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Light emission of plasmons during ion bombardment of silver surfaces
448
h’uclcar
Light emission of plasmons Yu.A. Bandurin,
Instruments
and Methods
in Physics Research
during ion bombardment
L.S. Belykh, I.E. Mitropolsky,
A.I. Dashchenko
IS58 (1991) 448-451 North-Holland
of silver surfaces
and S.S. Pop
,
photon emission during silver surface bombardment by H+, H; Hi , He’. Ne’ and Ar’ ions with energies from 7 to 21 keV was investigated. A continuum radiation was found in the spectral region from 200 to 630 nm with the energetic position of the maximum close to the plasmon energy of silver. The influence of the ion mass, energy and observation angle of radiation upon properties of this contmuum radiation has been studied. The observed continuum radiation is linearly polarized. It is supposed that the observed continuum radiation is a result of radiative relaxation of silver plasmons.
1. Introduction
2. Experiment
The bombardment of metal surfaces by ions with keV energies is accompanied by both emission of particles (electrons, ions, atoms, molecules) and photon emission. The spectrum of photon radiation in most cases consists of spectral lines emitted by sputtered and scattered particles. For some metals both molecular bands and continuum radiation (CR) have been observed [l]. In some experiments (see, e.g., ref. [2]), the nature of such molecular band emitters has been clarified. As a rule, these bands are due to the excited molecules of M,A,-type (where M and A are atoms of target and adsorbed gas, respectively, and n and m are numbers of atoms in corresponding molecules). The nature of CR emitters has so far not been established. The subject of our report is the observation of continuum radiation of silver surfaces bombarded by positive gas ions. The spectrum of this radiation has a maximum in the ultraviolet region. The observed radiation was interpreted as radiative relaxation of collective excitations of a target electronic subsystem. There are some reports (3-51 about the observation of CR which was supposed to be connected with one particle or collective excitations in metals. Zivitz and Thomas [3] considered that CR during Al bombardment recombinations. by H + ions was due to electron-hole Kobsev et al. [4] and Goldsmith and Jelley [S] observed the continuum radiation of silver surfaces during H’ bombardment with energies from 1 to 5 MeV. They established it to be a transition radiation. On the contrary, Chaudri et al. [6] found out that the position of the maximum in CR spectra of silver surfaces was very close to the energy of plasma oscillations of electrons in silver. However, so far there were no experimental data (in the literature) about the observation of CR in ionphoton spectra which could be explained by the radiative relaxation of plasmons. 0168-583X/91,%03.50
In our experiments we used H’, Hi. H;‘, He+. Ne’ and Ar ’ ion beams with energies from 7 to 21 keV. The magnetic mass-analyzer with high resolution (dispersion 5 mm for 1% change of mass) and effective stabilization of ion beam parameters gave us the opportunity to obtain the ion flux with high intensity and homogeneity on masses and energies. The ions current density j varied from a minimal value j = 1 A/m2 for 7 keV protons to a maximal value j = 20 A/m’ for 21 keV helium ions. During bombardment by Hf and H; ions with energies of 14-21 keV the values j were - 10 A/m’. Fig. 1 shows the geometry of the experiments. The ion bombardment of the target was carried out at the angles a = lo’-80” from the surface normal. The directions of the primary ion beam and collection of radiation were arranged on one plane with the surface normal. The glow which was produced by ion bombardment of the surface was focused on the entrance slit of a grating monochromator and detected by a photomultiplier tube in the spectral region from 200 to 630 nm.
4J
8 1991 - Elsevier Science Publishers B.V. (North-Holland)
,i.'
IFYS
.,.J-l a L_J rent Z’M
WINDOWPrlLAQliFG
~ONO;HROMATO~
Fig. 1. Scheme of the experiment.
