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The L2,3 X-ray emission spectrum of argon
Volume 4 1A, number 2
PHYSICS LETTERS
11 September 1972
THE L2 3 X-RAY EMISSION SPECTRUM OF ARGON L.O. WERME, B. GRENNBERG,
J.NORDGREN,
C. NORDLING
and K. SIEGBAHN
Uppsala University, Institute of Physics, Uppsala, Sweden
Received 27 June 1972 The Lz s X-ray emission spectrum of argon has been recorded. The resolution is sufficiently high to separate the Lz and’Ls lines as well as the satellite spectrum. The low energy satellites can be attributed to shake-up processes accompanying the X-ray emission.
In a recent investigation of the Ar L2,s emission X-ray spectrum by Cooper and LaVilla [l] a not previously reported low energy satellite was found. A comparison with the work of Mehlhorn [2] on L, Coster-Kronig processes rules out X-ray transitions in doubly ionized Ar as an explanation for this low energy satellite. Such transitions would give rise to lines with energies between 217.6 and 227.4 eV [ 11, i.e. mainly high energy satellites of the Ar L,,,-M X-ray lines. Since initial and final states in singly ionized, excited atoms will have approximately the same energy difference as doubly ionized states, it is also unlikely that the low energy satellite is due to such transitions. Cooper and LaVilla suggest that this satellite corresponds to a two-electron or shakeup process with a simultaneous filing of an inner shell vacancy and an excitation of a valence shell electron, resulting in emission of a single photon. However, their resolution was insufficient to present conclusive evidence. The soft X-ray grating spectrometer for gaseous samples [3,4] which has recently been built in our laboratory offers the high resolution that is ncessary to elucidate the origin of this low energy satellite. Fig. 1 shows the L2,s X-ray spectrum from argon recorded on our instrument. The spectrum obtained by Cooper and LaVilla is also shown in the same energy scale to make a direct comparison possible. Fig. 1 clearly demonstrates the different resolutions obtained with on one hand a grating and on the other a soap-film crystal (lead myristate in this particular case). Several low energy satellites as well as high energy satellites are well resolved in the grating spectrum. The stronger of these lines have been measured on a comparator and the corresponding photon energies have been calculated using the main X-ray
At-
LEAD MYRISTATE
h’
I
I
210
220
I
1
60
58
I
b
lV
56
Fig. 1. The emission X-ray spectrum from gaseous argon. The expected line positions are indicated as bars at the bottom of the figure. At the top an X-ray spectrum recorded with a soap-film analyser is shown [ 1] .
lines for calibration. The energies of the L2-M I and L,-M 1 transitions can be derived from ESCA binding energies. (See e.g. ref. [5] and references therein). We have used the energies 221.3 eV and 219.2 eV respectively. The measured photon energies are given in table 1, together with the assignments we propose. The high energy satellites most likely correspond 113
Volume
4 IA. number
PHYSICS
Energies
Table I and assignments of the 6 most mtense lines in the L 2 3 X-ray spectrum of argon. Photon energres were calculated the energies of the initial [S] and final [6] state;. Estimated errors for measured energies are + 0.2 eV.
2
Measured energy (eV)
Assignment
223.4 (221.3) (219.2) 212.1 209.8
Transrtion in doubly ionized Ar ls22s22p53s23p6(2~,,2)+1s22s22p63s’3p6(2S,i2) 1. 31 ‘Y3ir” .. J-ls22s22p~3s23p43d(2S,,,2) ]2ppI:2 7 312’.’ (-PI i,) - 1s22s22p63s23p44d(2S, 11 ., (“P$$-
207.2
to transitions in doubly ionized Ar. This satellite spectrum could consist of up to 44 lines. Without accurate calculations of transition probabilities and a better knowledge of the energy levels of the initial states of these transitions, an assignment of these lines is hardly possible. (The energies obtained by Mehlhorn ]2] are not accurate enough for this purpose). Only the strongest line in this satellite group is given in table 1. From the comparison in table 1 it is clear that the low energy satellites can be attributed to shake-up processes accompanying the X-ray emission. The tinal state of the X-ray transition, 3s’ ~P~(~S,,~ ). can mix strongly with states in other configurations where one of the 3p6 electrons is removed while another one is excited. These levels must have the same parity and only the same terms can mix through configuration interaction. (Shake-up processes are monopole transitions). The levels which are allowed to mix belong to either of the two series: a) 3s2 3p4 ns (2S) and b) 3s2 3p2 3p4 n’d (2S). Anomalies in optical spectra indicate strong mixing with configuration b) but only weak mixing with a) [ 71 This is in agreement with our results. As can be seen from the table, the line corresponding to the transition ls22s22p53s23p6(21’,,2) + ls22s22p63s23p44d(2S,,2) seems to be missing. One would expect lines in the doublet corresponding to a 4d excitation to have the same intensity ratio as the 3d doublet. This line should therefore be very weak and furthermore he
114
LETTERS
11 September
19’72
from
Calculated energy (eV)
,?)
