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Radiative auger transitions of iron compounds
Volume53; number 3
RADIATIVE
CHEhIICALPHYSICS LETTERS
AUGER
TRANSITIONS
1 February 1978
OF IRON COMPOUNDS
M. KASRAI and D.S. URCH Chemistry Department. Queen Mary cbllege. London El 4NS. UK
Received31 August 1977 Revised manuscript received 3 November1977
WeakX-ray emissionlinesfrom a series of iron compounds with energies in the range 5900-6970 eV can be rational&d as due to the K - M2 radiativeAuger process. A peak was also observedat ca. 6930 eV, of unknown origin which had a very low intensity in some low-spin compounds.
1_ Introduction
bound electronic state (sometimes referred to as a semiAuger transition).
Within the past ten years many authors @berg and Utrianinen [I], Linkoaho et al. [2], Servomaa and z Keski-Rahkonen 131, Keski-Rahkonen and Utriainen [4] ) have reported groups of weak X-ray peaks on the low energy side of Kac or K& 3 emission lines which appear to correspond to the radiative Auger process first proposed by Bloch and Ross IS] _ In their relative intensities and in their energies, these peaks agree well with the theoretical predictions for such a process by Aberg [6,7] . A single core vacancy can relax either by the emission of a photon or by simultaneous electron excitation and photon emission. In the latter process the excited electron may still be bound or it may be promoted to the continuum. If the excitation energy of the electron is e,, its binding energy in the initial ion e,., and the energy of the photon e, then a total energy E may be defined as E = ex + e. If the electron is excited to the continuum (ex > eb) then the kinetic energy of this electron will be ek = e, - eb_ In the simple Auger process where there is no photon emission e = 0 and ek = E - eb. Simultaneous photon and electron emission gives rise to the radiative Auger effect and a continuum of photon energies should be observed from zero up to e = E - eb (i-e_ up to the energy of the electron in simpIe Auger emission). At higher photon energies discrete peaks might be observed corresponding to the promotion of the electron to a particular
In the course of our study of the Kpl 3 X-ray emission spectra of iron compounds a long t&l was observed which extended for about 160 eV on the low energy side of the Fe Kpt 3 peak. Careful measurements re-
vealed detailed fink structure and several maxima in the 6900-6970 cV region. Some of these peaks correspond in energy to :he KM1 Mup, and KM, M,,, Auger electron energiec that can be calculated from known L,,3 MM Auger energies and the iron Kcr,, z X-ray energy. This stiongly suggests that some of the peaks corresponded to the high energy end of a radiative Auger continuum and some associated but unresolved discrete states as well.
2. Experimental All the compounds studied were reagent grade and were used without further treatment. Except iron metal which was in the form of a disc, the other samples were introduced to the spectrometer in the form of pressed disc backed with terephthalic acid. A Philips PW 1410 plane-crystal vacuum spectrometer coupled to Harwell2000 series electronic equipment was used to record the data. The specimens were excited by the radiations from a tungsten anode Xray tube operating at 50 kV, 50 mA. The floorescent radiation was analysed using a germanium crystal (1 I I) 539
Volume53, number3
1 Februar)_r978 _
CHEMICALPHYSICSLETTEPS
(2d = 653.2 pm), in the third order and detected. .by a scintillation counter. Some spectra were also analysed using Si( 111), 2d = 627.1 pm, in the third order to _ check any extra reflection due to the analysing crystal [8] _ In order to obtain the best resolution the fine !Soller cohirnator (angular divergence *O-l”, 26) was used. The angular range of interest (106-l 12O, 219) was step-scanned at intervals of O-02”, 20. Each step was counted for 5-10 mm. The relative energy scale was calculated using the wavelength of 1.75661 A
191for FeK&,3-
3. Results and discussion Fig_ 1 shows a typical spectrum obtained on the low energy tail of K/31,3 of iron metal. A background curve is drawn manually to show the significance of the spectrum on the tail. The typicaI standard deviations are indicated in the figure by a vertical bar. The spectra were smoothed by a computer programme using tenfold repetition of quadratic polynomial fitting to moving sets of five points [lo] _ Thus the circles represent the smoothed data. Fig. 2 shows the spectra obtained for a series of iron compounds: there are at least four prominent peaks which are common to all spectra. These are designated as A, B, C and D and their energies are listed in table 1_ As the fti state configurations of the radiative Auger and of the ordinary Auger process are the same, the energies of the latter is included in the bottom of the table for comparison. As is apparent from the table, values for energies for metallic iron agree K”IK1 and UilM2,3 remarkably well with radiative Auger values. No information is available for other iron compounds. The only other experimental data concerning the radiative Auger of iron are by Servomaa and Keski-Rahkonen [3]. The Iow energy tail in their spectra is rather broad and does not give the discrete levels. However, their value corresponding to the M1M2,3 transition is 6940 eV which seems to be an average v&e for peaks B and C of our data. These authors aIso report the relative intensity of radiative Auger peaks to the K@1,3 peak as being 1.6% but in this work,_%tb increased resolution, this value is found to be only 0.4%. Following Goad’s [ 1 I ] interpretation of L+MMAuger spectra it seems reasonable to associate peak A wrth the high-energy limit of the KMrM, radiative Auger process 540.
