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Excited-state formation model for metal-oxide sputtered molecules
65
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 65-67 North-Holland
Excited-state
formation model for metal-oxide sputtered molecules
S.F. Belykh, I.V. Redina and V.Kh. Ferleger Institute of Electronics,
Uzbek Academy of Sciences, Academgorodok,
700143 Tashkent,
USSR
At present it has been well established that the continuum and the bands observed in radiation spectra under ion bombardment of refractory and rare-earth metal surfaces are emitted by metal-oxide sputtered molecules. An important feature of this radiation is the anomalously high photon yield, Y,: Y,,=1O-2-1O-3 photon/ion. This value exceeds Y for sputtered excited atoms: Y,JY 2 102. In tbis article, we propose the excited-state formation model for sputtered molecules based on the following assumptions: (1) a molecule is nondissociatively sputtered if the energy of the relative motion of its atom in the center-of-mass system is less than its bond energy; and (2) the nuclei energy of relative motion of the nuclei is transferred to the electron subsystem excitation by a Landau-Zener-type transition. The model permits us to understand the large value of Yx, because a great number of the molecules undergoes excitation beyond the electron exchange region, Sa: S, = 5-10 A where molecule-surface exchange processes take place.
1. Introduction
It is well known that under ion bombardment of refractory and rare-earth metal surfaces continuum radiation (CR) is observed together with line spectra over a wide range of wavelengths [1,2]. This continuum spectrum is emitted not by atoms at the target surface but by some excited sputtered particles. The intensity of continuum emission, Y, = 10-2-10-3 photon/ion, is anomalously high, i.e., it is two to three orders of magnitude greater than the typical intensity values for line spectra. At present many investigators suppose that the CR is emitted by sputtered excited metal oxide molecules (MO) [2]. However, establishment of the CR origin is only the first step to understanding the CR. In order to complete the picture of this phenomenon it is necessary to investigate: - the mechanism of MO-type molecule formation and emission; _ the elementary processes leading to MO excitation; _ the possibility of this excitation conservation beyond the region of MO-surface effective electron exchange; - the ways of excitation relaxation providing the observed CR spectra. For the first time the possibility of the continuum emission of excited molecules has been discussed in ref. [3]. The authors supposed that the continuum spectra are caused by the processes of associative recombination of metal atoms with adsorbed gas atoms. This mechanism should result in a continuum photon emis0168-583X/91/$03.50
sion spectrum. However, the associative model gives a square dependence of the continuum emission intensity (Y,) on the ion current density (j,): Y, - ji, in disagreement with experiments [2]. In addition, it follows from associative models that the yield of the sputtered excited (or even not excited molecules) should be much less than the sputtered atomic yield, because formation of molecules by coupling of the sputtered atoms occurs quite rarely due to rigid restrictions imposed on both magnitude and direction of atomic momenta. Another model for excited molecule formation has been considered by Bazhin et al. [4]. In this model the metal oxide molecule (MO) is supposed to be formed by chemical reactions on the surface and then to be knocked out in the excited state due to energy transfer from the incident ion to the metal atom M of the MO molecule. Such excited molecules can emit both band spectra and the continuum. The band spectra are formed due to the relaxation of the excited molecule states while the continuum is a result of the repulsive predissociative state relaxation. It is clear that this model leads to the dependence Y, -j,,, in agreement with experiments In our opinion, the mechanism proposed in refs. [3,4] cannot explain the high intensity of continuum spectra (experimental value Y, L 10e3 photon/ion [2]) even if nonradiative de-excitation processes are neglected as in ref. [4], because excitation of these molecules due to violent collisions with fast recoils is an event with small probability. Such high values of Y, for continuum spectra mean, in addition, that the survival probability of the excited molecule states (P) in the nomadiative de-excitation
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
I. THEORY/FUNDAMENTALS
S.F. Beiykh et al. / Excited-state formation model
66
We consider the mechanism of formation of such survival states and estimate the light emission intensity within the framework of the following simple model.
