- Email: [email protected]
Towards a precision spectroscopy of very highly charged ions by employing dielectronic recombination
*H ckL3
Nuclear Instruments and Methods in Physics Research B 98 (1995) 158-161
__
NONiI
__
B
Beam Interactions with Materials&Atoms
I!!z
ELSEVIER
Towards a precision spectroscopy of very highly charged ions by employing dielectronic recombination * W. Spies a, 0. Uwira a, A. Mdler aT*, J. Linkemann a, L. Empacher a, A. Frank ‘, C. Kozhuharov b, P.H. Mokler b, F. Bosch b, 0. Klepper b, B. Franzke b, M. Steck b a Institutjiir Strahlenphysik,
Uniwrsitiit ’ Geselischaft fir Schwerionenforschung
Stuttgart. Stuttgart, German) (GSII. Darmstadt, Germany
Abstract The measurement of dielectronic recombination (DR) with high energy resolution provides a unique tool for precision spectroscopy of intermediate doubly excited states of very highly charged ions. One of our experimental goals at the experimental storage ring (ESR) of GSI is the determination of the 2s ,,?-2pt,? energy splitting 6 in high-Z Li-like ions. This requires electron-ion center-of-mass energies for DR measurements up to about 300 eV, which is not easily accessible with the cooling device of the ESR as long as it has to guarantee the cooling of the stored ion beam. In a new experiment with 150 MeV/u Au’~+ we were able to extend the accessible energy range to a maximum of about 300 eV. which requires a variation of the electron laboratory energy of up to 10 keV. New experimental data for recombination of U’“+ and Au’~+ ions were obtained. In particular, the group of (2p,,,61) resonances in e + AU”+ (1~~2s) collisions could be completely observed with good energy resolution. In addition, the first resonances of the (2p ,,2n1) Rydberg sequence with n = 20 to n = 26 were measured. By an extrapolation of the resonance energies to n = s, i.e. to the series limit, the energy F can be determined.
Atomic structure theories can be tested most stringently by studying simple atomic systems, i.e. ions and atoms with few electrons. For a correct description of these systems QED and, particularly in the case of heavy ions, also non-QED corrections, e.g. from nuclear size effects, have to be added to the relativistically correct solution of the Dirac equation. The sum of these corrections is called the Lamb shift. With the experimental storage ring ESR of GSI, several new kinds of Lamb-shift experiments in atomic physics using heavy ions have become feasible [1,2]. Among these, our previous measurements of dielectronic recombination [3] have demonstrated the potential for a new high-precision spectroscopy. In the present measurements the experimental procedures were similar to those of the previous experiment [3]. The ESR was fed from the heavy ion synchrotron @IS) with He-like AU”+ ions. The desired Li-like Au’~+ ions for the experiment were produced by electron-ion recombination in the cooler with simultaneous storage of both
* Corresponding author. Tel. +49 711 685 3875, fax +49 711 685 3866, e-mail: [email protected]. ’ The work of the Stuttgart team is supported by the university-collaboration program of GSI and by Bundesministerium ftir Forschung und Technologie (BMFT), Bonn. 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. Ail rights reserved SSDI 0168-583X(95)00096-8
the Li-like and the He-like ions. For our measurements the ESR cooler served three purposes simultaneously. It was used to breed the desired Au’~+ ions from Au”+ by electron-ion recombination, it kept the ion beam cold and stable in energy during the experiments and it served as an electron target providing defined energies E,, in the electron-ion center-of-mass frame. The present boundary conditions for the experiments are that the potential along the ESR cooler axis is determined by two separate drift tubes, and that one of the drift tubes can be set to potentials between 0 and f 5 kV (it thus defines the actual electron target) while the other is fixed at ground potential. Compared to our previous recombination experiments at the ESR we extended the accessible electron-ion centerof-mass energy range from 50 to 300 eV. This is not straightforward at the ESR, where a change of the cooler electron energy within +500 V from cooling conditions provides center-of-mass energies of typically less than 1 eV. The necessary intermittent cooling of the ion beam during a measurement requires fast switching of the cooler electron beam between the cooling energy and the energy needed for the experiment. The present achievements were made possible by the installation of a new fast power supply [4] for the drift tube that determines the target electron energy. Now, the drift-tube potential can be varied within 2 to 3 ms from - 5 kV to +5 kV with respect to
159
W. Spies et al. /Nucl. Instr. and Meth. in Phys. Rex B 98 (1995) 158-161
