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Mechanisms of formation of sputtered particles in excited states at Ar+ ion bombardment of oxide targets
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 256 (2007) 501–505 www.elsevier.com/locate/nimb
Mechanisms of formation of sputtered particles in excited states at Ar+ ion bombardment of oxide targets V.V. Bobkov
a,*
, S.P. Gokov a, V.V. Gritsyna a, V.T. Gritsyna a, D.I. Shevchenko a, S.S. Alimov a,b a
V. Karazin Kharkiv National University, Kharkiv, Ukraine b Hahn-Meitner-Institute, Berlin, Germany Available online 16 December 2006
Abstract Investigations of the main parameters of the ion–photon emission during the Ar+ ions bombardment of the complex oxides (MgO Æ nAl2O3) were provided. Spectral composition of the emitted light, relative intensities of different lines, and dependences of lines intensity on the distance from the target surface give information on process of sputtering of the particles and their formation in different exited states. It was shown that under the bombardment with Ar+ ions the excited atoms of metal (Al and Mg) are formed as in the process of multiple collisions so under the development of linear cascades. Formation of the excited sputtered ions (Mg+) takes place only during the process of multiple collisions. For deeply situated levels an essential contribution in formation of the excited states of sputtered particles gives mechanism of cascade population. 2006 Elsevier B.V. All rights reserved. PACS: 32.30.Jc; 71.20.Ps; 79.20.Rf Keywords: Sputtering; Spectra of atoms and ions; Oxides; Ion–photon emission
1. Introduction It was shown before [1] that the phenomenon of ion– photon emission (IPE), i.e. the emission of photons by exited particles knocked-out from solid surface by ions of keV-energy is caused by following processes. At first, these are the processes leading to departure of the particles: multiple collisions of incident ion or recoil atom with surface atoms of the target and development of linear cascades in solids. At second, these are the processes leading to formation of knocked-out particles in a given charge and excited state. There exist several mechanisms of formation of excited particles. In kinetic model an excited particle is formed during the separation of common electron cloud of pair particles. The model includes also the mechanism
*
Corresponding author. Tel.: +380 573 35 2734. E-mail address: [email protected] (V.V. Bobkov).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.049
of breaking chemical bonds [2]). Another mechanism is based on the process of electron exchange, in which formation of the particle in the excited state (i) is determined by decay of the system solid – departing atom [1,3]. Besides, for free particles one should take into account the additional mechanism based on the influence of cascade transitions from the upper excited state (k) to the exited state (i). This changes the number of the particles decaying to the lower excited state (j) due to the transition i–j [4]. The intensity of spectral line (I) of the given transition i–j is determined by the number of particles (Ni) in the excited state i c I ¼ Aij N i h ð1Þ kij where Aij is the probability of i–j transition by emitting photon of wavelength kij, h and c is Plank constant and light velocity, respectively. Using spontaneous decay law for the excited state i and the relation between the effective
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velocity (veff) of excited particles, the distance (l) of emitting particle from surface of target and the time (t = l/veff) we obtain t l ¼ N 0i exp ; ð2Þ N i ðlÞ ¼ N 0i exp si veff si where N0i is the number of particles in i-state leaving the surface at t = 0, si is the lifetime of particle in ith state. In case of additional population from the upper excited states (k) to the investigated (i) via cascade transitions the population will be defined by expression X l 1 N i ðlÞ ¼ N 0i exp þ N 0k si ðsk si Þ veff si k l l exp exp ; ð3Þ veff sk veff si where N0k is the population of kth level at t = 0, sk is the lifetime of this level. Summation should be provided over all levels k from which the cascade transitions to level i are possible. As it was already shown [1], despite of the departing particles with wide energy distribution we can distinguish the group of comparatively slow particles which arises in linear cascade collisions (several tenths eV) and the group of fast particles created by multiple collisions and having energy comparable to energy of primary beam. Thus taking into account the influence of cascade population and the existence of two energetic groups of departing excited particles the total number of particles in i-state at distance l is defined by the sum of number of particles calculated according to Eq. (3) for fast and slow particles. In this paper we investigated relative intensity and spatial distribution of several lines emitted by exited atoms and ions of aluminum and magnesium sputtered from magnesium aluminate spinel of different compositions. 