Yu.A. Bandwin
et RI. / 1Light emission of plusmons
The residual gas pressure in the vacuum chamber near the target surface was 7 x lops Pa. By means of a mass-spectrometer it was found that about 90% of this value could be ascribed to the working gas of the ion source. The partial pressure of oxygen and oxygen-contained-molecules near the targets was no more than 1 x lob6 Pa. Under such conditions, the incident flux on the surface target area of 1 cm2 had been - 3 X 1012 atoms and molecules per second. The minimal ion flux was 6 X 1014 ions/cm* s in the case of 7 keV protons. Taking into account the value of sputtered yield of silver by protons S - 10e2 [7], the surface coverage by adsorbed oxygen atoms can reach 0.6 of monolayer (if the sticking coefficient is recognized as a 1). During bombardment of silver by inert gas ions, their fluxes were about 1 X lOi ions/cm2 s. Therefore, inert gas ions, unlike hydrogen ions, had provided practically clean target surfaces. When the linear polarization of radiation was studied, the polarizer prism was put between two quartz lenses of the condenser. The deter~nation of the degree of polarization of the radiation was made, taking into account the instrumental polarization of the optical monochromator in an identical manner to the previous work of Andersen et al. 181.
3. Results Silver targets have been chosen as a subject of our study not by chance. It is known [9] that during bombardment of silver surfaces by electron beams a continuum radiation is observed. Such photon emission is resulting from radiative relaxation of the collective oscillations of free electrons (plasmons). The interest in studying of the role of such radiation during ion bombardment is prompted by the fact that the maximum of silver plasmon radiation is located in the convenient ultraviolet region. For all ion-target combinations studied, the continuum radiation was found (see figs. 2-4), with the general form of the spectra being similar to spectra of electron induced photon emission of Ag surface 191. First of all we established that the observed radiation came from the surface of the metal and was not connected with escaping sputtered or scattered particles (e.g., clusters). This conclusion has been drawn after the experiments on observations of CR at different angles, We did not observe the continuum radiation when B = 90”, i.e. this kind of radiation was absent in the glowing region above the silver surface. In this case the spectrum consisted of spectral lines only, emitted by sputtered excited silver atoms and by scattered excited particles of the primary beam. At all other angles (10” < 6 < 800), besides spectral lines, the spectrum included the continuum radiation. Detailed high-resolu-
449
Fig. 2. Emission spectra of Ag surface bombarded by H+ (curve 1) and He’ (curve 2) ions with 15 keV energy (0 = 30 o ).
tion analysis (with AX = 0.2 nm) of the continuum radiation showed that it had no fine structure and varied smoothly with wavelength as shown in figs. 2-4. The spectral intensity distribution and position of the ma~mum on the wavelength scale were practically independent from leaking into the vacuum chamber of gases such as H,, He and 0, up to a pressure of about 1.3 x lOK* Pa, when Ag targets were bombarded by inert gas ions. The intensities and band shapes of the observed continuum radiation depended on the kind of bombarding ions and the observation angle 0. In the cases of inert gas ions CR had the form of a broad band with one maximum at the wavelength X = 360 nm. its position varied insignificantly at small 0. For the hydrogen ions the band had one maximum only at 0 < 45 O. We
Fig. 3. Emission spectra of Ag bombarded by ions with 15 keV energy and at the observation angle B = 10 o Solid line: H’ ions; dashed line: Hz ions; dashed-dotted lines: He’ ions. III. SPUTTERING AND DESORPTION
450
Yu.A. Bandurin et al. / Light emission ofplasmons
_-
Ok
,
I
300
400
/
A\,nm
500
Fig. 4. Emission spectra of Ag obtained at the observation angle 8 = 60 O, Solid line: H+ ions with the energy 7 keV; dashed line: HT ions with the energy 14 keV; dashed-dotted line: H: ions with the energy 21 keV.
observed two maxima of the band when 9 > 45 O. Their intensity relationship essentially depended on 0 and the silver surface coverage degree. 41 The wavelength positions of these two maxima varied from 325 to 365 nm, while 0 changed from 45 o to 80 O. The comparison of CR intensities per incidence ion at a maximum made sense only among the inert gas ions, because they produced the same shape of CR spectra. Moreover, the spectral distribution of the broad band intensity and the position of the maximum depended insignificantly on the ion energy from 7 to 21 keV. At the same bombardment energy the largest intensity of the band per incident ion was observed for He+ ions. So, for He+, Ne+ an Ar+ ions with an energy of 10 keV and B = 45 “, the relationship of intensities at a maximum (h = 360 nm) was 10.5 : 6 : 1, correspondingly. Another situation is the comparison of the CR intensities, when hydrogen ions bombarded silver surfaces. In these cases we obtained different shapes of CR spectra. For example, in fig. 4 CR spectra are shown, measured during the bombarding of the silver surfaces with H+, H: and Hl ions with equal velocity V0 = 1.2 X 10’ cm/s (or with equal energy per incident nucleon E, = 7 keV). In this experiment we used ion beams with equal current density j = 1 A/m*. The relation of the CR intensities at a maximum (per incident nuclon) was 1.3: 1.1: 1 for Hf, Hl and H:, correspondingly. Further experiments have shown that the increase of current density up to 5 A/m* leads to a
*’ Recently, the authors have obtained new results which made it possible to interpret a short-wave maximum of the observed CR with the influence the Ag surfaces.
