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Study of ion-impact oxygen desorption by computer simulation
Surface and Coatings Technology 158 – 159 (2002) 277–280
Study of ion-impact oxygen desorption by computer simulation A.A. Dzhurakhalov*, S. Rahmatov Arifov Institute of Electronics, F. Khodjaev Str. 33, 700187 Tashkent, Uzbekistan
Abstract The ion-impact desorption processes of oxygen molecules adsorbed with a C(2=2) structure onto a Ag (110) surface at a parallel alignment of an axis of molecules along the N110M direction have been investigated by computer simulation. The angular distributions and desorption yields of dissociative and non-dissociative desorbed molecules have been calculated at a grazing 5 keV Heq, Nq and Neq ion bombardment. It is shown that in the case of a grazing incidence, the non-dissociative (molecular) desorption yield is a noticeable magnitude. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion-impact desorption; Grazing incidence; Computer simulation
1. Introduction Recently, many researchers have exhibited concern about so-called ion-impact desorption processes of the adsorbed particles, i.e. an immediate knocking-out of adsorbed particles caused by incident ions. The results of investigation of the ion-impact desorption are valuable, not only for study of desorption processes as an effective method of clearing a surface from contamination by ion bombardment, but also for the determination of some characters of adsorption states. In Onsgaard et al. w1x and Taglauer et al. w2,3x, the ion-impact desorption of adsorbed layers on metal surfaces has been studied by low energy ion scattering. When the degree of adsorption covering of a surface is no more than a monolayer, the characteristics of ionimpact desorption can be studied by measuring time dependencies of the intensities of ion peaks scattered from the substrate and adsorbate atoms. The decrease in intensity of adsorbate peak with time, observed by Taglauer et al. w2x, testifies to a desorption of adsorbed atoms as a result of an ion bombardment. In Taglauer et al. w3x, the processes of ion-impact desorption of H and CO particles adsorbed on the Ni and W single crystal surface have been studied experimentally and by computer simulation as a function of mass ratio, ion *Corresponding author. Tel.: q998-71-162-7331; fax: q998-71162-8767. E-mail address: [email protected] (A.A. Dzhurakhalov).
energy and angle of incidence. The measured crosssections of a desorption were in the range from 10y16 to 10y14 cm2, partly in agreement with the results from numerical calculations. In this report, it is also shown that the main part of adsorbed CO at Heq ion bombardment in the range of energy 200–2000 eV desorbs by the way of non-dissociative molecules. In Kapur and Garrison w4x, O’Connor et al. w5x and Dzhurakhalov et al. w6x, the process of ion-impact desorption has been investigated by the immediate detection of the desorbed particles (basically, an atomic adsorption of these particles), i.e. by measuring their angular and energy distributions. In O’Connor et al. w5x, from the detection of negatively charged recoil atoms, the character of a deposition of adsorbed oxygen atoms relative to a unit cell of a Ni (110) surface has been defined at 2 keV Neq ion bombardment along various crystallographic directions. This method is highly sensitive and allows us to determine the character of a submonolayer coverage with a concentration less than 10y4 monolayers. The aim of our investigations was to determine the orientation dependencies of desorption yields of both dissociative and non-dissociative desorbed O2 molecules at grazing ion bombardment of the Ag (110) surface with oxygen coverage, the elucidation of optimum conditions for non-dissociative desorption, and the calculation of angular distributions of desorbed particles.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 1 8 3 - 4
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A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
Fig. 1. Projections of the adsorbed molecules O2 and atoms of a matrix on the Ag (110) surface plane and angles used in calculations (a); and the scheme of non-dissociative desorption of O2 molecules (b).