ground. In addition, we changed the voltage of the cooler electron gun by up to 5 kV with respect to the cooling potential. By the combination of potential variations both at the drift tube and the electron gun, energies in the center-of-mass system from 0 to 300 eV (depending on the ion energy) are accessible to the experiment. Different modes of applying voltages to the drift tubes were tested to provide best experimental conditions. The Au’~+ recombination products were collected by a position-sensitive multi-wire proportional counter behind the first dipole magnet down-beam from the cooler. The detector pulses triggered the event mode recording of numerous experimental parameters including the instantaneous ring current, energy of the electrons and time. Absolute recombination rates were determined. Statistical uncertainties of the experimental data are typically 3%, the total uncertainty amounts to about 20%. The present experimental energy calibration is estimated to be correct within 0.5 eV. Beside radiative recombination (RR) our prime point of interest was dielectronic recombination (DR) following the collision scheme Au’~+(~s?~s)
+ e + (A~~~‘(ls’2p~,:,~,~~~,))*
*
--) Au’~+ + photons.
(1)
In comparison with our first measurement the energy resolution could be improved in the new experiments with 150 MeV/u Li-like Au’~+ ions. All states of the 2p,,,61, manifold could be observed (see Fig. 1) including j = 9/2 and j = 11/2 states which were outside the energy range of the previous experiment (O-50 eV). In addition, several resonances of the 2p,,z nl series of intermediate states with (n = 20,23,24,. . . ) can be seen in Fig. 1. Their cross sections (and rate coefficients) are one to two orders of
0
25
50
energy
75
100
(eV)
Fig. 1. Comparison of convoluted theoretical recombination rates and experimental data for recombination of Au’~+ ions (see text). The experimental spectrum was normalized to a previous absolute measurement [3].
energy
(eV)
Fig. 2. Rate spectrum of the Zp,,,nI intermediate states in the energy range 60-110 eV. States with principal quantum numbers n = 23, 24, 25, and 26 can be distinguished. The solid line is the theory curve from Fig. 1.
magnitude smaller than those of the 2p,,z series (see also Fig. 2). A fully relativistic DR calculation by Zimmerer et al. is available for the 2p,,,6/, resonances [3,5]. It can be combined with a semi-relativistic calculation by Pindzola and Badnell for the 2p,,,n 1 resonances [3,6] to cover all DR contributions in the present experimental energy range. In an independent-processes approximation neglecting possible interference of DR and RR the DR resonances are added to a calculation of the RR contribution. Our approach for RR is based on the theory of Bethe and Salpeter [7] for completely stripped parent ions. This theory was extended to the Li-like Au’~+ ions by taking into account a correction based on Stobbe’s theory [8] and corrections due to the population of the innermost subshells and due to the cut-off limit for field ionization of Rydberg states beyond n = 120. The independent sum of all contributions was then convoluted with the electron energy distribution resulting from the different voltages on the two cooler drift tubes and the related potential distribution along the cooler axis [3,9]. The result of this calculation is shown as a solid line in Fig. 1 and apparently provides a good representation of the experimental results by theory. Note that due to the convolutions involved in the experiment both the energies and the rates displayed in the figure do not directly reflect the rate quantities usually displayed in connection with such experiments. In particular, the necessary modeling of theory to experiment does not conserve resonance peak areas. For more details see Ref. [3]. Although the cross sections for the 2pl,,nZ series of Be-like resonant states are relatively small and the statistics of the present experiment is not yet adequate, five Rydberg states with n = 20, 23, 24, 25, and 26 could be resolved and identified (see Fig. 2) and their energies determined. Representing these energies E, by a modified Rydberg formula E, = E - 13.6 eV
Z&/n’
3.1. COLLISIONS
(2)
WITH ELECTRONS
160
W. Spies et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (I 995) 158-161
.o
0.5
1.5
1.0
2.0
2.5
3.0
1000/n’ Extrapolation to the 2p,,, series limit by fitting two different types of Rydberg formulas to the measured 2p,,,nl resonance energies; dashed line: fit of the experimental data with an adjusted effective ion charge state Z,,, (see Eq. (2)); dotted line: fit of the experimental data with an adjusted quantum defect wn (see Eq. (3)). The uncertainties of the experimental energies for the 2p,,,nI Rydberg states with n = 20, 23, 24, 25, and 26 are
given by the size of the symbols.