of the oxygen
coverage
on
dramatic decrease of the short-wave maximum, and the CR shape tends to that which is observed during inert gas ions bombardment. The continuum radiation of silver surfaces was studied very carefully to find out whether the light was polarized. The result was that the observed CR was linearly polarized. In fig. 5 s- and p-components of silver surface radiation are shown for 15 keV He+ bombardment and 0 = 45 O. For an s-component the oscillation plane of the electric vector component of the electromagnetic wave was perpendicular to the plane, which had been formed by the bombardment direction and normal to the surface. It was parallel to the entrance slit of monochromator. For a p-component of radiation the oscillation plane of the electric vector was in the same plane with bombardment direction and normal to the target surface. It was perpendicular to the entrance slit. The variation of the observation angle 0, like in the case of electron bombardment of silver, leads to a change of the spectral distribution of the degree of polarization. During ion bombardment, as well as during electron impact [9], the degree of polarization at a maximum of the CR decreased with observation angle decrease. In our experiments the largest value of the degree of polarization at a maximum of the CR was about 50%.
4. Discussion The analysis of the obtained results gives the possibility to suppose that the observed broad band in the spectra of ion-photon emission from silver targets is due to radiative relaxation of plasmons. Other known mechanisms of continuum radiation such as transition radiation, Bremsstrahlung and recombination radiation do not allow to interpret the measured characteristics of the observed CR. At present time it is not quite clear which processes give the main contribution to plasmon
LOO Anm 600 200 Fig. 5. Spectra of s- (curve a) and p- (curve b) polarized components of the surface continuum radiation. The energy of the He+ ions was 15 keV and the observation angle 0 = 45 o
Yu.A. Bandwin
et al. / Lighr emission of plasmons
excitation during ion bombardment of silver. The excitation of electron gas oscillations can be caused by different interactions such as (1) neutralization of impact ions by electrons from conduction band of silver with transfer of the released energy to the electron gas [lo], (2) formation of excited atoms (during scattering and sputtering processes), with the following transfer of excitation energy to the electrons of the metal, if the excitation energy is more than the energy of plasmons [ll], and (3) formation of fast secondary electrons including Auger electrons which come to the metal surface causing the plasmons excitation. In order to determine which process causes the observed continuum radiation, it is necessary to carry out further investigations of the ion-photon emission of metals. Nevertheless, the analysis of the obtained experimental data enables us to make a conclusion about a rather high probability of the first process mentioned above. In this supposition favor has been given to the following. The shapes of the CR spectra, including the maximum position, were identical during bombardment of silver surfaces by inert gas ions, i.e. in cases when surface cleaning had been ensured from the adsorbed particles. In such conditions the intensities of CR at a maximum linearly increase with the increase of the (E, - 2e~)~ parameter, where E, is the ionization energy of the inert gas atoms, and ep, is the work function of a silver surface. Exactly such a kind of dependence is typical for the neutralization processes 1121. It is to be expected, that during cleaning of the silver surface from the adsorbed oxygen atoms, the CR shapes and its maximum positions during hydrogen ions bombardment will be the same as in the cases of the inert gas ions bombardment. We suggest that the observed two maxima in the CR spectra under certain conditions are connected with excitation and relaxation of plasmons both on the clean silver surface and at the presence of adsorbed oxygen atoms.
5. Conclusion
It was found that the bombardment of silver targets by II+, H: >Hz, He+, Ne+ and Ar+ ions in the energy
451
range from 7 to 21 keV is accompanied by photon emission. In the emission spectra both spectral lines from scattered and sputtered particles and a broad band (continuum) radiation are observed. The intensity and its spectral distribution depended on the ion mass, energy and observation angle. At certain conditions of observation (8 > 45 o ) the spectrum of the CR had two maxima, moreover the short-wave m~mum was observed at the presence of adsorbed oxygen atoms. The observed continuum radiation was linearly polarized. The similarity of spectral characteristics of CR during ion and electron bombardment of silver gives the possibility to conclude that the observed continuum radiation results from the radiative relaxation of plasmons in silver.
