- Email: [email protected]
Surface ionization of the lanthanides
ARTICLE IN PRESS
Vacuum 74 (2004) 301–304
Surface ionization of the lanthanides a ! L. G"adyszewskia,*, G. G"adyszewskib, T. Pienkos b
a Institute of Physics, Maria Curie-Sk!odowska University, pl. M.Curie-Sk!odowskiej 1, 20-031 Lublin, Poland Department of Experimental Physics, Institute of Physics, Technical University of Lublin, ul. Nadbystrzycka 38, 20-618 Lublin, Poland
Accepted 22 December 2003
Abstract The surface ionization of Lanthanides: Ce, Pr, Nd, Sm and Eu, and its fluctuations (noises) have been studied using a double filament ion source in a 90 magnetic mass spectrometer. Using the Saha-Langmuir equation there were measured ionization potentials of LnO molecules as the dissociation products of the oxides Ln2O3 (the Ln symbol denotes the arbitrary rare earth elements: Ce, Pr, Nd, Sm, Eu). Ionic desorption energies of LnO molecules were determined using the fluctuation (noise) method. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ion emission; Mass spectrometry; Surface ionization
1. Introduction The surface ionization effect consists in desorption of atoms or molecules from metallic surface in the form of positive ions. The temperature dependence of this effect is given by the SahaLangmuir equation i0 ¼ enA exp½eðV jÞ=kT ;
ð1Þ
The fluctuation in the average ion emission current i0 due to a work function fluctuation dðejÞ is attributable to a time variation of the average number n of the adsorbed molecules LnO such that dðejÞ ¼ mdn=e0 ; (m is the dipole moment of adsorbed molecules and e0 is the dielectric constant of free space) [1,2]. To describe the ion current fluctuations, autocorrelation functions were used
where i0 is the intensity of ion current, e the electron charge, n the number of molecules coming in a unit time to the ionizing surface, A the coefficient depending on the geometry of the ion source, V the ionization potential of atom or molecule, ej the work function of ionizing surface, k the Boltzmann’s constant, T the temperature of metal surface.
where iðtÞ is the ion current and t the delay time of the correlator. The autocorrelation function can be approximated by the exponential expression
*Corresponding author. Fax: +48-81-537-6191. E-mail address: [email protected] (L. G"adyszewski).
where t0 is the characteristic relaxation time for the stochastic process iðtÞ: By fitting RðtÞ curves to the experimental data we determine t0 :
RðtÞ ¼ /iðtÞ iðt þ tÞS;
RðtÞ ¼ /di2 S expðt=t0 Þ;
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.143
ð2Þ
ARTICLE IN PRESS L. G!adyszewski et al. / Vacuum 74 (2004) 301–304
302
The desorption energy Q was estimated [3] from the plots of logðt0 Þ as a function of reciprocal temperature t0 ¼ t00 expðQ=kTÞ:
ð3Þ
chosen for studies: in the case of LaO molecule it was the line of the mass number 155; PrO—line 157; NdO—line 158; SmO—163; EuO—167. In the case of simultaneous ionization of two molecules on the same surface, we can obtain such formulae i1 ¼ A1 exp½ðj V1 Þ11; 600=T; i2 ¼ A2 exp½ðj V2 Þ11; 600=T
2. Experimental The measurements were performed using a magnetic mass spectrometer with 90 deflection and a resolution of 300. There was used a twofilament ion source [4], in which from the first tungsten band (so-called evaporator) evaporation of products of oxide dissociation placed on this band took place. Another identical band of the size (10 1 0.1) mm heated to higher temperatures was an ionizer. The molecules vaporizing from the evaporator underwent adsorption on the ionizer surface and after a very short lifetime on the surface, desorbed as ions and neutral molecules. In the evaporator temperature range 1000– 1400 K the dissociation process proceeded according to the reaction: Ln2O3-2LnO+O; however, at T > 1500 K, Ln+ ions appear in the mass spectrum which means that at high temperatures dissociation proceeds also according to the reaction: Ln2O3-LnO+Ln+O2. With a very small coverage of ionizer surface that is with a small beam of the LnO molecules from the evaporator, ionization potentials of molecule can be determined. In this method there are studied the ratio of two ionic current intensities corresponding to two molecules ionized simultaneously on the same surface. In our case the model molecule was LaO for which the ionization potential was determined earlier [5]. Of mass spectrum rich in isotopic lines, the isotopes of the highest percentage content were
and lnði1 =i2 Þ ¼ ðV2 V1 Þ11; 600=T þ A0 ;
ð4Þ
where the constant 11; 600 ¼ e=k using the units: coulomb and eV=K respectively. The ion thermoemission noise was amplified by a wide-band DC electrometer. The time correlation function was investigated using a stochastic analyser the ‘‘NSA-1000’’ type.
