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Bremsstrahlung isochromat spectroscopy in the study of surfaces
504
Surface Science 200 (1988) 504-511 North-Holland, Amsterdam
BREMSSTRAHLUNG ISOCHROMAT SPECTROSCOPY IN THE STUDY OF SURFACES J. A U L E Y T N E R Institute of Physics of the Polish Academy of Sciences, Warsaw Poland Received 25 June 1987; accepted for publication 25 November 1987
In the last ten years a significant progress in bremsstrahlung isochromat spectroscopy has been achieved° The factors which influence the isochromat spectrum have been defined with greater precision. This opens better possibilities for applying this method to stud) the electron band structure and the atomic structure of solids. Some examples of the applications are discussed in the paper. Bremsstrahlung spectroscopy can be regarded as one of the methods which can be applied to the study of surfaces.
1. Introduction The method of the bremsstrahlung isochromat (BI) is one of the X-ray emission techniques which can be used in the studies of the empty electron valence states in solids. The sampling depth of X-ray emission spectroscopies is of the order of micrometers and these methods are not surface sensitive. In the case of BI methods, however, this problem is different. The main aim of this paper is to show that the bremsstrahlung isochromat method, even in the case of an electron energy of 5 keV, gives information on unoccupied electron states from a depth approximately the same as the sampling depth of the XPS method, It has been shown that the structure of the low energy isochromat edge is created by electrons which do not lose their energy in any inelastic collision [i]. Thus, the material thickness examined by the isochromat method in the first part of the spectra is determined by the inelastic mean free path of an electron in a solid. So, this method is very surface sensitive. To the most important developments in b~'emsstrahlung isochromat spectroscopy in the last few years we can include the following: (1) proof that for the full interpretation of shape and features of isochromats not or.ly is required the knowledge of the part,,al unoccupied density of states (DOS) distributions for different symmetries Nx(E ) and the transition probability P(E), but also the function Ep determining the contribution of elastic and inelastic scattering of electrons, (2) develol~ment of special experimental tech0039-6028/88/$03.50 © Elsevier Sciem',~ Publishers B.V. (North-Holland Physics Publishing Div:sion)
J. Auleytner / Bremsstrahlung isc~:hromatspectroscopy
505
niques - ultra-soft BIS, which opens new possibilities in the field of electron structure investigation using angle resolved inverse photoemission, and (3) construction of an apparatus for measuring BIS and XPS spectra under the same conditions.
2. The principle of the method A bremsstrahlung isochromat represents the dependence of the X-ray intensi~y at constant frequency on varying the voltage applied to the X-ray tube. During the retarding process a part of the electron energy is transformed into continuous X-ray radiation. Using a monochromator we can choose quasi-monochromatic radiation with energy hpo (see fig. 1). During the experiment the energy of incident electrons is varied step by step, and after each step the intensity of the monochromatic X-ray beam is recorded. When the electron energy reaches a quantum energy, hs,0, then the condition of hvo = eVo is satisfied and the measured X-ray intensity increases rapidly above the background level. The linear increase of intensity in the initial range is typical for all isochromats. The knowledge of the threshold voltage and the frequency of the measured radiation allow the Planck constant to be calculated using the formula eV0 + ~,= hp, where ~ is the work function of an electron for given cathode material. More than 40 years ago in 1941 P. Ohlin in Uppsala discovered at a distance of about 5 eV from the threshold an intensity maximum for a tungsten target (see fig. 2) [2]. This discovery initiated the interpretation of BI in terms of an energy band structure of sohds. The explanation was given first by Nijboer [3] and developed by Ulmer [4] and other authors. When an electron has been almost completely stopped in one single process and an X-ray quantum in the neighbourhood of the quantum limit has been emitted, the remaining energy of the electron, relative to the Fermi level of the target (see fig. 3), fits it into the corresponding unoccupied level in the MONOCHROMATOR
1 DETFrTOR
Fig. 1. Diagram of the Bl-methcd.