References
PI S.S. Pop. SF. Belych, V.G. Drobnich
and V.H. Ferleger, Ion-photon Emission of Metals (Fan, Tashkent, 1989) in Russian. PI V.V. Braslavets, S.A. Evdokimov, Yu.A. Bandurin, AI. Dashchenko and S.S. Pop, Pis’ma Zh. Tekh. Fiz. 12 (1986) 543. in Russian. 131 M. Zivitz, and E.W. Thomas, Atomic Collisions in Solids (Amsterdam, 1975) 225. 141 A.P. Kobsev, S. Mikhalyak, E. Rutkowsky and I.M. Frank, Yad. Fiz.. 15 (1972) 326, in Russian. PI P. Goldsmith and J.V. Jelley, Philos. Mag. 4 (1959) 836. [f4 R.M. Chaudri, M.Y. Khan and M.M. Chaudri, Proc. 6th Int. Conf. Ionization Phenomena in Gases 2 (1963) 21 (SERMAT, Paris, 1963). by Particle Bombardment 171 R. Behrisch, ed., Sputtering Vol. 1, Topics in Applied Physics Vol. 47 (Springer, Berlin. 1981). 181 N. Andersen. K. Jensen, J. Jepsen, J. Melskens and E. Veje, Appl. Opt. 13 (1974) 1965. Pis’ma Zh. 191 S.S. Pop, ?‘.A. Kritsky and I.P. Zapesochnyi. Tekh. Fiz. 5 (1979) 1452, in Russian. [lOI A.A. Almulhem and M.D. Girardeau, Surf. Sci. 210 (1989) 138. 1111J.I. Gerstein and N. Tsoar, Phys. Rev., 9 (1974) 4038. PI N.N. Petrov and LA. Abroyan, Diagnostics of the Surfaces with Help of the Ion Beams (Leningrad State Univ. Publ. House, 1977) in Russian.
III. SPUTTERING
AND
DESORPTION
h’uclcar
Light emission of plasmons Yu.A. Bandurin,
Instruments
and Methods
in Physics Research
during ion bombardment
L.S. Belykh, I.E. Mitropolsky,
A.I. Dashchenko
IS58 (1991) 448-451 North-Holland
of silver surfaces
and S.S. Pop
,
photon emission during silver surface bombardment by H+, H; Hi , He’. Ne’ and Ar’ ions with energies from 7 to 21 keV was investigated. A continuum radiation was found in the spectral region from 200 to 630 nm with the energetic position of the maximum close to the plasmon energy of silver. The influence of the ion mass, energy and observation angle of radiation upon properties of this contmuum radiation has been studied. The observed continuum radiation is linearly polarized. It is supposed that the observed continuum radiation is a result of radiative relaxation of silver plasmons.
1. Introduction
2. Experiment
The bombardment of metal surfaces by ions with keV energies is accompanied by both emission of particles (electrons, ions, atoms, molecules) and photon emission. The spectrum of photon radiation in most cases consists of spectral lines emitted by sputtered and scattered particles. For some metals both molecular bands and continuum radiation (CR) have been observed [l]. In some experiments (see, e.g., ref. [2]), the nature of such molecular band emitters has been clarified. As a rule, these bands are due to the excited molecules of M,A,-type (where M and A are atoms of target and adsorbed gas, respectively, and n and m are numbers of atoms in corresponding molecules). The nature of CR emitters has so far not been established. The subject of our report is the observation of continuum radiation of silver surfaces bombarded by positive gas ions. The spectrum of this radiation has a maximum in the ultraviolet region. The observed radiation was interpreted as radiative relaxation of collective excitations of a target electronic subsystem. There are some reports (3-51 about the observation of CR which was supposed to be connected with one particle or collective excitations in metals. Zivitz and Thomas [3] considered that CR during Al bombardment recombinations. by H + ions was due to electron-hole Kobsev et al. [4] and Goldsmith and Jelley [S] observed the continuum radiation of silver surfaces during H’ bombardment with energies from 1 to 5 MeV. They established it to be a transition radiation. On the contrary, Chaudri et al. [6] found out that the position of the maximum in CR spectra of silver surfaces was very close to the energy of plasma oscillations of electrons in silver. However, so far there were no experimental data (in the literature) about the observation of CR in ionphoton spectra which could be explained by the radiative relaxation of plasmons. 0168-583X/91,%03.50
In our experiments we used H’, Hi. H;‘, He+. Ne’ and Ar ’ ion beams with energies from 7 to 21 keV. The magnetic mass-analyzer with high resolution (dispersion 5 mm for 1% change of mass) and effective stabilization of ion beam parameters gave us the opportunity to obtain the ion flux with high intensity and homogeneity on masses and energies. The ions current density j varied from a minimal value j = 1 A/m2 for 7 keV protons to a maximal value j = 20 A/m’ for 21 keV helium ions. During bombardment by Hf and H; ions with energies of 14-21 keV the values j were - 10 A/m’. Fig. 1 shows the geometry of the experiments. The ion bombardment of the target was carried out at the angles a = lo’-80” from the surface normal. The directions of the primary ion beam and collection of radiation were arranged on one plane with the surface normal. The glow which was produced by ion bombardment of the surface was focused on the entrance slit of a grating monochromator and detected by a photomultiplier tube in the spectral region from 200 to 630 nm.