2. Experimental details The investigation was carried out at the experimental setup which allows to use the mass separated Ar+ ion beam of 20 keV and current density of 10–20 lA cm2. The incident angle of ion beam to the target surface was 45. The photon emission of excited particles was measured in the direction perpendicular to plain, created by the bombarding beam and normal to target surface. The spectra of emission was analyzed with monochromator MDR-3 in the wavelength range of 250–800 nm and registered with cooled photomultiplier FEU-106 in photon counting mode. All emissions with intensity two times higher than background level were recorded. The full number of photons emitted by ejected excited particles was determined by using the method of entire collection of radiation described in [5]. For all observed emissions the quantum yield ck was defined as the number of photons of given wavelength k related to one incident ion. Magnesium aluminate spinel single crystals of different compositions
MgO Æ nAl2O3 (n = 1.0, 2.5) grown by Verneuil method were used as a target. 3. Results and discussion The spectra of ion–photon emission consist of the lines which are connected with transitions in metal particles: Mg, Mg+, Al, Al+ and Al++. Dependences I(l)of the emitted lines with intensity of 10 times higher than background level were measured according to the method described earlier [5]. The dependences I(l) observed in this work for the different lines in spectra of Al I, Mg I, and Mg II can be divided in four types. Despite of difference in the quantum yield of emitted lines registered from spinel crystals of different compositions MgO Æ nAl2O3 (n = 1.0 and 2.5) [1], the spatial distributions of studied emissions are the same. (1) For the lines k 473.0 nm Mg I and k 669.8 nm Al I the dependences Ln I = f(l) can be represented by the straight line. Effective velocity of these atoms was estimated from the slope angle. It was concluded that both excited atoms Mg (6s 1S0) and Al (5p 2P1/2,3/2) are the fast particles with kinetic energy 2000– 3000 eV. (2) For the lines kk 518.3, 382.9 and 383.8 nm Mg I and kk 396.1, 394.4, 308.2 and 309.2 nm Al I the dependences Ln I = f(l) are much complicated. In Fig. 1 we present the experimental results of dependences of intensities for two lines on the distance from the target surface. The curves were calculated according to expression (3). For the line k 396.1 nm Al I, transition 4s 2S1/2 ! 3p 2P3/2 (s4 = 1.6 · 108 s; Ni0 c = 9 · 104 ph/ion [5]) it is possible the existence of cascade transition from level 5p 2P1/2;3/2 (sk = 1.7 · 108 s;Nk0 c = 2.6 · 105 ph/ion), i.e. Nk0 < < Ni0. Comparison of the calculation (line) with experimental dependence Ln I = f(l) has shown that the jog in experimental dependence at l = 0.1 cm (Fig. 1a) could be explained by introducing only two groups of atoms: slow (Es = 50 eV) and fast (Ef = 1500 eV) ones in the ratio of number particles N0s:N0f = 6:1. For the line k 518.3 nm Mg I, transition 4 s 3 S1 ! 3p 2P02 (s4 = 1.0 · 108 s; N40 c = 2.5 · 104 ph/ion), the best fit of calculated dependence with experimental points was obtained when including two groups of atoms of different energies (slow Es = 50 eV and fast Ef = 3500 eV; N0s:N0f = 4:1). The same results were obtained for the other lines of this group. (3) Dependences Ln I = f(l) for the line k 285.2 nm Mg I, transition 3p 1P01 ! 3s21S0 (s3 = 0.2 · 108 s) are presented in Fig. 2(a). Because of small value of s3 the intensity of emission of slow particles decreases by two orders of magnitude at the distance l < 0.02 cm. Therefore the observed jog at the experimental dependence Ln I = f(l) cannot be explained by the emission of two energetic groups of particles. We
V.V. Bobkov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 501–505
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Fig. 1. Dependences of intensities of lines k 396.1 nm Al I (a) and k 518.3 nm Mg I (b) on the distance from the target surface. Points are experimental data, lines is calculation from Eq. (3).
