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Space Charging in 11 eV

Space Charging in 11 eV. Yu He SC Meeting Jul 25, 2013. Numbers and Conventions. UV Power vs 11eV Power. EF Position and Width vs ‘Counts’. Questions to be answered. The Laser. Ionization Chamber (IC-P). Gas Conversion Module (GCM). ARPES Manifold. Dispersion Chamber (DC).

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Space Charging in 11 eV

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  1. Space Charging in 11 eV Yu He SC Meeting Jul 25, 2013 • Numbers and Conventions • UV Power vs 11eV Power • EF Position and Width vs ‘Counts’ • Questions to be answered

  2. The Laser Ionization Chamber (IC-P) Gas Conversion Module (GCM) ARPES Manifold Dispersion Chamber (DC) Amplifier, SHG and FHG Gas Control Panel (GCP) Seed laser, 1 MHz, IR

  3. Numbers and Conventions IC-P current I0 (nA) IC-P vapor pressure (mV) Amplifier current (A) SHG X’tal temperature UV Power (×10mW) FHG X’tal temperature Laser diode current Idiode(A)

  4. Numbers and Conventions • UV power almost scales linearly with seed laser diode current between 4.5A and 7A – confirms the working range Andrew suggested in the manual • 11eV power (~ 11eV photon count) scales roughly with cubic(UVPower)

  5. On Gold – all data taken at T=8K PE=2 Counts = 55K 260K 670K 150K 970K 210K 390K 80K 110K 580K 230K 1.2M 300K 1.4M 1.7M 340K 1.9M 400K 970K 210K PE=5

  6. Numbers and Conventions • EF position drifts linearly in ‘electron count (near EF)’, but clearly picks up drifting rate ~ 400uW UV power • To get more count, PE=5 performs better in that charging issue develops slower than PE=2 (expected)

  7. Numbers and Conventions • EF broadening drifts linearly in ‘electron count (near EF)’, and also clearly picks up drifting rate ~ 400uW UV power • Resolution curves almost overlap as a function of UV power between PE=2 and PE=5 – charging issue taking place upstream from detector; • Also resolution converges to ~10meV taking to zero flux limit both in PE2 and PE5 case - something else is bottlenecking resolution other than space charging issue

  8. FeTeSe – blur band and space charging UV power 411mW 334mW 223mW 140mW

  9. FeTeSe – blur band and space charging • Strong evidence for space charging • Down to 150k counts/second at EF the effect is still visible • What’s wrong?

  10. The END

  11. Milestones Jul 10 Jul 16 Jul 12 Jul 07 Jul 20 Jul 11 Jul 15

  12. The ‘Gold’ Standard By proper beam alignment, two step puzzle solved Best Au resolution ~10meV

  13. UD92 Bi2212 – half wave plate rotation • Polarization changes intensity by 2 folds • Signal-background ratio modulated • To be motorized • To add quarter-wave plate

  14. UD92 Bi2212 – reaching zone boundary • Reaching zone boundary! • Need better stats and even longer time per scan

  15. Bi2Se3 – characterize polarization?

  16. Bi2Se3 – linear dichroism -0.1eV 0 eV Z.-H. Zhu et al, PRL 110, 216401 (2013) 0.1eV • P-polarization dominates • What exactly is the half-wave plate doing to the linear polarization of 11eV? 0.2eV

  17. Bi2Se3 – Bi2Se3 – circular dichroism 0.1eV 0.2eV Z.-H. Zhu et al, PRL 110, 216401 (2013)

  18. Bi2Se3 – Bi2Se3 – circular dichroism 0.1eV 0.2eV • Small circular dichroism exists • Linear dichroism dominates • Rotating half wave plate by θ doesn’t simply rotate linear polarization by 2θ Z.-H. Zhu et al, PRL 110, 216401 (2013)

  19. FeSe film – reaching zone boundary M Γ Γ 18K hv = 10.897eV 1024nm+256nm+256nm hv = 21.218eV Helium plasma discharge

  20. FeSe film – reaching zone boundary • Back bending clearly observed below Tc • Less background at low binding energy • Fully reaching zone boundary – powerful tool to study fine structures and subtle changes at M point

  21. Problems! Space Charging • Increase rep rate to 10MHz or more • The limiting factor: power or spot size? • Pass energy to get around? • How keen are we now perusing 100uW power? Polarization • Motorize • What does the wave plate exactly do upstream the GCM? • Quarter wave plate for circular dichroism Resolution • Go beyond 10meV how about 1meV?

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