J. Auleymer / Bremsstrahlung isochromat spectroscopy
506
C0UNTS/MIN. 250
D
-
200 -
C
15{
I0{
50 ~
;
+ |
l A
t
,o 2; 3; ~
so
I
m v°t-+s
Fig. 2. The isochromat of tungsten [2].
E
h~,
~v~
,_,
hv°
spech'um
al Fig. 3. One-electron picture of the bremsstrahlung process explaining the Ohlin maximum.
conduction band. From this model it follows immediately that the intensity distribution at the quantum limit reflects the density of unoccupied states above the Fermi level in the target material.
3. Near edge structure of high energy bremsstrahlung isochromats According to the results of the consideration presented by LawniczakJablonska the intensity distribution of a bremsstrahlung isochromat in the case of a thick target can be defined by the following formula [5]. /(Ekin,
hvo) = foAeF(Ekin- E ) P(hvo + E) N(E) dE,
(1)
J. Auleytner / Bremsstrahlung isochromat spectroscopy
507
where h~,0 is the energy of measured X-ray spectra, E k i n the kinetic energy of incident electrons impinging on the target, A E the energy increase of electrons, and E the electron energy in the final state. Assuming that the electron kinetic energy at the Fermi level is equal to 0, the limit of integration alters from 0 to AE. The function F(Eki n - - E ) determines the energy distribution of incident electrons and P(h~, 0 + E ) is the electron transition probability from the state with initial energy E k i n t O the final state E under simultaneous emission of an X-ray quantum hp. Monoenergetic electrons responsible for producing the bremsstrahlung penetrate the target in principle, without any energy losses at a distance equal to the mean free path. The initial isochromat structure results from electrons which do not undergo any inelastic collisions. Thus, the ixfforraation depth is in this ease of the order of 50 .~, or less [5]. The electrons which lose their energy inelastieally influence the secondary structure. The primary electrons penetrating the target have a certain energy and angular distribution in relation to the normal to the target surface. Integration with angular variables results in the electron energy distribution function F(E). In general we can distinguish three groups of electrons, namely the electrons scattered elastically, the electrons which are individually scattered inelastically by valence electrons and the electrons which lose the energy quanta for plasma excitation or core electron excitation. Taking into account these three processes we can write the general formula for the isochromat intensity distribution: I(Ekin,
hvo)= faAEg(Ekin- E) P(hvo+ E ) N(E) dE +[aEh(Ekin-E) P('.o +E) N(E) d E ,
(2)
"o
where g(Eki n
--
E) is the elastic scattered electron distribution functioii, and
Ni Z m o,,,-,,,
®C oP
I.A,.I' I,-4
S
/ 0
10
20
30
eV
Fig. 4. The nickel isochromat for electron energy Eo = 5420 eV, the PN,,(E) distribution for x = d, p and s symmetries of states, and the total distribution F.xPNx(E) [5,6].
508
J. A uleytner / Bremsstrahlung isochromat spectroscopy
the inelastic scattered electron distribution function. The first term of this formula describes the intensity distribution for thin target approximation and the second one includes the contribution of inelastic scattered electrons to the isochromat intensity. As an example fig. 4 shows the partial densities of unoccupied states in ttae case of Ni for different symmetries multiplied by the transition probabilities PN,,(E) and the total density of states PN(E) as well as the experimental isochromat obtained by Lawniczak-Jab~onska [6]. The comparison shows that in this case the fine structure of Ni isoc~omat is connected with states of various symmetries. The maxima of the density of states distribution correspond to the maxima in isochromat structure. The first maximum corresponds to the unoccupied 3d states. BIS can be applied to the characterization of the electron structure of metallic alloys and to investigate some physical properties connected with this structure [4,5].