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Fig. 2. Dependences of intensity of line k 285.2 nm Mg I (a) and line k 279.5 nm Mg II (b) on the distance from target surface. Points are experimental data, lines is calculation from Eq. (3).
expect that the level 3p 1P01 could be additionally populated from levels 3d 1D2 (sk = 8.0 · 108 s), 4d 1D2 (sk = 5.5 · 108 s), 4s 1S0 (sk = 4.7 · 108 s), and 5s 1 S0 (sk = 10 · 108 s). The wavelength of emission lines corresponding to these transitions (except line k 552.8 nm Mg I) situated out of the investigated spectral range. Nevertheless according to the spectral tables they are rather intense. Therefore calculated curve for the line k 285.2 nm shows the best fit to experimental data for such a case: the emission from near surface region (l < 0.05 cm) is caused by radiation of fast particles (Ef = 1700 eV) which were formed in state 3p 1P01 directly at the process of ion bombardment. The dependence Ln I = f(l) in the range l > 0.05 cm is related to the process of cascade population of the investigated level by slow particles
(Es = 100 eV). The calculated ratio is N0i:N0Rk = 0.3:1. (4) Finally, the lines of kk 279.5, 280.2 nm Mg II (s3 = 0.4 · 108 s) show essentially different dependences Ln I = f(l) compared to the other lines. The upper level 3p 2P3/2 of this transition situated essentially below the vacuum level. Therefore in this case we can expect contribution in the experimental dependence Ln I = f(l) from the emission as slow so fast ions. Moreover it is also possible that the emission contributes from the ions which were formed in the observed state by the transition from the higher excited states: 4s 2S1/2 (sk = 0.28 · 108 s), 4d 2D5/2 (sk = 0.81 · 108 s) and 5d 2D5/2 (sk = 2 · 108 s). In this case these could be both fast and slow particles. Calculation according to the Eq. (3) shows the
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best accordance between experimental Ln I = f(l) and calculated dependences Ln Ni = f(l) in case of influence of cascade population of the level 3p 2P3/2 only by fast ions Mg+ (Ef = 1700 eV). An attempt to take into account the contribution of slow Mg+ ions makes calculated curve Ni = f(l) apart from experimental data. This indicates probably that during the ion bombardment of magnesium aluminate spinel only the fast ions Mg+ are formed. To analyze the obtained results let us examine the scheme of energy levels of investigated transitions in aluminum atoms and magnesium atoms and ions, and energy bands in spinel (Fig. 3). The first type of dependences Ln I = f(l) is stipulated by the fast sputtered particles with levels of excited states 6s 1 S0 and 5p 2P1/2,3/2 for both aluminum and magnesium atoms situated resonantly on unoccupied levels of conduction band (CB) in spinel. Due to this energy position, the process of the resonant ionization can take place with the high probability. This makes possible conservation of the excitation during departing only for the fast group of excited particles formed as a result of multiple collisions of incident ion with particles of the target. The upper levels of the transition responsible for the emission lines of the second type dependence Ln I = f(l) are situated resonantly in forbidden band of spinel. Therefore the influence of such a process as resonance ionization or resonance neutralization is practically negligible. On the other hand these levels are situated rather high. That is why
the contribution of mechanisms of cascade population from the higher excited state could be neglected. Therefore we observe the emission of particles of only two velocity groups: fast and slow particles. The third type of spatial distribution of emission line k 285.2 nm Mg I includes transition from the 3p 3P01 level. This level is situated resonantly in the forbidden band of spinel. Its position is so low in respect to the vacuum level (LV) that the additional population could be realized due to the transitions from the upper levels which also situated resonantly in forbidden band of spinel. The influence of these transitions is becoming evident in second part (l > 0.05 cm) of dependence Ln I = f(l). In the first part of this dependence due to short lifetime of excited state we can observe the residual decay of this level for the fast group of Mg atoms. Finally, the forth type of dependence Ln I = f(l) was observed for emission line k 279.5 nm Mg II. It can be seen that 3p 2P1/2 level of excited state in Mg+ ions is situated very deep in respect to LV and resonantly to valence band (VB). According to the energy band structure of oxides the influence of charge exchange processes such as resonance ionization is not possible, i.e. we can expect the emission of slow and fast ions. On the other hand it is quite probable the cascade population of this level due to transitions from some higher energy states. However the comparison of experimental data and the calculated dependence Ni(l) suggests that cascade population of this level realizes only for the fast particles (Ef 1700 eV). This is in accordance with our previous conclusions [6,7] that ions and highly excited
Fig. 3. Scheme of the energy bands of magnesium aluminate spinel and energy levels of the isolated magnesium and aluminum atom and ion. Depicted transitions were studied in this paper.
