- Email: [email protected]
Far-UV spectroscopy of highly ionised argon using complementary techniques
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B56/57
309
(1991) 309-312
Far-UV spectroscopy of highly ionised argon using complementary techniques Emile J. Knystautas D&partement de physique,
and Norman
Schlader
UniuersitP LavaI, QuCbec, Canada GIK 7P4
Samuel L. Bliman S.Ph.At./DRFG,
CENG 85X, 38041 Grenoble, France
Far-UV spectroscopy Excitation by foil-excited
of highly ionised argon has been studied with particular attention paid to core-excited MeV beams is compared with single- and double- electron capture into excited states
metastable states. during low-energy
charge exchange of highly ionised argon ions in helium and hydrogen targets. Complementary techniques are used for a) line identification by the high charge-state selectivity of the charge-exchange process, b) lifetime measurements, c) considerations concerning different excitation and cascade processes involved, d) systematics along isoelectronic sequences, and e) model-potential and MCDF calculations. A number of new lines are reported.
1. Introduction
Spectroscopy of highly ionised atoms has long been used as a probe of astrophysical plasmas. The problems related to impurity transport and control in controlled thermonuclear fusion devices [l], as well as to the search for energy-level schemes suitable for X-ray lasers [2] have rekindled interest in this area. While few-electron systems are now quite well known (although even here, multiply excited states have only recently been studied), the more complex ions, such as those of the neon, sodium and magnesium isoelectronic sequences, remain incompletely determined. Where higher principal quantum numbers or core-excited states are involved information is even more scarce. Such information is vital if collisional excitation processes are to be used for studying fusion plasmas, X-ray laser schemes or low-energy electron capture by multicharged ions [3]. Line identification is greatly assisted by energy-level calculations, using parametric potentials, or multiconfigurational Dirac-Fock (MCDF) methods. However, such work is often a compromise between good accuracy for fewer levels and modest accuracy for a great number of levels, hence ambiguities remain when experimental results must be interpreted. Recent approaches even involve studying general features of transition arrays, rather than identifying many individual lines (see, for example, ref. [4], in which fig. 4 shows the case for Ar VI). In this paper we present previously unpublished far-UV spectra of highly ionised argon using two different kinds of emission spectroscopy, that of foil-excited 0168-583X/9i/$O3.50
0 1991 - Elsevier Science Publishers
MeV ion beams and that of low-velocity multicharged ions following electron capture in gases. We show how the use of such complementary techniques can help in the unraveling of complex ion spectra.
2. Experimental conditions Descriptions of our experimental setups can be found in previous works [3,5]. Beam-foil spectroscopy was carried out at Universite Laval, where a modified 7.5 MV model CN Van de Graaff was used to produce Ar+, Ar2+, and Ar 3+ ion beams of 6, 12, and 18 MeV energy, respectively, which then impinged onto thin carbon foils. Photons from the spontaneously radiating beam particles were dispersed with a McPherson 2.2-m grazing-incidence monochromator and counted with a Galileo ChanneltronTM. Entrance and exit slit widths were 50 pm (100 pm for the 18 MeV spectrum), and the grating (600 l/mm with a 2O4’ blaze angle) was used at a nominal angle of incidence of 88O, with a telescopic slit. The low-energy electron capture experiments took place at CEN Grenoble using the MINIMAFIOS electron cyclotron resonance ion source [6] which produced substantial beams of Ar’+ and Arat ions at an energy of 10 keV/q. The ions impinged on a target gas (hydrogen or helium) at a pressure of 5 x 1O-5 mbar (as measured by a capacitance manometer), thus ensuring single-collision conditions. The radiation emitted by the ions following single- (and sometimes double-) electron capture into excited states was dispersed by a l-m
B.V. (North-Holland)
I. ATOMIC/MOLECULAR
PHYSICS
310
E.J. Knystauras el al. / Far- UV spectroscapy
GISMO grazing-incidence monochromator. With a 1200 I/mm grating at 85” incidence and 400 km slits, photons emitted at 25” to the forward direction were detected with a channel electron multiplier.
