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High-resolution electron-excited Ag Lα emission spectra below and above the L1 ionisation threshold
Volume 78A, number 4
PHYSICS LETTERS
18 August 1980
ifiGH-RESOLUTION ELECTRON-EXCITED Ag La EMISSION SPECTRA BELOW AND ABOVE THE L1 IONISATION THRESHOLD C.F. HAGUE, J.-M. MARIOT and G. DUFOUR Laboratoire de Chimie Physique 1 Université Pierre et Marie Curie, 75231 Paris Cedex 05, France Received 19 March 1980
Experimental evidence shows that all high-energy satellites in the Ag La spectrum are due to multiplet splitting of the’ initial L3M4,s and final M4,5M4,5 two-hole configurations. The La1 line narrows by 0.13 eV when observed below the L1 ionisation threshold.
Much attention has been paid to the high-energy satellites of the La line (L3—M4,5) in silver. From the results of ab initio calculations, indicating that the L1 —L3M4 ~ Coster—Kronig process is energetically allowed in silver [1], Chen et al. [2] predicted that the L3M4 5—M45M45 satellite emissions are situated on the high-energy side of the La line and extend over an energy range compatible with experimental observations [3,4}. Doyle and Shafroth [5,6] measured the relative intensities of the La parent line and its satellites produced by 2.5 MeV proton bombardment and showed experimentally that the L1 —L3M45 Coster— Kronig transitions do occur in silver. Similarly, evidence for the existence of this Coster—Kronig process in silver has been provided by Kostroun et a!. [71in an experiment where the La emission was excited by monochromatised synchrotron radiation: an increase in intensity in the satellite region was observed as the energy of the primary excitation was increased to beyond the L1 ionisation energy. In both the above experiments, however, the satellite fine structure was not resolved because of the low-resolution conditions. The object of this letter is to define in more detail the role played by the L1 —L3M45 Coster—Kronig process in the Ag La spectrum by means of high-resolution measurements. The spectrometer used is a Johann type mounting Laboratoire associé au CNRS.
328
to which an entrance slit is added for improved resolution [8]. It is fitted with a 0.5 m radius bent quartz (1 l’~0)crystal. A bulk silver target is bombarded by a 150 mA beam of 3.7 keV or 5.5 keV electrons, i.e. below and above the L1 ionisation threshold (3.8 keV). The resolution setting gives an uncorrected full-width at half maximum (FWHM) of Ag La1 of 2.57 eV at 5.5 keV excitation. Under these conditions, 160 counts/s and 1200 counts/s at the La emission peak were obtained for the low and high excitation energy measurements and with a proportional counter of 80% efficiency as detector. The La spectra at the two excitation energies are presented in fig. 1 with an enlarged view of the high~ energy part of the spectral region. Two main features are brought out by a comparison between the spectra: a 0.13 eV broadening of the La1 line (FWHM) and the appearance of all the satellite fine structure at an excitation energy above the L1 ionisation threshold. The measured FWHM of 2.57 ±0.05 eV for the 5.5 keV electron excited La1 line is in agreement with previously reported values (see table 1). This FWHM drops to 2.44 ±0.05 eV when the excitation energy is below the L1 ionisation energy. A similar reduction in width has been reported for Fe Ka observed at the K ionisation threshold [9]. This narrowing can be attributed to the disappearance of satellite emissions superimposed on the parent line [2,10,111. The purity of the La line observed in the low excita-
PHYSICS LETTERS
Volume 78A, number 4
Ag La 01 i .x10
0
-10 Relative
10
20
energy(eV)
Fig. 1. Electron excited Ag Lo! emission spectra below and above the Lr ionisation threshold. The notations for the satellites are those used by Parratt [ 31. The dashed curve is the lorentzian doublet which fits best the experimental data (see text).
