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Charge exchange cross sections for hydrogen and deuterium ions incident on a Cs vapor target
Volume 54A, number 4
PHYSICS LETTERS
22 September 1975
CHARGE EXCHANGE CROSS SECTIONS FOR HYDROGEN AND DEUTERIUM IONS I N C I D E N T O N A Cs V A P O R T A R G E T *
F.W. MEYER and L.W. ANDERSON
Department of Physics, University of Wisconsin 53706, USA Received 18 July 1975 The total charge exchange cross section, o-to, has been measured for H+, D+, H+, and D+ incident on Cs in the energy range 1 to 30 keV. The cross sections are found to decrease with increasing energy. For a given velocity of the incident ion, the cross sections for the atomic, diatomic, and triatomic ions of hydrogen and deuterium are observed to be the same. Measurements of the equilibrium fractions when D~ is incident on Cs are also reported. There have been a number of experimental determinations of the total charge exchange cross section, o+0, for I-F and D + incident on Cs [ 1 - 4 ] . The cross section o+0 for D~ incident on Cs has also been measured [5]. In this paper we report new measurements of o+0 for H +, D+, I~2 and D~ incident on a Cs vapor target. The apparatus, procedure, and analysis used in our measurements have been described in connection with previous experiments [5,6]. For measurements of o+0 we have used a hot wire gauge in the target to determine the Cs density, and we have made sure the transmitted incident beam is not elastically scattered so that it misses the detector Faraday cups. The results of our measurements of the various cross sections are shown in fig. 1. Also shown in fig. 1 are the results from ref. [5] for o+0 when D~ is incident on Cs. The remarkable feature of these cross sections is that for a given velocity of the incident ion, the cross sections for the various reactions are the same. It is expected that o+0 for I-I+ and D+ incident on Cs are the same for the same incident ion velocity [1,4]. However an explanation is required to describe why the electron capture cross sections for atomic, diatomic, and triatomic ions incident on Cs should be identical functions of the velocity. When I-I+, D+, I-I~2,DE, or D~ is incident on Cs, o+0 is very large, being more than 10 -14 cm 2 at an incident ion velocity of 5 X 105 m/see. The electron capture occurs at a very large internuclear separation (R 12a0) in a nearly resonant collision, forming excited states of the neutral atom or molecule and leaving the * Research supported in part by ERDA Contract AT-11-Gen 7.
Cs+ ion in the ground state. Using the Demkov model which is appropriate under these conditions, Olson [7] calculates that in the high velocity region the charge transfer reactions behave like resonant processes, and o+0 is a function only of the velocity, v, and a parameter ~. The parameter ?~is proportional to [1}/2 + I~/2], where 11 is the ionization potential of the Cs target atom and 12 is the ionization potential of the excited atom or molecule that is produced in the reaction [8]. The H ° and D ° atoms and the H~, D~, and D~ molecules all have excited states whose ionization energy is within ½ eV of the 3.9 eV ionization energy of a Cs atom. The electron capture is into these states so that I 1 ~ 12 ~- 3.9 eV. Consequently ;k is nearly the same for all the reactions, and the cross sections depend onINCIDENT ION VELOCITY ( ~ m / s e c ) 2.5 0.5 1.0 1.5 ' 2.0 ÷
= H+Cs 1.0
• D**Cs a
+ Cs
•
H2+ Cs
o-c .
'o ,
• He++ Cs
0.1 I
i
t
,
~
I0 15 20 25 30 INCIDENT ION ENERGY (keV/NUCLEON)
Fig. 1. The total electron capture cross sections for H+, D+, I~2, D +, D~, and He+ incident on a Cs vapor target as a function of the energy per nucleon.