3. Results and discussion Fig. 1 shows the far-UV spectra obtained at incident-ion energies of 6, 12 and 18 MeV. By varying the energy, the charge-state distribution of the foil-excited beam particles changes in a known fashion [7] and by following the intensity variation of a given line over this range, one can deduce approximately from which charge state it arises [S].
of highly ion&d argon
The results of the low-energy charge-exchange experiments are displayed in figs. 2 and 3. These show far-WV spectra stemming from the collision of Ar’+ and A?’ ions respectively, with helium and hydrogen targets. The monochromator-detector system response has been previously measured over the entire wavelength range using the branching-ratio technique with various ions; the spectra shown are normalised in order to show relative intensities. (The apparent rise in background at the shortest wavelengths appears due to this process.) New assignments are based on MCDF calculations and systematic trends along isoelectronic sequences. The spectral purity is striking: fig. 2 contains almost entirely Ar VII, while fig. 3 has Ar VIII lines. Further-
18 MeV
Fig. 1. Far-UV spectrum of foil-excited argon beams at incident ion energies of 6,12 and 18 MeV.
E.J. Knystautas
et al. / Far-UV spectroscopy
more, in passing from hydrogen to helium targets, one can also see an additional high degree of selectivity due to the greatly differing ionisation potentials of the two target gases. In fact, a simple but reliable model [9] gives the following expression for the most probable principal quantum number into which capture takes place:
ofhighly ionised argon
Ar8+ -
311
He
i np’
21 (eV) rIq+ q-l 27.2 2&+1 I
’ 1
where I is the ionisation potential of the target in eV, and q is the incident ion charge. One can see from fig. 1 that, although higher charge states grow at the cost of lower ones as the incident ion energy increases, it is still difficult to assign with certainty a given line to a particular charge state. For example (see table 2 of ref. [S]), a line at 659 A, unknown at the time, was assigned to an ion of charge about +6. In fact, by its presence in fig. 2 and its absence from fig. 3, one can definitely exclude this line from the Ar VIII spectrum. Note also how the strong
)
I !I I
Ar7+
-
He
c
x (3
1 Fig. 3. Far-UV in low-velocity
Fig. 2. Far-UV spectrum of Ar’+ ions following capture in low-velocity (10 keV/q) single collisions drogen and helium targets.
electron with hy-
spectra of Ar s+ ions following electron capture (10 keV/q) single collisions with hydrogen and helium targets.
“signature” lines of the Ar VIII spectrum at 700/714 A (corresponding to the yellow D-lines in neutral sodium) are present in figs. 3 and 5, but not in fig. 2 which “cuts off” completely all lines from charge states 7 and above. Another significant difference in the excitation by the two methods is seen in the spectra of figs. 4 and 5 which show an enlarged view of the beam-foil spectrum in the region 450 A. Of particular interest here are the lines from the core-excited metastable levels such as 3s3d4F,0/, (e.g. 471.4 A). Such lines, also recently seen by JupCn et al. [lo], do not show up on the charge-exchange spectra, indicating that the corresponding upper states do not play a role in such collisions. In summary, beam-foil spectra provide spectra lines whose ionic origin can only be approximately ascertained by varying the incident ion energy, while low-energy charge-exchange experiments provide a “cut-off” for charge states above a given one. Of course, the latter experiments also provide information about the collision process itself by studying the relative intensities of lines and relating them to initial excited-state populaI. ATOMIC/MOLECULAR
PHYSICS
312
E.J. Knystautas
Ar++_, 10 MeV
c
et al. / Far- UV spectroscopy of highly ionised argon
“:“’
A:++
c
gi
600
700
Fig. 5. Core-excited
states of Ar VIII in the beam-foil trum of argon at 10 MeV.
spec-
VIII prior to publication. We also thank Joanne Miller and Serge Knystautas for their assistance in the data reduction. The support of NATO (via a research grant to E.J.K. and S.B.) facilitated collaborative aspects of this work.