with an adjusted effective charge Z,, = 76.48 of the ionic core or by E, = E - 13.6
eV .76’/(
n - CL,)’
with an adjusted quantum defect CL,,= 0.098, we have made an attempt to extrapolate from the measured level energies of the 2p,,, nl series to n = ~0, i.e. to the 2p,,, series limit (see Fig. 3). The extrapolations result in E = E(2%,* + 2p,,*) = (219.0 f 3) eV when Eq. (2) is used and in E(2s,,, --) 2p,,,) = (218.4 f 3) eV when Eq. (3) is employed. The experimental precision of the present determination energy splitting in Li-like of the E= E(2s,,, + 2p,,,) Au’~+ is not competitive by far with that accomplished by Schweppe et al. for U89t (with an uncertainty of 0.1 eV> [lo]. However, a new technique has been demonstrated by our experiment and our analysis, which promises to have the potential for a similar precision not only for Li-like ions and relatively small energies c but also for H-like ions and transition energies up to about 100 keV. The necessary improvements envisaged are the following: _ Improvement of the counting statistics for the 2p,,,nl resonances. This will allow the observation of Rydberg states with much larger principal quantum numbers and, hence, the extrapolation to n = 00 will be much less uncertain. Good statistics for each resonance peak will allow the peak energy betermination with an accuracy which can be a factor of at least 10 better than the energy resolution of the experiment (which is presently of the order of 2 eV at 100 eV). _ Improvement of the energy resolution of the DR experiments. Part of the present limitation of the energy resolution results from the fact that the electron cooler of
the storage ring has to serve several purposes at the same time. During a measurement with a center-of-mass energy E,, # 0 eV the ion beam is not efficiently cooled (even if we use the trick of keeping one drift tube of the cooler on cooling potential, which is not possible at energies E,, > 50 eV). A solution to this problem is the installation of a separate electron target in the ring for experiments, additional to the available electron cooler. Such an electron target can be made quite versatile and adjustable to the experimental requirements. In particular, the possibility of adiabatic expansion of the electron beam in a decreasing magnetic field, which has recently been demonstrated by Danared et al. [ll] and successfully employed also at the Heidelberg test storage ring (TSR) [12,13], can provide very low electron beam temperatures and, hence, energy spreads as low as 25 meV have become reality. Further reduction of beam temperatures appears to be within reach. - Improvement of the absolute energy calibration. The very high energy resolution which is technically possible for DR measurements and therefore also for the determination of energy splittings can only be fully exploited if the absolute laboratory energies of the ion beam and the electron beam are known to within relative deviations of 10m5. By the use of laser spectroscopy, e.g. in connection with laser induced electron-ion recombination, the ion (and thus indirectly also the electron) velocity under cooling conditions can be determined from the Doppler effect even with a better precision than required to exploit the energy resolution of DR resonances. With the cooling potential as a reference point the absolute center-of-mass energy is determined by a measurement of the voltage by which the electron energy is detuned from cooling conditions. With the present experiment we have shown a new route towards high precision spectroscopy of very highly charged ions by DR measurements. The improvements of experimental conditions necessary to reach the goal of a precision of the order of 0.1 eV appear to be technically possible. Our experiments with much lighter ions at the storage ring TSR in Heidelberg have provided energy resolutions at the 0.025 eV FWHM level [12,13]. For the very heavy few-electron ions discussed here, such a resolution is not easily obtained, however the necessary technical improvements are under development at GSI.
References [I] H.F. Beyer, D. Liesen, K.D. Finlayson,
F. Bosch, M. Jung, 0. Klepper, R. Moshammer, K. Becker& H. Eickhoff, B. Franzke, F. Nolden, P. SpBdtke, M. Steck, G. Menzel and R.D. Deslattes, Phys. Lett. A 184 (1994) 435. [2] T. Stiihlker, P.H. Mokler, K. Beckert, F. Bosch, H. Eickhoff, B. Franzke, M. Jung, T. Kandler, 0. Klepper, C. Kozhuharov, R. Moshammer, F. Nolden, H. Reich, P. Rymuza, P. Spldtke and M. Steck, Phys. Rev. Lett. 71 (1993) 2184.
