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Electron heated high temperature atomic beam source for VUV photoelectron spectroscopy
Nuclear Instruments and Methods in Physics Research A254 (1987) 627-629 North-Holland, Amsterdam
627
ELECTRON HEATED HIGH TEMPERATURE ATOMIC BEAM SOURCE F O R VUV P H O T O E L E C T R O N S P E C T R O S C O P Y T. P R E S C H E R , M. R I C H T E R , B. S O N N T A G a n d H.E. W E T Z E L IL lnstitut fftr Experimentalphysik, Universitiit Hamburg, D-2000 Hamburg 50, FRG
Received 25 September 1986
A high temperature atomic beam source heated by electron bombardment is described.
1. Introduction Vacuum ultraviolet photoelectron spectroscopy has been established as an excellent tool for probing the electronic structure of atoms in great detail [1-5]. In recent years these investigations have been extended to high temperature open shell metal atoms [6-8]. Great progress has been made in overcoming the serious difficulties posed by the reactivity of the molten metals and high temperatures required for evaporation. The atomic beam source has to 1) provide a well collimated atomic beam of sufficient density ( > 1012 atoms/cm3), 2) operate under stable conditions for several hours, 3) be easily removable and rechargeable, 4) cause a low heat load in the environment, especially the electron energy analyser, 5) cause no background of electrons, 6) cause low electric and magnetic stray fields.
2. Description of the source In fig. 1 the electron beam heated atomic beam source is presented. The molten metal is contained in a tungsten, tantalum or graphite crucible (1). The choice of the material depends on the sample. The atomic beam formed in the crucible emanates through the orifice in the Cu cap (4) of the water cooled Cu cylinder (5), which surrounds the source. A lid (2) made out of Nb, Mo, Ta or C tightly seals the Cu cylinder. It is crucial that this lid prevents any electron created inside the cylinder from escaping into the outer region. The crucible (1) rests in a tubular extension of this lid. Four W filaments (6), separated by 90 °, are mounted between a Nb ring (3) and an inner cyhnder made out of Ta sheet (8). An outer Ta sheet cylinder (7) surrounds the filaments and the inner cylinder. Both cylinders are supported by electrically insulated stainless-steel rings 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(9, 10). For heating the filaments, a current (7 V, < 60 A) is passed through the outer and the inner cylinder. Electrons emitted from the hot filaments are accelerated towards the tubular extension of the hd, which contains the crucible. A voltage up to 1500 V can be applied between the filaments and the grounded lid. By this electron bombardment the temperature of the crucible has been raised up to 1800°C. The power required was less than 2.5 kW. Three outer stainless-steel rods (11) and one center rod (12) provide the electrical connections. Metal shields (13) protect the ceramic insulators (14) against metal contamination. The cylindrical geometry minimizes electrical and magnetical stray fields, which could deteriorate the energy resolution of the electron energy analyzer. The beam source is pumped from below by a turbomolecular pump or a cryopump.
3. VUV photoelectron spectra of atomic Ti VUV electron spectroscopy on atomic Ti forms a severe test for the source of the atomic beam. The Ti photoelectron spectra presented in fig. 2 clearly demonstrate that the atomic beam source described above very successfully passed this test. Ti was evaporated from a graphite crucible. At 1300 V an electron current of 1.5 A kept the crucible at a temperature of 1700°C. A charge of 3 g Ti allowed for a stable operation of the atomic beam for more than 4 h. The atomic beam crossed the photon beam a few mm above the Cu cap (4 in fig. 1). The photons were emitted by the electron storage ring BESSY. After passing through a toroidal grating monochromator the photon beam (band-width A E < 0.3 eV; approximately 10 it photons/s) was focused on the interaction zone. The kinetic energy of the electrons emerging from the interaction zone was determined by a cylindrical mirror analyser (angular acceptance 0.8% of 4~r; energy resolution A E = 0.74% of the pass energy). A more detailed description of the
628
T. Prescher et al. / Electron heated high temperature atomic beam source .-2
section A-A _.