460
480
50
References
x(H)
Fig. 4. Beam-foil spectra of argon at 6 and 10 MeV showing emission lines from core-excited quartet states of Ar VIII. tions via various cascade pathways [3]. On the other hand, beam-foil spectroscopy strongly populates multiply excited states which are not present in other sources. In addition, it provides lifetime measurements which also assist in line identification. Thus both techniques complement each other, and combined with modern calculational methods such as MCDF, are of considerable importance in unraveling the increasingly complex ionic spectra which are coming under study today.
,Acknowledgements
We wish to thank Christer JupCn and the Lund spectroscopy group for kindly communicating to us the results of their work on the core-excited states of Ar
G.C. Crume, C.E. Bush, J.L. 111 R.C. Isler, L.E. Murray, Dunlap, P.H. Edmonds, S. Kasai, E.A. Lazarus, M. Murakami, G.H. Neilson, V.K. Pare, S.D. Scott, C.E. Thomas and A.J. Wootton, Nucl. Fus. 23 (1983) 1017. PI U. Feldman, J.F. Seely and G.A. Doschek, J. Phys. (Paris) 47 C6 (1986) 187. [31 M. Comille, T. Ludac, D. Hitz, S. Bliman, G.A. Heckman and E.J. Knystautas, Phys. Rev. A (1991) in press. and M. Klapisch, Nucl. 141 J. Bauche, C. Bauche-Arnoult Instr. and Meth. B31 (1988) 139. and M. Druetta, Phys. Rev. A31 (1985) [51 E.J. Knystautas 2279. 161 R. Geller and B. Jacquot, Phys. Scripta T3 (1983) 63. and M. Jomphe, Phys. Rev. A23 (1981) I71 E.J. Knystautas 679. R. Drouin and M. Druetta, J. Phys. PI E.J. Knystautas, (Paris) 40 Cl (1979) 186. and H.F. Beyer, J. Phys. B14 [91 R. Mann, F. Folkmann (1981) 1161. R. Hutton, N. Reistad and M. [lOI C. JupCn, L. Engstrom, Westerlind, Phys. Scripta 42 (1990) 44.
and Methods
in Physics
Research
B56/57
309
(1991) 309-312
Far-UV spectroscopy of highly ionised argon using complementary techniques Emile J. Knystautas D&partement de physique,
and Norman
Schlader
UniuersitP LavaI, QuCbec, Canada GIK 7P4
Samuel L. Bliman S.Ph.At./DRFG,
CENG 85X, 38041 Grenoble, France
Far-UV spectroscopy Excitation by foil-excited
of highly ionised argon has been studied with particular attention paid to core-excited MeV beams is compared with single- and double- electron capture into excited states
metastable states. during low-energy
charge exchange of highly ionised argon ions in helium and hydrogen targets. Complementary techniques are used for a) line identification by the high charge-state selectivity of the charge-exchange process, b) lifetime measurements, c) considerations concerning different excitation and cascade processes involved, d) systematics along isoelectronic sequences, and e) model-potential and MCDF calculations. A number of new lines are reported.
1. Introduction
Spectroscopy of highly ionised atoms has long been used as a probe of astrophysical plasmas. The problems related to impurity transport and control in controlled thermonuclear fusion devices [l], as well as to the search for energy-level schemes suitable for X-ray lasers [2] have rekindled interest in this area. While few-electron systems are now quite well known (although even here, multiply excited states have only recently been studied), the more complex ions, such as those of the neon, sodium and magnesium isoelectronic sequences, remain incompletely determined. Where higher principal quantum numbers or core-excited states are involved information is even more scarce. Such information is vital if collisional excitation processes are to be used for studying fusion plasmas, X-ray laser schemes or low-energy electron capture by multicharged ions [3]. Line identification is greatly assisted by energy-level calculations, using parametric potentials, or multiconfigurational Dirac-Fock (MCDF) methods. However, such work is often a compromise between good accuracy for fewer levels and modest accuracy for a great number of levels, hence ambiguities remain when experimental results must be interpreted. Recent approaches even involve studying general features of transition arrays, rather than identifying many individual lines (see, for example, ref. [4], in which fig. 4 shows the case for Ar VI). In this paper we present previously unpublished far-UV spectra of highly ionised argon using two different kinds of emission spectroscopy, that of foil-excited 0168-583X/9i/$O3.50
0 1991 - Elsevier Science Publishers
MeV ion beams and that of low-velocity multicharged ions following electron capture in gases. We show how the use of such complementary techniques can help in the unraveling of complex ion spectra.