h(Eki n - E )
4. Ultra-soft BIS
An other modem direction of BIS is ultra-soft bremsstrah!ung isochromat spectroscopy. The bremsstrahlung investigation of solid state electronic structures at UV energies represents a technique with an information potential comparable to UV photoelectron spectroscopy. The bremsstrahlung and photoelectron emission processes can be regarded as each other's time reversals. Therefore UV-bremsstrahlung spectroscopy is often named inverse photoemission spectroscopy. The principle of a vacuum ultraviolet isochromat spectrometer has been described by Dose [7], Denninger, Dose and Scheidt [8], Kovacs, Nilsson and Kanski [9] and also by Schiilke, Mourikis and Liedtke [10]. The mean energy of the spectrometer is about 9.7 eV. In this ener~,y range the radiation is analyzed with the use of a band pass filter which is composed of a UV window and a photocathode. The electrons from the photoeathode are counted with a channeltron. CaF 2 or LiF can be used as filter. With a CaF 2 filter one obtains a pass energy of about 9.8 eV with a resolution of 0.8 eV F W H M and with LiF the transtifission has a maximum at about 10.8 eV and the resolution is 1.3 eV FWHM. In the experiments reported by Kovacs, Nilsson and Kanski [9] the analyzer is located close to the sample with the aim to collect the bremsstrahiung efficiently. Primary electrons were obtained from a DC heated pure tungsten filament. The reasonable signal intensity is obtained with a primary current in the range of 100 ~A. An other spectrometer-type has been proposed by Schiilke et al. As a band pass filter GeOz-Ge was used (fig. 5). Dose compared the tantalum isochromat obtained by using his VUV-spectrometer with the results of an earlier measured isochromat at an energy
J. Auleytner /Bremsstrahlungisochromat spectroscopy
to " ~._lk GeO __z__ ',r~
!
Ie(E)~ f
/
\
EK6e E
E
x
EK6eO 2
E
Fig. 5. Principle of a GeO2-Ge X-ray band filter [10].
i
Pd(111)
~--~
60 c
j
j
.~
-
55c 50 °
=,
i,5° /,0° 35 ° =. 30~ 250 20= -
IO°L
~ , ,~, I ~ , , , 1 ~ ~ 0 10 20 Energy obcve Fermi level (eV)
Fig. 6. 9.8 eV isochromats from Pd (111) in the FLUX plane [12].
509
510
J. Auleytner / Bremsstrahlung isochromat spectroscopy
hvo = 1.240 keV by Merz and Ulmer [11]. The general agreement between the two isochromats was satisfactory but the sensitivity of VUV-BSI is higher by a factor of ten. This result indicated that the BIS method can be regarded as a technique useful for surface studies. In this case ultra-high vacuum is necessary. A new direction of VUV-BIS is angle resolved inverse photoemission spectroscopy (ARIPES). By using a band pass filter the isochromats can be measured for different angles of incident electrons on a single crystal. Fig. 6 shows as an example the angle resolved inverse photoemission spectra from the Pd(111) surface recorded by Ilver, Kovacs, Kanski, Nilsson and Sobczak in the isochromat mode at 9.8 and 10.8 eV photon energies [12]. Unusually large spectral peaks were found in the energy range of 10-20 eV above EF. Very interesting information concerning inverse photoemission can be found in a review paper by Straub and Himpsel [13].
5. Combined XPS-BIS apparatus From the experimental point of view applications of high energy isochromats are limited to materials with good electrical and thermal condt:etivity which do not melt or decompose during experiments. High vacuum is also required to avoid contamination of the target materials. The target composition should be under c¢,ntrol. Lang and Baer combined BIS and XPS techniques in one apparatus wkich gave several important advantages [14]. The investigations could be performed on the same sample without taking the sample out of the spectrometer, the state of surface contamination could be controlled with XPS at any time, and the Fermi level position with respect to the isochromat edge could be estimated. Fig. 7 shows the scheme of the combined XPS-BIS isochromat apparatus. Without disturbing the vacuum the.
Fig. 7. Combined XPS-BIS apparatus: (1) electron gun, (2) sample, (3) X-ray monochromator, (4) X-ray photon detector, (5) electror, gun power supply, (6) counting electronics and data output, (7) photoelectron energy analyzer, (8) photoelectron detector, (9) X-ray tube [14].