333
Volume 54A, number 4
PttYSICS LETTERS
ly on the velocity [9]. This feature of Olson's calculations agrees with our observations. In addition our measured values of o÷0 for H + on Cs agree well in the range 0.7 to 2.5 keV with a calculation o f o+0 b y Olson, Shipsey and Browne [10] using ab initio potential curves. Charge transfer cross sections for He +, Ne +, Ar +, Kr +, and Xe + incident on a Cs vapor target have been reported previously [6,11,12,13 ]. Only for He + + Cs has o+0 been measured in the same velocity range as for the atomic and molecular ions of hydrogen and deuterium [10]. Since there are excited states o f He ° for which 12 ~ 3.9 eV one would expect on the basis o f the argument in the previous paragraph that o+0 for He + + Cs would be the same as that for incident atomic and molecular ions o f hydrogen and deuterium in the high velocity limit. Fig. 1 shows o+0 for He + + Cs in addition to o+0 for the various atomic and molecular ions o f hydrogen incident on Cs. As can be seen a÷0 for He + + Cs is similar to o+0 for the various ions o f hydrogen a n d deuterium incident on Cs. As shown in fig. 1 o+0 for H + + Cs as a function o f energy has two regions with distinctly different slopes. The cross section falls more rapidly with increasing energy for energies less than 15 keV than for energies greater than 15 keV. This behavior o f o÷0 was observed previously b y II'in et al. [2] for I-I+ incident on various alkali vapor targets including Cs. II'in et al. suggested that in the low energy range pick up of the valence electron from the alkali was dominant and in the high energy range pick up o f a core electron was the dominant process [2]. There have been a number o f different measurements o f o+0 for I-I+ and D + incident on Cs. Our measurements o f o+0 lie somewhat below (~25%) those o f Ii'in et al. [2] in the energy range 10 to 30 keV and somewhat above (~25%) those of Schlachter et al. [1] in the energy range 1 to 20 keV. Our measurements of a+0 lie somewhat above ('~25%) the measurements of various other workers at energies below 5 keV [3,4]. We do not know why our cross section measurements differ from those o f others. The principal difference between our measurements and those o f other workers is that we use a h o t wire detector inside the target chamber as an absolute gauge for the Cs density whereas other have used the Cs vapor pressure as a function of the temperature to obtain an absolute calibation of their Cs gauge [ 1 - 4 ] . 334
22 September 1975
We have also measured the equilibrium fractions for + D 3 incident on a thick Cs target. The experiment on equilibrium fractions was carried out and analyzed in a manner similar to that used for our previous experi+ ments on the equilibrium fractions when D 2 is incident + on a Cs vapor target [5]. As the D 3 beam passes through + the target the D 3 molecular ions are broken up into fast atoms and atomic ions. Since the atoms and atomic ions do not recombine to form molecules the equilibrium fractions are expected to be identical to the equilibrium fractions when D + is incident on Cs with the same velocity [5]. It was experimentally confirmed that the equilibrium fractions for D~ incident on Cs are the same as those for D + incident on Cs [5]. We call the target thickness at which the D - fraction emerging from the target has reached 90% o f the equilibrium fraction n = . + For D 3 incident on Cs we find zr~ ~ 5 × 1014 atoms/ + cm 2 at energies from 1 to 20 keV. Since 7r°~ for+ D 2 incident on Cs is slightly higher than 7r~0 when D 3 is in+ + cident [5] we infer that D 3 does not break up into D 2 to any appreciable extent. We have crudely measured the D +2 yield when D~ is incident and find it to be small, which is consistent with this inference. Although we have not accurately measured the angular distribution + o f atoms and atomic ions when D 3 picks up an electron and breaks up, we have observed, using methods similar to those discussed in ref. [5], that the distribution is wider than the 1° angle subtended by the Faraday cups as seen from the target. [1] A.S. Schlachter, et al., Phys. Rev. 177 (1969) 184. [2] R.N. II'in et al., Zh. Tech. Fiz. 36 (1966) 1241 [Sov. Phys. Tech. Phys. 11 (1967) 921]. [3] G. Spiess et al., Phys. Rev. A6 (1972) 746. [4] W. Gruebler et al., Helv. Phys. Acta 43 (1970) 254. [51 F.W. Meyer and L.W. Anderson, Phys. Rev. A 11 (1975) 589. [6] F.W. Meyer and L.W. Anderson, Phys. Rev. A 9 (1974) 1909. [7] R.E. Olson, Phys. Rev. A 6 (1972) 1822. [8] R.E. Olson, F.T. Smith, and E. Bauer, Appl. Optics 10 (1971) 1848. [9] We wish to thank R.E. Olson and G. Spiess for pointing out to us why O+o should be the same for atomic, diatomic and triatomic molecular ions of hydrogen and deuterium incident on Cs. [ 10] R.E. Olson, private communication. [ 11] F.W. Meyer and L.W. Anderson, Physics Lett. 49 A (1974) 441. [12] F.W. Meyer and L.W. Anderson, Phys. Rev. A 11 (1975) 586. [13] J.R. Peterson and D.C. Lorents, Phys. Rev. 182 (1969) 152.
