CESIUM ELECTRON IMPACT CROSS SECTIONS

1. CESIUM ELECTRON IMPACT CROSS SECTIONS Fig. 1Cs energy diagram 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 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 ELECTRON - MOTSPECTROSCOPY ( TRAP-LOSS TECHNIQUE) 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 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. e = Ejection rate due to electron collisionse = Electron scattering cross sectionJe = Electron current densitye = Electron charge COOLING and TRAPPING 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-vacuum 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. 4electron gun Fig. 7electron – beam probes Fig. 5electron beam simulation Fig. 6Typical beam profiles 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. Fig. 21D MOT for asimple two level atom Fig. 3B - field contours TIMING SCHEMES AND TRAP FLUORESCENCE MEAUSREMENTS CROSS SECTION RESULTS 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. 9Typical scan and measured loses Fig. 8Beam pulsing arrangement for 62S total  measurements Fig. 10Beam pulsing arrangement for ionisation measurements Fig. 11Typical scan and measured loses The measured TICS out of the Cs 6 2P3/2 state compared to SICS from CCC, RMPS and Born calculations The ratios of the 2P / 2S ionization cross sections for Cesium and Rubidium ExCITED STATE POPULATION In the ionisation experiment, the atoms in the trap are in a mixture of 62S1/2 and 62P3/2 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 62P3/2 state cesium, with the remaining 74% in the 62S1/2 ground state. = 32.7686 MHz I = Laser intensity, Is = Saturation Intensity ACKNOWLEDGEMENTS SUMMARY AND CONCLUSIONS • Abroad 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-monitoringtechniques. • The trap contained a known mixture of cesium atoms of in the 62S1/2 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. 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) www.uwindsor.ca/physics/atomic