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Radiative-lifetime measurements: Effects of configuration mixing and singlet-triplet interaction in singly-ionized nitrogen
Volume 55A, number 4
PHYSICS LETTERS
15 December 1975
RADIATIVE-LIFETIME MEASUREMENTS: EFFECTS OF CONFIGURATION MIXING AND SINGLET-TRIPLET INTERACTION IN SINGLY-IONIZED NITROGEN A.E. LIVINGSTON, Y. BAUDINET-ROBINET and P.D. DUMONT Institut de Physique Nucléaire, Université de Liege, Sort Tilman, B-4000 Liege 1, Belgium Received 12 September 1975 We have used the beam-foil technique to measure lifetimes for the resolved transitions 745.8, 747.0, and 748,4 A 3 and 2s22p3s in N II. The results reflect effects of configuration mixing and singlet-triplet interaction involving the 2s2p configurations.
The measurement of radiative lifetimes using the beam-foil excitation technique allows the establishment of reliable oscillator-strength (f-value) trends along isoelectronic sequences [1—4].Deviations from regularity within f-value trends frequently represent complexities of atomic structure that are difficult to treat theoretically [5]. In the C I isoelectronic sequence, configuration mixing is observed among 2s2p3, 2s22p3s, and 2s22p3d in their 1P°terms, with the effects being particularly strong for 2s2p3 and 2s22p3s in N II [61. Calculations that include configuration mixing in these terms have been performed in NIl only for the transition probabilities of the two branches involving 2s2p3 1P°as upper level [7, 8]. Beam-foil measurements [9, 10] for one branch (660.3 A) yield a lifetime that is twice the theoretical values. Confirmation of this lifetime using the other branch transition (745.8 A) was not possible during previous studies owing to blending with the NI! transitions at 747.0 A and 748.4 A. The 747.0 A transition represents the only strong branch from 2p3s 1P°,for which the lifetime also will reflect the effects of the mixing between 2s2p3 and 2s22p3s. The 748.4 A feature is an “intercombination” transition that results from interaction between the ‘~?and 3P? states in the 2s22p3s configuration. These two energy levels are coincidentally very close together in N II, owing to a depression in energy of the 1P°state by the interaction involving 2s2p3 and 2s22p3s [6]. During previous attempts to perform lifetime measurements near 747 A [10—12], the three transitions have not been resolved, so that the reliability of the reported results is difficult to estimate.
N fl 2s 2 p
2s22p3s1
747,0
2
~
745,8
ipO
7~ 7458 748,4
66O~
572 102
1iI~I~II 748,4
l.0
Fig. 1. A section of the observed beam-foil spectrum of nitrogen (N~500 keV) and a schematic partial level diagram for N II.
By improving the wavelength resolution of our experimental system [13],we have resolved the transitions at 745.8, 747.0, and 748.4 A for the first time in beam-foil work (see fig. 1). We report here lifetime results for these transitions and we discuss briefly their significance with regard to the interactions in their respective upper states. The basic experimental setup has been described previously [14]. Intensity decays were measured by pulse-counting techniques, and decay data were analyzedusing the computer program HOMER [15]. We list below our lifetime results for four terms in N II: 207
Volume 55A, number 4
PHYSICS LETTERS
no CI.
ci x
cr.
o •
This work Other beorn -fot
and a configuration-interaction 2s2p3 1P°lifetime calculation for the 2s22p3s 1P°lifetime are needed to explain the effects responsible for our observed life-
028
Ci sequence 2p2 1D -2p3s 1p°
o,~’.
Theory
0 20
~
f
0,12 0
0
15 December 1975
008 004
times. In the triplet system, we find the 2p3s 3P~lifetime, measured using the 748.4 A “intercombination” transition, to be shorter than the 2p3s 3P°lifetime measured at the unresolved 672 A multiplet. This reveals that, as a result of mixing between the 3P? and 1~?levels of the 2s22p3s configuration, the transition probability for 2p2 1D 3P? affects significantly the 3P~lifetime.2 For2p3s this reason, the lifetime observed for 672 A is probably some mean value of —
C
0
OIINU
PT
1
0
CI
/z
the 3P? and 3P~,
2 ‘D-Fig. 2. Plot of oscillator versus sequence. l/Z for theThe 2p 0111 2p3s’PO transition in thestrength,f, CI isoelectronic measurement is from ref. 116]. See ref. [5], fig. 7, for the other experimental and theoretical results. No branching correction has been made for our N II result. (C I denotes configuration interaction.)
2lifetimes. No theoretical transition probability is available for 748.4 A. We thank Professor L. Winand, the “Université de Liege”, and the “Institut Interuniversitaire des Sciences Nucléaires” for encouragement and financial support.