2. Model
+i 0
4
8
-’ po; ‘O
12
16
(Torr)
Fig. 1. The Y and Y, dependence on Paz: (1) for Ho0 band, X = 527 run; (2) for HOI line, X = 598.3 run.
processes is anomalously high and exceeds the corresponding value of P for atomic ion-photon emission by
over two to three orders of magnitude. To determine the mechanism of continuum ion-photon emission it is necessary to understand the origin of the small contribution from deexcitation of the nonradiative molecules. Interesting information on the P-functions is contained in the experimental data [2]. In this article the photon emission intensity dependence on the partial oxygen pressure PO, for the molecule bands, YA(Po2), was compared with that for atomic lines, Y( PO,). Typical results are shown in fig. 1. It is seen that the Y( PO,) for atoms and Y,( PO,) for molecules are quite different. At small values of Paz, the yield Y for atoms is nearly constant and then drastically increases with increasing P02, which is explained by the changing of the surface electron structure for sufficiently large oxygen coverage. On the contrary, the yield Y, for molecules increases linearly with increasing PO, within a wide PO, region and is proportional to the oxygen coverage 8. The Yx-8 dependence testifies only to the fact that the probability of molecule formation increases linearly with the increasing 8, not that Y,,depends on the surface electronic structure. The independence of the intensity of the ion-photon emission by molecules on the surface state is a peculiarity of the continuum spectra only. The large values of the survival probability of excited molecules, together with the independence Y,, of the surface state, can be understood only by assuming that formation of a considerable number of excited molecules occurs beyond the near surface efficient electron exchange region.
The molecule is formed on the surface and its axis is perpendicular to the surface (fig. 2). As the binding energy of the atoms in the MO molecules for refractory and rare-earth metals is sufficiently high: E, = 6-8 eV, then the bond of the metal atom of this molecule with its neighbors in the lattice becomes reduced. Therefore the binding energy of the MO molecule with the surface is supposed to be small: U 5 1 eV. The sputtering of the MO molecules under ion bombardment is considered to occur by a cascade mechanism [S], which means that the molecules leave the surface when the M atom gets an energy E 2 I-Jfrom the collision cascade. It is clear that under these conditions the momentum directions of the M and 0 atoms are not correlated. As a result, the molecules are emitted in high vibrational and rotational excited states. The relaxation of this excitation occurs either by molecule destruction or, if the excitation transfers from the nuclear subsystem to the electron one, by ionisation or by light-emission of the molecule. The destruction of the molecule does not occur if the energy of relative motion of its atoms in the center-of-mass system (E,) is less than the binding energy of these atoms: E, < E,. It is easy to show that E, is proportional to the molecules kinetic energy: E,=Em/(M+m),
(1)
where m and M are the masses of oxygen and metal atoms respectively. We suppose also that if E, < E,, the entire energy E, transfers to the molecule electron subsystem due to the promotion and crossing of the terms by the Landau-Ziner mechanism [6], then the entire energy E, transfers to the excitation energy E, E = E,. In
Fig. 2. The scheme of the excited sputtered molecule formation.
S.F. Belykh et al. / Excited-state formation model
order for the molecule to emit light in the visible region, two conditions must be fulfilled: (1) The excitation energy of this molecule must be within the range: cl
(2)
where Ei=e,(M+m)/m, and i=l, 2. (2) The obtained excitation must be conserved. We assume that the survival probability, P, of an excited molecule can be written as follows:
Sl 1,
-~;‘j~W(s)
where st is the distance from the surface where excitation occurs. For further estimates we assume that:
P(sd =
i
0
if st < sa (se = 5-10 A),
1
ifs,>s,.