4J
8 1991 - Elsevier Science Publishers B.V. (North-Holland)
,i.'
IFYS
.,.J-l a L_J rent Z’M
WINDOWPrlLAQliFG
~ONO;HROMATO~
Fig. 1. Scheme of the experiment.
Yu.A. Bandwin
et RI. / 1Light emission of plusmons
The residual gas pressure in the vacuum chamber near the target surface was 7 x lops Pa. By means of a mass-spectrometer it was found that about 90% of this value could be ascribed to the working gas of the ion source. The partial pressure of oxygen and oxygen-contained-molecules near the targets was no more than 1 x lob6 Pa. Under such conditions, the incident flux on the surface target area of 1 cm2 had been - 3 X 1012 atoms and molecules per second. The minimal ion flux was 6 X 1014 ions/cm* s in the case of 7 keV protons. Taking into account the value of sputtered yield of silver by protons S - 10e2 [7], the surface coverage by adsorbed oxygen atoms can reach 0.6 of monolayer (if the sticking coefficient is recognized as a 1). During bombardment of silver by inert gas ions, their fluxes were about 1 X lOi ions/cm2 s. Therefore, inert gas ions, unlike hydrogen ions, had provided practically clean target surfaces. When the linear polarization of radiation was studied, the polarizer prism was put between two quartz lenses of the condenser. The deter~nation of the degree of polarization of the radiation was made, taking into account the instrumental polarization of the optical monochromator in an identical manner to the previous work of Andersen et al. 181.
3. Results Silver targets have been chosen as a subject of our study not by chance. It is known [9] that during bombardment of silver surfaces by electron beams a continuum radiation is observed. Such photon emission is resulting from radiative relaxation of the collective oscillations of free electrons (plasmons). The interest in studying of the role of such radiation during ion bombardment is prompted by the fact that the maximum of silver plasmon radiation is located in the convenient ultraviolet region. For all ion-target combinations studied, the continuum radiation was found (see figs. 2-4), with the general form of the spectra being similar to spectra of electron induced photon emission of Ag surface 191. First of all we established that the observed radiation came from the surface of the metal and was not connected with escaping sputtered or scattered particles (e.g., clusters). This conclusion has been drawn after the experiments on observations of CR at different angles, We did not observe the continuum radiation when B = 90”, i.e. this kind of radiation was absent in the glowing region above the silver surface. In this case the spectrum consisted of spectral lines only, emitted by sputtered excited silver atoms and by scattered excited particles of the primary beam. At all other angles (10” < 6 < 800), besides spectral lines, the spectrum included the continuum radiation. Detailed high-resolu-
449
Fig. 2. Emission spectra of Ag surface bombarded by H+ (curve 1) and He’ (curve 2) ions with 15 keV energy (0 = 30 o ).
