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Foil-excited rydberg states of fast oxygen and sulfur ions
Nuclear Instruments and Methods 194 (1982) 341-343 North-Holland Publishing Company
341
FOIL-EXCITED RYDBERG STATES OF FAST OXYGEN AND SULFUR IONS J. R O T H E R M E L , H.-D. BETZ, F. B E L L
Sektion Physik der Umversitiit MSnchen, D-8046 Garching, Fed. Rep. Germany and V. Z A C E K
Phvsik Department. Technische Universitiit Miinchen. D-8046 Garching, Fed. Rep. Germany
Delayed emission of prompt Ly-a and Ly-fl radiation has been used to investigate the initial population of excited states of fast H-like oxygen and sulfur ions emerging from foil targets. We can distinguish between low- and high-angular momentum states and identify ions with orbital dimensions of t 1000 a 0, It is shown that electron pick-up known from single ion-atom collisions is not sufficient to explain foil-induced excitation of high-angular momentum states and additional mechanisms must be considered. Capture due to last layer interaction is commented on,
Formation of Rydberg states of fast projectile ions due to interaction with solid targets represents an interesting process which, though investigated by many authors, remains unclarified in its essential features. Two assumptions are usually considered: 1) Highly excited projectile states which cannot exist inside a dense solid are produced due to collisional interaction with the last layer of the traversed target. 2) Rydberg states are formed by electron capture similar to single ion-atom capture which, for large collision velocity v, is approximated by the OBKformalism [1]. In this contribution we discuss evidence that neither assumption is necessarily true and additional mechanisms will be required. Experimentally, we have chosen to study very fast ions. This allows us to observe one-electron ions so that multi-electron effects (multiple excitation, Auger processes) are avoided and deexcitation of Rydberg states proceeds only via exactly known radiative transitions. Furthermore, fast collisions allow easier theoretical treatment such as application of the OBK-formalism for single ion-atom encounters. 127 MeV sulfur and 60 MeV oxygen ions were produced at the Munich tandem Van de Graaff facility. Fully stripped ions were selected behind a post-stripper and directed onto a 5/~g/cm 2 carbon target which could be moved along the beam direction. X-ray detectors were employed to observe radiative K-shell transitions both directly at the foil and at distances from the foil, x, ranging up to 22.5 cm, i.e. over up to 3 × 105 2p-decay lengths of S. A slit collimation system ensured that the detector viewed 1 cm along the beam direction and was 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
shielded against background photons. Beam intensity was monitored by charge collection in a Faraday cup and the charge state distribution of the beams was measured by magnetic deflection. Measured quantities are the absolute number of photons, N(x), for specific prompt transitions emitted per hydrogen-like ion after delay time t over unit distance traveling length. Fig. 1 shows decay curves N~(x) and N~(x) observed for Ly-a and Ly-fl radiation of hydrogen-like sulfur ions, respectively, and for hydrogen-like oxygen as a function of distance between exciter foil and detector. The behaviour N ( x ) is understood to result from cascading processes [2-4]: highly excited states have long lifetimes and decay mostly via several intermediate levels until, after delay time t, the 2p or 3p state is reached so that prompt Ly-a or Ly-fl transitions are detectable. Consequently, decay curves N,, and N~ reflect in some integral way the initial distribution P(n, I) of excited states. Data analysis, thus, can be performed by finding those initial distributions P(n, l) which lead via cascading to the observed curves N(x). For the case of sulfur, a surprising result follows without any sophisticated analysis: inspection of radiative transition rates shows that low-/states, independent of n, give rise to Ly-a/Ly-fl intensity ratios between - 3 and 5, whereas high-/ states decay mostly via the 2p level and give almost no contribution to Ly-fl lines. Our experimental Ly-a/Ly-fl ratio lies between 30 and 50 and, thus, leads us to conclude: (l) Most of the delayed Ly-a intensity ( ~ 90%) arises from initial Rydberg states with large angular momentuna (l >>0). (2) Most of the delayed Ly-fl intensity ( ~ 90%) arises VII. PROJECTILE EXCITATIONS
J. Rothermel et al. / Foil-exuted Rydberg states
342 AX [crn] I
10
(
' 103 b '-5
100
I q
~ Ly-o~ S-15 ÷ Ly-p J ~ Ly-%13 0 - 7 +
~ []
~h
c]
o
oA
tO O tr", -4" i N
102 b b
~DA
C
c
I01 ~-
I X
[]
'°°r
% ++7
i I0-~ ..........,
1'
A|[ns] ....... I~0 ........