V.V. Bobkov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 501–505
atoms can be formed only during the process of multiple collisions with large energy transfer for each collision event.
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essential contribution in formation of excited states of sputtered particles gives the mechanism of cascade population. References
4. Conclusions There were investigated the emission spectra of sputtered particles during Ar+ ion bombardment of magnesium aluminate spinel MgO Æ nAl2O3. The experimental dependences of intensities of certain emissions of spectra Al I, Mg I and Mg II on the distance from the target surface I(l) were obtained. The comparison of experimental dependences I(l) with calculations which takes into account different mechanisms of formation of excited states were provided. It was shown that for deeply situated levels the
[1] V.V. Bobkov, S.P. Gokov, V.V. Gritsyna, V.T. Gritsyna, D.I. Shevchenko, Nucl. Instr. and Meth. B 218 (2004) 46. [2] Ming L. Yu, Nucl. Instr. and Meth. B 18 (1987) 542. [3] R. Hippler, W. Kruger, A. Scharmann, K.-H. Schartner, Nucl. Instr. and Meth. 132 (1976) 439. [4] T.S. Kijan, V.V. Gritsyna, Izv. AN. SSSR. Ser. Fis. 43 (1979) 595. [5] V.V. Gritsyna, A.G. Koval’, V.T. Koppe, S.P. Gokov, Opt. Spectrosc. 79 (1995) 212. [6] T.S. Kijan, V.V. Gritsyna, Ya.M. Fogel’, Nucl. Instr. and Meth. 132 (1976) 435. [7] V.V. Bobkov, S.P. Gokov, V.V. Gritsyna, D.I. Shevchenko, SIMS, Europe, 2004, Munster, p. 64.
Nuclear Instruments and Methods in Physics Research B 256 (2007) 501–505 www.elsevier.com/locate/nimb
Mechanisms of formation of sputtered particles in excited states at Ar+ ion bombardment of oxide targets V.V. Bobkov
a,*
, S.P. Gokov a, V.V. Gritsyna a, V.T. Gritsyna a, D.I. Shevchenko a, S.S. Alimov a,b a
V. Karazin Kharkiv National University, Kharkiv, Ukraine b Hahn-Meitner-Institute, Berlin, Germany Available online 16 December 2006
Abstract Investigations of the main parameters of the ion–photon emission during the Ar+ ions bombardment of the complex oxides (MgO Æ nAl2O3) were provided. Spectral composition of the emitted light, relative intensities of different lines, and dependences of lines intensity on the distance from the target surface give information on process of sputtering of the particles and their formation in different exited states. It was shown that under the bombardment with Ar+ ions the excited atoms of metal (Al and Mg) are formed as in the process of multiple collisions so under the development of linear cascades. Formation of the excited sputtered ions (Mg+) takes place only during the process of multiple collisions. For deeply situated levels an essential contribution in formation of the excited states of sputtered particles gives mechanism of cascade population. 2006 Elsevier B.V. All rights reserved. PACS: 32.30.Jc; 71.20.Ps; 79.20.Rf Keywords: Sputtering; Spectra of atoms and ions; Oxides; Ion–photon emission
1. Introduction It was shown before [1] that the phenomenon of ion– photon emission (IPE), i.e. the emission of photons by exited particles knocked-out from solid surface by ions of keV-energy is caused by following processes. At first, these are the processes leading to departure of the particles: multiple collisions of incident ion or recoil atom with surface atoms of the target and development of linear cascades in solids. At second, these are the processes leading to formation of knocked-out particles in a given charge and excited state. There exist several mechanisms of formation of excited particles. In kinetic model an excited particle is formed during the separation of common electron cloud of pair particles. The model includes also the mechanism
