that the counting rate of the sample 107mAg gammas, R(s), is given by: R(s)=e c(1−P )C DV/4p, 0 des 0
est, dP /dv, is given by: d 1−P R(c) dP des d= . dv 4pe R(s)
L1003
(3)
Since the differential probability density depends, through the multiplicative constant (1−P ), on the total desorption probability, the des two have to be determined self-consistently. In the present experiment only the polar inclination angle h was varied. The sum (R(s)+R(c)) was measured directly with the sample kept at the central point with asymptotic equilibrium established with the collector. R(c) was extracted from the measurement of the collector activity with the sample removed, taking into account the exponential decay of the activity with the 107mAg lifetime. The measured values of the ratio g=
R(c) 4peR(s)
(4)
which is dP /dh expressed in units of (1−P ), d des are shown in Fig. 2 as a function of the inclination angle h. The graph is symmetric as expected and displays a pronounced minimum at h=0 (upward direction).
(1)
where c is the gamma branching ratio of the 107mAg decay and e is the intrinsic detector effi0 ciency for the 93 keV gammas. Similarly, for the collector, one has dP d DV. R(c)=e ecC 0 0 dv
(2)
where e is the geometric efficiency for counting the collector activity, which can be calculated with relatively high accuracy. Note, that in forming the ratio of the two counting rates, e , c, C and DV 0 0 cancel out. Thus, the experimental entity of inter-
Fig. 2. The measured values of g, defined by Eq. (4), as a function of h. The statistical uncertainty in the plotted values is very small, but it represents an average over an angular range of ca. 10°. The corresponding computed distributions from a substitutional step site and from a terrace site are given by the dashed curve and the dotted curve respectively. Note, that both experimental and theoretical curves do not involve any adjustable parameters.
SU RF A LE CE TT S ER CIE S N CE
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Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
It is interesting to compare the observed distribution with predictions of model calculations. The theoretical results are obtained through molecular dynamics simulations, which treat the actual physical system as an ensemble of classically interacting atoms. The interaction between atoms is described by the semi-empirical potential derived from the second moment approximation (SMA) [10]. The total cohesive energy is written as the sum of an attractive term representing the tight-binding band energy due to the d-electrons, and a pairwise repulsive interaction term of the Born–Mayer type. Details of the integration algorithm and of the specific choice of parameters are described in Ref. [7]. Note that the palladium parameters were used to describe the interaction between the desorbing, neutral silver atom and the substrate. This approximation is motivated by the similarity of the interaction potential parameters of the different species [11]. The dashed curve in Fig. 2 gives the result of the calculation for a substitutional step site, taking into account the realistic conditions of the experiment. Namely, an average was taken over all contributing ledge directions and the finite solid angle subtended by the collector was folded into the distribution. Due to this averaging procedure there is little difference between the substitutional step site curve and the corresponding curves for step and kink sites. This common curve, however, is distinctly different from the one for a terrace site, also shown in Fig. 2 for demonstrative purposes. The terrace site distribution is essentially flat for h between 0 and 30°. We note, without presenting explicit quantitative results, that these conclusions are not very sensitive to the details of the employed theoretical model. It is instructive to point out that past PAC work [5] shows that under the conditions of the present experiment most Cd atoms which contribute to the PAC signal occupy substitutional step sites. Although a different substrate (a palladium crystal ) was used in that experiment, we expect the surface mobility of the cadmium atoms to be similar at room temperature for both cases. The fact that a sizable fraction of the surface is covered by palladium atoms in the present case makes this a more plausible assumption. Thus, the two experiments, utilizing totally
different physical processes, seem to be at least qualitatively consistent. Clearly, to draw more quantitative conclusions from such a comparison and to obtain detailed information about the actual site distribution, the two experiments must be performed concurrently on the same substrate. In order to extract the total desorption probability the observed differential curve was integrated and Eq. (3) solved. Using for the counter geometric efficiency the computed value e= 0.33±0.01, one gets P =0.22±0.01. This value des means that almost half of the atoms which recoil in the outward direction actually manage to desorb from the surface. The total desorption probability was stable throughout the experiment. This, incidentally, rules out the occurrence of diffusion of cadmium atoms into the bulk, which would have manifested itself as a reduction of that probability. The uncertainty in the ratio R(c)/R(s) is essentially determined by the statistical error in R(c). Since the total number of net counts from the collector was above 104 in the present case, this error was of the order of 1%. To check the consistency and the stability of the results we have repeated the measurement at h=0 periodically over a total time span of ca. 8 h. The sample activity C(s) remained unchanged within the statistical errors, except for the exponential time dependence of the decay. This simply implies that no 107Cd atoms were lost during this time. This was also the case for the ratio g. This demonstrates that the physical arrangement of the 107Cd atoms on the sample (i.e. the sites occupied by them) as well as the overall conditions of the surface were unchanged throughout the experiment. A further property of the desorbing species (107mAg atoms) which was investigated was their charge state. To this end the standard collection procedure, again for h=0, was followed, but this time with an electric bias applied between the sample and the collector. A bias of up to 50 V in both polarities had no measurable effect on the collected activity, implying that all collected species were electrically neutral. This, in fact, is to be explained by the fact that image charges in the metallic substrate would bind the 107mAg ions (if they are formed ) with more energy than available to them as a result of their recoil. This observation
CE N IE SC RS CE E A TT RF LE SU
Y. Ashkenazy et al. / Surface Science 442 (1999) L1001–L1005
is significant since the theoretical calculations used in the comparison with the experimental results actually assume the desorbed species to be neutral [7]. To conclude, we have performed for the first time an experiment on NSD from well-characterized surfaces. We have measured the total desorption probability and its polar angle dependence with high precision and have established the stability of the observed patterns over long times. The observations are consistent with realistic molecular dynamics simulations and with independent findings from PAC experiments. Although NSD measurements are inherently very accurate, the work presented here should only be regarded as a preliminary, semi-quantitative demonstration of principle. A number of experimental tests and improvements are required in order to turn this method into a useful tool. First, appropriately designed single crystals should be used. Nickel or palladium crystals, for example, vicinally cut at a small angle, would have steps facing predominantly in a given direction. Second, the desorption pattern should be monitored under varying conditions (e.g. temperature), which are known to modify the distribution of sites occupied by the active species. This would allow a more direct correlation between the site population and the NSD phenomenology, independently of theoretical modelling. Third and technically perhaps most important, a position sensitive device (e.g. a micro-channel plate) should be used to collect and measure the desorbing species. This would greatly improve the overall statistics and the two-dimensional angular resolution, allowing one to distinguish between sites with similar gross desorption characteristics [7]. Finally, we wish to express the
L1005
hope that the ensemble of improved NSD measurements might be used to study properties of the underlying interatomic interactions.
Acknowledgements This work was partially supported by the United States–Israel Binational Science Foundation and by the Israeli Ministry of Science. One of the authors (I.K ) acknowledges the hospitality of ISOLDE/CERN. The authors are indebted to H. Haas for many useful discussions.
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