f
! IOOJ
Fig. 1. Absolute decay curves for Ly-a and Ly-J~ transitions in 127 MeV sulfur, and for Ly X-ray transitions in 60 MeV oxygen, as a function of time elapsed between excitation and photon detection. Since the two beam velocities are approximately equal, distances from exciter foil are given on top. For better comparison, observed transition intensities have been scaled by Z 4 according to the strength of dipole transition rates for different hydrogen-like ions.
from initial Rydberg states with small angular momentum (l~< 2). It becomes obvious that an OBK-type interaction can not explain (1) since it leads to population of low-/ states only. We note that our data are sensitive up to n ~ 120 for low l and n ~ 25 for / =- n - 1. These are the highest Rydberg states seen in beam-foil type ion excitation. A m o n g the mechanism which could perhaps explain the observed large production of high-/states in sulfur ions, we mention three partly well-known possibilities: (I) Coulomb capture of cusp electrons. It is known that a large fraction of ions are trailed by continuum electrons which emerge with velocities ve ~vion, and these electrons may be captured into high-/states [5].
(2) Radiative electron capture of continuum electrons trailing the ions. Here, capture into orbits with radius 6, is not limited to foil distances r ~ 6, and can take place at much larger separations. Capture occurs with high probability for ve ~ v ( O R E C C £ 1 [ e - - U 1 - 2 ) whereby high-/states are favoured [6]. (3) Final-state interaction via Stark quenching redistributes, for each n and with a certain probability, initial l o w - / t o high-/states. In a more quantitative analysis of the decay curves of sulfur we solved the cascading numerically [7,8] with some 50000 exact radiative transition rates for various trial functions PL and PH for the initial populations of l o w - / a n d high-/states, respectively. It is found that a variation P ~ n -3 is not at variance with the present data N ( x ) , and that the populations PL and PH, summed over l, are approximately equal for each n. Absolute reproduction of N~(x) is achieved with ~/ PL(n, /) = (30--+ 15)/n 3 for n >> 1, whereby the /dependence in PL was taken from the OBK-formalism [1]. When the present system is treated in terms of single i o n - a t o m collisions, capture cross sections amount of %(1 s ~ n) ~ 5 × 10 ~ 17/n3 cm 2 for n >> 1. A single carbon layer with 2 × 10 t5 a t o m s / c m 2 would then given rise to an excited state population of the order of O . l / n 3, which is more than 2 orders of magnitude smaller than our present result. It is unlikely, therefore, that highly exited states are formed by OBK-type electron capture acting on last-layer atoms. It may be concluded that many layers contribute to capture, but then the concept of single ion-atoms it not strictly applicable. We conclude that the production neither of low- nor high-/ Rydberg states is understood. The situation becomes even more intricate when the oxygen data is analyzed together with results from other authors [3,9]. Both low Ly-a/Ly-/~ ratios [3] and our evaluation of data from optical transitions [9] indicate that in case of oxygen the population PH is relatively small, in sharp contrast to the high-I population in sulfur. These findings, along with experimental studies of Rydberg states from single i o n - a t o m collisions will be communicated separately. We expect that the technique to measure delayed emission of prompt radiation will be of further usefulness for the investigation of Rydberg state production mechanisms. This work has been partially supported by B M F T (Bundesministerium fiar Forschung und Technologie).
References [1] K. Omidvar, Phys. Rev. A12 (1975) 911; F.T. Chan and J. Eichler, Phys. Rev. A20 (1979) 1841. [2] P. Richard, Phys. Lett. 45A (19" " 13.
J. Rothermel et al. / Foil-excited Rydberg states
[3] W.J. Braithwaite, D.L. Matthews and C.F. Moore, Phys. Rev. AII (1975) 465. [4] R.M. Schectman, Phys. Rev. AI2 (1975) 1717. [5] M. Day and M. Ebel, Phys. Rev. BI9 (1979) 3434. [6] A. Burgess, Mem. Astr. Soc. 69 (1964) I; E. Spindler, H.-D. Betz and F. Bell, J. Phys. B10 (1977) L561.
343
[7] L.J. Curtis, R.M. Schectman, J.L. Kohl, D.A. Chojnacki and D.R. Shoffstall, Nucl. Instr. and Meth. 90 (1970) 207. [8] H.-D. Betz, J. Rothermel and F. Bell, Nucl. Instr. and Meth. 170 (1980) 243. [9] G. Dehmelt, to be published.