W. Spies et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (199s) 158-161 [3] W. Spies, A. Miller, J. Linkemann, A. Frank, M. Wagner, C.
[4] [5] [6] [7] [8] [9]
Kozhuharov, B. Franzke, K. Beckert, F. Bosch, H. Eickhoff, M. Jung, 0. Klepper, W. Kiinig. P.H. Mokler, R. Moshammer, F. Nolden, U. Schaaf, P. SpIdtke, M. Steck, P. Zimmerer, N. Griin, W. Scheid, MS. Pindzola and N.R. Badnell, Phys. Rev. Lett. 69 (1992) 2768. H. Horneff, Patentschrift DE 4040164 C2, Deutsches Patentamt, 1992. P. Zimmerer, Ph.D. Thesis, Giessen D26, 1992. M.S. Pindzola and N.R. Badnell, private communication. H.A. Bethe and E.E. Salpeter, in: Handbuch der Physik, ed. S. Fliigge (Springer, Berlin. 1957). M. Stobbe, Ann. Phys. 7 (1930) 661. W. Spies, A. Miiller, J. Linkemann, A. Frank, M. Wagner, C. Kozhuharov, B. Franzke, K. Beckert, F. Bosch, H. Eickhoff, M. Jung, 0. Klepper, W. Kdnig, P.H. MokIer, R. Moshammer, F. Nolden, U. Schaaf, P. Spadtke, M. Steck, P. Zimmerer, N. Grim, W. Scheid, M.S. Pindzola and N.R. Badnell, AIP Conf. Proc., vol. 274, VIth Int. Conf. on the Physics of Highly Charged Ions, Manhattan, Kansas, USA, 28.9-
161
2.10.1992, Atomic Physics of Highly-Charged Ions, eds. P. Richard, M. Stiickli, C.L. Cocke and C.D. Lin (American Institute of Physics, New York, 1993) p. 541. [lo] J. Schweppe, A. Belkacem, L. Blumenfeld, N. Claytor, B. Feinberg, H. Gould, V.E. Kostroun, L. Levy, S. Misawa, J.R. Mowat and M.H. Prior, Phys. Rev. Lett. 66 (1991) 1434. [ll] H. Danared, G. Andler, L. Bagge, C.J. Herrlander, J. Hilke, J. Jeansson, A. Kallberg, A. Nilsson, A. Paal, K.-G. Rensfelt, U. Rosengird, J. Starker and M. af Ugglas, Phys. Rev. Lett. 72 (1994) 3775. [12] J. Linkemann, J. Kenntner, A. Miiller, A. Wolf, D. Habs, D. Schwalm, W. Spies, 0. Uwira, A. Frank, A. Liedtke, G. Hofmann, E. Salzborn, M.S. Pindzola and N.R. Badnell, these Proceedings (7th Int. Conf. on the Physics of Highly Charged Ions (HCI-941, Vienna, Austria, 1994) Nucl. Instr. and Meth. B 98 (1995) 154. [13] J. Kenntner, J. Linkemann, J. Broude, D. Habs, G. Hofmann, A. Miller, E. Salzborn, D. Schwalm and A. Wolf, Ref. [12], p. 142.