a/-
211.3 219.2 2 12.0 209.9 209.4 207.2
very close to the strongest satellite in this region (cf. fig. I), which would tend to obscure the line. Apart from these three satellites at the low energy side of the main Ar L2,3 X-ray lines, there are indications of more lines which have not been measured so far. Recent experiments indicate that there is often a partial probability for shake-up and shake-off processes when an inner-shell vacancy is deexcited. Very weak such effects have been seen in K X-ray spectra [g] ; much stronger ones were seen in Auger spectra of the rare gases [9] Our results give further evidence of the effect in X-ray emission and ascertain that the tentative explanation given in [l] was correct. References 25 Ill J.W. Cooper and R.E. LaVilla, Phys.Rev.Lett. (1970) 1745. PI W. Mehlhorn, Z.Physik 208 (1967) 1. J. Nordgren 131 K. Siegbahn, L.O. Werme, B. Grennberg, and C. Nordling, Phys.Lett. 41A (1972) 111, ed. D.A. 141 K. Siegbahn, in Electron spectroscopy, Shirley, Proc.Intern.Conf. Asilomar, Pacific Grove, California, USA, 197 1 (North-Holland, Amsterdam, 1972). I51 K. Siegbahn et al., ESCA applied to free molecules (North-Holland, Amsterdam, 1969). of 161 C.E. Moore, Atomic energy levels, Nat.Bureau Standards, Circ.467 (Washington 1949, 1952 and 1958). Ark.Fys.25 (1963) 203. [71 L. Minnhagen, J.Phys.Rev.Lett. 22 (1969) [81 T.Aberg and J. Utriainen, 1346. [91 L.O. Werme, T. Bergmark and K. Siegbahn, Uppsala University, Institute of Physics, Report no.UUIP-783.
PHYSICS LETTERS
11 September 1972
THE L2 3 X-RAY EMISSION SPECTRUM OF ARGON L.O. WERME, B. GRENNBERG,
J.NORDGREN,
C. NORDLING
and K. SIEGBAHN
Uppsala University, Institute of Physics, Uppsala, Sweden
Received 27 June 1972 The Lz s X-ray emission spectrum of argon has been recorded. The resolution is sufficiently high to separate the Lz and’Ls lines as well as the satellite spectrum. The low energy satellites can be attributed to shake-up processes accompanying the X-ray emission.
In a recent investigation of the Ar L2,s emission X-ray spectrum by Cooper and LaVilla [l] a not previously reported low energy satellite was found. A comparison with the work of Mehlhorn [2] on L, Coster-Kronig processes rules out X-ray transitions in doubly ionized Ar as an explanation for this low energy satellite. Such transitions would give rise to lines with energies between 217.6 and 227.4 eV [ 11, i.e. mainly high energy satellites of the Ar L,,,-M X-ray lines. Since initial and final states in singly ionized, excited atoms will have approximately the same energy difference as doubly ionized states, it is also unlikely that the low energy satellite is due to such transitions. Cooper and LaVilla suggest that this satellite corresponds to a two-electron or shakeup process with a simultaneous filing of an inner shell vacancy and an excitation of a valence shell electron, resulting in emission of a single photon. However, their resolution was insufficient to present conclusive evidence. The soft X-ray grating spectrometer for gaseous samples [3,4] which has recently been built in our laboratory offers the high resolution that is ncessary to elucidate the origin of this low energy satellite. Fig. 1 shows the L2,s X-ray spectrum from argon recorded on our instrument. The spectrum obtained by Cooper and LaVilla is also shown in the same energy scale to make a direct comparison possible. Fig. 1 clearly demonstrates the different resolutions obtained with on one hand a grating and on the other a soap-film crystal (lead myristate in this particular case). Several low energy satellites as well as high energy satellites are well resolved in the grating spectrum. The stronger of these lines have been measured on a comparator and the corresponding photon energies have been calculated using the main X-ray
At-
LEAD MYRISTATE
h’
I
I
210
220
I
1
60
58
I
b
lV
56
Fig. 1. The emission X-ray spectrum from gaseous argon. The expected line positions are indicated as bars at the bottom of the figure. At the top an X-ray spectrum recorded with a soap-film analyser is shown [ 1] .