tion energy spectrum remains difficult to determine because although, as discussed below, Lol transitions in the presence of an extra M,,, hole have disappeared, a certain amount of multiple ionisation in outer shells Table 1 FWHM of the Lor line and natural width of the Ls level in silver (eVJ. La1
this work (>Lr) (CL11
L3
measured
corrected
2.57 + 0.05 2.44+0.05
2.29 f 0.13 2.16iO.13
1.9 t 0.3
2.79
2.34to.19
-
2.91 f 0.10
2.54 f 0.15
-
Paratt [3] OLl)
Jusldn et al. [4] (>LlJ Krause and Oliver 1141
-
18 August 1980
could be present (see for instance ref. [ 121). In fact, the hump still visible above the tail of the lorentzian doublet which fits best with the low excitation energy measurement in the range -20 eV to t2 eV may be attributed to such a process (see fig. 1). If we assume that satellites have a negligible influence on the FWHM of the low energy excited La, line, the natural width of the L3 level can be deduced by correcting our experimental value for instrumental broadening and by subtracting the M4,, level width. The instrumental broadening has been estimated by folding a lorentzian lineshape with the instrumental function. It is roughly triangular in shape and has a FWHM of 0.28 + 0.08 eV. It leads to a corrected FWHM of 2.16 f 0.13 eV. Using a width of 0.3 kO.2 eV for the M4,5 levels as given by X-ray photoelectron spectrometry [ 131, a natural width of 1.9 ?r 0.3 eV is obtained for the L3 level. This value is smaller than the semi-empirical 2.40 eV width determined by Krause and Oliver from radiative rates and fluorescence yields [ 141. The disappearance of all the fine structure corresponding to the high-energy Lo satellite emissions observed when the excitation energy is =lOO eV below the L, threshold indicates that they are due to L3M4 5-M4,5M4,5 transitions and that the multiplet splitting of the initial L3M4,5 and final M4,, M,,, two-hole configurations is responsible for the observed fine structure. This result is in agreement with the calculations of Chen et al. [2] which predicts that the LsM~,~-M~,~M~,~ emissions extend between -7 eV and +26 eV with respect to the parent b, line maximum. Our experiment also shows that double-vacancy LN states play a minor role in the Lo satellites of silver, contrary to a previous assumption made to interpret the relative intensities of the L series X-ray lines [15]. Having now ascertained that the L3M4 5 and M,,,M,,, two-hole configurations play ad important -part in the La satellite emissions of silver, a more detailed interpretation of these emissions based on intensity calculations remains to be proposed. Such a study is now in progress. The authors wish to thank Professor C. Bonnelle for a critical reading of the manuscript.
2.40
--329
Volume 78A, number 4
PHYSICS LETTERS
References [11 M.H. Chen, B. Crasemann, K.-N. Huang, M. Aoyagi and H. Mark, At. Data Nucl. Data Tables 19(1977)97. [2] M.H. Chen, B. Crasemann, M. Aoyagi and H. Mark, Phys. Rev. A15 (1977) 2312. [3] L.G. Parratt, Phys. Rev. 54(1938)99. [4] H. Juslén, M. Pessa and G. Graeffe, Phys. Rev. A19 (1979) 196. [51 B.L. Doyle and S.M. Shafroth, in: Abstracts of contri~buted papers, Second Intern. Conf. on inner-shell ion!sation phenomena (Univ. Freiburg, Germany, 1976) p. 211. [6] B.L. Doyle and S.M. Shafroth, Phys. Rev. Al9 (1979) 1433. [7] V.0. Kostroun, 3.-P. Briand, A. Chetioui, P. Chevallier and A. Touati, Poster presented at the Stanford Synchrotwn Radiation Laboratory Users Group Meeting (Stanford, CA, USA, 1978), unpublished; and private communication.
330
18 August 1980
[8] C.F. Hague and D. Laporte, Rev. Sc Instrum. 51(1980) 621. [9] P. Chevallier, M. Tavernier and J.-P. Briand, J. Phys. Bll (1978) Ll7l [10] M.O. Krause, F. Wuilleumier and C.W. Nestor Jr, Phys. Rev. A6 (1972) 871. [11] J.-P. Briand, P. Chevallier, A. Chetioui, J.-P. Rozet, M. Tavernier and A. Touati, in: Extended abstracts of the Intern. Conf. on the Physics of X-ray spectra (NBS, Gaithersburg, MD, USA, 1976) p. 335. [12] W.C. Sauder, J.R. Huddle, J.D. Wilson and R.E. La Villa, Phys. Lett. 63A (1977) 313. [13] J.-M. Mariot and G. Dufour, J. Electron Spectrosc. 13 (1978) 403, and references cited therein. [14] M.O. Krause and J.H. Oliver, J. Phys. Chem. Ref. Data 8 (1979) 329. [151E.J. McGuire, Phys. Rev. A5 (1972) 2313.