(3)
As the transition probability at the crossing of the terms is - f then for the molecules moving with energy E 2 10 eV in the region st < s0 the excitation can occur not more than once or twice. So nonradiative de-excitation cannot reduce the number of excited molecules drastically. The emission intensity (the number of photons per incident ion) may be written as: Y, = &Q/K,
the relative number of molecules sputtered with energy intherangeE,
Q=
(4)
where S, is the molecule sputtering yield, Q is the probability of its excitation, and K is the excited molecule reducing coefficient (due to nonradiative de-excitation). K is assumed to equal 2-4. We assume that S, = SB, where S is the sputtering yield of the metal atoms with binding energy U and 0 is the oxygen coverage rate. The probability Q is equal to
jE;f(E)W(h)
dE.
Taking into account that f(E) obtain:
Q=
U(E2 - E,)(2E,E,+ (E,
+ U)‘(E,+
E,U+
(5)
= AE/( E + U)3 [5] we
E&J)
U)’
.
(6)
As UC+ E,, E,, then:
Q=
ds
61
2mU(e2
- q)
r,e,(M+
m)
.
When m/(M+ m) = 0.1, S < 10, U = 1 eV the photon yield: Y, > lop3 photon/ion, even if B 5 0.1, in qualitative agreement with the experiments [2]. More careful analysis of this problem requires knowledge of the real form of the different MO molecule terms.
References
[II G.E. Thomas, Surf. Sci. 90 (1979) 381.
VI S.S. Pop, S.F. Belykh, V.G. Drobnich and V.Kh. Ferleger, Ion-Photon Emission of Metals (Fan, Tashkent, 1989) in Russian. [31 E.O. Rausch, AI. Bazhin and E.W. Thomas, J. Chem. Phys. 65 (1976) 4441. [41 A.I. Bazhin, M. Suchanska and S.V. Teplov, Nucl. Instr. and Meth. B48 (1990) 639. I, ed. R. Behrisch 151 Sputtering by Particle Bombardment (Springer, Berlin, 1981). VI L.D. Landau and E.M. Lifshitz, Quantum Mechanics (Moscow, 1963) in Russian.
I. THEORY/FUNDAMENTALS
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 65-67 North-Holland
Excited-state
formation model for metal-oxide sputtered molecules
S.F. Belykh, I.V. Redina and V.Kh. Ferleger Institute of Electronics,
Uzbek Academy of Sciences, Academgorodok,
700143 Tashkent,
USSR
At present it has been well established that the continuum and the bands observed in radiation spectra under ion bombardment of refractory and rare-earth metal surfaces are emitted by metal-oxide sputtered molecules. An important feature of this radiation is the anomalously high photon yield, Y,: Y,,=1O-2-1O-3 photon/ion. This value exceeds Y for sputtered excited atoms: Y,JY 2 102. In tbis article, we propose the excited-state formation model for sputtered molecules based on the following assumptions: (1) a molecule is nondissociatively sputtered if the energy of the relative motion of its atom in the center-of-mass system is less than its bond energy; and (2) the nuclei energy of relative motion of the nuclei is transferred to the electron subsystem excitation by a Landau-Zener-type transition. The model permits us to understand the large value of Yx, because a great number of the molecules undergoes excitation beyond the electron exchange region, Sa: S, = 5-10 A where molecule-surface exchange processes take place.
