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Observation of fine structure in the 1s2s2p2 5P-1s2p3 5S transition of core-excited O V
Volume 80A, number 5,6
PHYSICS LETTERS
22 December 1980
OBSERVATION OF FINE STRUCTURE IN THE 1s2s2p2 5P—1s2p3 5S TRANSITION OF CORE-EXCITED0 V A.E. LIVINGSTON and S.J. HINTERLONG Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Received 3 September 1980
We have measured the absolute wavelengths of the fine structure components of the 1s2s2p2 5P—1s2p3 5S transition in beryllium-like oxygen using the method of fast ion spectroscopy. These results represent the first measurement of fine structure in any core-excited 4-electron ion. We have also measured the lifetime of the metastable 1 s2p3 ~ state. The structure and lifetime results are compared with recent relativistic MCHF calculations.
Beryllium-like ions represent the simplest atomic systems for which quintet spin states may be formed. The energetically lowest-lying (or quintet states are expected to be the core-excited doubly excited) 1s2s2p2 5P and 1s2p3 5S states. These states are metastable against rapid Coulomb autoionization as well as against electric dipole (El) radiative decay to lower lying triplet and singlet levels. For this reason, the El transition from 1s2p3 ~S to 1s2s2p2 5P should provide a favorable decay route for the 5S level at low Z. Very recently, Bunge [1] has shown theoretically that these two quintet states are (the only) bound excited states in the Li— ion and he has suggested that a specific line observed previously in fast-ion spectra of lithium is the 1s2s2p2 5P—1s2p3 55 transition of Li—. Brooks et al. [2], and Mannervik et al. [3], have experimentally verified that this line originates from the U ion and the former authors have also suggested the identification of this transition multiplet in heavier ions for z = 4—9. We note that deexcitation of the metastable 1s2s2p2 5P state has been observed previously [4] in Auger-electron spectra for Z = 5,6, and 8. In beryllium-like 0 V we have measured the absolute wavelengths of the resolved fine-structure cornponents of the 1s2s2p2 5~3,2, 1 —1s2p3 5S 2 multiplet near 69.5 mm. This represents the first measurement of core-excited quintet state fine structure for 372
any four-electron system. We have also measured the lifetime of the 1s2p3 5S 2 state. This lifetime is ex2 5P pected determined the 1s2s2p —1s2p3to5Sbetransition rate,primarily althoughbyit may also be sensitive to the effects of spin—orbit or spin—spin induced autoionization rates. We produced the spectrum of 0 V by carbon foil excitation of 2.5 MeV 160— ions using the 4 MV Van de Graaff accelerator at the Notre Dame Injector! Tandem Accelerator Laboratory. The fast-ion spectrum was observed using a 1 m normal incidence monochromator that was Doppler-compensated for the moving source and was equipped with a stepping motor controlled wavelength drive, as described previously [5]. Detection of the extreme ultraviolet photons was performed by photon counting using a windowless Channeltron detector. The spectrum was scanned by stepping the monochromator wavelength drive in increments of 0.0025 nm. The lifetime was measured by stepping the exciter foil in increments of 0.08 mm, which is equivalent to about 0.015 ns, and thus monitoring the decay-in-flight of the transition intensity over a distance of about 20mm. The stepping motor drives and the data acquisition were controlled by an on-line computer. In fig. 1 we show the result of a single wavelength scan through the three fine structurein components of 2 5P—1s2p3 5S transition OV. This specthe 1s2s2p trum was recordedin the second order of dispersion
Volume 80A, number 5,6
PHYSICS LETTERS
0 V 2 5
Is 2P
3
Is 2s 2p
P
1-2
2-2
—
several nearby oxygen lines [6] in our spectra. Our results the absolute transition wavelengths and 1. the finefor structure splittings are summarized in table
S
3-2
~
I
I I
69.4
69.5
69.6
22 December 1980
X (n m) Fig.! A single wavelength scan of the resolved fine structure of the 1s2s2p2 5P—1s2p3 5S transition in OV. The observed llnewidths (FWHM) are 0.02 nm.