tion analysis (with AX = 0.2 nm) of the continuum radiation showed that it had no fine structure and varied smoothly with wavelength as shown in figs. 2-4. The spectral intensity distribution and position of the ma~mum on the wavelength scale were practically independent from leaking into the vacuum chamber of gases such as H,, He and 0, up to a pressure of about 1.3 x lOK* Pa, when Ag targets were bombarded by inert gas ions. The intensities and band shapes of the observed continuum radiation depended on the kind of bombarding ions and the observation angle 0. In the cases of inert gas ions CR had the form of a broad band with one maximum at the wavelength X = 360 nm. its position varied insignificantly at small 0. For the hydrogen ions the band had one maximum only at 0 < 45 O. We
Fig. 3. Emission spectra of Ag bombarded by ions with 15 keV energy and at the observation angle B = 10 o Solid line: H’ ions; dashed line: Hz ions; dashed-dotted lines: He’ ions. III. SPUTTERING AND DESORPTION
450
Yu.A. Bandurin et al. / Light emission ofplasmons
_-
Ok
,
I
300
400
/
A\,nm
500
Fig. 4. Emission spectra of Ag obtained at the observation angle 8 = 60 O, Solid line: H+ ions with the energy 7 keV; dashed line: HT ions with the energy 14 keV; dashed-dotted line: H: ions with the energy 21 keV.
observed two maxima of the band when 9 > 45 O. Their intensity relationship essentially depended on 0 and the silver surface coverage degree. 41 The wavelength positions of these two maxima varied from 325 to 365 nm, while 0 changed from 45 o to 80 O. The comparison of CR intensities per incidence ion at a maximum made sense only among the inert gas ions, because they produced the same shape of CR spectra. Moreover, the spectral distribution of the broad band intensity and the position of the maximum depended insignificantly on the ion energy from 7 to 21 keV. At the same bombardment energy the largest intensity of the band per incident ion was observed for He+ ions. So, for He+, Ne+ an Ar+ ions with an energy of 10 keV and B = 45 “, the relationship of intensities at a maximum (h = 360 nm) was 10.5 : 6 : 1, correspondingly. Another situation is the comparison of the CR intensities, when hydrogen ions bombarded silver surfaces. In these cases we obtained different shapes of CR spectra. For example, in fig. 4 CR spectra are shown, measured during the bombarding of the silver surfaces with H+, H: and Hl ions with equal velocity V0 = 1.2 X 10’ cm/s (or with equal energy per incident nucleon E, = 7 keV). In this experiment we used ion beams with equal current density j = 1 A/m*. The relation of the CR intensities at a maximum (per incident nuclon) was 1.3: 1.1: 1 for Hf, Hl and H:, correspondingly. Further experiments have shown that the increase of current density up to 5 A/m* leads to a
*’ Recently, the authors have obtained new results which made it possible to interpret a short-wave maximum of the observed CR with the influence the Ag surfaces.
of the oxygen
coverage
on
dramatic decrease of the short-wave maximum, and the CR shape tends to that which is observed during inert gas ions bombardment. The continuum radiation of silver surfaces was studied very carefully to find out whether the light was polarized. The result was that the observed CR was linearly polarized. In fig. 5 s- and p-components of silver surface radiation are shown for 15 keV He+ bombardment and 0 = 45 O. For an s-component the oscillation plane of the electric vector component of the electromagnetic wave was perpendicular to the plane, which had been formed by the bombardment direction and normal to the surface. It was parallel to the entrance slit of monochromator. For a p-component of radiation the oscillation plane of the electric vector was in the same plane with bombardment direction and normal to the target surface. It was perpendicular to the entrance slit. The variation of the observation angle 0, like in the case of electron bombardment of silver, leads to a change of the spectral distribution of the degree of polarization. During ion bombardment, as well as during electron impact [9], the degree of polarization at a maximum of the CR decreased with observation angle decrease. In our experiments the largest value of the degree of polarization at a maximum of the CR was about 50%.