3.1. COLLISIONS
WITH ELECTRONS
Nuclear Instruments and Methods in Physics Research B 98 (1995) 158-161
__
NONiI
__
B
Beam Interactions with Materials&Atoms
I!!z
ELSEVIER
Towards a precision spectroscopy of very highly charged ions by employing dielectronic recombination * W. Spies a, 0. Uwira a, A. Mdler aT*, J. Linkemann a, L. Empacher a, A. Frank ‘, C. Kozhuharov b, P.H. Mokler b, F. Bosch b, 0. Klepper b, B. Franzke b, M. Steck b a Institutjiir Strahlenphysik,
Uniwrsitiit ’ Geselischaft fir Schwerionenforschung
Stuttgart. Stuttgart, German) (GSII. Darmstadt, Germany
Abstract The measurement of dielectronic recombination (DR) with high energy resolution provides a unique tool for precision spectroscopy of intermediate doubly excited states of very highly charged ions. One of our experimental goals at the experimental storage ring (ESR) of GSI is the determination of the 2s ,,?-2pt,? energy splitting 6 in high-Z Li-like ions. This requires electron-ion center-of-mass energies for DR measurements up to about 300 eV, which is not easily accessible with the cooling device of the ESR as long as it has to guarantee the cooling of the stored ion beam. In a new experiment with 150 MeV/u Au’~+ we were able to extend the accessible energy range to a maximum of about 300 eV. which requires a variation of the electron laboratory energy of up to 10 keV. New experimental data for recombination of U’“+ and Au’~+ ions were obtained. In particular, the group of (2p,,,61) resonances in e + AU”+ (1~~2s) collisions could be completely observed with good energy resolution. In addition, the first resonances of the (2p ,,2n1) Rydberg sequence with n = 20 to n = 26 were measured. By an extrapolation of the resonance energies to n = s, i.e. to the series limit, the energy F can be determined.
Atomic structure theories can be tested most stringently by studying simple atomic systems, i.e. ions and atoms with few electrons. For a correct description of these systems QED and, particularly in the case of heavy ions, also non-QED corrections, e.g. from nuclear size effects, have to be added to the relativistically correct solution of the Dirac equation. The sum of these corrections is called the Lamb shift. With the experimental storage ring ESR of GSI, several new kinds of Lamb-shift experiments in atomic physics using heavy ions have become feasible [1,2]. Among these, our previous measurements of dielectronic recombination [3] have demonstrated the potential for a new high-precision spectroscopy. In the present measurements the experimental procedures were similar to those of the previous experiment [3]. The ESR was fed from the heavy ion synchrotron @IS) with He-like AU”+ ions. The desired Li-like Au’~+ ions for the experiment were produced by electron-ion recombination in the cooler with simultaneous storage of both
* Corresponding author. Tel. +49 711 685 3875, fax +49 711 685 3866, e-mail: [email protected]. ’ The work of the Stuttgart team is supported by the university-collaboration program of GSI and by Bundesministerium ftir Forschung und Technologie (BMFT), Bonn. 0168-583X/95/$09.50 0 1995 Elsevier Science B.V. Ail rights reserved SSDI 0168-583X(95)00096-8
the Li-like and the He-like ions. For our measurements the ESR cooler served three purposes simultaneously. It was used to breed the desired Au’~+ ions from Au”+ by electron-ion recombination, it kept the ion beam cold and stable in energy during the experiments and it served as an electron target providing defined energies E,, in the electron-ion center-of-mass frame. The present boundary conditions for the experiments are that the potential along the ESR cooler axis is determined by two separate drift tubes, and that one of the drift tubes can be set to potentials between 0 and f 5 kV (it thus defines the actual electron target) while the other is fixed at ground potential. Compared to our previous recombination experiments at the ESR we extended the accessible electron-ion centerof-mass energy range from 50 to 300 eV. This is not straightforward at the ESR, where a change of the cooler electron energy within +500 V from cooling conditions provides center-of-mass energies of typically less than 1 eV. The necessary intermittent cooling of the ion beam during a measurement requires fast switching of the cooler electron beam between the cooling energy and the energy needed for the experiment. The present achievements were made possible by the installation of a new fast power supply [4] for the drift tube that determines the target electron energy. Now, the drift-tube potential can be varied within 2 to 3 ms from - 5 kV to +5 kV with respect to
159
W. Spies et al. /Nucl. Instr. and Meth. in Phys. Rex B 98 (1995) 158-161
ground. In addition, we changed the voltage of the cooler electron gun by up to 5 kV with respect to the cooling potential. By the combination of potential variations both at the drift tube and the electron gun, energies in the center-of-mass system from 0 to 300 eV (depending on the ion energy) are accessible to the experiment. Different modes of applying voltages to the drift tubes were tested to provide best experimental conditions. The Au’~+ recombination products were collected by a position-sensitive multi-wire proportional counter behind the first dipole magnet down-beam from the cooler. The detector pulses triggered the event mode recording of numerous experimental parameters including the instantaneous ring current, energy of the electrons and time. Absolute recombination rates were determined. Statistical uncertainties of the experimental data are typically 3%, the total uncertainty amounts to about 20%. The present experimental energy calibration is estimated to be correct within 0.5 eV. Beside radiative recombination (RR) our prime point of interest was dielectronic recombination (DR) following the collision scheme Au’~+(~s?~s)
+ e + (A~~~‘(ls’2p~,:,~,~~~,))*
*
--) Au’~+ + photons.