~
I 41
__.1(9) _0o,
--(11) ~(1~)
section B - B j (12) (5) (14) (11) (15)
section C- C
./(12)
i1,)
40059
Fig. 1. Sectional views of the electron beam heated atomic beam source. 1, crucible; 2, lid; 3, Nb ring; 4, Cu cap, 5, Cu cylinder; 6, W filaments; 7, outer Ta sheet cylinder; 8, inner Ta sheet cylinder; 9, 10, stainless steel rings; 11, 12, stainless steel rods; 13; metal shields; 14, ceramic insulators; 15, stainlees steel ring; 16, stainless steel support.
?•
I
33.36 eV
Ti
33.4, 36.6, 38.9, 41.2 a n d 43.5 eV. T h e p h o t o n energy of hto = 45.7 eV lies a b o v e the highest observed 3pS3d24s 2 ionization limit [7,12]. The assignment of the photoelectron lines 1 - 7 is s u m m a r i z e d in table 1. W e a k satellite lines are discernible in the b i n d i n g energy range between 15 a n d 20 eV. F o r p h o t o n energies above 40 eV A u g e r lines c o n t r i b u t e to the high energy part of the spectra. The electron b a c k g r o u n d a m o u n t e d to less t h a n
iVn t~cJ=3662ev "E::} t'i v >..I'--
~co = 3 8 8 6 e V
.y,,.,..
Z W
experimental setup is given in refs. [7-11]. The spectra (fig. 2) were taken at the energies of the 3p63d24s 2 3F--* 3p53d34s2 3G, 3F, 3D absorption maxima ho9 =
Z
Table 1 Experimentally observed Ti photoelectron lines and their assignment
~(~= 4 3 . / . 5
5
eV
15
25
BINDING ENERGY (eV) Fig. 2. VUV photoelectron spectra of atomic Ti taken at different photon energies.
Line
Initial state of Ti I
inal state of Ti II
Binding energy (eV)
1 2 3 4 5 6 7
3d34s 5F 3d24s 2 3F 3d24s 2 3F 3d24s 2 3g 3d24s 2 3F 3d24s 2 3F 3d24s 2 3F
3d24s 4F 3d24s 4F 3d24s 2F 3d24s 2D 3d24s 2G 3d4s 2 2D 3d24p 2D
6.0 ± 0.1 6.8 + 0.1 7.4+ 0.1 7.9 _+0.1 8.7 + 0.1 9.9_+0.1 10.8 + 0.1
T. Prescher et al. / Electron heated high temperature atomic beam source
1% of the count rate in the dominant 3d photoelectron fine. F o r a presentation and discussion of all our results on atomic Ti the reader is referred to refs. [7,12].
Acknowledgement The authors gratefully acknowledge the financial support of the Bundesministerium fiir Forschung und Technologic and the Deutsche Forschungsgemeinschaft.
References [1] J.A.R. Samson, in: Handbuch der Physik, vol. 31, ed., W. Mehlhorn (Springer, Berlin, 1982) p. 123. [2] A.F. Starace, ibid., p. 1.
629
[3] B. Crasemann and F. Wuilleumier, in: Atomic Inner-Shell Physics, ed., B. Crasemann (Plenum, New York, 1985) p. 281. [4] H. Siegbahn and L. Karlsson, in: Handbuch der Physik, vol. 31, ed., W. Mehlhorn (Springer, Berlin, 1982) p. 215. [5] M.O. Krause, in: Atomic Physics, vol. 9, eds., R.S. Van Dyck and E.N. Fortson (World Scientific, 1984) p. 414. [6] J.M. Dyke, B.W.J. Gravenor, R.A. Lewis and A. Morris, Mol. Phys. 53 (1984) 465; J. Phys. B15 (1982) 4523. [7] M. Meyer, Th. Prescher, E. yon Raven, M. Richter, E. Schmidt, B. Sonntag and H.E. Wetzel, Z. Phys. D 2 (1986) 347. [8] B. Sonntag and F. Wuilleumier, Nucl. Instr. and Meth. 208 (1983) 735. [9] E. Schmidt, H. Schr~der, B. Sonntag, H. Voss and H.E. Wetzel, J. Phys. B17 (1984) 707. [10] H. SchriSder, Thesis, Universit~it Hamburg (1982). [11] E. Schmidt, Thesis, Universitiit Hamburg (1985). [12] H.E. Wetzel, Thesis, Universit~it Hamburg (1987).