lines for calibration. The energies of the L2-M I and L,-M 1 transitions can be derived from ESCA binding energies. (See e.g. ref. [5] and references therein). We have used the energies 221.3 eV and 219.2 eV respectively. The measured photon energies are given in table 1, together with the assignments we propose. The high energy satellites most likely correspond 113
Volume
4 IA. number
PHYSICS
Energies
Table I and assignments of the 6 most mtense lines in the L 2 3 X-ray spectrum of argon. Photon energres were calculated the energies of the initial [S] and final [6] state;. Estimated errors for measured energies are + 0.2 eV.
2
Measured energy (eV)
Assignment
223.4 (221.3) (219.2) 212.1 209.8
Transrtion in doubly ionized Ar ls22s22p53s23p6(2~,,2)+1s22s22p63s’3p6(2S,i2) 1. 31 ‘Y3ir” .. J-ls22s22p~3s23p43d(2S,,,2) ]2ppI:2 7 312’.’ (-PI i,) - 1s22s22p63s23p44d(2S, 11 ., (“P$$-
207.2
to transitions in doubly ionized Ar. This satellite spectrum could consist of up to 44 lines. Without accurate calculations of transition probabilities and a better knowledge of the energy levels of the initial states of these transitions, an assignment of these lines is hardly possible. (The energies obtained by Mehlhorn ]2] are not accurate enough for this purpose). Only the strongest line in this satellite group is given in table 1. From the comparison in table 1 it is clear that the low energy satellites can be attributed to shake-up processes accompanying the X-ray emission. The tinal state of the X-ray transition, 3s’ ~P~(~S,,~ ). can mix strongly with states in other configurations where one of the 3p6 electrons is removed while another one is excited. These levels must have the same parity and only the same terms can mix through configuration interaction. (Shake-up processes are monopole transitions). The levels which are allowed to mix belong to either of the two series: a) 3s2 3p4 ns (2S) and b) 3s2 3p2 3p4 n’d (2S). Anomalies in optical spectra indicate strong mixing with configuration b) but only weak mixing with a) [ 71 This is in agreement with our results. As can be seen from the table, the line corresponding to the transition ls22s22p53s23p6(21’,,2) + ls22s22p63s23p44d(2S,,2) seems to be missing. One would expect lines in the doublet corresponding to a 4d excitation to have the same intensity ratio as the 3d doublet. This line should therefore be very weak and furthermore he
114
LETTERS
11 September
19’72
from
Calculated energy (eV)
,?)
a/-
211.3 219.2 2 12.0 209.9 209.4 207.2
very close to the strongest satellite in this region (cf. fig. I), which would tend to obscure the line. Apart from these three satellites at the low energy side of the main Ar L2,3 X-ray lines, there are indications of more lines which have not been measured so far. Recent experiments indicate that there is often a partial probability for shake-up and shake-off processes when an inner-shell vacancy is deexcited. Very weak such effects have been seen in K X-ray spectra [g] ; much stronger ones were seen in Auger spectra of the rare gases [9] Our results give further evidence of the effect in X-ray emission and ascertain that the tentative explanation given in [l] was correct. References 25 Ill J.W. Cooper and R.E. LaVilla, Phys.Rev.Lett. (1970) 1745. PI W. Mehlhorn, Z.Physik 208 (1967) 1. J. Nordgren 131 K. Siegbahn, L.O. Werme, B. Grennberg, and C. Nordling, Phys.Lett. 41A (1972) 111, ed. D.A. 141 K. Siegbahn, in Electron spectroscopy, Shirley, Proc.Intern.Conf. Asilomar, Pacific Grove, California, USA, 197 1 (North-Holland, Amsterdam, 1972). I51 K. Siegbahn et al., ESCA applied to free molecules (North-Holland, Amsterdam, 1969). of 161 C.E. Moore, Atomic energy levels, Nat.Bureau Standards, Circ.467 (Washington 1949, 1952 and 1958). Ark.Fys.25 (1963) 203. [71 L. Minnhagen, J.Phys.Rev.Lett. 22 (1969) [81 T.Aberg and J. Utriainen, 1346. [91 L.O. Werme, T. Bergmark and K. Siegbahn, Uppsala University, Institute of Physics, Report no.UUIP-783.