J. Auleytner / Bremsstrahlung isochromat spectroscopy
511
X-ray detector (4) can be retracted from the X-ray beam and the sample (2) turned into the horizontal position. For XPS measurements the monochromator (3) filtres the radiation produced by the rotating anode X-ray tube (9). The photoelectron energy is measured by the hemispherical analyzer (7). Summarizing, one can say that the new approach to the problem of BI
interpretation as well as development of experimental techniques make the BI-method an important tool for investigation of solid state electronic structures including the surface.
References
[11 K. Lawniczak-Jablonska, K. Glegoh and J. Auleytner, J. Microsc. Spectrosc. Electron. 8 (1983) 81. [21 P. Ohlin, Uppsala Univ. Arsskr. 1 (1941) 1. [31 B.R.A. Nijbocr, Physica 12 (1946) 461. [41 K. Ulmer, Phys. Rev. Letters 3 (1959) 519. [51 K. Lawniczak-Jablonska and J. Auleytner, J. Phys. F (Metal Phys.) 12 (1982) 2523. [61 K. Lawniczak-Jablonska, in: Inner-shell and X-ray Physics of Atoms and Solids, Eds. Fabian, Kleinpoppen and Watson (Plenum, New York, 1981). [71 V. Dose, J. Appl. Phys. 14 (1977) 117. i81 G. Denninger, V. Dose and H. Scheidt, Appl. Phys. 18 (1979) 375. [91 A. Kovacs, P.O. Nilsson and J. Kanski, Phys. Scripta 25 (1982) 791. [101 H. Schtilke, S. Mourikis and K.S. Liedtke, Nucl. Instr. Methods 222 (1984) 266. [111 H. Merz and K. Ulmer, Z. Phys. 197 (1966) 409. [121 L. Ilver, A. Kovacs, J. Kanski, P.O. Nilsson and E. Sobczak, Phys. Scripta, to be pub!ished. [131 D. Straub and F.J. Himpsel, Phys. Rev. B33 (1986) 2256. [14] J.K. Lang and Y. Baer, Rev. Sci. Instr. 50 (1979) 221.
Surface Science 200 (1988) 504-511 North-Holland, Amsterdam
BREMSSTRAHLUNG ISOCHROMAT SPECTROSCOPY IN THE STUDY OF SURFACES J. A U L E Y T N E R Institute of Physics of the Polish Academy of Sciences, Warsaw Poland Received 25 June 1987; accepted for publication 25 November 1987
In the last ten years a significant progress in bremsstrahlung isochromat spectroscopy has been achieved° The factors which influence the isochromat spectrum have been defined with greater precision. This opens better possibilities for applying this method to stud) the electron band structure and the atomic structure of solids. Some examples of the applications are discussed in the paper. Bremsstrahlung spectroscopy can be regarded as one of the methods which can be applied to the study of surfaces.
1. Introduction The method of the bremsstrahlung isochromat (BI) is one of the X-ray emission techniques which can be used in the studies of the empty electron valence states in solids. The sampling depth of X-ray emission spectroscopies is of the order of micrometers and these methods are not surface sensitive. In the case of BI methods, however, this problem is different. The main aim of this paper is to show that the bremsstrahlung isochromat method, even in the case of an electron energy of 5 keV, gives information on unoccupied electron states from a depth approximately the same as the sampling depth of the XPS method, It has been shown that the structure of the low energy isochromat edge is created by electrons which do not lose their energy in any inelastic collision [i]. Thus, the material thickness examined by the isochromat method in the first part of the spectra is determined by the inelastic mean free path of an electron in a solid. So, this method is very surface sensitive. To the most important developments in b~'emsstrahlung isochromat spectroscopy in the last few years we can include the following: (1) proof that for the full interpretation of shape and features of isochromats not or.ly is required the knowledge of the part,,al unoccupied density of states (DOS) distributions for different symmetries Nx(E ) and the transition probability P(E), but also the function Ep determining the contribution of elastic and inelastic scattering of electrons, (2) develol~ment of special experimental tech0039-6028/88/$03.50 © Elsevier Sciem',~ Publishers B.V. (North-Holland Physics Publishing Div:sion)
J. Auleytner / Bremsstrahlung isc~:hromatspectroscopy
505
niques - ultra-soft BIS, which opens new possibilities in the field of electron structure investigation using angle resolved inverse photoemission, and (3) construction of an apparatus for measuring BIS and XPS spectra under the same conditions.