2s2 2p2 1D
References
2p2 1S
—
2s2p3 1P° 660.3 A
0.23 ns
—
2s2p3 2P~ 745.8
0.25
2p2 1D
—
2p3s
lpO
747.0
0.22
2p2 1D
—
2p3s
3~? 748.4
0.60
2p2 3P
—
2p3s
3p~ 672
0.90
We estimate our maximum uncertainties to be about 15% Our result for 745.8 A confirms the 2s2p3 1P°lifetime value reported here and previously [9, 10], using the 660.3 A branch. The theoretical lifetime for this level is low by a factor of two, in contrast to the good agreement between theory and experiment found for 2s2p3 iPO in ispelectronic 0111 [16]. The reversal in energy ordering of 2s2p3 l}30 and 2s22p3s 1P°between NI! and 0 III may be responsible for this disagreement. The f-value for the 2p2 1D 2p3s 1po transition, derived from the 2p3s 1P°lifetime (747.0 A), is anomalously high when compared with the f-values for this transition in C land 0111 (see fig. 2). An improved theoretical value for the -
—
208
[1] Beam-foil spectroscopy, ed. S.
Bashkin, Nucl. Instrum.
Meth. 110 (1973). [2] M.W. Smith and W.L.Wiese,Astrophys.J. Supp. Ser. 23(1971)103. [3] A.E. Livingston, D.J.G. Irwin and E.H. Pinnington, J. Opt. Soc. Am. 62 (1972) 1303. [4] A.E. Livingston, Ph.D. thesis, Univ. of Alberta (1974). [5] M.W. Smith, G.A. Martin and W.L. Wiese, Nucl. Instnim. Meth. 110 (1973) 219. [6] Eriksson, Fysik 13(1958)303. [7] K.B.S. P. Westhaus and Arkiv 0. Sinano~lu, Phys. Rev. 183 (1969) 56. [8] C.A. Nicolaides,Chem. Phys. Lett. 21(1973)242. [91 J.P. Buchet, M.C. Poulizac and M. Carré, J. Opt. Soc. Am. 62(1972)623. [10] J.A. Kernahan, A.E. Livingston and E.H. Pinnington, Can. J. Phys. 52(1974)1895. [11] J.E. Hesser and B.L. Lutz, J. Opt. Soc. Am. 58 (1968) 1513. [12] E.J. Knystautas, M. Brochu and R. Drouin, Can. J. Spectrosc. 18 (1973) 153. [13] P.D. Dumont, Y. Baudinet-Robinet and A.E. Livingston, to be published. [14] P.D. Dumont, Physica 62 (1972) 104. [15] DJ.G. Irwin and A.E. Livingston, Comput. Phys. Cornmun. 7 (1974) 95. [16] E.H. Pinnington, DJ.G. Irwin, A.E. Livingston and J.A. Kernahan, Can. J. Phys. 52 (1974) 1961.
PHYSICS LETTERS
15 December 1975
RADIATIVE-LIFETIME MEASUREMENTS: EFFECTS OF CONFIGURATION MIXING AND SINGLET-TRIPLET INTERACTION IN SINGLY-IONIZED NITROGEN A.E. LIVINGSTON, Y. BAUDINET-ROBINET and P.D. DUMONT Institut de Physique Nucléaire, Université de Liege, Sort Tilman, B-4000 Liege 1, Belgium Received 12 September 1975 We have used the beam-foil technique to measure lifetimes for the resolved transitions 745.8, 747.0, and 748,4 A 3 and 2s22p3s in N II. The results reflect effects of configuration mixing and singlet-triplet interaction involving the 2s2p configurations.
The measurement of radiative lifetimes using the beam-foil excitation technique allows the establishment of reliable oscillator-strength (f-value) trends along isoelectronic sequences [1—4].Deviations from regularity within f-value trends frequently represent complexities of atomic structure that are difficult to treat theoretically [5]. In the C I isoelectronic sequence, configuration mixing is observed among 2s2p3, 2s22p3s, and 2s22p3d in their 1P°terms, with the effects being particularly strong for 2s2p3 and 2s22p3s in N II [61. Calculations that include configuration mixing in these terms have been performed in NIl only for the transition probabilities of the two branches involving 2s2p3 1P°as upper level [7, 8]. Beam-foil measurements [9, 10] for one branch (660.3 A) yield a lifetime that is twice the theoretical values. Confirmation of this lifetime using the other branch transition (745.8 A) was not possible during previous studies owing to blending with the NI! transitions at 747.0 A and 748.4 A. The 747.0 A transition represents the only strong branch from 2p3s 1P°,for which the lifetime also will reflect the effects of the mixing between 2s2p3 and 2s22p3s. The 748.4 A feature is an “intercombination” transition that results from interaction between the ‘~?and 3P? states in the 2s22p3s configuration. These two energy levels are coincidentally very close together in N II, owing to a depression in energy of the 1P°state by the interaction involving 2s2p3 and 2s22p3s [6]. During previous attempts to perform lifetime measurements near 747 A [10—12], the three transitions have not been resolved, so that the reliability of the reported results is difficult to estimate.
N fl 2s 2 p
2s22p3s1
747,0
2
~
745,8
ipO
7~ 7458 748,4
66O~
572 102
1iI~I~II 748,4
l.0
Fig. 1. A section of the observed beam-foil spectrum of nitrogen (N~500 keV) and a schematic partial level diagram for N II.