3. Results (1) Work function of the ion emitter surface and its surface structure. In the additional experiment [6] a very small beam of sodium atoms was directed on the surface of polycrystalline tungsten. The obtained value was ej ¼ 4:6170:05 eV. The surface structure was determined by means of X-ray diffraction method finding out domination of (0 0 1) planes. (2) Ionization potentials of the LnO molecules. In the ionizer temperature range 1500–2500 K the linear functions of 1=T are obtained according to Eq. (4). The obtained values of the ionization potentials are presented in Table 1 (see an example in Fig. 1).
Table 1 Ionization potentials V (in Volts) and desorption energies Q of LnO molecules (in eV)
V Q
LaO
CeO
PrO
NdO
SmO
EuO
4.8 [5] —
6.0270.08 1.5670.10
5.0470.04 1.6870.08
5.3770.06 1.4270.11
5.9070.05 1.7670.12
5.1070.05 1.8770.09
ARTICLE IN PRESS L. G!adyszewski et al. / Vacuum 74 (2004) 301–304 8
NdO
+
SmO
7
+
+
+
PrO
SmO
6
∆V =1.10V
6
5.5
5
NdO
+
5
4 ∆V =0. 57V
3
4.5 +
2
PrO
∆V =0. 24V
4
1 2500°C
2250°C
0 4.5
2000°C
5
5.5
1750°C
6
6.5
7
ln (τ0)
ln(i0/i)
303
3.5 3
11600/T
2.5
Fig. 1. Logarithm of the ratio of LaO+ ion current intensity i0 to LnO+ ion current intensity i: DV is difference between the ionization potentials of the molecules.
2 1.5
T=1750 K
0
T=2010 K
1 4.5
T=2300 K
5
5.5
6
6.5
7
11600/T
ln [normalized R(τ)]
-0.5
Fig. 3. Mean residence time t0 for PrO, NdO and SmO molecules on the tungsten surface as a function of 1=T:
-1 -1.5 -2 -2.5 -3 -3.5 -4 -4.5 0
20
40
60
80
100
120
140
160
180
τ [µs]
Fig. 2. Normalized autocorrelation functions RðtÞ versus delay time t for PrO+ current noise.
(2) (3) Autocorrelation functions and energy of desorption. The time autocorrelation function can be approximated by an exponential one (Fig. 2): RðtÞ ¼ /di2 S expðt=t0 Þ: By fitting RðtÞ to the experimental data we determine t0 : The desorption energy was estimated from the plots of logðt0 Þ as a function of 1=T (Fig. 3).
(3) (4)
4. Conclusions (1) We found out that with a sufficiently small beam of molecules directed from the evaporator on the ionizer and with great sensitivity of measuring ionic current intensity on the level
(5)
1016 A it is possible to evaporate mixtures of all studied oxides simultaneously and to choose suitable pairs of LaO/LnO for measurement by means of mass spectrometer. The data in the error limit are identical as in the case of laborious evaporation and ionization of chosen pairs of compounds: LaO/CeO, LaO/PrO etc. Of the studied molecules, those of PrO and EuO, which in the migration process and statistical process of desorption generate particularly strong noises, stand out. They probably have a large dipole moment generating large changes of local work function. For all molecules distributions of instantaneous noise amplitudes were Gaussian. Spectral density functions of the noise for all molecules were of the Lorentzian form which is characteristic for physical processes with one relaxation time characteristic for a given process. In our model this time is identified with the mean residence time of molecules on the ionizer surface. The obtained data are fundamental quantities of atomic and molecular physics.
ARTICLE IN PRESS 304
L. G!adyszewski et al. / Vacuum 74 (2004) 301–304
References [1] Gomer R. Surf. Sci 1973;38:373–6. [2] Holzl J, Schulte F. In: Solid surface physics. Berlin: Springer; 1979. [3] G"adyszewski L. Surf. Sci. 1990;231:120–4.
[4] G"adyszewski L. Annales UMCS, sect. AAA, XXXIV/ XXXV, 1979/1980:55–63. [5] Chupka W, Inghram M. J. Chem. Phys. 1956;24:792–5. [6] G"adyszewski L, G"adyszewski G. Surf. Sci, 1991; 247:274–8.