2. Experimental conditions Descriptions of our experimental setups can be found in previous works [3,5]. Beam-foil spectroscopy was carried out at Universite Laval, where a modified 7.5 MV model CN Van de Graaff was used to produce Ar+, Ar2+, and Ar 3+ ion beams of 6, 12, and 18 MeV energy, respectively, which then impinged onto thin carbon foils. Photons from the spontaneously radiating beam particles were dispersed with a McPherson 2.2-m grazing-incidence monochromator and counted with a Galileo ChanneltronTM. Entrance and exit slit widths were 50 pm (100 pm for the 18 MeV spectrum), and the grating (600 l/mm with a 2O4’ blaze angle) was used at a nominal angle of incidence of 88O, with a telescopic slit. The low-energy electron capture experiments took place at CEN Grenoble using the MINIMAFIOS electron cyclotron resonance ion source [6] which produced substantial beams of Ar’+ and Arat ions at an energy of 10 keV/q. The ions impinged on a target gas (hydrogen or helium) at a pressure of 5 x 1O-5 mbar (as measured by a capacitance manometer), thus ensuring single-collision conditions. The radiation emitted by the ions following single- (and sometimes double-) electron capture into excited states was dispersed by a l-m
B.V. (North-Holland)
I. ATOMIC/MOLECULAR
PHYSICS
310
E.J. Knystauras el al. / Far- UV spectroscapy
GISMO grazing-incidence monochromator. With a 1200 I/mm grating at 85” incidence and 400 km slits, photons emitted at 25” to the forward direction were detected with a channel electron multiplier.
3. Results and discussion Fig. 1 shows the far-UV spectra obtained at incident-ion energies of 6, 12 and 18 MeV. By varying the energy, the charge-state distribution of the foil-excited beam particles changes in a known fashion [7] and by following the intensity variation of a given line over this range, one can deduce approximately from which charge state it arises [S].
of highly ion&d argon
The results of the low-energy charge-exchange experiments are displayed in figs. 2 and 3. These show far-WV spectra stemming from the collision of Ar’+ and A?’ ions respectively, with helium and hydrogen targets. The monochromator-detector system response has been previously measured over the entire wavelength range using the branching-ratio technique with various ions; the spectra shown are normalised in order to show relative intensities. (The apparent rise in background at the shortest wavelengths appears due to this process.) New assignments are based on MCDF calculations and systematic trends along isoelectronic sequences. The spectral purity is striking: fig. 2 contains almost entirely Ar VII, while fig. 3 has Ar VIII lines. Further-
18 MeV
Fig. 1. Far-UV spectrum of foil-excited argon beams at incident ion energies of 6,12 and 18 MeV.
E.J. Knystautas
et al. / Far-UV spectroscopy
more, in passing from hydrogen to helium targets, one can also see an additional high degree of selectivity due to the greatly differing ionisation potentials of the two target gases. In fact, a simple but reliable model [9] gives the following expression for the most probable principal quantum number into which capture takes place:
ofhighly ionised argon
Ar8+ -
311
He
i np’
21 (eV) rIq+ q-l 27.2 2&+1 I
’ 1
where I is the ionisation potential of the target in eV, and q is the incident ion charge. One can see from fig. 1 that, although higher charge states grow at the cost of lower ones as the incident ion energy increases, it is still difficult to assign with certainty a given line to a particular charge state. For example (see table 2 of ref. [S]), a line at 659 A, unknown at the time, was assigned to an ion of charge about +6. In fact, by its presence in fig. 2 and its absence from fig. 3, one can definitely exclude this line from the Ar VIII spectrum. Note also how the strong
)
I !I I
Ar7+
-
He
c
x (3
1 Fig. 3. Far-UV in low-velocity
Fig. 2. Far-UV spectrum of Ar’+ ions following capture in low-velocity (10 keV/q) single collisions drogen and helium targets.
electron with hy-
spectra of Ar s+ ions following electron capture (10 keV/q) single collisions with hydrogen and helium targets.