(1)
In comparison with our first measurement the energy resolution could be improved in the new experiments with 150 MeV/u Li-like Au’~+ ions. All states of the 2p,,,61, manifold could be observed (see Fig. 1) including j = 9/2 and j = 11/2 states which were outside the energy range of the previous experiment (O-50 eV). In addition, several resonances of the 2p,,z nl series of intermediate states with (n = 20,23,24,. . . ) can be seen in Fig. 1. Their cross sections (and rate coefficients) are one to two orders of
0
25
50
energy
75
100
(eV)
Fig. 1. Comparison of convoluted theoretical recombination rates and experimental data for recombination of Au’~+ ions (see text). The experimental spectrum was normalized to a previous absolute measurement [3].
energy
(eV)
Fig. 2. Rate spectrum of the Zp,,,nI intermediate states in the energy range 60-110 eV. States with principal quantum numbers n = 23, 24, 25, and 26 can be distinguished. The solid line is the theory curve from Fig. 1.
magnitude smaller than those of the 2p,,z series (see also Fig. 2). A fully relativistic DR calculation by Zimmerer et al. is available for the 2p,,,6/, resonances [3,5]. It can be combined with a semi-relativistic calculation by Pindzola and Badnell for the 2p,,,n 1 resonances [3,6] to cover all DR contributions in the present experimental energy range. In an independent-processes approximation neglecting possible interference of DR and RR the DR resonances are added to a calculation of the RR contribution. Our approach for RR is based on the theory of Bethe and Salpeter [7] for completely stripped parent ions. This theory was extended to the Li-like Au’~+ ions by taking into account a correction based on Stobbe’s theory [8] and corrections due to the population of the innermost subshells and due to the cut-off limit for field ionization of Rydberg states beyond n = 120. The independent sum of all contributions was then convoluted with the electron energy distribution resulting from the different voltages on the two cooler drift tubes and the related potential distribution along the cooler axis [3,9]. The result of this calculation is shown as a solid line in Fig. 1 and apparently provides a good representation of the experimental results by theory. Note that due to the convolutions involved in the experiment both the energies and the rates displayed in the figure do not directly reflect the rate quantities usually displayed in connection with such experiments. In particular, the necessary modeling of theory to experiment does not conserve resonance peak areas. For more details see Ref. [3]. Although the cross sections for the 2pl,,nZ series of Be-like resonant states are relatively small and the statistics of the present experiment is not yet adequate, five Rydberg states with n = 20, 23, 24, 25, and 26 could be resolved and identified (see Fig. 2) and their energies determined. Representing these energies E, by a modified Rydberg formula E, = E - 13.6 eV
Z&/n’
3.1. COLLISIONS
(2)
WITH ELECTRONS
160
W. Spies et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (I 995) 158-161
.o
0.5
1.5
1.0
2.0
2.5
3.0
1000/n’ Extrapolation to the 2p,,, series limit by fitting two different types of Rydberg formulas to the measured 2p,,,nl resonance energies; dashed line: fit of the experimental data with an adjusted effective ion charge state Z,,, (see Eq. (2)); dotted line: fit of the experimental data with an adjusted quantum defect wn (see Eq. (3)). The uncertainties of the experimental energies for the 2p,,,nI Rydberg states with n = 20, 23, 24, 25, and 26 are
given by the size of the symbols.