VII. PROJECTILE EXCITATIONS
341
FOIL-EXCITED RYDBERG STATES OF FAST OXYGEN AND SULFUR IONS J. R O T H E R M E L , H.-D. BETZ, F. B E L L
Sektion Physik der Umversitiit MSnchen, D-8046 Garching, Fed. Rep. Germany and V. Z A C E K
Phvsik Department. Technische Universitiit Miinchen. D-8046 Garching, Fed. Rep. Germany
Delayed emission of prompt Ly-a and Ly-fl radiation has been used to investigate the initial population of excited states of fast H-like oxygen and sulfur ions emerging from foil targets. We can distinguish between low- and high-angular momentum states and identify ions with orbital dimensions of t 1000 a 0, It is shown that electron pick-up known from single ion-atom collisions is not sufficient to explain foil-induced excitation of high-angular momentum states and additional mechanisms must be considered. Capture due to last layer interaction is commented on,
Formation of Rydberg states of fast projectile ions due to interaction with solid targets represents an interesting process which, though investigated by many authors, remains unclarified in its essential features. Two assumptions are usually considered: 1) Highly excited projectile states which cannot exist inside a dense solid are produced due to collisional interaction with the last layer of the traversed target. 2) Rydberg states are formed by electron capture similar to single ion-atom capture which, for large collision velocity v, is approximated by the OBKformalism [1]. In this contribution we discuss evidence that neither assumption is necessarily true and additional mechanisms will be required. Experimentally, we have chosen to study very fast ions. This allows us to observe one-electron ions so that multi-electron effects (multiple excitation, Auger processes) are avoided and deexcitation of Rydberg states proceeds only via exactly known radiative transitions. Furthermore, fast collisions allow easier theoretical treatment such as application of the OBK-formalism for single ion-atom encounters. 127 MeV sulfur and 60 MeV oxygen ions were produced at the Munich tandem Van de Graaff facility. Fully stripped ions were selected behind a post-stripper and directed onto a 5/~g/cm 2 carbon target which could be moved along the beam direction. X-ray detectors were employed to observe radiative K-shell transitions both directly at the foil and at distances from the foil, x, ranging up to 22.5 cm, i.e. over up to 3 × 105 2p-decay lengths of S. A slit collimation system ensured that the detector viewed 1 cm along the beam direction and was 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
shielded against background photons. Beam intensity was monitored by charge collection in a Faraday cup and the charge state distribution of the beams was measured by magnetic deflection. Measured quantities are the absolute number of photons, N(x), for specific prompt transitions emitted per hydrogen-like ion after delay time t over unit distance traveling length. Fig. 1 shows decay curves N~(x) and N~(x) observed for Ly-a and Ly-fl radiation of hydrogen-like sulfur ions, respectively, and for hydrogen-like oxygen as a function of distance between exciter foil and detector. The behaviour N ( x ) is understood to result from cascading processes [2-4]: highly excited states have long lifetimes and decay mostly via several intermediate levels until, after delay time t, the 2p or 3p state is reached so that prompt Ly-a or Ly-fl transitions are detectable. Consequently, decay curves N,, and N~ reflect in some integral way the initial distribution P(n, I) of excited states. Data analysis, thus, can be performed by finding those initial distributions P(n, l) which lead via cascading to the observed curves N(x). For the case of sulfur, a surprising result follows without any sophisticated analysis: inspection of radiative transition rates shows that low-/states, independent of n, give rise to Ly-a/Ly-fl intensity ratios between - 3 and 5, whereas high-/ states decay mostly via the 2p level and give almost no contribution to Ly-fl lines. Our experimental Ly-a/Ly-fl ratio lies between 30 and 50 and, thus, leads us to conclude: (l) Most of the delayed Ly-a intensity ( ~ 90%) arises from initial Rydberg states with large angular momentuna (l >>0). (2) Most of the delayed Ly-fl intensity ( ~ 90%) arises VII. PROJECTILE EXCITATIONS
J. Rothermel et al. / Foil-exuted Rydberg states
342 AX [crn] I
10
(
' 103 b '-5
100
I q
~ Ly-o~ S-15 ÷ Ly-p J ~ Ly-%13 0 - 7 +
~ []
~h
c]
o
oA
tO O tr", -4" i N
102 b b
~DA
C
c
I01 ~-
I X
[]
'°°r
% ++7
i I0-~ ..........,
1'
A|[ns] ....... I~0 ........