“signature” lines of the Ar VIII spectrum at 700/714 A (corresponding to the yellow D-lines in neutral sodium) are present in figs. 3 and 5, but not in fig. 2 which “cuts off” completely all lines from charge states 7 and above. Another significant difference in the excitation by the two methods is seen in the spectra of figs. 4 and 5 which show an enlarged view of the beam-foil spectrum in the region 450 A. Of particular interest here are the lines from the core-excited metastable levels such as 3s3d4F,0/, (e.g. 471.4 A). Such lines, also recently seen by JupCn et al. [lo], do not show up on the charge-exchange spectra, indicating that the corresponding upper states do not play a role in such collisions. In summary, beam-foil spectra provide spectra lines whose ionic origin can only be approximately ascertained by varying the incident ion energy, while low-energy charge-exchange experiments provide a “cut-off” for charge states above a given one. Of course, the latter experiments also provide information about the collision process itself by studying the relative intensities of lines and relating them to initial excited-state populaI. ATOMIC/MOLECULAR
PHYSICS
312
E.J. Knystautas
Ar++_, 10 MeV
c
et al. / Far- UV spectroscopy of highly ionised argon
“:“’
A:++
c
gi
600
700
Fig. 5. Core-excited
states of Ar VIII in the beam-foil trum of argon at 10 MeV.
spec-
VIII prior to publication. We also thank Joanne Miller and Serge Knystautas for their assistance in the data reduction. The support of NATO (via a research grant to E.J.K. and S.B.) facilitated collaborative aspects of this work.
460
480
50
References
x(H)
Fig. 4. Beam-foil spectra of argon at 6 and 10 MeV showing emission lines from core-excited quartet states of Ar VIII. tions via various cascade pathways [3]. On the other hand, beam-foil spectroscopy strongly populates multiply excited states which are not present in other sources. In addition, it provides lifetime measurements which also assist in line identification. Thus both techniques complement each other, and combined with modern calculational methods such as MCDF, are of considerable importance in unraveling the increasingly complex ionic spectra which are coming under study today.
,Acknowledgements
We wish to thank Christer JupCn and the Lund spectroscopy group for kindly communicating to us the results of their work on the core-excited states of Ar
G.C. Crume, C.E. Bush, J.L. 111 R.C. Isler, L.E. Murray, Dunlap, P.H. Edmonds, S. Kasai, E.A. Lazarus, M. Murakami, G.H. Neilson, V.K. Pare, S.D. Scott, C.E. Thomas and A.J. Wootton, Nucl. Fus. 23 (1983) 1017. PI U. Feldman, J.F. Seely and G.A. Doschek, J. Phys. (Paris) 47 C6 (1986) 187. [31 M. Comille, T. Ludac, D. Hitz, S. Bliman, G.A. Heckman and E.J. Knystautas, Phys. Rev. A (1991) in press. and M. Klapisch, Nucl. 141 J. Bauche, C. Bauche-Arnoult Instr. and Meth. B31 (1988) 139. and M. Druetta, Phys. Rev. A31 (1985) [51 E.J. Knystautas 2279. 161 R. Geller and B. Jacquot, Phys. Scripta T3 (1983) 63. and M. Jomphe, Phys. Rev. A23 (1981) I71 E.J. Knystautas 679. R. Drouin and M. Druetta, J. Phys. PI E.J. Knystautas, (Paris) 40 Cl (1979) 186. and H.F. Beyer, J. Phys. B14 [91 R. Mann, F. Folkmann (1981) 1161. R. Hutton, N. Reistad and M. [lOI C. JupCn, L. Engstrom, Westerlind, Phys. Scripta 42 (1990) 44.