to increase the wavelength resolution. The FWHM linewidths obtained are about 0.02 nm. The peaks of several such scans were fitted to gaussian profiles. The peakseparations represent the 1s2s2p2 5P 32,1 fine 5S structure directly, since the 2 upper state is a single level. The absolute wavelengths of these features are determined by the use of the known wavelengths for
These results are compared with multi-configuration Dirac—Hartree—Fock (MCDHF) values calculated by Cheng[7]. We have measured the absolute transition wavelengths with a precision of better than I part in I 0~ and our results show the MCDHF wavelength values to be low by about 2% for 0 V. This discrepancy is not surprising at low Z. The theoretical accuracy is expected to be better for more highly ionized systems where electron correlation effects are less important. Our measurements confirm the suggestion by Brooks et al. [2] that this transition multiplet lies near 69.5 nm in 0 V. We have determined the fine structure splittings with a precision of about 3%, which corresponds to about 9 X iO—4 eV. The 5P-state fine structure is seen to differ greatly from the Landé interval rule, which reflects the importance of such relativistic effects as the spin—spin and spin—other-orbit interactions. Our fine structuremeasurements suggest that the MCDHF splittings are slightly too low. The same trend was observed in recent measurements [8] of the doubly excited state fine structure in the ls2s2p4P —ls2p2 4P transitions for three-electron CIV, NV, and 0 VI. A possible source of these very small remaining discrepancies between experiment and theory is an approximation employed in the calculation of the retardation term in the 3Breit operator 5S state for U[7].has atThe lifetime of the 1s2p tracted considerable interest [2, 3] since that state is expected to be totally cascade free [1], owing to the
Table 1 The transition energies and splittings of the 1 s2s2p2 5P— 1 s2p3 5S fine structure in 0 V. The theoretical values are from ref. [7]. 1s2s2p2 5P—1s2p3 5S transition
!s2s2p2 5P fine structure
fine structure component
interval
1—2 2—2 3—2
wavelength (nm) experiment
theory
69.36 69A8
±0.01
69.57
±0.01
68.010 68.123 68.205
±0.01
1—2 2—3
separation (cm1) experiment
theory
249 192
242 176
±7 ±7
373
Volume 80A, number 5,6
PHYSICS LETTERS
absence of higher-lying bound states in Li— For heavier 4-electron systems, however, numerous metastable core-excited quintet states lie energetically above 1s2p3 5S, so that cascade repopulation of this state is expected to contribute to the decay characteristics of the 1 s2s2p2 5P— 1 s2p3 5S transition in the fast-ion source. Our decay measurement for this transition in O V has yielded a multiexponential form that is well fit by a 2-exponential decomposition. The values of the decay components are 0.21 ±0.2 ns and 2.0 ns. The weak 2.0 ns component probably represents cascade contributions from higher-lying quintet states along with weak blends of unknown lines. We believe that the strong 0.21 ns component represents the 1s2p3 5S 2 lifetime. This value maybe compared with the theoretical MCDHF value [7] of 0.24 ns. The theoretical result does not take into account con3 5Spossible from Auger tributions to the deexcitation of 1s2p processes, which would reduce the theoretical lifetime. The good agreement between our measured value and the calculated value from ref. [7] suggests that such radiationless processes are small for this state in OV. -
374
22 December 1980
We are grateful to H.G. Berry and K.T. Cheng for providing us with their results in advance of publication. This work has been supported in part by the University of Notre Dame and by the National Science Foundation under Grant No. PHY78-27635.
References [1] CF. Bunge, Phys. Rev. Lett. 44 (1980) 1450; Phys. Rev. A22 (1980) 1. [2] R.L. Brooks et al., Phys. Rev. Lett. 45 (1980) 1318. [3] (1980) S. Mannervik, L44!. G. Astner and M. Kisielinski, J. Phys. B13 [4] M. R~dbro,R. Bruch and P. Bisgaard, J. Phys. B12 (1979) 2413; R. Bruch et al., Phys. Rev. A!9 (1979) 587. [5] A.E. Livingston, S.J. Hinterlong, J.A. Poirier, R. DeSerio H.G. Berry, Phys.Johansson, B13 (1980) L319. [6] and K. Bockasten andJ. K.B. Ark. Fys. 38 (1968) 563; B. Edlén, Rep. Prog. Phys. 26 (1963) 181. [7] K.T. Cheng, private communication; see also K.T. Cheng, J.P. Desclaux and Y. -K. Kim, J. Phys. Bit (1978) L359. [8] A.E. Livingston and H.G. Berry, Phys. Rev. All (1978) 1966.