2. Computational methods In our calculation, an adsorption site of O2 molecules corresponds to the top-most second layer site with the O–O axis parallel to the Ag (110) surface along a N110M direction at the C(2=2) adsorption structure (Fig. 1a). The height of its center of mass above the surface plane is 0.094 nm, the O–O bond length is 0.155 nm, the binding energy Eb of O2 with a surface is 0.53 eV, and molecular binding (dissociation) energy ´ of O2 is 5 eV. Heq, Nq and Neq ions were used as bombarding particles, which has enabled us to consider essentially different kinematics of scattering relevant to the cases m2 ym1-1 and m2 ym1)1 (where m1—mass of an incident particle, m2—mass of atom of a molecule). The interaction of potential used is a screened Coulomb potential with a Ziegler–Biersack–Littmark approximation w7x to the Thomas–Fermi screening function. The inelastic energy losses were considered as local and calculated by the Firsov formula modified by Kishinevsky w8x. The angle of incidence of the ion beam relative to the surface was changed in the range cs5–208, the azimuth angle of incidence changed in the range js0– 908 by rotation of a target around its normal was counted from N001M direction, the polar and azimuth angle of ejection of desorbed particles are marked in d and w, respectively (see Fig. 1a). Dissociative (at the gap of intramolecular bond) and non-dissociative desorption of adsorbed molecules were simulated. In the case of a dissociative desorption, the possible collisions of an ejected atom of a molecule with neighbor molecules and with surface atoms have been taken into account. The atom or molecule was
Fig. 2. The polar diagrams of an angular distribution of the dissociative desorbed particles at 5 keV Heq , Nq and Neq ion bombardment of Ag (110) surface covered by O2 molecules, for cs168, js908.
considered as desorbed if its impulse after all possible collisions was directed to a vacuum and its energy was sufficient for overcoming a surface barrier. For calculations of non-dissociative desorption of molecules from a surface of single crystals, an approach similar to the ‘cut-off’ model w9x suggested for the description of the sputtered particles as dimers was used. According to Bitenskiy and Parilis w9x, the ion as a result of the series correlated collisions can eject a molecule without a bond gap between its atoms, if the relative kinetic energy of atoms does not exceed a binding (dissociation) energy ´ of molecule (Fig. 1b): EMOp s r
m2 ™ ™ 2 Žv1yv2. F´ 4
(1)
and the energy of a center of mass is sufficient for overcoming a binding energy of molecule with a surface E b: Ecs
m2 ™ ™ 2 Žv1yv2. GEb 4
(2)
where m2—mass of atom of a molecule, v1,2—velocities of atoms of molecule. The molecule was considered as non-dissociative desorbed, if an impulse of its center of mass is directed to a vacuum at a fulfillment of the conditions of Eqs. (1) and (2). For such molecules, their polar d and azimuth w angular distributions, as
A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
Fig. 3. The yield of non-dissociative desorbed molecules in dependence on azimuth angle of incidence j at 5 keV Neq ion bombardment of O2yAg (110) surface. Number of incident particles is 2=104.
well as the yield of a non-dissociative desorption have been calculated. 3. Results and discussion In Fig. 2, the polar diagrams of an angular distribution of particles dissociative desorbed in a plane of incidence at 5 keV Heq, Nq and Neq ion bombardment of Ag (110) surface covered by adsorbed O2 molecules are presented for case of cs168, js908. The angle of ejection d was counted from the surface plane; d)908 corresponds to a knocking-out of particles in that part of a hemisphere, where the bombarding ions impinge. It can be seen that, in the case of Heq ions, the main peak of the desorbed particles is formed near the df908, i.e. the main part of particles is ejected near the normal of surface. Additionally to this, the rather small peaks are observed at df958 and df1058. The limiting of an angular distribution on the part of large (d)1608) and small (d-208) angles of ejection is caused by a blocking effect created by atoms of neighbor molecules. In the case of Nq and Neq ion bombardment, the angular distribution of the desorbed particles contains a number of intensive peaks. In these cases, additionally to the peak at df908, rather intensive peaks can be observed at other angles of ejection. The enrichment of an angular distribution with additional peaks in the case of Nq or Neq ions is explained by the occurrence of additional different possibilities of a knocking-out of atoms of a molecule because of heavier masses of Nq and Neq in comparison with Heq ions. Besides, the obtained results demonstrate that, with increasing mass of bombarding particles, the desorption yield increases. The non-dissociative desorption of the adsorbed molecules under the conditions of grazing ion bombardment have been also investigated. At a grazing incidence, the number of bombarding particles channeled under an adsorbed layer increase. Thus, the projectile transmits a part of its energy to a great number of molecules. As a result, the atoms of these molecules desorb intensively.
