CESIUM ELECTRON IMPACT CROSS SECTIONS 1 Department of Physics, University of Windsor, Ontario N9B...
1
CESIUM ELECTRON IMPACT CROSS SECTIONS 1 Department of Physics, University of Windsor, Ontario N9B 3P4, Canada 2 The ARC Centre for Antimatter-Matter Studies, Murdoch University, Perth 6150, Australia 3 Department of Physics, Drake University, Des Moines, Iowa, 50311, USA Absolute electron impact cross sections are measured using a magnet-optical trap (MOT) by employing the trap-loss technique. This method monitors the fluorescence decays of the trapped atoms, with and without an electron beam present. The loss rate of atoms from the trap due to electron collisions, e , is related directly to the cross section, , and electron flux, J, through the trap. Hence measurements of e and J yield directly; knowledge of the absolute target density is not required. We have used this approach to measure absolute total cross sections for both the ground, 6 2 S 1/2 and first excited state, 6 2 P 3/2 of Cesium. By then altering the timing sequence for the M. Łukomski 1 , J.A. MacAskill 1 , S. Sutton 1 , W. Kedzierski 1 , T.J. Reddish 1 , J.W. McConkey 1 , P.L. Bartlett 2 , I. Bray 2 , A.T. Stelbovics 2 , P.L. Bartlett 2 , K. Bartschat 3 e = Ejection rate due to electron collisions e = Electron scattering cross section Je = Electron current density e = Electron charge e J e e e COOLING and TRAPPING Our conventional MOT creates optical molasses using IR laser light that is slightly red detuned (~19MHz) to the cesium 6 2 S 1/2 (F = 4) 6 2 P 3/2 (F = 5) hyperfine transition. Three pairs (incident and retroflected) of orthogonal lasers, ~17mm in diameter, are used so that within their overlap region the atoms are cooled in all directions. A second repumping laser is required to pump the electrons out of the 6 2 S 1/2 (F=3) dark ground state, so that the cooling/trapping laser can access these atoms. If the repumper is turned off the trap disappears in <500 ns, as all the atoms fall to the F = 3 ground state, and cannot partake in the cooling sequence. A spherical quadrupole magnetic field is used to provide a position-dependent force which, in conjunction with the circularly polarised trapping laser beams, causes the atoms to be pushed towards the trap centre. Relevant Zeeman sub-levels are shown in the simplified diagram for a two level atom moving in the direction of increasing z, away from the trap centre. The B field was produced by a pair of in-vacuum anti-Helmholtz coils, each of 40 turns, 3 cm in radius and separated by 4.5 cm, and whose centre coincides with that of the bichromatic orthogonal laser beams. The B field gradient is approximately 10 G/cm in the horizontal plane and 20 G/cm in the vertical plane. ELECTRON - MOT SPECTROSCOPY ( TRAP-LOSS TECHNIQUE) Fig. 2 1D MOT for a simple two level atom CROSS SECTION RESULTS Fig. 1 Cs energy diagram Total Cross Section for Electron Scattering from Cesium: Theoretical and Experimental Results (6 2 P3/2 excited state) Total Cross Section for Electron Scattering from Cesium: Theoretical and Experimental Results (6 2 S 1/2 ground state) Fig. 3 B - field contours ELECTRON BEAM PRODUCTION & PROFILING A 7-element electron gun with a BaO disc cathode is used to produce a near parallel ~10 mm diameter electron beam of uniform current density over the entire 7-400 eV energy range. Two orthogonal, thin wire probes are used to measure the spatial current distribution of the beam. These 0.010 inch diameter wire probes are micrometer- driven on linear motion feedthroughs and are arranged to both intersect the electron beam at right angles and pass through the trap centre. During trapping they are retracted from the laser paths. The e-beam energy was calibrated by detecting the threshold production of He + ions using a time-of-flight system. Fig. 4 electron gun Fig. 5 electron beam simulation Fig. 6 Typical beam profiles pulsed magnetic field and electron beam, together with the trapping and repumping lasers, we have been able to determine the loss of atoms from the trap due solely to ionization. ACKNOWLEDGEMENTS PUBLICATIONS [1] J. A. MacAskill, W Kedzierski, J W McConkey, J Domyslawska, and I Bray, J Elect Spect Rel Phen , 123, 173, (2002). [2] M. Łukomski, J. A. MacAskill, D P Seccombe, C McGrath, S Sutton, J Teeuwen, W Kedzierski, T J Reddish, J W McConkey, and W A van Wijngaarden, J. Phys. B. 38 3535 (2005) [3] M. Łukomski, S. Sutton, W. Kedzierski, T.J. Reddish, K. Bartschat, P.L. Bartlett, I. Bray, A.T. Stelbovics and J.W. McConkey, Phys Rev. A. (submitted 2006) Fig. 8 Beam pulsing arrangement for 62S total measurements Fig. 9 Typical scan and measured loses The ratios of the 2 P / 2 S ionization cross sections for Cesium and Rubidium The measured TICS out of the Cs 6 2 P3/2 