2. The principle of the method A bremsstrahlung isochromat represents the dependence of the X-ray intensi~y at constant frequency on varying the voltage applied to the X-ray tube. During the retarding process a part of the electron energy is transformed into continuous X-ray radiation. Using a monochromator we can choose quasi-monochromatic radiation with energy hpo (see fig. 1). During the experiment the energy of incident electrons is varied step by step, and after each step the intensity of the monochromatic X-ray beam is recorded. When the electron energy reaches a quantum energy, hs,0, then the condition of hvo = eVo is satisfied and the measured X-ray intensity increases rapidly above the background level. The linear increase of intensity in the initial range is typical for all isochromats. The knowledge of the threshold voltage and the frequency of the measured radiation allow the Planck constant to be calculated using the formula eV0 + ~,= hp, where ~ is the work function of an electron for given cathode material. More than 40 years ago in 1941 P. Ohlin in Uppsala discovered at a distance of about 5 eV from the threshold an intensity maximum for a tungsten target (see fig. 2) [2]. This discovery initiated the interpretation of BI in terms of an energy band structure of sohds. The explanation was given first by Nijboer [3] and developed by Ulmer [4] and other authors. When an electron has been almost completely stopped in one single process and an X-ray quantum in the neighbourhood of the quantum limit has been emitted, the remaining energy of the electron, relative to the Fermi level of the target (see fig. 3), fits it into the corresponding unoccupied level in the MONOCHROMATOR
1 DETFrTOR
Fig. 1. Diagram of the Bl-methcd.
J. Auleymer / Bremsstrahlung isochromat spectroscopy
506
C0UNTS/MIN. 250
D
-
200 -
C
15{
I0{
50 ~
;
+ |
l A
t
,o 2; 3; ~
so
I
m v°t-+s
Fig. 2. The isochromat of tungsten [2].
E
h~,
~v~
,_,
hv°
spech'um
al Fig. 3. One-electron picture of the bremsstrahlung process explaining the Ohlin maximum.
conduction band. From this model it follows immediately that the intensity distribution at the quantum limit reflects the density of unoccupied states above the Fermi level in the target material.
3. Near edge structure of high energy bremsstrahlung isochromats According to the results of the consideration presented by LawniczakJablonska the intensity distribution of a bremsstrahlung isochromat in the case of a thick target can be defined by the following formula [5]. /(Ekin,
hvo) = foAeF(Ekin- E ) P(hvo + E) N(E) dE,
(1)
J. Auleytner / Bremsstrahlung isochromat spectroscopy
507
where h~,0 is the energy of measured X-ray spectra, E k i n the kinetic energy of incident electrons impinging on the target, A E the energy increase of electrons, and E the electron energy in the final state. Assuming that the electron kinetic energy at the Fermi level is equal to 0, the limit of integration alters from 0 to AE. The function F(Eki n - - E ) determines the energy distribution of incident electrons and P(h~, 0 + E ) is the electron transition probability from the state with initial energy E k i n t O the final state E under simultaneous emission of an X-ray quantum hp. Monoenergetic electrons responsible for producing the bremsstrahlung penetrate the target in principle, without any energy losses at a distance equal to the mean free path. The initial isochromat structure results from electrons which do not undergo any inelastic collisions. Thus, the ixfforraation depth is in this ease of the order of 50 .~, or less [5]. The electrons which lose their energy inelastieally influence the secondary structure. The primary electrons penetrating the target have a certain energy and angular distribution in relation to the normal to the target surface. Integration with angular variables results in the electron energy distribution function F(E). In general we can distinguish three groups of electrons, namely the electrons scattered elastically, the electrons which are individually scattered inelastically by valence electrons and the electrons which lose the energy quanta for plasma excitation or core electron excitation. Taking into account these three processes we can write the general formula for the isochromat intensity distribution: I(Ekin,
hvo)= faAEg(Ekin- E) P(hvo+ E ) N(E) dE +[aEh(Ekin-E) P('.o +E) N(E) d E ,
(2)
"o
where g(Eki n
--
E) is the elastic scattered electron distribution functioii, and
Ni Z m o,,,-,,,
®C oP
I.A,.I' I,-4
S
/ 0
10
20
30
eV
Fig. 4. The nickel isochromat for electron energy Eo = 5420 eV, the PN,,(E) distribution for x = d, p and s symmetries of states, and the total distribution F.xPNx(E) [5,6].