279
Besides, in this case, the probability of realization of non-dissociative desorption increases, because, at a channeling of incident ions under the adsorbed layer, the impulses of both atoms of a molecule are not significantly different (see Fig. 1b). Fig. 3 shows the yield of non-dissociative desorption Snd vs. the azimuth angle of incidence j for two values of an angle c at 5 keV Neq ion bombardment of a O2 y Ag (110) surface. It is seen that at cs68, the dependencies have maxima at js0, 35 and 908; the formation of these maxima was conditioned by the presence of favorable conditions for non-dissociative desorption of adsorbed molecules due to the possibility of a channeling of incident particles under an adsorption layer in these directions. The most intensive maximum at js 908 also testifies to favorable realization of conditions (1) and (2), as long as an axis of molecules is parallel to the N110M direction. With increasing angle of incidence c, the shape of dependence varies a little. For example, at cs108, instead of maxima, minima are observed at cs0 and 358, and additionally at cs658. It is caused by increasing in these directions the possibility of penetration of incident particles in deeper layers with increasing angle of incidence. In Fig. 4, the total (dissociativeqnon-dissociative) desorption yield Sd and the non-dissociative desorption yield Snd of the adsorbed O2 molecules vs. the angle of incidence c at 5 keV Heq ion bombardment of O2 yAg (110) surfaces along N110M direction have been presented. It can be seen that Sd (c) Snd (c) dependencies are not monotone, the intensive formations of desorbed particles are observed at some values of c. In this case, the most effective dissociative desorption is observed at cs108, and non-dissociative desorption at cs148. Because of an increase in a non-dissociative desorption
Fig. 4. The total (dissociativeqnon-dissociative) desorption yield Sd and the non-dissociative desorption yield Snd of the adsorbed O2 molecules vs. the angle of incidence c at 5 keV Heq ion bombardment of O2yAg (110) surfaces along the N110M direction.
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A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
transmission to each atom of a molecule increases, which results in a decrease of probability of realization of condition of non-dissociative desorption. 4. Conclusions
Fig. 5. The angular (polar d and azimuth w) distribution of the nondissociative desorbed molecules at 5 keV Neq ion bombardment of O2yAg(110) surface for cs108 and js908. Number of incident particles is 2=104.
Thus, the calculations demonstrate that, in the case of a grazing ion bombardment owing to a channeling of ions between adsorption layer surface, there is an effective desorption of adsorbed particles. The desorption yield increases with increasing mass of bombarding particles. In this case, the yield of non-dissociative desorption is of noticeable magnitude and strongly depends on the azimuth angle of incidence of bombarding beam, and on the tilt angle of an axis of a molecule relative to the surface. Acknowledgments
contribution, there is a relevant peak in the curve of total desorption yield. The Snd (c) dependencies demonstrates that, in the case of grazing incidence, the yield of a non-dissociative desorption is of a noticeable magnitude. In Fig. 5, the angular distribution of the non-dissociative desorbed molecules on polar d and azimuth w angles of ejection has been presented at 5 keV Neq ion bombardment of O2 yAg(110) surface for cs108 and js908. At not-so-large values of a polar angle of ejection d, the molecules are desorbed mainly under azimuth angles wf908 and 2708. The molecules ejected near to the normal of a surface have wide distribution on azimuth angles of ejection w. The influence of a tilt angle of axis of a molecule relative to a surface on magnitude of yield of nondissociative desorption also has been studied. With increasing a from 0 up to 458, the magnitudes of Snd decrease monotonically. The results show that the most effective non-dissociative desorption is observed at a parallel disposition of an axis of a molecule to a surface. With increasing tilt angle a, the difference of energy
This work was partially supported by the Fundamental Research Foundations of the Academy of Sciences and of the State Committee for Science and Technology of the Republic of Uzbekistan. We thank Dr S. Khakimov for his assistance in some calculations. References w1x J. Onsgaard, W. Heiland, E. Taglauer, Surf. Sci. 99 (1980) 112. w2x E. Taglauer, G. Marin, W. Heiland, U. Beitat, Surf. Sci. 63 (1977) 507. w3x E. Taglauer, U. Beitat, W. Heiland, Nucl. Instrum. Meth. 149 (1978) 605. w4x Sh. Kapur, B.J. Garrison, Chem. Phys. 75 (1981) 445. w5x D.J. O’Connor, R.J. MacDonald, W. Eckstein, P.R. Higginbottom, Nucl. Instrum. Meth. Phys. Res. B13 (1986) 235. w6x A.A. Dzhurakhalov, N.A. Teshabaeva, V.Kh. Ferleger, Uzb. Fiz. Zhurn. 4 (1993) 65. w7x D.J. O’Connor, J.P. Biersack, Nucl. Instr. Meth. Phys. Res. B15 (1986) 14. w8x E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, et al., Atomic Collisions on Solid Surfaces, North-Holland, 1993. w9x I.S. Bitenskiy, E.S. Parilis, Zhur. Tekh. Fiz. 51 (1981) 2134.