state compared to SICS from CCC, RMPS and Born calculations TIMING SCHEMES AND TRAP FLUORESCENCE MEAUSREMENTS Fig. 11 Typical scan and measured loses Fig. 10 Beam pulsing arrangement for ionisation measurements www.uwindsor.ca/physics/atomic • A broad 7- 400eV incident electron energy range is covered in these experiments. Excellent agreement is found for total electron scattering cross sections from 6 2 S Cs with earlier experimental work obtained by very different methods. CCC and RMPS calculations appear to overestimate below ~75 eV. • A novel feature of our MOT method is in utilizing Doppler cooling of neutral atoms to determine total ionization cross sections (TICS) via fluorescence-monitoring techniques. • The trap contained a known mixture of cesium atoms of in the 6 2 S 1/2 ground and 6 2 P 3/2 excited states. From subsequent data analysis, the TICS out of excited Cs atoms was obtained. • We demonstrate that autoionisation, core and multiple ionization make a significant contributions to the ground and excited state TICS. We also identify a SUMMARY AND CONCLUSIONS Fig. 7 electron – beam probes ExCITED STATE POPULATION In the ionisation experiment, the atoms in the trap are in a mixture of 6 2 S 1/2 and 6 2 P 3/2 states, as both trapping and repumping lasers are present in the electron-atom interaction. The excited state fraction, e , can be estimated using the given two level atom approximation. We used a Pockels cell to rapidly rotate the polarization of the trapping laser beam to control the intensity while maintaining a constant number of atoms in the trap and a constant detuning ( = 19 MHz). From this method we estimate the trap to contain 26(±1)% of excited 6 2 P 3/2 state cesium, with the remaining 74% in the 6 2 S 1/2 ground state. 2 2 / 4 / 1 2 / s s e I I I I f = 32.7686 MHz I = Laser intensity, I s = Saturation Intensity
-
Upload
augusta-cole -
Category
Documents
-
view
213 -
download
1
Transcript of CESIUM ELECTRON IMPACT CROSS SECTIONS 1 Department of Physics, University of Windsor, Ontario N9B...
CESIUM ELECTRON IMPACT CROSS SECTIONS
1 Department of Physics, University of Windsor, Ontario N9B 3P4, Canada 2 The ARC Centre for Antimatter-Matter Studies, Murdoch University, Perth 6150, Australia
3 Department of Physics, Drake University, Des Moines, Iowa, 50311, USA
Absolute electron impact cross sections are measured using a magnet-optical trap (MOT) by employing the trap-loss technique. This method monitors the fluorescence decays of the trapped atoms, with and without an electron beam present. The loss rate of atoms from the trap due to electron collisions, e, is related directly to the cross section, , and electron flux, J, through the trap. Hence measurements of e and J yield directly; knowledge of the absolute target density is not required. We have used this approach to measure absolute total cross sections for both the ground, 62S1/2 and first excited state, 62P3/2 of Cesium. By then altering the timing sequence for the
Absolute electron impact cross sections are measured using a magnet-optical trap (MOT) by employing the trap-loss technique. This method monitors the fluorescence decays of the trapped atoms, with and without an electron beam present. The loss rate of atoms from the trap due to electron collisions, e, is related directly to the cross section, , and electron flux, J, through the trap. Hence measurements of e and J yield directly; knowledge of the absolute target density is not required. We have used this approach to measure absolute total cross sections for both the ground, 62S1/2 and first excited state, 62P3/2 of Cesium. By then altering the timing sequence for the
M. Łukomski1, J.A. MacAskill1, S. Sutton1, W. Kedzierski1, T.J. Reddish1, J.W. McConkey1, P.L. Bartlett2, I. Bray2, A.T. Stelbovics2, P.L. Bartlett2, K. Bartschat3
e = Ejection rate due to electron collisionse = Electron scattering cross sectionJe = Electron current densitye = Electron charge
e = Ejection rate due to electron collisionse = Electron scattering cross sectionJe = Electron current densitye = Electron charge
e
J eee
COOLING and TRAPPINGCOOLING and TRAPPINGOur conventional MOT creates optical molasses using IR laser light that is slightly red detuned (~19MHz) to the cesium 62S1/2(F = 4) 62P3/2(F = 5) hyperfine transition. Three pairs (incident and retroflected) of orthogonal lasers, ~17mm in diameter, are used so that within their overlap region the atoms are cooled in all directions. A second repumping laser is required to pump the electrons out of the 62S1/2(F=3) dark ground state, so that the cooling/trapping laser can access these atoms. If the repumper is turned off the trap disappears in <500 ns, as all the atoms fall to the F = 3 ground state, and cannot partake in the cooling sequence.