PHYSICS LETTERS
18 August 1980
ifiGH-RESOLUTION ELECTRON-EXCITED Ag La EMISSION SPECTRA BELOW AND ABOVE THE L1 IONISATION THRESHOLD C.F. HAGUE, J.-M. MARIOT and G. DUFOUR Laboratoire de Chimie Physique 1 Université Pierre et Marie Curie, 75231 Paris Cedex 05, France Received 19 March 1980
Experimental evidence shows that all high-energy satellites in the Ag La spectrum are due to multiplet splitting of the’ initial L3M4,s and final M4,5M4,5 two-hole configurations. The La1 line narrows by 0.13 eV when observed below the L1 ionisation threshold.
Much attention has been paid to the high-energy satellites of the La line (L3—M4,5) in silver. From the results of ab initio calculations, indicating that the L1 —L3M4 ~ Coster—Kronig process is energetically allowed in silver [1], Chen et al. [2] predicted that the L3M4 5—M45M45 satellite emissions are situated on the high-energy side of the La line and extend over an energy range compatible with experimental observations [3,4}. Doyle and Shafroth [5,6] measured the relative intensities of the La parent line and its satellites produced by 2.5 MeV proton bombardment and showed experimentally that the L1 —L3M45 Coster— Kronig transitions do occur in silver. Similarly, evidence for the existence of this Coster—Kronig process in silver has been provided by Kostroun et a!. [71in an experiment where the La emission was excited by monochromatised synchrotron radiation: an increase in intensity in the satellite region was observed as the energy of the primary excitation was increased to beyond the L1 ionisation energy. In both the above experiments, however, the satellite fine structure was not resolved because of the low-resolution conditions. The object of this letter is to define in more detail the role played by the L1 —L3M45 Coster—Kronig process in the Ag La spectrum by means of high-resolution measurements. The spectrometer used is a Johann type mounting Laboratoire associé au CNRS.
328
to which an entrance slit is added for improved resolution [8]. It is fitted with a 0.5 m radius bent quartz (1 l’~0)crystal. A bulk silver target is bombarded by a 150 mA beam of 3.7 keV or 5.5 keV electrons, i.e. below and above the L1 ionisation threshold (3.8 keV). The resolution setting gives an uncorrected full-width at half maximum (FWHM) of Ag La1 of 2.57 eV at 5.5 keV excitation. Under these conditions, 160 counts/s and 1200 counts/s at the La emission peak were obtained for the low and high excitation energy measurements and with a proportional counter of 80% efficiency as detector. The La spectra at the two excitation energies are presented in fig. 1 with an enlarged view of the high~ energy part of the spectral region. Two main features are brought out by a comparison between the spectra: a 0.13 eV broadening of the La1 line (FWHM) and the appearance of all the satellite fine structure at an excitation energy above the L1 ionisation threshold. The measured FWHM of 2.57 ±0.05 eV for the 5.5 keV electron excited La1 line is in agreement with previously reported values (see table 1). This FWHM drops to 2.44 ±0.05 eV when the excitation energy is below the L1 ionisation energy. A similar reduction in width has been reported for Fe Ka observed at the K ionisation threshold [9]. This narrowing can be attributed to the disappearance of satellite emissions superimposed on the parent line [2,10,111. The purity of the La line observed in the low excita-
PHYSICS LETTERS
Volume 78A, number 4
Ag La 01 i .x10
0
-10 Relative
10
20
energy(eV)
Fig. 1. Electron excited Ag Lo! emission spectra below and above the Lr ionisation threshold. The notations for the satellites are those used by Parratt [ 31. The dashed curve is the lorentzian doublet which fits best the experimental data (see text).