627
ELECTRON HEATED HIGH TEMPERATURE ATOMIC BEAM SOURCE F O R VUV P H O T O E L E C T R O N S P E C T R O S C O P Y T. P R E S C H E R , M. R I C H T E R , B. S O N N T A G a n d H.E. W E T Z E L IL lnstitut fftr Experimentalphysik, Universitiit Hamburg, D-2000 Hamburg 50, FRG
Received 25 September 1986
A high temperature atomic beam source heated by electron bombardment is described.
1. Introduction Vacuum ultraviolet photoelectron spectroscopy has been established as an excellent tool for probing the electronic structure of atoms in great detail [1-5]. In recent years these investigations have been extended to high temperature open shell metal atoms [6-8]. Great progress has been made in overcoming the serious difficulties posed by the reactivity of the molten metals and high temperatures required for evaporation. The atomic beam source has to 1) provide a well collimated atomic beam of sufficient density ( > 1012 atoms/cm3), 2) operate under stable conditions for several hours, 3) be easily removable and rechargeable, 4) cause a low heat load in the environment, especially the electron energy analyser, 5) cause no background of electrons, 6) cause low electric and magnetic stray fields.
2. Description of the source In fig. 1 the electron beam heated atomic beam source is presented. The molten metal is contained in a tungsten, tantalum or graphite crucible (1). The choice of the material depends on the sample. The atomic beam formed in the crucible emanates through the orifice in the Cu cap (4) of the water cooled Cu cylinder (5), which surrounds the source. A lid (2) made out of Nb, Mo, Ta or C tightly seals the Cu cylinder. It is crucial that this lid prevents any electron created inside the cylinder from escaping into the outer region. The crucible (1) rests in a tubular extension of this lid. Four W filaments (6), separated by 90 °, are mounted between a Nb ring (3) and an inner cyhnder made out of Ta sheet (8). An outer Ta sheet cylinder (7) surrounds the filaments and the inner cylinder. Both cylinders are supported by electrically insulated stainless-steel rings 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(9, 10). For heating the filaments, a current (7 V, < 60 A) is passed through the outer and the inner cylinder. Electrons emitted from the hot filaments are accelerated towards the tubular extension of the hd, which contains the crucible. A voltage up to 1500 V can be applied between the filaments and the grounded lid. By this electron bombardment the temperature of the crucible has been raised up to 1800°C. The power required was less than 2.5 kW. Three outer stainless-steel rods (11) and one center rod (12) provide the electrical connections. Metal shields (13) protect the ceramic insulators (14) against metal contamination. The cylindrical geometry minimizes electrical and magnetical stray fields, which could deteriorate the energy resolution of the electron energy analyzer. The beam source is pumped from below by a turbomolecular pump or a cryopump.
3. VUV photoelectron spectra of atomic Ti VUV electron spectroscopy on atomic Ti forms a severe test for the source of the atomic beam. The Ti photoelectron spectra presented in fig. 2 clearly demonstrate that the atomic beam source described above very successfully passed this test. Ti was evaporated from a graphite crucible. At 1300 V an electron current of 1.5 A kept the crucible at a temperature of 1700°C. A charge of 3 g Ti allowed for a stable operation of the atomic beam for more than 4 h. The atomic beam crossed the photon beam a few mm above the Cu cap (4 in fig. 1). The photons were emitted by the electron storage ring BESSY. After passing through a toroidal grating monochromator the photon beam (band-width A E < 0.3 eV; approximately 10 it photons/s) was focused on the interaction zone. The kinetic energy of the electrons emerging from the interaction zone was determined by a cylindrical mirror analyser (angular acceptance 0.8% of 4~r; energy resolution A E = 0.74% of the pass energy). A more detailed description of the
628
T. Prescher et al. / Electron heated high temperature atomic beam source .-2
section A-A _.