A spherical quadrupole magnetic field is used to provide a position-dependent force which, in conjunction with the circularly polarised trapping laser beams, causes the atoms to be pushed towards the trap centre. Relevant Zeeman sub-levels are shown in the simplified diagram for a two level atom moving in the direction of increasing z, away from the trap centre. The B field was produced by a pair of in-vacuum
Our conventional MOT creates optical molasses using IR laser light that is slightly red detuned (~19MHz) to the cesium 62S1/2(F = 4) 62P3/2(F = 5) hyperfine transition. Three pairs (incident and retroflected) of orthogonal lasers, ~17mm in diameter, are used so that within their overlap region the atoms are cooled in all directions. A second repumping laser is required to pump the electrons out of the 62S1/2(F=3) dark ground state, so that the cooling/trapping laser can access these atoms. If the repumper is turned off the trap disappears in <500 ns, as all the atoms fall to the F = 3 ground state, and cannot partake in the cooling sequence.
A spherical quadrupole magnetic field is used to provide a position-dependent force which, in conjunction with the circularly polarised trapping laser beams, causes the atoms to be pushed towards the trap centre. Relevant Zeeman sub-levels are shown in the simplified diagram for a two level atom moving in the direction of increasing z, away from the trap centre. The B field was produced by a pair of in-vacuumanti-Helmholtz coils, each of 40 turns, 3 cm in radius and separated by 4.5 cm, and whose centre coincides with that of the bichromatic orthogonal laser beams. The B field gradient is approximately 10 G/cm in the horizontal plane and 20 G/cm in the vertical plane.
anti-Helmholtz coils, each of 40 turns, 3 cm in radius and separated by 4.5 cm, and whose centre coincides with that of the bichromatic orthogonal laser beams. The B field gradient is approximately 10 G/cm in the horizontal plane and 20 G/cm in the vertical plane.
ELECTRON - MOT SPECTROSCOPY ( TRAP-LOSS TECHNIQUE)ELECTRON - MOT SPECTROSCOPY ( TRAP-LOSS TECHNIQUE)
Fig. 21D MOT for asimple two level atom
CROSS SECTION RESULTSCROSS SECTION RESULTS
Fig. 1Cs energy diagram
Total Cross Section for Electron Scattering from Cesium:
Theoretical and Experimental Results (62P3/2 excited state)
Total Cross Section for Electron Scattering from Cesium:
Theoretical and Experimental Results (62S1/2 ground state)
Fig. 3B - field contours
ELECTRON BEAM PRODUCTION & PROFILINGELECTRON BEAM PRODUCTION & PROFILINGA 7-element electron gun with a BaO disc cathode is used to produce a near parallel ~10 mm diameter electron beam of uniform current density over the entire 7-400 eV energy range. Two orthogonal, thin wire probes are used to measure the spatial current distribution of the beam. These 0.010 inch diameter wire probes are micrometer-driven on linear motion feedthroughs and are arranged to both intersect the electron beam at right angles and pass through the trap centre. During trapping they are retracted from the laser paths.
The e-beam energy was calibrated by detecting the threshold production of He+ ions using a time-of-flight system.