%369.15
&*.s5
6539.95
ENERGY
[ELECTRON
d965.35 6;390.75 VOLTS1
-
Fig. 1. Fe, X-ray emissionfrom iron disc 6890-7000 eV; circles indicatedata points. Crossesabove a3d below data points at one standarcidevi&on. Backgroundis estimatedby the dashed line.
and peaks C and D with tire transition KM, M2,3 ; the splitting~being due to spin-orbit coupling effects between the unfilled 3d shell and the 3s1 3p5 configuration. The energy difference between peaks C &rd D is about 1 l-13 V, similar to tbat observed by Slater and Urch [ 123 between Kp’ tid K&,3 in a series of iron comppunds. ‘I&is splitting was also rationalised as due to spin-orbit intemc$ons between the final 3p5 state and the spin state of the 3d electrons. Ld the Auger process the final configuration, being doubly ionised,
-
CHEMICAL PHYSICS LETTERS
Volume 53,~nuinber 3
1 February 1978
is more complex. A greater number of final states is, therefore, possible in which the ‘MO spin states that arise from 3d-3p5 interactions will both be split by the two possible spin states of 3st. This effect does not seem to manifest itself in the pure LMM Auger spectra of Coad but it may provide some explanation for the origin of the otherwise mysterious peak B. That this peak may in some way be connected with spin-state interactions i; indicated by its relative weakness in the diamagnetic complex Na4 [Fe(CN)6]. However, detailed calculations of possible radiative Auger X-ray energies from a series of possible valence and spin state are required before the origin of B can be established_
&a)
Acknowledgement
w
-163
-138
(eV less than FeKB,,,) -113 -88 -63
6895
6920
6945
6970
6995
The authors are grateful to the Royal Society, the Science Research Council and the Central Research Fund of London University for funds for the purchase of equipment. M. Xasrai is indebted to the Institute of Nuclear Science and Technology, University of Tehran for leave of a5sence and for financial support. The authors also +sh to acknowledge with gratitude the help given by Mr.D. Haycock in the development of computing techniques_
eV
Fig. 2. Fe, X-ray emission spectra from the following comPounds (a) Fe SZ (pyrites), (b) Nzb ]Fe(CN)e 1, (C) K3
IFeW%
I , Cd)
FeF3,
FeFt , Q FezOs,
6)
Fe (metal).
c
D
&I
:V)
(ev)
(ev)
6915 6909 6912 6914 6913 6916 6916
6930 6930 6932 6930 6932 6932 6928
6954
6964 6962
[S] &loch and P.A. Ross, Phys. Rev. 47 (1935) [6] T. Aberg, Phys. Rev. 4 (1971) 1735;
Compound
KMM Auger tmnsitionsa)
_
[1] T- Abeg and J. Utriainen,Phys.
Rev. Letters 22 (1969) 1346. [2] hL Linkoaho, J. Utriainen and J. Valjakka, Solid State Cornmu; _ 19 (1976) 399. [3] A. Servomaa and 0. Keski-Rahkonen, J. Phys. C8 (1975) 4124. [4] 0. Keski-Rahkonen and J. Utriainen, J. Phys. B 7 (1974)
Table 1 Experimental radiative Auger energies
Fe& (pyrites) Na4 IFe (CN)6 I K3 i Fe(CN)e I FeFs FeFs Fe&s Fe (metal disc)
References
KMrMr 6915
6952 6951 6954 6953 6954 6954
6965 6964 6960 6961
6961
KaM23
-
6950
6963
3) Calculated using L&f&fAuger { 11] and Fe Ko,+. energies_
[ 71 [S] [9] [lo] ] 11) [ 121
884.
T. Aberg and J_ Utriaiien, J. Phys. (Paris) 32, Suppl. c4 (1971) 295. T. Abeg, in: Atomic inner-shell processes, Vol. 1, ed. B. Cruse (Academic Press, New York, 197.5). N. Spielberg and J. Ladell, J. AppL Phys. 31 (1960) 1659. J-A- Bearden. Rev. Mod. Phys. 39 (1967) 78. A. Savitsky and AlJ.E. Golaz, Anal. Chem. 36 (1964) 1627. J.P. Coad, Phys. Letters 3?A (19’71) 437. R.A. Slater and D.S. Urch. J. Chem. Sot. Chem- Commun(1972) 564.