~
I 41
__.1(9) _0o,
--(11) ~(1~)
section B - B j (12) (5) (14) (11) (15)
section C- C
./(12)
i1,)
40059
Fig. 1. Sectional views of the electron beam heated atomic beam source. 1, crucible; 2, lid; 3, Nb ring; 4, Cu cap, 5, Cu cylinder; 6, W filaments; 7, outer Ta sheet cylinder; 8, inner Ta sheet cylinder; 9, 10, stainless steel rings; 11, 12, stainless steel rods; 13; metal shields; 14, ceramic insulators; 15, stainlees steel ring; 16, stainless steel support.
?•
I
33.36 eV
Ti
33.4, 36.6, 38.9, 41.2 a n d 43.5 eV. T h e p h o t o n energy of hto = 45.7 eV lies a b o v e the highest observed 3pS3d24s 2 ionization limit [7,12]. The assignment of the photoelectron lines 1 - 7 is s u m m a r i z e d in table 1. W e a k satellite lines are discernible in the b i n d i n g energy range between 15 a n d 20 eV. F o r p h o t o n energies above 40 eV A u g e r lines c o n t r i b u t e to the high energy part of the spectra. The electron b a c k g r o u n d a m o u n t e d to less t h a n
iVn t~cJ=3662ev "E::} t'i v >..I'--
~co = 3 8 8 6 e V
.y,,.,..
Z W
experimental setup is given in refs. [7-11]. The spectra (fig. 2) were taken at the energies of the 3p63d24s 2 3F--* 3p53d34s2 3G, 3F, 3D absorption maxima ho9 =
Z
Table 1 Experimentally observed Ti photoelectron lines and their assignment
~(~= 4 3 . / . 5
5
eV
15
25
BINDING ENERGY (eV) Fig. 2. VUV photoelectron spectra of atomic Ti taken at different photon energies.
Line
Initial state of Ti I
inal state of Ti II
Binding energy (eV)
1 2 3 4 5 6 7
3d34s 5F 3d24s 2 3F 3d24s 2 3F 3d24s 2 3g 3d24s 2 3F 3d24s 2 3F 3d24s 2 3F
3d24s 4F 3d24s 4F 3d24s 2F 3d24s 2D 3d24s 2G 3d4s 2 2D 3d24p 2D
6.0 ± 0.1 6.8 + 0.1 7.4+ 0.1 7.9 _+0.1 8.7 + 0.1 9.9_+0.1 10.8 + 0.1
T. Prescher et al. / Electron heated high temperature atomic beam source
1% of the count rate in the dominant 3d photoelectron fine. F o r a presentation and discussion of all our results on atomic Ti the reader is referred to refs. [7,12].
Acknowledgement The authors gratefully acknowledge the financial support of the Bundesministerium fiir Forschung und Technologic and the Deutsche Forschungsgemeinschaft.
References [1] J.A.R. Samson, in: Handbuch der Physik, vol. 31, ed., W. Mehlhorn (Springer, Berlin, 1982) p. 123. [2] A.F. Starace, ibid., p. 1.
629
[3] B. Crasemann and F. Wuilleumier, in: Atomic Inner-Shell Physics, ed., B. Crasemann (Plenum, New York, 1985) p. 281. [4] H. Siegbahn and L. Karlsson, in: Handbuch der Physik, vol. 31, ed., W. Mehlhorn (Springer, Berlin, 1982) p. 215. [5] M.O. Krause, in: Atomic Physics, vol. 9, eds., R.S. Van Dyck and E.N. Fortson (World Scientific, 1984) p. 414. [6] J.M. Dyke, B.W.J. Gravenor, R.A. Lewis and A. Morris, Mol. Phys. 53 (1984) 465; J. Phys. B15 (1982) 4523. [7] M. Meyer, Th. Prescher, E. yon Raven, M. Richter, E. Schmidt, B. Sonntag and H.E. Wetzel, Z. Phys. D 2 (1986) 347. [8] B. Sonntag and F. Wuilleumier, Nucl. Instr. and Meth. 208 (1983) 735. [9] E. Schmidt, H. Schr~der, B. Sonntag, H. Voss and H.E. Wetzel, J. Phys. B17 (1984) 707. [10] H. SchriSder, Thesis, Universit~it Hamburg (1982). [11] E. Schmidt, Thesis, Universitiit Hamburg (1985). [12] H.E. Wetzel, Thesis, Universit~it Hamburg (1987).