*
Corresponding author. Tel.: +380 573 35 2734. E-mail address: [email protected] (V.V. Bobkov).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.049
of breaking chemical bonds [2]). Another mechanism is based on the process of electron exchange, in which formation of the particle in the excited state (i) is determined by decay of the system solid – departing atom [1,3]. Besides, for free particles one should take into account the additional mechanism based on the influence of cascade transitions from the upper excited state (k) to the exited state (i). This changes the number of the particles decaying to the lower excited state (j) due to the transition i–j [4]. The intensity of spectral line (I) of the given transition i–j is determined by the number of particles (Ni) in the excited state i c I ¼ Aij N i h ð1Þ kij where Aij is the probability of i–j transition by emitting photon of wavelength kij, h and c is Plank constant and light velocity, respectively. Using spontaneous decay law for the excited state i and the relation between the effective
502
V.V. Bobkov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 501–505
velocity (veff) of excited particles, the distance (l) of emitting particle from surface of target and the time (t = l/veff) we obtain t l ¼ N 0i exp ; ð2Þ N i ðlÞ ¼ N 0i exp si veff si where N0i is the number of particles in i-state leaving the surface at t = 0, si is the lifetime of particle in ith state. In case of additional population from the upper excited states (k) to the investigated (i) via cascade transitions the population will be defined by expression X l 1 N i ðlÞ ¼ N 0i exp þ N 0k si ðsk si Þ veff si k l l exp exp ; ð3Þ veff sk veff si where N0k is the population of kth level at t = 0, sk is the lifetime of this level. Summation should be provided over all levels k from which the cascade transitions to level i are possible. As it was already shown [1], despite of the departing particles with wide energy distribution we can distinguish the group of comparatively slow particles which arises in linear cascade collisions (several tenths eV) and the group of fast particles created by multiple collisions and having energy comparable to energy of primary beam. Thus taking into account the influence of cascade population and the existence of two energetic groups of departing excited particles the total number of particles in i-state at distance l is defined by the sum of number of particles calculated according to Eq. (3) for fast and slow particles. In this paper we investigated relative intensity and spatial distribution of several lines emitted by exited atoms and ions of aluminum and magnesium sputtered from magnesium aluminate spinel of different compositions. 2. Experimental details The investigation was carried out at the experimental setup which allows to use the mass separated Ar+ ion beam of 20 keV and current density of 10–20 lA cm2. The incident angle of ion beam to the target surface was 45. The photon emission of excited particles was measured in the direction perpendicular to plain, created by the bombarding beam and normal to target surface. The spectra of emission was analyzed with monochromator MDR-3 in the wavelength range of 250–800 nm and registered with cooled photomultiplier FEU-106 in photon counting mode. All emissions with intensity two times higher than background level were recorded. The full number of photons emitted by ejected excited particles was determined by using the method of entire collection of radiation described in [5]. For all observed emissions the quantum yield ck was defined as the number of photons of given wavelength k related to one incident ion. Magnesium aluminate spinel single crystals of different compositions