Study of ion-impact oxygen desorption by computer simulation A.A. Dzhurakhalov*, S. Rahmatov Arifov Institute of Electronics, F. Khodjaev Str. 33, 700187 Tashkent, Uzbekistan
Abstract The ion-impact desorption processes of oxygen molecules adsorbed with a C(2=2) structure onto a Ag (110) surface at a parallel alignment of an axis of molecules along the N110M direction have been investigated by computer simulation. The angular distributions and desorption yields of dissociative and non-dissociative desorbed molecules have been calculated at a grazing 5 keV Heq, Nq and Neq ion bombardment. It is shown that in the case of a grazing incidence, the non-dissociative (molecular) desorption yield is a noticeable magnitude. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion-impact desorption; Grazing incidence; Computer simulation
1. Introduction Recently, many researchers have exhibited concern about so-called ion-impact desorption processes of the adsorbed particles, i.e. an immediate knocking-out of adsorbed particles caused by incident ions. The results of investigation of the ion-impact desorption are valuable, not only for study of desorption processes as an effective method of clearing a surface from contamination by ion bombardment, but also for the determination of some characters of adsorption states. In Onsgaard et al. w1x and Taglauer et al. w2,3x, the ion-impact desorption of adsorbed layers on metal surfaces has been studied by low energy ion scattering. When the degree of adsorption covering of a surface is no more than a monolayer, the characteristics of ionimpact desorption can be studied by measuring time dependencies of the intensities of ion peaks scattered from the substrate and adsorbate atoms. The decrease in intensity of adsorbate peak with time, observed by Taglauer et al. w2x, testifies to a desorption of adsorbed atoms as a result of an ion bombardment. In Taglauer et al. w3x, the processes of ion-impact desorption of H and CO particles adsorbed on the Ni and W single crystal surface have been studied experimentally and by computer simulation as a function of mass ratio, ion *Corresponding author. Tel.: q998-71-162-7331; fax: q998-71162-8767. E-mail address: [email protected] (A.A. Dzhurakhalov).
energy and angle of incidence. The measured crosssections of a desorption were in the range from 10y16 to 10y14 cm2, partly in agreement with the results from numerical calculations. In this report, it is also shown that the main part of adsorbed CO at Heq ion bombardment in the range of energy 200–2000 eV desorbs by the way of non-dissociative molecules. In Kapur and Garrison w4x, O’Connor et al. w5x and Dzhurakhalov et al. w6x, the process of ion-impact desorption has been investigated by the immediate detection of the desorbed particles (basically, an atomic adsorption of these particles), i.e. by measuring their angular and energy distributions. In O’Connor et al. w5x, from the detection of negatively charged recoil atoms, the character of a deposition of adsorbed oxygen atoms relative to a unit cell of a Ni (110) surface has been defined at 2 keV Neq ion bombardment along various crystallographic directions. This method is highly sensitive and allows us to determine the character of a submonolayer coverage with a concentration less than 10y4 monolayers. The aim of our investigations was to determine the orientation dependencies of desorption yields of both dissociative and non-dissociative desorbed O2 molecules at grazing ion bombardment of the Ag (110) surface with oxygen coverage, the elucidation of optimum conditions for non-dissociative desorption, and the calculation of angular distributions of desorbed particles.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 1 8 3 - 4
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A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
Fig. 1. Projections of the adsorbed molecules O2 and atoms of a matrix on the Ag (110) surface plane and angles used in calculations (a); and the scheme of non-dissociative desorption of O2 molecules (b).
2. Computational methods In our calculation, an adsorption site of O2 molecules corresponds to the top-most second layer site with the O–O axis parallel to the Ag (110) surface along a N110M direction at the C(2=2) adsorption structure (Fig. 1a). The height of its center of mass above the surface plane is 0.094 nm, the O–O bond length is 0.155 nm, the binding energy Eb of O2 with a surface is 0.53 eV, and molecular binding (dissociation) energy ´ of O2 is 5 eV. Heq, Nq and Neq ions were used as bombarding particles, which has enabled us to consider essentially different kinematics of scattering relevant to the cases m2 ym1-1 and m2 ym1)1 (where m1—mass of an incident particle, m2—mass of atom of a molecule). The interaction of potential used is a screened Coulomb potential with a Ziegler–Biersack–Littmark approximation w7x to the Thomas–Fermi screening function. The inelastic energy losses were considered as local and calculated by the Firsov formula modified by Kishinevsky w8x. The angle of incidence of the ion beam relative to the surface was changed in the range cs5–208, the azimuth angle of incidence changed in the range js0– 908 by rotation of a target around its normal was counted from N001M direction, the polar and azimuth angle of ejection of desorbed particles are marked in d and w, respectively (see Fig. 1a). Dissociative (at the gap of intramolecular bond) and non-dissociative desorption of adsorbed molecules were simulated. In the case of a dissociative desorption, the possible collisions of an ejected atom of a molecule with neighbor molecules and with surface atoms have been taken into account. The atom or molecule was
Fig. 2. The polar diagrams of an angular distribution of the dissociative desorbed particles at 5 keV Heq , Nq and Neq ion bombardment of Ag (110) surface covered by O2 molecules, for cs168, js908.