By improving the wavelength resolution of our experimental system [13],we have resolved the transitions at 745.8, 747.0, and 748.4 A for the first time in beam-foil work (see fig. 1). We report here lifetime results for these transitions and we discuss briefly their significance with regard to the interactions in their respective upper states. The basic experimental setup has been described previously [14]. Intensity decays were measured by pulse-counting techniques, and decay data were analyzedusing the computer program HOMER [15]. We list below our lifetime results for four terms in N II: 207
Volume 55A, number 4
PHYSICS LETTERS
no CI.
ci x
cr.
o •
This work Other beorn -fot
and a configuration-interaction 2s2p3 1P°lifetime calculation for the 2s22p3s 1P°lifetime are needed to explain the effects responsible for our observed life-
028
Ci sequence 2p2 1D -2p3s 1p°
o,~’.
Theory
0 20
~
f
0,12 0
0
15 December 1975
008 004
times. In the triplet system, we find the 2p3s 3P~lifetime, measured using the 748.4 A “intercombination” transition, to be shorter than the 2p3s 3P°lifetime measured at the unresolved 672 A multiplet. This reveals that, as a result of mixing between the 3P? and 1~?levels of the 2s22p3s configuration, the transition probability for 2p2 1D 3P? affects significantly the 3P~lifetime.2 For2p3s this reason, the lifetime observed for 672 A is probably some mean value of —
C
0
OIINU
PT
1
0
CI
/z
the 3P? and 3P~,
2 ‘D-Fig. 2. Plot of oscillator versus sequence. l/Z for theThe 2p 0111 2p3s’PO transition in thestrength,f, CI isoelectronic measurement is from ref. 116]. See ref. [5], fig. 7, for the other experimental and theoretical results. No branching correction has been made for our N II result. (C I denotes configuration interaction.)
2lifetimes. No theoretical transition probability is available for 748.4 A. We thank Professor L. Winand, the “Université de Liege”, and the “Institut Interuniversitaire des Sciences Nucléaires” for encouragement and financial support.
2s2 2p2 1D
References
2p2 1S
—
2s2p3 1P° 660.3 A
0.23 ns
—
2s2p3 2P~ 745.8
0.25
2p2 1D
—
2p3s
lpO
747.0
0.22
2p2 1D
—
2p3s
3~? 748.4
0.60
2p2 3P
—
2p3s
3p~ 672
0.90
We estimate our maximum uncertainties to be about 15% Our result for 745.8 A confirms the 2s2p3 1P°lifetime value reported here and previously [9, 10], using the 660.3 A branch. The theoretical lifetime for this level is low by a factor of two, in contrast to the good agreement between theory and experiment found for 2s2p3 iPO in ispelectronic 0111 [16]. The reversal in energy ordering of 2s2p3 l}30 and 2s22p3s 1P°between NI! and 0 III may be responsible for this disagreement. The f-value for the 2p2 1D 2p3s 1po transition, derived from the 2p3s 1P°lifetime (747.0 A), is anomalously high when compared with the f-values for this transition in C land 0111 (see fig. 2). An improved theoretical value for the -
—
208
[1] Beam-foil spectroscopy, ed. S.
Bashkin, Nucl. Instrum.
Meth. 110 (1973). [2] M.W. Smith and W.L.Wiese,Astrophys.J. Supp. Ser. 23(1971)103. [3] A.E. Livingston, D.J.G. Irwin and E.H. Pinnington, J. Opt. Soc. Am. 62 (1972) 1303. [4] A.E. Livingston, Ph.D. thesis, Univ. of Alberta (1974). [5] M.W. Smith, G.A. Martin and W.L. Wiese, Nucl. Instnim. Meth. 110 (1973) 219. [6] Eriksson, Fysik 13(1958)303. [7] K.B.S. P. Westhaus and Arkiv 0. Sinano~lu, Phys. Rev. 183 (1969) 56. [8] C.A. Nicolaides,Chem. Phys. Lett. 21(1973)242. [91 J.P. Buchet, M.C. Poulizac and M. Carré, J. Opt. Soc. Am. 62(1972)623. [10] J.A. Kernahan, A.E. Livingston and E.H. Pinnington, Can. J. Phys. 52(1974)1895. [11] J.E. Hesser and B.L. Lutz, J. Opt. Soc. Am. 58 (1968) 1513. [12] E.J. Knystautas, M. Brochu and R. Drouin, Can. J. Spectrosc. 18 (1973) 153. [13] P.D. Dumont, Y. Baudinet-Robinet and A.E. Livingston, to be published. [14] P.D. Dumont, Physica 62 (1972) 104. [15] DJ.G. Irwin and A.E. Livingston, Comput. Phys. Cornmun. 7 (1974) 95. [16] E.H. Pinnington, DJ.G. Irwin, A.E. Livingston and J.A. Kernahan, Can. J. Phys. 52 (1974) 1961.