MgO Æ nAl2O3 (n = 1.0, 2.5) grown by Verneuil method were used as a target. 3. Results and discussion The spectra of ion–photon emission consist of the lines which are connected with transitions in metal particles: Mg, Mg+, Al, Al+ and Al++. Dependences I(l)of the emitted lines with intensity of 10 times higher than background level were measured according to the method described earlier [5]. The dependences I(l) observed in this work for the different lines in spectra of Al I, Mg I, and Mg II can be divided in four types. Despite of difference in the quantum yield of emitted lines registered from spinel crystals of different compositions MgO Æ nAl2O3 (n = 1.0 and 2.5) [1], the spatial distributions of studied emissions are the same. (1) For the lines k 473.0 nm Mg I and k 669.8 nm Al I the dependences Ln I = f(l) can be represented by the straight line. Effective velocity of these atoms was estimated from the slope angle. It was concluded that both excited atoms Mg (6s 1S0) and Al (5p 2P1/2,3/2) are the fast particles with kinetic energy 2000– 3000 eV. (2) For the lines kk 518.3, 382.9 and 383.8 nm Mg I and kk 396.1, 394.4, 308.2 and 309.2 nm Al I the dependences Ln I = f(l) are much complicated. In Fig. 1 we present the experimental results of dependences of intensities for two lines on the distance from the target surface. The curves were calculated according to expression (3). For the line k 396.1 nm Al I, transition 4s 2S1/2 ! 3p 2P3/2 (s4 = 1.6 · 108 s; Ni0 c = 9 · 104 ph/ion [5]) it is possible the existence of cascade transition from level 5p 2P1/2;3/2 (sk = 1.7 · 108 s;Nk0 c = 2.6 · 105 ph/ion), i.e. Nk0 < < Ni0. Comparison of the calculation (line) with experimental dependence Ln I = f(l) has shown that the jog in experimental dependence at l = 0.1 cm (Fig. 1a) could be explained by introducing only two groups of atoms: slow (Es = 50 eV) and fast (Ef = 1500 eV) ones in the ratio of number particles N0s:N0f = 6:1. For the line k 518.3 nm Mg I, transition 4 s 3 S1 ! 3p 2P02 (s4 = 1.0 · 108 s; N40 c = 2.5 · 104 ph/ion), the best fit of calculated dependence with experimental points was obtained when including two groups of atoms of different energies (slow Es = 50 eV and fast Ef = 3500 eV; N0s:N0f = 4:1). The same results were obtained for the other lines of this group. (3) Dependences Ln I = f(l) for the line k 285.2 nm Mg I, transition 3p 1P01 ! 3s21S0 (s3 = 0.2 · 108 s) are presented in Fig. 2(a). Because of small value of s3 the intensity of emission of slow particles decreases by two orders of magnitude at the distance l < 0.02 cm. Therefore the observed jog at the experimental dependence Ln I = f(l) cannot be explained by the emission of two energetic groups of particles. We
V.V. Bobkov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 501–505
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Fig. 1. Dependences of intensities of lines k 396.1 nm Al I (a) and k 518.3 nm Mg I (b) on the distance from the target surface. Points are experimental data, lines is calculation from Eq. (3).
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Fig. 2. Dependences of intensity of line k 285.2 nm Mg I (a) and line k 279.5 nm Mg II (b) on the distance from target surface. Points are experimental data, lines is calculation from Eq. (3).