considered as desorbed if its impulse after all possible collisions was directed to a vacuum and its energy was sufficient for overcoming a surface barrier. For calculations of non-dissociative desorption of molecules from a surface of single crystals, an approach similar to the ‘cut-off’ model w9x suggested for the description of the sputtered particles as dimers was used. According to Bitenskiy and Parilis w9x, the ion as a result of the series correlated collisions can eject a molecule without a bond gap between its atoms, if the relative kinetic energy of atoms does not exceed a binding (dissociation) energy ´ of molecule (Fig. 1b): EMOp s r
m2 ™ ™ 2 Žv1yv2. F´ 4
(1)
and the energy of a center of mass is sufficient for overcoming a binding energy of molecule with a surface E b: Ecs
m2 ™ ™ 2 Žv1yv2. GEb 4
(2)
where m2—mass of atom of a molecule, v1,2—velocities of atoms of molecule. The molecule was considered as non-dissociative desorbed, if an impulse of its center of mass is directed to a vacuum at a fulfillment of the conditions of Eqs. (1) and (2). For such molecules, their polar d and azimuth w angular distributions, as
A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
Fig. 3. The yield of non-dissociative desorbed molecules in dependence on azimuth angle of incidence j at 5 keV Neq ion bombardment of O2yAg (110) surface. Number of incident particles is 2=104.
well as the yield of a non-dissociative desorption have been calculated. 3. Results and discussion In Fig. 2, the polar diagrams of an angular distribution of particles dissociative desorbed in a plane of incidence at 5 keV Heq, Nq and Neq ion bombardment of Ag (110) surface covered by adsorbed O2 molecules are presented for case of cs168, js908. The angle of ejection d was counted from the surface plane; d)908 corresponds to a knocking-out of particles in that part of a hemisphere, where the bombarding ions impinge. It can be seen that, in the case of Heq ions, the main peak of the desorbed particles is formed near the df908, i.e. the main part of particles is ejected near the normal of surface. Additionally to this, the rather small peaks are observed at df958 and df1058. The limiting of an angular distribution on the part of large (d)1608) and small (d-208) angles of ejection is caused by a blocking effect created by atoms of neighbor molecules. In the case of Nq and Neq ion bombardment, the angular distribution of the desorbed particles contains a number of intensive peaks. In these cases, additionally to the peak at df908, rather intensive peaks can be observed at other angles of ejection. The enrichment of an angular distribution with additional peaks in the case of Nq or Neq ions is explained by the occurrence of additional different possibilities of a knocking-out of atoms of a molecule because of heavier masses of Nq and Neq in comparison with Heq ions. Besides, the obtained results demonstrate that, with increasing mass of bombarding particles, the desorption yield increases. The non-dissociative desorption of the adsorbed molecules under the conditions of grazing ion bombardment have been also investigated. At a grazing incidence, the number of bombarding particles channeled under an adsorbed layer increase. Thus, the projectile transmits a part of its energy to a great number of molecules. As a result, the atoms of these molecules desorb intensively.