1. Introduction
It is well known that under ion bombardment of refractory and rare-earth metal surfaces continuum radiation (CR) is observed together with line spectra over a wide range of wavelengths [1,2]. This continuum spectrum is emitted not by atoms at the target surface but by some excited sputtered particles. The intensity of continuum emission, Y, = 10-2-10-3 photon/ion, is anomalously high, i.e., it is two to three orders of magnitude greater than the typical intensity values for line spectra. At present many investigators suppose that the CR is emitted by sputtered excited metal oxide molecules (MO) [2]. However, establishment of the CR origin is only the first step to understanding the CR. In order to complete the picture of this phenomenon it is necessary to investigate: - the mechanism of MO-type molecule formation and emission; _ the elementary processes leading to MO excitation; _ the possibility of this excitation conservation beyond the region of MO-surface effective electron exchange; - the ways of excitation relaxation providing the observed CR spectra. For the first time the possibility of the continuum emission of excited molecules has been discussed in ref. [3]. The authors supposed that the continuum spectra are caused by the processes of associative recombination of metal atoms with adsorbed gas atoms. This mechanism should result in a continuum photon emis0168-583X/91/$03.50
sion spectrum. However, the associative model gives a square dependence of the continuum emission intensity (Y,) on the ion current density (j,): Y, - ji, in disagreement with experiments [2]. In addition, it follows from associative models that the yield of the sputtered excited (or even not excited molecules) should be much less than the sputtered atomic yield, because formation of molecules by coupling of the sputtered atoms occurs quite rarely due to rigid restrictions imposed on both magnitude and direction of atomic momenta. Another model for excited molecule formation has been considered by Bazhin et al. [4]. In this model the metal oxide molecule (MO) is supposed to be formed by chemical reactions on the surface and then to be knocked out in the excited state due to energy transfer from the incident ion to the metal atom M of the MO molecule. Such excited molecules can emit both band spectra and the continuum. The band spectra are formed due to the relaxation of the excited molecule states while the continuum is a result of the repulsive predissociative state relaxation. It is clear that this model leads to the dependence Y, -j,,, in agreement with experiments In our opinion, the mechanism proposed in refs. [3,4] cannot explain the high intensity of continuum spectra (experimental value Y, L 10e3 photon/ion [2]) even if nonradiative de-excitation processes are neglected as in ref. [4], because excitation of these molecules due to violent collisions with fast recoils is an event with small probability. Such high values of Y, for continuum spectra mean, in addition, that the survival probability of the excited molecule states (P) in the nomadiative de-excitation
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
I. THEORY/FUNDAMENTALS
S.F. Beiykh et al. / Excited-state formation model
66
We consider the mechanism of formation of such survival states and estimate the light emission intensity within the framework of the following simple model.
2. Model
+i 0
4
8
-’ po; ‘O
12
16
(Torr)
Fig. 1. The Y and Y, dependence on Paz: (1) for Ho0 band, X = 527 run; (2) for HOI line, X = 598.3 run.
processes is anomalously high and exceeds the corresponding value of P for atomic ion-photon emission by
over two to three orders of magnitude. To determine the mechanism of continuum ion-photon emission it is necessary to understand the origin of the small contribution from deexcitation of the nonradiative molecules. Interesting information on the P-functions is contained in the experimental data [2]. In this article the photon emission intensity dependence on the partial oxygen pressure PO, for the molecule bands, YA(Po2), was compared with that for atomic lines, Y( PO,). Typical results are shown in fig. 1. It is seen that the Y( PO,) for atoms and Y,( PO,) for molecules are quite different. At small values of Paz, the yield Y for atoms is nearly constant and then drastically increases with increasing P02, which is explained by the changing of the surface electron structure for sufficiently large oxygen coverage. On the contrary, the yield Y, for molecules increases linearly with increasing PO, within a wide PO, region and is proportional to the oxygen coverage 8. The Yx-8 dependence testifies only to the fact that the probability of molecule formation increases linearly with the increasing 8, not that Y,,depends on the surface electronic structure. The independence of the intensity of the ion-photon emission by molecules on the surface state is a peculiarity of the continuum spectra only. The large values of the survival probability of excited molecules, together with the independence Y,, of the surface state, can be understood only by assuming that formation of a considerable number of excited molecules occurs beyond the near surface efficient electron exchange region.