A 7-element electron gun with a BaO disc cathode is used to produce a near parallel ~10 mm diameter electron beam of uniform current density over the entire 7-400 eV energy range. Two orthogonal, thin wire probes are used to measure the spatial current distribution of the beam. These 0.010 inch diameter wire probes are micrometer-driven on linear motion feedthroughs and are arranged to both intersect the electron beam at right angles and pass through the trap centre. During trapping they are retracted from the laser paths.
The e-beam energy was calibrated by detecting the threshold production of He+ ions using a time-of-flight system.
Fig. 4electron gun
Fig. 5electron beam simulation
Fig. 6Typical beam
profiles
pulsed magnetic field and electron beam, together with the trapping and repumping lasers, we have been able to determine the loss of atoms from the trap due solely to ionization.
ACKNOWLEDGEMENTS
PUBLICATIONS[1] J. A. MacAskill, W Kedzierski, J W McConkey, J Domyslawska, and I Bray, J Elect Spect Rel Phen , 123, 173, (2002).
[2] M. Łukomski, J. A. MacAskill, D P Seccombe, C McGrath, S Sutton, J Teeuwen, W Kedzierski, T J Reddish, J W McConkey, and W A van Wijngaarden, J. Phys. B. 38 3535 (2005)
[3] M. Łukomski, S. Sutton, W. Kedzierski, T.J. Reddish, K. Bartschat, P.L. Bartlett, I. Bray, A.T. Stelbovics and J.W. McConkey, Phys Rev. A. (submitted 2006)
Fig. 8Beam pulsing arrangement for
62S total measurements
Fig. 9Typical scan and measured loses
The ratios of the 2P / 2S ionization cross sections for Cesium and Rubidium
The measured TICS out of the Cs 6 2P3/2 state compared to SICS
from CCC, RMPS and Born calculations
TIMING SCHEMES AND TRAP FLUORESCENCE MEAUSREMENTS
Fig. 11Typical scan and measured loses
Fig. 10Beam pulsing arrangement for
ionisation measurements
www.uwindsor.ca/physics/atomicwww.uwindsor.ca/physics/atomic
• A broad 7- 400eV incident electron energy range is covered in these experiments. Excellent agreement is found for total electron scattering cross sections from 62S Cs with earlier experimental work obtained by very different methods. CCC and RMPS calculations appear to overestimate below ~75 eV.
• A novel feature of our MOT method is in utilizing Doppler cooling of neutral atoms to determine total ionization cross sections (TICS) via fluorescence-monitoring techniques.
• The trap contained a known mixture of cesium atoms of in the 6 2S1/2 ground and 62P3/2 excited states. From subsequent data analysis, the TICS out of excited Cs atoms was obtained.
• We demonstrate that autoionisation, core and multiple ionization make a significant contributions to the ground and excited state TICS. We also identify a significant and as yet unexplainable discrepancy between theory and experiment below ~ 12eV in the excited state ionisation cross section.
• A broad 7- 400eV incident electron energy range is covered in these experiments. Excellent agreement is found for total electron scattering cross sections from 62S Cs with earlier experimental work obtained by very different methods. CCC and RMPS calculations appear to overestimate below ~75 eV.
• A novel feature of our MOT method is in utilizing Doppler cooling of neutral atoms to determine total ionization cross sections (TICS) via fluorescence-monitoring techniques.
• The trap contained a known mixture of cesium atoms of in the 6 2S1/2 ground and 62P3/2 excited states. From subsequent data analysis, the TICS out of excited Cs atoms was obtained.
• We demonstrate that autoionisation, core and multiple ionization make a significant contributions to the ground and excited state TICS. We also identify a significant and as yet unexplainable discrepancy between theory and experiment below ~ 12eV in the excited state ionisation cross section.
SUMMARY AND CONCLUSIONS
Fig. 7electron – beam
probes
ExCITED STATE POPULATION
In the ionisation experiment, the atoms in the trap are in a mixture of 6 2S1/2 and 62P3/2 states, as both trapping and repumping lasers are present in the electron-atom interaction. The excited statefraction, e, can be estimated using the given two level atom approximation. We used a Pockels cell to rapidly rotate the polarization of the trapping laser beam to control the intensity while maintaining a constant number of atoms in the trap and a constant detuning ( = 19 MHz). From this method we estimate the trap to contain 26(±1)% of excited 62P3/2 state cesium, with the remaining 74% in the 62S1/2 ground state.
22 /4/1
2/
s
se
II
IIf
= 32.7686 MHzI = Laser intensity,
Is = Saturation Intensity