508
J. A uleytner / Bremsstrahlung isochromat spectroscopy
the inelastic scattered electron distribution function. The first term of this formula describes the intensity distribution for thin target approximation and the second one includes the contribution of inelastic scattered electrons to the isochromat intensity. As an example fig. 4 shows the partial densities of unoccupied states in ttae case of Ni for different symmetries multiplied by the transition probabilities PN,,(E) and the total density of states PN(E) as well as the experimental isochromat obtained by Lawniczak-Jab~onska [6]. The comparison shows that in this case the fine structure of Ni isoc~omat is connected with states of various symmetries. The maxima of the density of states distribution correspond to the maxima in isochromat structure. The first maximum corresponds to the unoccupied 3d states. BIS can be applied to the characterization of the electron structure of metallic alloys and to investigate some physical properties connected with this structure [4,5].
h(Eki n - E )
4. Ultra-soft BIS
An other modem direction of BIS is ultra-soft bremsstrah!ung isochromat spectroscopy. The bremsstrahlung investigation of solid state electronic structures at UV energies represents a technique with an information potential comparable to UV photoelectron spectroscopy. The bremsstrahlung and photoelectron emission processes can be regarded as each other's time reversals. Therefore UV-bremsstrahlung spectroscopy is often named inverse photoemission spectroscopy. The principle of a vacuum ultraviolet isochromat spectrometer has been described by Dose [7], Denninger, Dose and Scheidt [8], Kovacs, Nilsson and Kanski [9] and also by Schiilke, Mourikis and Liedtke [10]. The mean energy of the spectrometer is about 9.7 eV. In this ener~,y range the radiation is analyzed with the use of a band pass filter which is composed of a UV window and a photocathode. The electrons from the photoeathode are counted with a channeltron. CaF 2 or LiF can be used as filter. With a CaF 2 filter one obtains a pass energy of about 9.8 eV with a resolution of 0.8 eV F W H M and with LiF the transtifission has a maximum at about 10.8 eV and the resolution is 1.3 eV FWHM. In the experiments reported by Kovacs, Nilsson and Kanski [9] the analyzer is located close to the sample with the aim to collect the bremsstrahiung efficiently. Primary electrons were obtained from a DC heated pure tungsten filament. The reasonable signal intensity is obtained with a primary current in the range of 100 ~A. An other spectrometer-type has been proposed by Schiilke et al. As a band pass filter GeOz-Ge was used (fig. 5). Dose compared the tantalum isochromat obtained by using his VUV-spectrometer with the results of an earlier measured isochromat at an energy
J. Auleytner /Bremsstrahlungisochromat spectroscopy
to " ~._lk GeO __z__ ',r~
!
Ie(E)~ f
/
\
EK6e E
E
x
EK6eO 2
E
Fig. 5. Principle of a GeO2-Ge X-ray band filter [10].
i
Pd(111)
~--~
60 c
j
j
.~
-
55c 50 °
=,
i,5° /,0° 35 ° =. 30~ 250 20= -
IO°L
~ , ,~, I ~ , , , 1 ~ ~ 0 10 20 Energy obcve Fermi level (eV)
Fig. 6. 9.8 eV isochromats from Pd (111) in the FLUX plane [12].