PHYSICS LETTERS
22 December 1980
OBSERVATION OF FINE STRUCTURE IN THE 1s2s2p2 5P—1s2p3 5S TRANSITION OF CORE-EXCITED0 V A.E. LIVINGSTON and S.J. HINTERLONG Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Received 3 September 1980
We have measured the absolute wavelengths of the fine structure components of the 1s2s2p2 5P—1s2p3 5S transition in beryllium-like oxygen using the method of fast ion spectroscopy. These results represent the first measurement of fine structure in any core-excited 4-electron ion. We have also measured the lifetime of the metastable 1 s2p3 ~ state. The structure and lifetime results are compared with recent relativistic MCHF calculations.
Beryllium-like ions represent the simplest atomic systems for which quintet spin states may be formed. The energetically lowest-lying (or quintet states are expected to be the core-excited doubly excited) 1s2s2p2 5P and 1s2p3 5S states. These states are metastable against rapid Coulomb autoionization as well as against electric dipole (El) radiative decay to lower lying triplet and singlet levels. For this reason, the El transition from 1s2p3 ~S to 1s2s2p2 5P should provide a favorable decay route for the 5S level at low Z. Very recently, Bunge [1] has shown theoretically that these two quintet states are (the only) bound excited states in the Li— ion and he has suggested that a specific line observed previously in fast-ion spectra of lithium is the 1s2s2p2 5P—1s2p3 55 transition of Li—. Brooks et al. [2], and Mannervik et al. [3], have experimentally verified that this line originates from the U ion and the former authors have also suggested the identification of this transition multiplet in heavier ions for z = 4—9. We note that deexcitation of the metastable 1s2s2p2 5P state has been observed previously [4] in Auger-electron spectra for Z = 5,6, and 8. In beryllium-like 0 V we have measured the absolute wavelengths of the resolved fine-structure cornponents of the 1s2s2p2 5~3,2, 1 —1s2p3 5S 2 multiplet near 69.5 mm. This represents the first measurement of core-excited quintet state fine structure for 372
any four-electron system. We have also measured the lifetime of the 1s2p3 5S 2 state. This lifetime is ex2 5P pected determined the 1s2s2p —1s2p3to5Sbetransition rate,primarily althoughbyit may also be sensitive to the effects of spin—orbit or spin—spin induced autoionization rates. We produced the spectrum of 0 V by carbon foil excitation of 2.5 MeV 160— ions using the 4 MV Van de Graaff accelerator at the Notre Dame Injector! Tandem Accelerator Laboratory. The fast-ion spectrum was observed using a 1 m normal incidence monochromator that was Doppler-compensated for the moving source and was equipped with a stepping motor controlled wavelength drive, as described previously [5]. Detection of the extreme ultraviolet photons was performed by photon counting using a windowless Channeltron detector. The spectrum was scanned by stepping the monochromator wavelength drive in increments of 0.0025 nm. The lifetime was measured by stepping the exciter foil in increments of 0.08 mm, which is equivalent to about 0.015 ns, and thus monitoring the decay-in-flight of the transition intensity over a distance of about 20mm. The stepping motor drives and the data acquisition were controlled by an on-line computer. In fig. 1 we show the result of a single wavelength scan through the three fine structurein components of 2 5P—1s2p3 5S transition OV. This specthe 1s2s2p trum was recordedin the second order of dispersion
Volume 80A, number 5,6
PHYSICS LETTERS
0 V 2 5
Is 2P
3
Is 2s 2p
P
1-2
2-2
—
several nearby oxygen lines [6] in our spectra. Our results the absolute transition wavelengths and 1. the finefor structure splittings are summarized in table
S
3-2
~
I
I I
69.4
69.5
69.6
22 December 1980
X (n m) Fig.! A single wavelength scan of the resolved fine structure of the 1s2s2p2 5P—1s2p3 5S transition in OV. The observed llnewidths (FWHM) are 0.02 nm.