expect that the level 3p 1P01 could be additionally populated from levels 3d 1D2 (sk = 8.0 · 108 s), 4d 1D2 (sk = 5.5 · 108 s), 4s 1S0 (sk = 4.7 · 108 s), and 5s 1 S0 (sk = 10 · 108 s). The wavelength of emission lines corresponding to these transitions (except line k 552.8 nm Mg I) situated out of the investigated spectral range. Nevertheless according to the spectral tables they are rather intense. Therefore calculated curve for the line k 285.2 nm shows the best fit to experimental data for such a case: the emission from near surface region (l < 0.05 cm) is caused by radiation of fast particles (Ef = 1700 eV) which were formed in state 3p 1P01 directly at the process of ion bombardment. The dependence Ln I = f(l) in the range l > 0.05 cm is related to the process of cascade population of the investigated level by slow particles
(Es = 100 eV). The calculated ratio is N0i:N0Rk = 0.3:1. (4) Finally, the lines of kk 279.5, 280.2 nm Mg II (s3 = 0.4 · 108 s) show essentially different dependences Ln I = f(l) compared to the other lines. The upper level 3p 2P3/2 of this transition situated essentially below the vacuum level. Therefore in this case we can expect contribution in the experimental dependence Ln I = f(l) from the emission as slow so fast ions. Moreover it is also possible that the emission contributes from the ions which were formed in the observed state by the transition from the higher excited states: 4s 2S1/2 (sk = 0.28 · 108 s), 4d 2D5/2 (sk = 0.81 · 108 s) and 5d 2D5/2 (sk = 2 · 108 s). In this case these could be both fast and slow particles. Calculation according to the Eq. (3) shows the
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V.V. Bobkov et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 501–505
best accordance between experimental Ln I = f(l) and calculated dependences Ln Ni = f(l) in case of influence of cascade population of the level 3p 2P3/2 only by fast ions Mg+ (Ef = 1700 eV). An attempt to take into account the contribution of slow Mg+ ions makes calculated curve Ni = f(l) apart from experimental data. This indicates probably that during the ion bombardment of magnesium aluminate spinel only the fast ions Mg+ are formed. To analyze the obtained results let us examine the scheme of energy levels of investigated transitions in aluminum atoms and magnesium atoms and ions, and energy bands in spinel (Fig. 3). The first type of dependences Ln I = f(l) is stipulated by the fast sputtered particles with levels of excited states 6s 1 S0 and 5p 2P1/2,3/2 for both aluminum and magnesium atoms situated resonantly on unoccupied levels of conduction band (CB) in spinel. Due to this energy position, the process of the resonant ionization can take place with the high probability. This makes possible conservation of the excitation during departing only for the fast group of excited particles formed as a result of multiple collisions of incident ion with particles of the target. The upper levels of the transition responsible for the emission lines of the second type dependence Ln I = f(l) are situated resonantly in forbidden band of spinel. Therefore the influence of such a process as resonance ionization or resonance neutralization is practically negligible. On the other hand these levels are situated rather high. That is why
the contribution of mechanisms of cascade population from the higher excited state could be neglected. Therefore we observe the emission of particles of only two velocity groups: fast and slow particles. The third type of spatial distribution of emission line k 285.2 nm Mg I includes transition from the 3p 3P01 level. This level is situated resonantly in the forbidden band of spinel. Its position is so low in respect to the vacuum level (LV) that the additional population could be realized due to the transitions from the upper levels which also situated resonantly in forbidden band of spinel. The influence of these transitions is becoming evident in second part (l > 0.05 cm) of dependence Ln I = f(l). In the first part of this dependence due to short lifetime of excited state we can observe the residual decay of this level for the fast group of Mg atoms. Finally, the forth type of dependence Ln I = f(l) was observed for emission line k 279.5 nm Mg II. It can be seen that 3p 2P1/2 level of excited state in Mg+ ions is situated very deep in respect to LV and resonantly to valence band (VB). According to the energy band structure of oxides the influence of charge exchange processes such as resonance ionization is not possible, i.e. we can expect the emission of slow and fast ions. On the other hand it is quite probable the cascade population of this level due to transitions from some higher energy states. However the comparison of experimental data and the calculated dependence Ni(l) suggests that cascade population of this level realizes only for the fast particles (Ef 1700 eV). This is in accordance with our previous conclusions [6,7] that ions and highly excited
Fig. 3. Scheme of the energy bands of magnesium aluminate spinel and energy levels of the isolated magnesium and aluminum atom and ion. Depicted transitions were studied in this paper.
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atoms can be formed only during the process of multiple collisions with large energy transfer for each collision event.
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essential contribution in formation of excited states of sputtered particles gives the mechanism of cascade population. References
4. Conclusions There were investigated the emission spectra of sputtered particles during Ar+ ion bombardment of magnesium aluminate spinel MgO Æ nAl2O3. The experimental dependences of intensities of certain emissions of spectra Al I, Mg I and Mg II on the distance from the target surface I(l) were obtained. The comparison of experimental dependences I(l) with calculations which takes into account different mechanisms of formation of excited states were provided. It was shown that for deeply situated levels the
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