509
510
J. Auleytner / Bremsstrahlung isochromat spectroscopy
hvo = 1.240 keV by Merz and Ulmer [11]. The general agreement between the two isochromats was satisfactory but the sensitivity of VUV-BSI is higher by a factor of ten. This result indicated that the BIS method can be regarded as a technique useful for surface studies. In this case ultra-high vacuum is necessary. A new direction of VUV-BIS is angle resolved inverse photoemission spectroscopy (ARIPES). By using a band pass filter the isochromats can be measured for different angles of incident electrons on a single crystal. Fig. 6 shows as an example the angle resolved inverse photoemission spectra from the Pd(111) surface recorded by Ilver, Kovacs, Kanski, Nilsson and Sobczak in the isochromat mode at 9.8 and 10.8 eV photon energies [12]. Unusually large spectral peaks were found in the energy range of 10-20 eV above EF. Very interesting information concerning inverse photoemission can be found in a review paper by Straub and Himpsel [13].
5. Combined XPS-BIS apparatus From the experimental point of view applications of high energy isochromats are limited to materials with good electrical and thermal condt:etivity which do not melt or decompose during experiments. High vacuum is also required to avoid contamination of the target materials. The target composition should be under c¢,ntrol. Lang and Baer combined BIS and XPS techniques in one apparatus wkich gave several important advantages [14]. The investigations could be performed on the same sample without taking the sample out of the spectrometer, the state of surface contamination could be controlled with XPS at any time, and the Fermi level position with respect to the isochromat edge could be estimated. Fig. 7 shows the scheme of the combined XPS-BIS isochromat apparatus. Without disturbing the vacuum the.
Fig. 7. Combined XPS-BIS apparatus: (1) electron gun, (2) sample, (3) X-ray monochromator, (4) X-ray photon detector, (5) electror, gun power supply, (6) counting electronics and data output, (7) photoelectron energy analyzer, (8) photoelectron detector, (9) X-ray tube [14].
J. Auleytner / Bremsstrahlung isochromat spectroscopy
511
X-ray detector (4) can be retracted from the X-ray beam and the sample (2) turned into the horizontal position. For XPS measurements the monochromator (3) filtres the radiation produced by the rotating anode X-ray tube (9). The photoelectron energy is measured by the hemispherical analyzer (7). Summarizing, one can say that the new approach to the problem of BI
interpretation as well as development of experimental techniques make the BI-method an important tool for investigation of solid state electronic structures including the surface.
References
[11 K. Lawniczak-Jablonska, K. Glegoh and J. Auleytner, J. Microsc. Spectrosc. Electron. 8 (1983) 81. [21 P. Ohlin, Uppsala Univ. Arsskr. 1 (1941) 1. [31 B.R.A. Nijbocr, Physica 12 (1946) 461. [41 K. Ulmer, Phys. Rev. Letters 3 (1959) 519. [51 K. Lawniczak-Jablonska and J. Auleytner, J. Phys. F (Metal Phys.) 12 (1982) 2523. [61 K. Lawniczak-Jablonska, in: Inner-shell and X-ray Physics of Atoms and Solids, Eds. Fabian, Kleinpoppen and Watson (Plenum, New York, 1981). [71 V. Dose, J. Appl. Phys. 14 (1977) 117. i81 G. Denninger, V. Dose and H. Scheidt, Appl. Phys. 18 (1979) 375. [91 A. Kovacs, P.O. Nilsson and J. Kanski, Phys. Scripta 25 (1982) 791. [101 H. Schtilke, S. Mourikis and K.S. Liedtke, Nucl. Instr. Methods 222 (1984) 266. [111 H. Merz and K. Ulmer, Z. Phys. 197 (1966) 409. [121 L. Ilver, A. Kovacs, J. Kanski, P.O. Nilsson and E. Sobczak, Phys. Scripta, to be pub!ished. [131 D. Straub and F.J. Himpsel, Phys. Rev. B33 (1986) 2256. [14] J.K. Lang and Y. Baer, Rev. Sci. Instr. 50 (1979) 221.