f
! IOOJ
Fig. 1. Absolute decay curves for Ly-a and Ly-J~ transitions in 127 MeV sulfur, and for Ly X-ray transitions in 60 MeV oxygen, as a function of time elapsed between excitation and photon detection. Since the two beam velocities are approximately equal, distances from exciter foil are given on top. For better comparison, observed transition intensities have been scaled by Z 4 according to the strength of dipole transition rates for different hydrogen-like ions.
from initial Rydberg states with small angular momentum (l~< 2). It becomes obvious that an OBK-type interaction can not explain (1) since it leads to population of low-/ states only. We note that our data are sensitive up to n ~ 120 for low l and n ~ 25 for / =- n - 1. These are the highest Rydberg states seen in beam-foil type ion excitation. A m o n g the mechanism which could perhaps explain the observed large production of high-/states in sulfur ions, we mention three partly well-known possibilities: (I) Coulomb capture of cusp electrons. It is known that a large fraction of ions are trailed by continuum electrons which emerge with velocities ve ~vion, and these electrons may be captured into high-/states [5].
(2) Radiative electron capture of continuum electrons trailing the ions. Here, capture into orbits with radius 6, is not limited to foil distances r ~ 6, and can take place at much larger separations. Capture occurs with high probability for ve ~ v ( O R E C C £ 1 [ e - - U 1 - 2 ) whereby high-/states are favoured [6]. (3) Final-state interaction via Stark quenching redistributes, for each n and with a certain probability, initial l o w - / t o high-/states. In a more quantitative analysis of the decay curves of sulfur we solved the cascading numerically [7,8] with some 50000 exact radiative transition rates for various trial functions PL and PH for the initial populations of l o w - / a n d high-/states, respectively. It is found that a variation P ~ n -3 is not at variance with the present data N ( x ) , and that the populations PL and PH, summed over l, are approximately equal for each n. Absolute reproduction of N~(x) is achieved with ~/ PL(n, /) = (30--+ 15)/n 3 for n >> 1, whereby the /dependence in PL was taken from the OBK-formalism [1]. When the present system is treated in terms of single i o n - a t o m collisions, capture cross sections amount of %(1 s ~ n) ~ 5 × 10 ~ 17/n3 cm 2 for n >> 1. A single carbon layer with 2 × 10 t5 a t o m s / c m 2 would then given rise to an excited state population of the order of O . l / n 3, which is more than 2 orders of magnitude smaller than our present result. It is unlikely, therefore, that highly exited states are formed by OBK-type electron capture acting on last-layer atoms. It may be concluded that many layers contribute to capture, but then the concept of single ion-atoms it not strictly applicable. We conclude that the production neither of low- nor high-/ Rydberg states is understood. The situation becomes even more intricate when the oxygen data is analyzed together with results from other authors [3,9]. Both low Ly-a/Ly-/~ ratios [3] and our evaluation of data from optical transitions [9] indicate that in case of oxygen the population PH is relatively small, in sharp contrast to the high-I population in sulfur. These findings, along with experimental studies of Rydberg states from single i o n - a t o m collisions will be communicated separately. We expect that the technique to measure delayed emission of prompt radiation will be of further usefulness for the investigation of Rydberg state production mechanisms. This work has been partially supported by B M F T (Bundesministerium fiar Forschung und Technologie).
References [1] K. Omidvar, Phys. Rev. A12 (1975) 911; F.T. Chan and J. Eichler, Phys. Rev. A20 (1979) 1841. [2] P. Richard, Phys. Lett. 45A (19" " 13.
J. Rothermel et al. / Foil-excited Rydberg states
[3] W.J. Braithwaite, D.L. Matthews and C.F. Moore, Phys. Rev. AII (1975) 465. [4] R.M. Schectman, Phys. Rev. AI2 (1975) 1717. [5] M. Day and M. Ebel, Phys. Rev. BI9 (1979) 3434. [6] A. Burgess, Mem. Astr. Soc. 69 (1964) I; E. Spindler, H.-D. Betz and F. Bell, J. Phys. B10 (1977) L561.
343
[7] L.J. Curtis, R.M. Schectman, J.L. Kohl, D.A. Chojnacki and D.R. Shoffstall, Nucl. Instr. and Meth. 90 (1970) 207. [8] H.-D. Betz, J. Rothermel and F. Bell, Nucl. Instr. and Meth. 170 (1980) 243. [9] G. Dehmelt, to be published.
VII. PROJECTILE EXCITATIONS