541
RADIATIVE
CHEhIICALPHYSICS LETTERS
AUGER
TRANSITIONS
1 February 1978
OF IRON COMPOUNDS
M. KASRAI and D.S. URCH Chemistry Department. Queen Mary cbllege. London El 4NS. UK
Received31 August 1977 Revised manuscript received 3 November1977
WeakX-ray emissionlinesfrom a series of iron compounds with energies in the range 5900-6970 eV can be rational&d as due to the K - M2 radiativeAuger process. A peak was also observedat ca. 6930 eV, of unknown origin which had a very low intensity in some low-spin compounds.
1_ Introduction
bound electronic state (sometimes referred to as a semiAuger transition).
Within the past ten years many authors @berg and Utrianinen [I], Linkoaho et al. [2], Servomaa and z Keski-Rahkonen 131, Keski-Rahkonen and Utriainen [4] ) have reported groups of weak X-ray peaks on the low energy side of Kac or K& 3 emission lines which appear to correspond to the radiative Auger process first proposed by Bloch and Ross IS] _ In their relative intensities and in their energies, these peaks agree well with the theoretical predictions for such a process by Aberg [6,7] . A single core vacancy can relax either by the emission of a photon or by simultaneous electron excitation and photon emission. In the latter process the excited electron may still be bound or it may be promoted to the continuum. If the excitation energy of the electron is e,, its binding energy in the initial ion e,., and the energy of the photon e, then a total energy E may be defined as E = ex + e. If the electron is excited to the continuum (ex > eb) then the kinetic energy of this electron will be ek = e, - eb_ In the simple Auger process where there is no photon emission e = 0 and ek = E - eb. Simultaneous photon and electron emission gives rise to the radiative Auger effect and a continuum of photon energies should be observed from zero up to e = E - eb (i-e_ up to the energy of the electron in simpIe Auger emission). At higher photon energies discrete peaks might be observed corresponding to the promotion of the electron to a particular
In the course of our study of the Kpl 3 X-ray emission spectra of iron compounds a long t&l was observed which extended for about 160 eV on the low energy side of the Fe Kpt 3 peak. Careful measurements re-
vealed detailed fink structure and several maxima in the 6900-6970 cV region. Some of these peaks correspond in energy to :he KM1 Mup, and KM, M,,, Auger electron energiec that can be calculated from known L,,3 MM Auger energies and the iron Kcr,, z X-ray energy. This stiongly suggests that some of the peaks corresponded to the high energy end of a radiative Auger continuum and some associated but unresolved discrete states as well.
2. Experimental All the compounds studied were reagent grade and were used without further treatment. Except iron metal which was in the form of a disc, the other samples were introduced to the spectrometer in the form of pressed disc backed with terephthalic acid. A Philips PW 1410 plane-crystal vacuum spectrometer coupled to Harwell2000 series electronic equipment was used to record the data. The specimens were excited by the radiations from a tungsten anode Xray tube operating at 50 kV, 50 mA. The floorescent radiation was analysed using a germanium crystal (1 I I) 539
Volume53, number3
1 Februar)_r978 _
CHEMICALPHYSICSLETTEPS
(2d = 653.2 pm), in the third order and detected. .by a scintillation counter. Some spectra were also analysed using Si( 111), 2d = 627.1 pm, in the third order to _ check any extra reflection due to the analysing crystal [8] _ In order to obtain the best resolution the fine !Soller cohirnator (angular divergence *O-l”, 26) was used. The angular range of interest (106-l 12O, 219) was step-scanned at intervals of O-02”, 20. Each step was counted for 5-10 mm. The relative energy scale was calculated using the wavelength of 1.75661 A
191for FeK&,3-
3. Results and discussion Fig_ 1 shows a typical spectrum obtained on the low energy tail of K/31,3 of iron metal. A background curve is drawn manually to show the significance of the spectrum on the tail. The typicaI standard deviations are indicated in the figure by a vertical bar. The spectra were smoothed by a computer programme using tenfold repetition of quadratic polynomial fitting to moving sets of five points [lo] _ Thus the circles represent the smoothed data. Fig. 2 shows the spectra obtained for a series of iron compounds: there are at least four prominent peaks which are common to all spectra. These are designated as A, B, C and D and their energies are listed in table 1_ As the fti state configurations of the radiative Auger and of the ordinary Auger process are the same, the energies of the latter is included in the bottom of the table for comparison. As is apparent from the table, values for energies for metallic iron agree K”IK1 and UilM2,3 remarkably well with radiative Auger values. No information is available for other iron compounds. The only other experimental data concerning the radiative Auger of iron are by Servomaa and Keski-Rahkonen [3]. The Iow energy tail in their spectra is rather broad and does not give the discrete levels. However, their value corresponding to the M1M2,3 transition is 6940 eV which seems to be an average v&e for peaks B and C of our data. These authors aIso report the relative intensity of radiative Auger peaks to the K@1,3 peak as being 1.6% but in this work,_%tb increased resolution, this value is found to be only 0.4%. Following Goad’s [ 1 I ] interpretation of L+MMAuger spectra it seems reasonable to associate peak A wrth the high-energy limit of the KMrM, radiative Auger process 540.