PHYSICS LETTERS
22 September 1975
CHARGE EXCHANGE CROSS SECTIONS FOR HYDROGEN AND DEUTERIUM IONS I N C I D E N T O N A Cs V A P O R T A R G E T *
F.W. MEYER and L.W. ANDERSON
Department of Physics, University of Wisconsin 53706, USA Received 18 July 1975 The total charge exchange cross section, o-to, has been measured for H+, D+, H+, and D+ incident on Cs in the energy range 1 to 30 keV. The cross sections are found to decrease with increasing energy. For a given velocity of the incident ion, the cross sections for the atomic, diatomic, and triatomic ions of hydrogen and deuterium are observed to be the same. Measurements of the equilibrium fractions when D~ is incident on Cs are also reported. There have been a number of experimental determinations of the total charge exchange cross section, o+0, for I-F and D + incident on Cs [ 1 - 4 ] . The cross section o+0 for D~ incident on Cs has also been measured [5]. In this paper we report new measurements of o+0 for H +, D+, I~2 and D~ incident on a Cs vapor target. The apparatus, procedure, and analysis used in our measurements have been described in connection with previous experiments [5,6]. For measurements of o+0 we have used a hot wire gauge in the target to determine the Cs density, and we have made sure the transmitted incident beam is not elastically scattered so that it misses the detector Faraday cups. The results of our measurements of the various cross sections are shown in fig. 1. Also shown in fig. 1 are the results from ref. [5] for o+0 when D~ is incident on Cs. The remarkable feature of these cross sections is that for a given velocity of the incident ion, the cross sections for the various reactions are the same. It is expected that o+0 for I-I+ and D+ incident on Cs are the same for the same incident ion velocity [1,4]. However an explanation is required to describe why the electron capture cross sections for atomic, diatomic, and triatomic ions incident on Cs should be identical functions of the velocity. When I-I+, D+, I-I~2,DE, or D~ is incident on Cs, o+0 is very large, being more than 10 -14 cm 2 at an incident ion velocity of 5 X 105 m/see. The electron capture occurs at a very large internuclear separation (R 12a0) in a nearly resonant collision, forming excited states of the neutral atom or molecule and leaving the * Research supported in part by ERDA Contract AT-11-Gen 7.
Cs+ ion in the ground state. Using the Demkov model which is appropriate under these conditions, Olson [7] calculates that in the high velocity region the charge transfer reactions behave like resonant processes, and o+0 is a function only of the velocity, v, and a parameter ~. The parameter ?~is proportional to [1}/2 + I~/2], where 11 is the ionization potential of the Cs target atom and 12 is the ionization potential of the excited atom or molecule that is produced in the reaction [8]. The H ° and D ° atoms and the H~, D~, and D~ molecules all have excited states whose ionization energy is within ½ eV of the 3.9 eV ionization energy of a Cs atom. The electron capture is into these states so that I 1 ~ 12 ~- 3.9 eV. Consequently ;k is nearly the same for all the reactions, and the cross sections depend onINCIDENT ION VELOCITY ( ~ m / s e c ) 2.5 0.5 1.0 1.5 ' 2.0 ÷
= H+Cs 1.0
• D**Cs a
+ Cs
•
H2+ Cs
o-c .
'o ,
• He++ Cs
0.1 I
i
t
,
~
I0 15 20 25 30 INCIDENT ION ENERGY (keV/NUCLEON)
Fig. 1. The total electron capture cross sections for H+, D+, I~2, D +, D~, and He+ incident on a Cs vapor target as a function of the energy per nucleon.