4. Discussion The analysis of the obtained results gives the possibility to suppose that the observed broad band in the spectra of ion-photon emission from silver targets is due to radiative relaxation of plasmons. Other known mechanisms of continuum radiation such as transition radiation, Bremsstrahlung and recombination radiation do not allow to interpret the measured characteristics of the observed CR. At present time it is not quite clear which processes give the main contribution to plasmon
LOO Anm 600 200 Fig. 5. Spectra of s- (curve a) and p- (curve b) polarized components of the surface continuum radiation. The energy of the He+ ions was 15 keV and the observation angle 0 = 45 o
Yu.A. Bandwin
et al. / Lighr emission of plasmons
excitation during ion bombardment of silver. The excitation of electron gas oscillations can be caused by different interactions such as (1) neutralization of impact ions by electrons from conduction band of silver with transfer of the released energy to the electron gas [lo], (2) formation of excited atoms (during scattering and sputtering processes), with the following transfer of excitation energy to the electrons of the metal, if the excitation energy is more than the energy of plasmons [ll], and (3) formation of fast secondary electrons including Auger electrons which come to the metal surface causing the plasmons excitation. In order to determine which process causes the observed continuum radiation, it is necessary to carry out further investigations of the ion-photon emission of metals. Nevertheless, the analysis of the obtained experimental data enables us to make a conclusion about a rather high probability of the first process mentioned above. In this supposition favor has been given to the following. The shapes of the CR spectra, including the maximum position, were identical during bombardment of silver surfaces by inert gas ions, i.e. in cases when surface cleaning had been ensured from the adsorbed particles. In such conditions the intensities of CR at a maximum linearly increase with the increase of the (E, - 2e~)~ parameter, where E, is the ionization energy of the inert gas atoms, and ep, is the work function of a silver surface. Exactly such a kind of dependence is typical for the neutralization processes 1121. It is to be expected, that during cleaning of the silver surface from the adsorbed oxygen atoms, the CR shapes and its maximum positions during hydrogen ions bombardment will be the same as in the cases of the inert gas ions bombardment. We suggest that the observed two maxima in the CR spectra under certain conditions are connected with excitation and relaxation of plasmons both on the clean silver surface and at the presence of adsorbed oxygen atoms.
5. Conclusion
It was found that the bombardment of silver targets by II+, H: >Hz, He+, Ne+ and Ar+ ions in the energy
451
range from 7 to 21 keV is accompanied by photon emission. In the emission spectra both spectral lines from scattered and sputtered particles and a broad band (continuum) radiation are observed. The intensity and its spectral distribution depended on the ion mass, energy and observation angle. At certain conditions of observation (8 > 45 o ) the spectrum of the CR had two maxima, moreover the short-wave m~mum was observed at the presence of adsorbed oxygen atoms. The observed continuum radiation was linearly polarized. The similarity of spectral characteristics of CR during ion and electron bombardment of silver gives the possibility to conclude that the observed continuum radiation results from the radiative relaxation of plasmons in silver.
References
PI S.S. Pop. SF. Belych, V.G. Drobnich
and V.H. Ferleger, Ion-photon Emission of Metals (Fan, Tashkent, 1989) in Russian. PI V.V. Braslavets, S.A. Evdokimov, Yu.A. Bandurin, AI. Dashchenko and S.S. Pop, Pis’ma Zh. Tekh. Fiz. 12 (1986) 543. in Russian. 131 M. Zivitz, and E.W. Thomas, Atomic Collisions in Solids (Amsterdam, 1975) 225. 141 A.P. Kobsev, S. Mikhalyak, E. Rutkowsky and I.M. Frank, Yad. Fiz.. 15 (1972) 326, in Russian. PI P. Goldsmith and J.V. Jelley, Philos. Mag. 4 (1959) 836. [f4 R.M. Chaudri, M.Y. Khan and M.M. Chaudri, Proc. 6th Int. Conf. Ionization Phenomena in Gases 2 (1963) 21 (SERMAT, Paris, 1963). by Particle Bombardment 171 R. Behrisch, ed., Sputtering Vol. 1, Topics in Applied Physics Vol. 47 (Springer, Berlin. 1981). 181 N. Andersen. K. Jensen, J. Jepsen, J. Melskens and E. Veje, Appl. Opt. 13 (1974) 1965. Pis’ma Zh. 191 S.S. Pop, ?‘.A. Kritsky and I.P. Zapesochnyi. Tekh. Fiz. 5 (1979) 1452, in Russian. [lOI A.A. Almulhem and M.D. Girardeau, Surf. Sci. 210 (1989) 138. 1111J.I. Gerstein and N. Tsoar, Phys. Rev., 9 (1974) 4038. PI N.N. Petrov and LA. Abroyan, Diagnostics of the Surfaces with Help of the Ion Beams (Leningrad State Univ. Publ. House, 1977) in Russian.
III. SPUTTERING
AND
DESORPTION