Vacuum 74 (2004) 301–304
Surface ionization of the lanthanides a ! L. G"adyszewskia,*, G. G"adyszewskib, T. Pienkos b
a Institute of Physics, Maria Curie-Sk!odowska University, pl. M.Curie-Sk!odowskiej 1, 20-031 Lublin, Poland Department of Experimental Physics, Institute of Physics, Technical University of Lublin, ul. Nadbystrzycka 38, 20-618 Lublin, Poland
Accepted 22 December 2003
Abstract The surface ionization of Lanthanides: Ce, Pr, Nd, Sm and Eu, and its fluctuations (noises) have been studied using a double filament ion source in a 90 magnetic mass spectrometer. Using the Saha-Langmuir equation there were measured ionization potentials of LnO molecules as the dissociation products of the oxides Ln2O3 (the Ln symbol denotes the arbitrary rare earth elements: Ce, Pr, Nd, Sm, Eu). Ionic desorption energies of LnO molecules were determined using the fluctuation (noise) method. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ion emission; Mass spectrometry; Surface ionization
1. Introduction The surface ionization effect consists in desorption of atoms or molecules from metallic surface in the form of positive ions. The temperature dependence of this effect is given by the SahaLangmuir equation i0 ¼ enA exp½eðV jÞ=kT ;
ð1Þ
The fluctuation in the average ion emission current i0 due to a work function fluctuation dðejÞ is attributable to a time variation of the average number n of the adsorbed molecules LnO such that dðejÞ ¼ mdn=e0 ; (m is the dipole moment of adsorbed molecules and e0 is the dielectric constant of free space) [1,2]. To describe the ion current fluctuations, autocorrelation functions were used
where i0 is the intensity of ion current, e the electron charge, n the number of molecules coming in a unit time to the ionizing surface, A the coefficient depending on the geometry of the ion source, V the ionization potential of atom or molecule, ej the work function of ionizing surface, k the Boltzmann’s constant, T the temperature of metal surface.
where iðtÞ is the ion current and t the delay time of the correlator. The autocorrelation function can be approximated by the exponential expression
*Corresponding author. Fax: +48-81-537-6191. E-mail address: [email protected] (L. G"adyszewski).
where t0 is the characteristic relaxation time for the stochastic process iðtÞ: By fitting RðtÞ curves to the experimental data we determine t0 :
RðtÞ ¼ /iðtÞ iðt þ tÞS;
RðtÞ ¼ /di2 S expðt=t0 Þ;
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.143
ð2Þ
ARTICLE IN PRESS L. G!adyszewski et al. / Vacuum 74 (2004) 301–304
302
The desorption energy Q was estimated [3] from the plots of logðt0 Þ as a function of reciprocal temperature t0 ¼ t00 expðQ=kTÞ:
ð3Þ
chosen for studies: in the case of LaO molecule it was the line of the mass number 155; PrO—line 157; NdO—line 158; SmO—163; EuO—167. In the case of simultaneous ionization of two molecules on the same surface, we can obtain such formulae i1 ¼ A1 exp½ðj V1 Þ11; 600=T; i2 ¼ A2 exp½ðj V2 Þ11; 600=T
2. Experimental The measurements were performed using a magnetic mass spectrometer with 90 deflection and a resolution of 300. There was used a twofilament ion source [4], in which from the first tungsten band (so-called evaporator) evaporation of products of oxide dissociation placed on this band took place. Another identical band of the size (10 1 0.1) mm heated to higher temperatures was an ionizer. The molecules vaporizing from the evaporator underwent adsorption on the ionizer surface and after a very short lifetime on the surface, desorbed as ions and neutral molecules. In the evaporator temperature range 1000– 1400 K the dissociation process proceeded according to the reaction: Ln2O3-2LnO+O; however, at T > 1500 K, Ln+ ions appear in the mass spectrum which means that at high temperatures dissociation proceeds also according to the reaction: Ln2O3-LnO+Ln+O2. With a very small coverage of ionizer surface that is with a small beam of the LnO molecules from the evaporator, ionization potentials of molecule can be determined. In this method there are studied the ratio of two ionic current intensities corresponding to two molecules ionized simultaneously on the same surface. In our case the model molecule was LaO for which the ionization potential was determined earlier [5]. Of mass spectrum rich in isotopic lines, the isotopes of the highest percentage content were
and lnði1 =i2 Þ ¼ ðV2 V1 Þ11; 600=T þ A0 ;
ð4Þ
where the constant 11; 600 ¼ e=k using the units: coulomb and eV=K respectively. The ion thermoemission noise was amplified by a wide-band DC electrometer. The time correlation function was investigated using a stochastic analyser the ‘‘NSA-1000’’ type.