to increase the wavelength resolution. The FWHM linewidths obtained are about 0.02 nm. The peaks of several such scans were fitted to gaussian profiles. The peakseparations represent the 1s2s2p2 5P 32,1 fine 5S structure directly, since the 2 upper state is a single level. The absolute wavelengths of these features are determined by the use of the known wavelengths for
These results are compared with multi-configuration Dirac—Hartree—Fock (MCDHF) values calculated by Cheng[7]. We have measured the absolute transition wavelengths with a precision of better than I part in I 0~ and our results show the MCDHF wavelength values to be low by about 2% for 0 V. This discrepancy is not surprising at low Z. The theoretical accuracy is expected to be better for more highly ionized systems where electron correlation effects are less important. Our measurements confirm the suggestion by Brooks et al. [2] that this transition multiplet lies near 69.5 nm in 0 V. We have determined the fine structure splittings with a precision of about 3%, which corresponds to about 9 X iO—4 eV. The 5P-state fine structure is seen to differ greatly from the Landé interval rule, which reflects the importance of such relativistic effects as the spin—spin and spin—other-orbit interactions. Our fine structuremeasurements suggest that the MCDHF splittings are slightly too low. The same trend was observed in recent measurements [8] of the doubly excited state fine structure in the ls2s2p4P —ls2p2 4P transitions for three-electron CIV, NV, and 0 VI. A possible source of these very small remaining discrepancies between experiment and theory is an approximation employed in the calculation of the retardation term in the 3Breit operator 5S state for U[7].has atThe lifetime of the 1s2p tracted considerable interest [2, 3] since that state is expected to be totally cascade free [1], owing to the
Table 1 The transition energies and splittings of the 1 s2s2p2 5P— 1 s2p3 5S fine structure in 0 V. The theoretical values are from ref. [7]. 1s2s2p2 5P—1s2p3 5S transition
!s2s2p2 5P fine structure
fine structure component
interval
1—2 2—2 3—2
wavelength (nm) experiment
theory
69.36 69A8
±0.01
69.57
±0.01
68.010 68.123 68.205
±0.01
1—2 2—3
separation (cm1) experiment
theory
249 192
242 176
±7 ±7
373
Volume 80A, number 5,6
PHYSICS LETTERS
absence of higher-lying bound states in Li— For heavier 4-electron systems, however, numerous metastable core-excited quintet states lie energetically above 1s2p3 5S, so that cascade repopulation of this state is expected to contribute to the decay characteristics of the 1 s2s2p2 5P— 1 s2p3 5S transition in the fast-ion source. Our decay measurement for this transition in O V has yielded a multiexponential form that is well fit by a 2-exponential decomposition. The values of the decay components are 0.21 ±0.2 ns and 2.0 ns. The weak 2.0 ns component probably represents cascade contributions from higher-lying quintet states along with weak blends of unknown lines. We believe that the strong 0.21 ns component represents the 1s2p3 5S 2 lifetime. This value maybe compared with the theoretical MCDHF value [7] of 0.24 ns. The theoretical result does not take into account con3 5Spossible from Auger tributions to the deexcitation of 1s2p processes, which would reduce the theoretical lifetime. The good agreement between our measured value and the calculated value from ref. [7] suggests that such radiationless processes are small for this state in OV. -
374
22 December 1980
We are grateful to H.G. Berry and K.T. Cheng for providing us with their results in advance of publication. This work has been supported in part by the University of Notre Dame and by the National Science Foundation under Grant No. PHY78-27635.
References [1] CF. Bunge, Phys. Rev. Lett. 44 (1980) 1450; Phys. Rev. A22 (1980) 1. [2] R.L. Brooks et al., Phys. Rev. Lett. 45 (1980) 1318. [3] (1980) S. Mannervik, L44!. G. Astner and M. Kisielinski, J. Phys. B13 [4] M. R~dbro,R. Bruch and P. Bisgaard, J. Phys. B12 (1979) 2413; R. Bruch et al., Phys. Rev. A!9 (1979) 587. [5] A.E. Livingston, S.J. Hinterlong, J.A. Poirier, R. DeSerio H.G. Berry, Phys.Johansson, B13 (1980) L319. [6] and K. Bockasten andJ. K.B. Ark. Fys. 38 (1968) 563; B. Edlén, Rep. Prog. Phys. 26 (1963) 181. [7] K.T. Cheng, private communication; see also K.T. Cheng, J.P. Desclaux and Y. -K. Kim, J. Phys. Bit (1978) L359. [8] A.E. Livingston and H.G. Berry, Phys. Rev. All (1978) 1966.