The molecule is formed on the surface and its axis is perpendicular to the surface (fig. 2). As the binding energy of the atoms in the MO molecules for refractory and rare-earth metals is sufficiently high: E, = 6-8 eV, then the bond of the metal atom of this molecule with its neighbors in the lattice becomes reduced. Therefore the binding energy of the MO molecule with the surface is supposed to be small: U 5 1 eV. The sputtering of the MO molecules under ion bombardment is considered to occur by a cascade mechanism [S], which means that the molecules leave the surface when the M atom gets an energy E 2 I-Jfrom the collision cascade. It is clear that under these conditions the momentum directions of the M and 0 atoms are not correlated. As a result, the molecules are emitted in high vibrational and rotational excited states. The relaxation of this excitation occurs either by molecule destruction or, if the excitation transfers from the nuclear subsystem to the electron one, by ionisation or by light-emission of the molecule. The destruction of the molecule does not occur if the energy of relative motion of its atoms in the center-of-mass system (E,) is less than the binding energy of these atoms: E, < E,. It is easy to show that E, is proportional to the molecules kinetic energy: E,=Em/(M+m),
(1)
where m and M are the masses of oxygen and metal atoms respectively. We suppose also that if E, < E,, the entire energy E, transfers to the molecule electron subsystem due to the promotion and crossing of the terms by the Landau-Ziner mechanism [6], then the entire energy E, transfers to the excitation energy E, E = E,. In
Fig. 2. The scheme of the excited sputtered molecule formation.
S.F. Belykh et al. / Excited-state formation model
order for the molecule to emit light in the visible region, two conditions must be fulfilled: (1) The excitation energy of this molecule must be within the range: cl
(2)
where Ei=e,(M+m)/m, and i=l, 2. (2) The obtained excitation must be conserved. We assume that the survival probability, P, of an excited molecule can be written as follows:
Sl 1,
-~;‘j~W(s)
where st is the distance from the surface where excitation occurs. For further estimates we assume that:
P(sd =
i
0
if st < sa (se = 5-10 A),
1
ifs,>s,.
(3)
As the transition probability at the crossing of the terms is - f then for the molecules moving with energy E 2 10 eV in the region st < s0 the excitation can occur not more than once or twice. So nonradiative de-excitation cannot reduce the number of excited molecules drastically. The emission intensity (the number of photons per incident ion) may be written as: Y, = &Q/K,
the relative number of molecules sputtered with energy intherangeE,
Q=
(4)
where S, is the molecule sputtering yield, Q is the probability of its excitation, and K is the excited molecule reducing coefficient (due to nonradiative de-excitation). K is assumed to equal 2-4. We assume that S, = SB, where S is the sputtering yield of the metal atoms with binding energy U and 0 is the oxygen coverage rate. The probability Q is equal to
jE;f(E)W(h)
dE.
Taking into account that f(E) obtain:
Q=
U(E2 - E,)(2E,E,+ (E,
+ U)‘(E,+
E,U+
(5)
= AE/( E + U)3 [5] we
E&J)
U)’
.
(6)
As UC+ E,, E,, then:
Q=
ds
61
2mU(e2
- q)
r,e,(M+
m)
.
When m/(M+ m) = 0.1, S < 10, U = 1 eV the photon yield: Y, > lop3 photon/ion, even if B 5 0.1, in qualitative agreement with the experiments [2]. More careful analysis of this problem requires knowledge of the real form of the different MO molecule terms.
References
[II G.E. Thomas, Surf. Sci. 90 (1979) 381.
VI S.S. Pop, S.F. Belykh, V.G. Drobnich and V.Kh. Ferleger, Ion-Photon Emission of Metals (Fan, Tashkent, 1989) in Russian. [31 E.O. Rausch, AI. Bazhin and E.W. Thomas, J. Chem. Phys. 65 (1976) 4441. [41 A.I. Bazhin, M. Suchanska and S.V. Teplov, Nucl. Instr. and Meth. B48 (1990) 639. I, ed. R. Behrisch 151 Sputtering by Particle Bombardment (Springer, Berlin, 1981). VI L.D. Landau and E.M. Lifshitz, Quantum Mechanics (Moscow, 1963) in Russian.
I. THEORY/FUNDAMENTALS