tion energy spectrum remains difficult to determine because although, as discussed below, Lol transitions in the presence of an extra M,,, hole have disappeared, a certain amount of multiple ionisation in outer shells Table 1 FWHM of the Lor line and natural width of the Ls level in silver (eVJ. La1
this work (>Lr) (CL11
L3
measured
corrected
2.57 + 0.05 2.44+0.05
2.29 f 0.13 2.16iO.13
1.9 t 0.3
2.79
2.34to.19
-
2.91 f 0.10
2.54 f 0.15
-
Paratt [3] OLl)
Jusldn et al. [4] (>LlJ Krause and Oliver 1141
-
18 August 1980
could be present (see for instance ref. [ 121). In fact, the hump still visible above the tail of the lorentzian doublet which fits best with the low excitation energy measurement in the range -20 eV to t2 eV may be attributed to such a process (see fig. 1). If we assume that satellites have a negligible influence on the FWHM of the low energy excited La, line, the natural width of the L3 level can be deduced by correcting our experimental value for instrumental broadening and by subtracting the M4,, level width. The instrumental broadening has been estimated by folding a lorentzian lineshape with the instrumental function. It is roughly triangular in shape and has a FWHM of 0.28 + 0.08 eV. It leads to a corrected FWHM of 2.16 f 0.13 eV. Using a width of 0.3 kO.2 eV for the M4,5 levels as given by X-ray photoelectron spectrometry [ 131, a natural width of 1.9 ?r 0.3 eV is obtained for the L3 level. This value is smaller than the semi-empirical 2.40 eV width determined by Krause and Oliver from radiative rates and fluorescence yields [ 141. The disappearance of all the fine structure corresponding to the high-energy Lo satellite emissions observed when the excitation energy is =lOO eV below the L, threshold indicates that they are due to L3M4 5-M4,5M4,5 transitions and that the multiplet splitting of the initial L3M4,5 and final M4,, M,,, two-hole configurations is responsible for the observed fine structure. This result is in agreement with the calculations of Chen et al. [2] which predicts that the LsM~,~-M~,~M~,~ emissions extend between -7 eV and +26 eV with respect to the parent b, line maximum. Our experiment also shows that double-vacancy LN states play a minor role in the Lo satellites of silver, contrary to a previous assumption made to interpret the relative intensities of the L series X-ray lines [15]. Having now ascertained that the L3M4 5 and M,,,M,,, two-hole configurations play ad important -part in the La satellite emissions of silver, a more detailed interpretation of these emissions based on intensity calculations remains to be proposed. Such a study is now in progress. The authors wish to thank Professor C. Bonnelle for a critical reading of the manuscript.
2.40
--329
Volume 78A, number 4
PHYSICS LETTERS
References [11 M.H. Chen, B. Crasemann, K.-N. Huang, M. Aoyagi and H. Mark, At. Data Nucl. Data Tables 19(1977)97. [2] M.H. Chen, B. Crasemann, M. Aoyagi and H. Mark, Phys. Rev. A15 (1977) 2312. [3] L.G. Parratt, Phys. Rev. 54(1938)99. [4] H. Juslén, M. Pessa and G. Graeffe, Phys. Rev. A19 (1979) 196. [51 B.L. Doyle and S.M. Shafroth, in: Abstracts of contri~buted papers, Second Intern. Conf. on inner-shell ion!sation phenomena (Univ. Freiburg, Germany, 1976) p. 211. [6] B.L. Doyle and S.M. Shafroth, Phys. Rev. Al9 (1979) 1433. [7] V.0. Kostroun, 3.-P. Briand, A. Chetioui, P. Chevallier and A. Touati, Poster presented at the Stanford Synchrotwn Radiation Laboratory Users Group Meeting (Stanford, CA, USA, 1978), unpublished; and private communication.
330
18 August 1980
[8] C.F. Hague and D. Laporte, Rev. Sc Instrum. 51(1980) 621. [9] P. Chevallier, M. Tavernier and J.-P. Briand, J. Phys. Bll (1978) Ll7l [10] M.O. Krause, F. Wuilleumier and C.W. Nestor Jr, Phys. Rev. A6 (1972) 871. [11] J.-P. Briand, P. Chevallier, A. Chetioui, J.-P. Rozet, M. Tavernier and A. Touati, in: Extended abstracts of the Intern. Conf. on the Physics of X-ray spectra (NBS, Gaithersburg, MD, USA, 1976) p. 335. [12] W.C. Sauder, J.R. Huddle, J.D. Wilson and R.E. La Villa, Phys. Lett. 63A (1977) 313. [13] J.-M. Mariot and G. Dufour, J. Electron Spectrosc. 13 (1978) 403, and references cited therein. [14] M.O. Krause and J.H. Oliver, J. Phys. Chem. Ref. Data 8 (1979) 329. [151E.J. McGuire, Phys. Rev. A5 (1972) 2313.