with an adjusted effective charge Z,, = 76.48 of the ionic core or by E, = E - 13.6
eV .76’/(
n - CL,)’
with an adjusted quantum defect CL,,= 0.098, we have made an attempt to extrapolate from the measured level energies of the 2p,,, nl series to n = ~0, i.e. to the 2p,,, series limit (see Fig. 3). The extrapolations result in E = E(2%,* + 2p,,*) = (219.0 f 3) eV when Eq. (2) is used and in E(2s,,, --) 2p,,,) = (218.4 f 3) eV when Eq. (3) is employed. The experimental precision of the present determination energy splitting in Li-like of the E= E(2s,,, + 2p,,,) Au’~+ is not competitive by far with that accomplished by Schweppe et al. for U89t (with an uncertainty of 0.1 eV> [lo]. However, a new technique has been demonstrated by our experiment and our analysis, which promises to have the potential for a similar precision not only for Li-like ions and relatively small energies c but also for H-like ions and transition energies up to about 100 keV. The necessary improvements envisaged are the following: _ Improvement of the counting statistics for the 2p,,,nl resonances. This will allow the observation of Rydberg states with much larger principal quantum numbers and, hence, the extrapolation to n = 00 will be much less uncertain. Good statistics for each resonance peak will allow the peak energy betermination with an accuracy which can be a factor of at least 10 better than the energy resolution of the experiment (which is presently of the order of 2 eV at 100 eV). _ Improvement of the energy resolution of the DR experiments. Part of the present limitation of the energy resolution results from the fact that the electron cooler of
the storage ring has to serve several purposes at the same time. During a measurement with a center-of-mass energy E,, # 0 eV the ion beam is not efficiently cooled (even if we use the trick of keeping one drift tube of the cooler on cooling potential, which is not possible at energies E,, > 50 eV). A solution to this problem is the installation of a separate electron target in the ring for experiments, additional to the available electron cooler. Such an electron target can be made quite versatile and adjustable to the experimental requirements. In particular, the possibility of adiabatic expansion of the electron beam in a decreasing magnetic field, which has recently been demonstrated by Danared et al. [ll] and successfully employed also at the Heidelberg test storage ring (TSR) [12,13], can provide very low electron beam temperatures and, hence, energy spreads as low as 25 meV have become reality. Further reduction of beam temperatures appears to be within reach. - Improvement of the absolute energy calibration. The very high energy resolution which is technically possible for DR measurements and therefore also for the determination of energy splittings can only be fully exploited if the absolute laboratory energies of the ion beam and the electron beam are known to within relative deviations of 10m5. By the use of laser spectroscopy, e.g. in connection with laser induced electron-ion recombination, the ion (and thus indirectly also the electron) velocity under cooling conditions can be determined from the Doppler effect even with a better precision than required to exploit the energy resolution of DR resonances. With the cooling potential as a reference point the absolute center-of-mass energy is determined by a measurement of the voltage by which the electron energy is detuned from cooling conditions. With the present experiment we have shown a new route towards high precision spectroscopy of very highly charged ions by DR measurements. The improvements of experimental conditions necessary to reach the goal of a precision of the order of 0.1 eV appear to be technically possible. Our experiments with much lighter ions at the storage ring TSR in Heidelberg have provided energy resolutions at the 0.025 eV FWHM level [12,13]. For the very heavy few-electron ions discussed here, such a resolution is not easily obtained, however the necessary technical improvements are under development at GSI.
References [I] H.F. Beyer, D. Liesen, K.D. Finlayson,
F. Bosch, M. Jung, 0. Klepper, R. Moshammer, K. Becker& H. Eickhoff, B. Franzke, F. Nolden, P. SpBdtke, M. Steck, G. Menzel and R.D. Deslattes, Phys. Lett. A 184 (1994) 435. [2] T. Stiihlker, P.H. Mokler, K. Beckert, F. Bosch, H. Eickhoff, B. Franzke, M. Jung, T. Kandler, 0. Klepper, C. Kozhuharov, R. Moshammer, F. Nolden, H. Reich, P. Rymuza, P. Spldtke and M. Steck, Phys. Rev. Lett. 71 (1993) 2184.
W. Spies et al. / Nucl. Instr. and Meth. in Phys. Res. B 98 (199s) 158-161 [3] W. Spies, A. Miller, J. Linkemann, A. Frank, M. Wagner, C.
[4] [5] [6] [7] [8] [9]
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3.1. COLLISIONS
WITH ELECTRONS