%369.15
&*.s5
6539.95
ENERGY
[ELECTRON
d965.35 6;390.75 VOLTS1
-
Fig. 1. Fe, X-ray emissionfrom iron disc 6890-7000 eV; circles indicatedata points. Crossesabove a3d below data points at one standarcidevi&on. Backgroundis estimatedby the dashed line.
and peaks C and D with tire transition KM, M2,3 ; the splitting~being due to spin-orbit coupling effects between the unfilled 3d shell and the 3s1 3p5 configuration. The energy difference between peaks C &rd D is about 1 l-13 V, similar to tbat observed by Slater and Urch [ 123 between Kp’ tid K&,3 in a series of iron comppunds. ‘I&is splitting was also rationalised as due to spin-orbit intemc$ons between the final 3p5 state and the spin state of the 3d electrons. Ld the Auger process the final configuration, being doubly ionised,
-
CHEMICAL PHYSICS LETTERS
Volume 53,~nuinber 3
1 February 1978
is more complex. A greater number of final states is, therefore, possible in which the ‘MO spin states that arise from 3d-3p5 interactions will both be split by the two possible spin states of 3st. This effect does not seem to manifest itself in the pure LMM Auger spectra of Coad but it may provide some explanation for the origin of the otherwise mysterious peak B. That this peak may in some way be connected with spin-state interactions i; indicated by its relative weakness in the diamagnetic complex Na4 [Fe(CN)6]. However, detailed calculations of possible radiative Auger X-ray energies from a series of possible valence and spin state are required before the origin of B can be established_
&a)
Acknowledgement
w
-163
-138
(eV less than FeKB,,,) -113 -88 -63
6895
6920
6945
6970
6995
The authors are grateful to the Royal Society, the Science Research Council and the Central Research Fund of London University for funds for the purchase of equipment. M. Xasrai is indebted to the Institute of Nuclear Science and Technology, University of Tehran for leave of a5sence and for financial support. The authors also +sh to acknowledge with gratitude the help given by Mr.D. Haycock in the development of computing techniques_
eV
Fig. 2. Fe, X-ray emission spectra from the following comPounds (a) Fe SZ (pyrites), (b) Nzb ]Fe(CN)e 1, (C) K3
IFeW%
I , Cd)
FeF3,
FeFt , Q FezOs,
6)
Fe (metal).
c
D
&I
:V)
(ev)
(ev)
6915 6909 6912 6914 6913 6916 6916
6930 6930 6932 6930 6932 6932 6928
6954
6964 6962
[S] &loch and P.A. Ross, Phys. Rev. 47 (1935) [6] T. Aberg, Phys. Rev. 4 (1971) 1735;
Compound
KMM Auger tmnsitionsa)
_
[1] T- Abeg and J. Utriainen,Phys.
Rev. Letters 22 (1969) 1346. [2] hL Linkoaho, J. Utriainen and J. Valjakka, Solid State Cornmu; _ 19 (1976) 399. [3] A. Servomaa and 0. Keski-Rahkonen, J. Phys. C8 (1975) 4124. [4] 0. Keski-Rahkonen and J. Utriainen, J. Phys. B 7 (1974)
Table 1 Experimental radiative Auger energies
Fe& (pyrites) Na4 IFe (CN)6 I K3 i Fe(CN)e I FeFs FeFs Fe&s Fe (metal disc)
References
KMrMr 6915
6952 6951 6954 6953 6954 6954
6965 6964 6960 6961
6961
KaM23
-
6950
6963
3) Calculated using L&f&fAuger { 11] and Fe Ko,+. energies_
[ 71 [S] [9] [lo] ] 11) [ 121
884.
T. Aberg and J_ Utriaiien, J. Phys. (Paris) 32, Suppl. c4 (1971) 295. T. Abeg, in: Atomic inner-shell processes, Vol. 1, ed. B. Cruse (Academic Press, New York, 197.5). N. Spielberg and J. Ladell, J. AppL Phys. 31 (1960) 1659. J-A- Bearden. Rev. Mod. Phys. 39 (1967) 78. A. Savitsky and AlJ.E. Golaz, Anal. Chem. 36 (1964) 1627. J.P. Coad, Phys. Letters 3?A (19’71) 437. R.A. Slater and D.S. Urch. J. Chem. Sot. Chem- Commun(1972) 564.
541