279
Besides, in this case, the probability of realization of non-dissociative desorption increases, because, at a channeling of incident ions under the adsorbed layer, the impulses of both atoms of a molecule are not significantly different (see Fig. 1b). Fig. 3 shows the yield of non-dissociative desorption Snd vs. the azimuth angle of incidence j for two values of an angle c at 5 keV Neq ion bombardment of a O2 y Ag (110) surface. It is seen that at cs68, the dependencies have maxima at js0, 35 and 908; the formation of these maxima was conditioned by the presence of favorable conditions for non-dissociative desorption of adsorbed molecules due to the possibility of a channeling of incident particles under an adsorption layer in these directions. The most intensive maximum at js 908 also testifies to favorable realization of conditions (1) and (2), as long as an axis of molecules is parallel to the N110M direction. With increasing angle of incidence c, the shape of dependence varies a little. For example, at cs108, instead of maxima, minima are observed at cs0 and 358, and additionally at cs658. It is caused by increasing in these directions the possibility of penetration of incident particles in deeper layers with increasing angle of incidence. In Fig. 4, the total (dissociativeqnon-dissociative) desorption yield Sd and the non-dissociative desorption yield Snd of the adsorbed O2 molecules vs. the angle of incidence c at 5 keV Heq ion bombardment of O2 yAg (110) surfaces along N110M direction have been presented. It can be seen that Sd (c) Snd (c) dependencies are not monotone, the intensive formations of desorbed particles are observed at some values of c. In this case, the most effective dissociative desorption is observed at cs108, and non-dissociative desorption at cs148. Because of an increase in a non-dissociative desorption
Fig. 4. The total (dissociativeqnon-dissociative) desorption yield Sd and the non-dissociative desorption yield Snd of the adsorbed O2 molecules vs. the angle of incidence c at 5 keV Heq ion bombardment of O2yAg (110) surfaces along the N110M direction.
280
A.A. Dzhurakhalov, S. Rahmatov / Surface and Coatings Technology 158 – 159 (2002) 277–280
transmission to each atom of a molecule increases, which results in a decrease of probability of realization of condition of non-dissociative desorption. 4. Conclusions
Fig. 5. The angular (polar d and azimuth w) distribution of the nondissociative desorbed molecules at 5 keV Neq ion bombardment of O2yAg(110) surface for cs108 and js908. Number of incident particles is 2=104.
Thus, the calculations demonstrate that, in the case of a grazing ion bombardment owing to a channeling of ions between adsorption layer surface, there is an effective desorption of adsorbed particles. The desorption yield increases with increasing mass of bombarding particles. In this case, the yield of non-dissociative desorption is of noticeable magnitude and strongly depends on the azimuth angle of incidence of bombarding beam, and on the tilt angle of an axis of a molecule relative to the surface. Acknowledgments
contribution, there is a relevant peak in the curve of total desorption yield. The Snd (c) dependencies demonstrates that, in the case of grazing incidence, the yield of a non-dissociative desorption is of a noticeable magnitude. In Fig. 5, the angular distribution of the non-dissociative desorbed molecules on polar d and azimuth w angles of ejection has been presented at 5 keV Neq ion bombardment of O2 yAg(110) surface for cs108 and js908. At not-so-large values of a polar angle of ejection d, the molecules are desorbed mainly under azimuth angles wf908 and 2708. The molecules ejected near to the normal of a surface have wide distribution on azimuth angles of ejection w. The influence of a tilt angle of axis of a molecule relative to a surface on magnitude of yield of nondissociative desorption also has been studied. With increasing a from 0 up to 458, the magnitudes of Snd decrease monotonically. The results show that the most effective non-dissociative desorption is observed at a parallel disposition of an axis of a molecule to a surface. With increasing tilt angle a, the difference of energy
This work was partially supported by the Fundamental Research Foundations of the Academy of Sciences and of the State Committee for Science and Technology of the Republic of Uzbekistan. We thank Dr S. Khakimov for his assistance in some calculations. References w1x J. Onsgaard, W. Heiland, E. Taglauer, Surf. Sci. 99 (1980) 112. w2x E. Taglauer, G. Marin, W. Heiland, U. Beitat, Surf. Sci. 63 (1977) 507. w3x E. Taglauer, U. Beitat, W. Heiland, Nucl. Instrum. Meth. 149 (1978) 605. w4x Sh. Kapur, B.J. Garrison, Chem. Phys. 75 (1981) 445. w5x D.J. O’Connor, R.J. MacDonald, W. Eckstein, P.R. Higginbottom, Nucl. Instrum. Meth. Phys. Res. B13 (1986) 235. w6x A.A. Dzhurakhalov, N.A. Teshabaeva, V.Kh. Ferleger, Uzb. Fiz. Zhurn. 4 (1993) 65. w7x D.J. O’Connor, J.P. Biersack, Nucl. Instr. Meth. Phys. Res. B15 (1986) 14. w8x E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, et al., Atomic Collisions on Solid Surfaces, North-Holland, 1993. w9x I.S. Bitenskiy, E.S. Parilis, Zhur. Tekh. Fiz. 51 (1981) 2134.