333
Volume 54A, number 4
PttYSICS LETTERS
ly on the velocity [9]. This feature of Olson's calculations agrees with our observations. In addition our measured values of o÷0 for H + on Cs agree well in the range 0.7 to 2.5 keV with a calculation o f o+0 b y Olson, Shipsey and Browne [10] using ab initio potential curves. Charge transfer cross sections for He +, Ne +, Ar +, Kr +, and Xe + incident on a Cs vapor target have been reported previously [6,11,12,13 ]. Only for He + + Cs has o+0 been measured in the same velocity range as for the atomic and molecular ions of hydrogen and deuterium [10]. Since there are excited states o f He ° for which 12 ~ 3.9 eV one would expect on the basis o f the argument in the previous paragraph that o+0 for He + + Cs would be the same as that for incident atomic and molecular ions o f hydrogen and deuterium in the high velocity limit. Fig. 1 shows o+0 for He + + Cs in addition to o+0 for the various atomic and molecular ions o f hydrogen incident on Cs. As can be seen a÷0 for He + + Cs is similar to o+0 for the various ions o f hydrogen a n d deuterium incident on Cs. As shown in fig. 1 o+0 for H + + Cs as a function o f energy has two regions with distinctly different slopes. The cross section falls more rapidly with increasing energy for energies less than 15 keV than for energies greater than 15 keV. This behavior o f o÷0 was observed previously b y II'in et al. [2] for I-I+ incident on various alkali vapor targets including Cs. II'in et al. suggested that in the low energy range pick up of the valence electron from the alkali was dominant and in the high energy range pick up o f a core electron was the dominant process [2]. There have been a number o f different measurements o f o+0 for I-I+ and D + incident on Cs. Our measurements o f o+0 lie somewhat below (~25%) those o f Ii'in et al. [2] in the energy range 10 to 30 keV and somewhat above (~25%) those of Schlachter et al. [1] in the energy range 1 to 20 keV. Our measurements of a+0 lie somewhat above ('~25%) the measurements of various other workers at energies below 5 keV [3,4]. We do not know why our cross section measurements differ from those o f others. The principal difference between our measurements and those o f other workers is that we use a h o t wire detector inside the target chamber as an absolute gauge for the Cs density whereas other have used the Cs vapor pressure as a function of the temperature to obtain an absolute calibation of their Cs gauge [ 1 - 4 ] . 334
22 September 1975
We have also measured the equilibrium fractions for + D 3 incident on a thick Cs target. The experiment on equilibrium fractions was carried out and analyzed in a manner similar to that used for our previous experi+ ments on the equilibrium fractions when D 2 is incident + on a Cs vapor target [5]. As the D 3 beam passes through + the target the D 3 molecular ions are broken up into fast atoms and atomic ions. Since the atoms and atomic ions do not recombine to form molecules the equilibrium fractions are expected to be identical to the equilibrium fractions when D + is incident on Cs with the same velocity [5]. It was experimentally confirmed that the equilibrium fractions for D~ incident on Cs are the same as those for D + incident on Cs [5]. We call the target thickness at which the D - fraction emerging from the target has reached 90% o f the equilibrium fraction n = . + For D 3 incident on Cs we find zr~ ~ 5 × 1014 atoms/ + cm 2 at energies from 1 to 20 keV. Since 7r°~ for+ D 2 incident on Cs is slightly higher than 7r~0 when D 3 is in+ + cident [5] we infer that D 3 does not break up into D 2 to any appreciable extent. We have crudely measured the D +2 yield when D~ is incident and find it to be small, which is consistent with this inference. Although we have not accurately measured the angular distribution + o f atoms and atomic ions when D 3 picks up an electron and breaks up, we have observed, using methods similar to those discussed in ref. [5], that the distribution is wider than the 1° angle subtended by the Faraday cups as seen from the target. [1] A.S. Schlachter, et al., Phys. Rev. 177 (1969) 184. [2] R.N. II'in et al., Zh. Tech. Fiz. 36 (1966) 1241 [Sov. Phys. Tech. Phys. 11 (1967) 921]. [3] G. Spiess et al., Phys. Rev. A6 (1972) 746. [4] W. Gruebler et al., Helv. Phys. Acta 43 (1970) 254. [51 F.W. Meyer and L.W. Anderson, Phys. Rev. A 11 (1975) 589. [6] F.W. Meyer and L.W. Anderson, Phys. Rev. A 9 (1974) 1909. [7] R.E. Olson, Phys. Rev. A 6 (1972) 1822. [8] R.E. Olson, F.T. Smith, and E. Bauer, Appl. Optics 10 (1971) 1848. [9] We wish to thank R.E. Olson and G. Spiess for pointing out to us why O+o should be the same for atomic, diatomic and triatomic molecular ions of hydrogen and deuterium incident on Cs. [ 10] R.E. Olson, private communication. [ 11] F.W. Meyer and L.W. Anderson, Physics Lett. 49 A (1974) 441. [12] F.W. Meyer and L.W. Anderson, Phys. Rev. A 11 (1975) 586. [13] J.R. Peterson and D.C. Lorents, Phys. Rev. 182 (1969) 152.