3. Results (1) Work function of the ion emitter surface and its surface structure. In the additional experiment [6] a very small beam of sodium atoms was directed on the surface of polycrystalline tungsten. The obtained value was ej ¼ 4:6170:05 eV. The surface structure was determined by means of X-ray diffraction method finding out domination of (0 0 1) planes. (2) Ionization potentials of the LnO molecules. In the ionizer temperature range 1500–2500 K the linear functions of 1=T are obtained according to Eq. (4). The obtained values of the ionization potentials are presented in Table 1 (see an example in Fig. 1).
Table 1 Ionization potentials V (in Volts) and desorption energies Q of LnO molecules (in eV)
V Q
LaO
CeO
PrO
NdO
SmO
EuO
4.8 [5] —
6.0270.08 1.5670.10
5.0470.04 1.6870.08
5.3770.06 1.4270.11
5.9070.05 1.7670.12
5.1070.05 1.8770.09
ARTICLE IN PRESS L. G!adyszewski et al. / Vacuum 74 (2004) 301–304 8
NdO
+
SmO
7
+
+
+
PrO
SmO
6
∆V =1.10V
6
5.5
5
NdO
+
5
4 ∆V =0. 57V
3
4.5 +
2
PrO
∆V =0. 24V
4
1 2500°C
2250°C
0 4.5
2000°C
5
5.5
1750°C
6
6.5
7
ln (τ0)
ln(i0/i)
303
3.5 3
11600/T
2.5
Fig. 1. Logarithm of the ratio of LaO+ ion current intensity i0 to LnO+ ion current intensity i: DV is difference between the ionization potentials of the molecules.
2 1.5
T=1750 K
0
T=2010 K
1 4.5
T=2300 K
5
5.5
6
6.5
7
11600/T
ln [normalized R(τ)]
-0.5
Fig. 3. Mean residence time t0 for PrO, NdO and SmO molecules on the tungsten surface as a function of 1=T:
-1 -1.5 -2 -2.5 -3 -3.5 -4 -4.5 0
20
40
60
80
100
120
140
160
180
τ [µs]
Fig. 2. Normalized autocorrelation functions RðtÞ versus delay time t for PrO+ current noise.
(2) (3) Autocorrelation functions and energy of desorption. The time autocorrelation function can be approximated by an exponential one (Fig. 2): RðtÞ ¼ /di2 S expðt=t0 Þ: By fitting RðtÞ to the experimental data we determine t0 : The desorption energy was estimated from the plots of logðt0 Þ as a function of 1=T (Fig. 3).
(3) (4)
4. Conclusions (1) We found out that with a sufficiently small beam of molecules directed from the evaporator on the ionizer and with great sensitivity of measuring ionic current intensity on the level
(5)
1016 A it is possible to evaporate mixtures of all studied oxides simultaneously and to choose suitable pairs of LaO/LnO for measurement by means of mass spectrometer. The data in the error limit are identical as in the case of laborious evaporation and ionization of chosen pairs of compounds: LaO/CeO, LaO/PrO etc. Of the studied molecules, those of PrO and EuO, which in the migration process and statistical process of desorption generate particularly strong noises, stand out. They probably have a large dipole moment generating large changes of local work function. For all molecules distributions of instantaneous noise amplitudes were Gaussian. Spectral density functions of the noise for all molecules were of the Lorentzian form which is characteristic for physical processes with one relaxation time characteristic for a given process. In our model this time is identified with the mean residence time of molecules on the ionizer surface. The obtained data are fundamental quantities of atomic and molecular physics.
ARTICLE IN PRESS 304
L. G!adyszewski et al. / Vacuum 74 (2004) 301–304
References [1] Gomer R. Surf. Sci 1973;38:373–6. [2] Holzl J, Schulte F. In: Solid surface physics. Berlin: Springer; 1979. [3] G"adyszewski L. Surf. Sci. 1990;231:120–4.
[4] G"adyszewski L. Annales UMCS, sect. AAA, XXXIV/ XXXV, 1979/1980:55–63. [5] Chupka W, Inghram M. J. Chem. Phys. 1956;24:792–5. [6] G"adyszewski L, G"adyszewski G. Surf. Sci, 1991; 247:274–8.