ARPES (Angular Resolved Photoemission Spectroscopy) with a Gas Discharge Lamp

Transcription

1 ARPES (Angular Resolved Photoemission Spectroscopy) with a Gas Discharge Lamp

2 Outline - Experimental Setup - Individual Components - How to conduct an experiment - VUV lamps - Electron analyzer essentials - Some further remarks on perfomance

3 Experimental Setup sample manipulator and He-cryostat UV source UVS 300 UV source UVS 300 Monochromator TMM 304 with polarizer Main chamber Analyzer ETC, elliptical transfer capillary Detector Rotation by + 45 around ETC axis Turbo Pump Getter Pump Cryo-Pump

4 Experimental Setup UV source UVS 300 Monochromator TMM 304 with polarizer Analyzer Detector sample manipulator and He-cryostat Main chamber Transfer Rod Cryo-Pump (backside) Rotation by + 45 around ETC axis Getter Pump Turbo Pump (backside) Analyzer Voltage supply PC

5 View on load lock Transfer Rod

6 Analyzer UV source UVS 300 He-gas Bottle with pressure reducer Roughing pumps

7 UV source UVS 300 Pressure gauge Valve to main chamber Dose Valve Watercooling Turbopump

8 UV source UVS 300 HV plug

9 UV source UVS 300 duoplasmatron ) Principle: high power gas discharge in plasma microvalve Capillary to monochromator High purity 99,999%) He Tungsten filament = cathode (20.0 Amp.) Electrons accelerated to anode Impact ionization in plasma UV light emission by transition to ground state (next slide) Spectral lines from spontaneous emission

10 Duoplasmatron Two plasma clouds ( duo ) 1: He+ and He2+ cations move to cathode 2: electrons move to anode Both clouds are trapped by the external magnet on the axis like in a magnetic lens Enhance density of plasma Accelerated electrons make impact ionisation Relaxation to ground state High output of photons (water cooling necessary) I α /I β ~ 3% II α /I α ~ 30%

11 capillary to monochromator

12 Monochromator TMM 304 monochromator UV lamp (UVS 300) Vertical adjustment mirror Capillary (ETC) Blazed grating UHV chamber Horizontal adjustment UV radiation from UVS 300 on blazed grating (monochromatization) (blaze angle = specular reflection of first order (HeI) 2nd. reflection on toroidal mirror (displacement correction and focussing) Elliptically shaped Cu- transfer capillary (ETC) (Ø 1.6mm) focusses to 5 mm

13 Monochromator TMM 304 UV lamp (UVS 300) UHV chamber Horizontal adjustment Differential Pumped rotary Vertical adjustment rotation -45 to +45 around TFC capillary axis

14 Polarization: Two reflections under grazing incidence give 88% degree of polarization Whole polarizer setup can be rotated -45 to +45 around TFC capillary axis s-polarized, p-polarized VUV radiation source Grating to mirror in situ exchange: Grating: monochromatization to HeI α (=21.22 ev) Mirror: high flux

15 Focussing effect of elliptical transfer capillary (ETC) Quarz capillary Focussing ETC capillary Left: ordinary quarz capillary Right: elliptical focussing capillary (ETC), smaller linewidth, higher intensity

16 Main chamber V 2 A stainless steel recipient Ionivac vacuum gauge Turbo-, Ion getter- and cryopump, p= 5-7 exp -11 mbar 2 μ-metal (high permeability) shieldings inside (reducing extrernal magnetic fields by factor ~ axis manipulator for sample (x,y,z, rotation around cryostat axis and around sample normal) Liquid He flow cryostat, T =15 K with heater Connected to VUV source and by sample transfer to load lock (all under UHV conditions)

17 Magnetic shielding of main chamber For high resolution angular resolved photoemission spectroscopy, ARPES (energy resolution some mev) low magnetic fields (1% of earth magnetic field) in the ARPES chamber are essential. μ -metal: high magnetic permeability metal ( μ ~ ) B out Outer vacuum Chamber: V 2 A Stainless steel μ -metal ( μ ~ ) B in Two inner μ -metal liners

18 Ionivac vacuum gauge

19 Turbo-pump, p= 5-7 exp -10 mbar

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21 Ion getter- pump, p= 5-7 exp -11 mbar

22 cryopump, p= 5-7 exp -11 mbar

23 Manipulator

24 Y X He-cryostat heater Five degrees of freedom X, Y, Z, Θ, Φ Φ Θ Z Diode for Temperature measurement Y X

25 Sample transfer and load lock chamber

26 Load lock chamber V 2 A stainless steel cross Ionivac vacuum gauge Turbo- pump, p= 5-7 exp -10 mbar Sample storage for three sample holders Transfer rod with sample holder transfer system Au- evaporator Connected to main chamber, can be sealed off by valve

27 Tungsten filament with gold Au- evaporator evaporating

28 Sample on the sample holder In the load lock chamber Attached to the transfer mechanism

29 Electron detection (massive parallel detection) analyzer Entrance slit Exit slit Electron lens 2-dim. Channel Plate detector sample

30 Electron analyzer SES-100b Electron lens (8 elements): Focusses electrons on entrance slit, Angular or spatial resolution Retards to pass energy Entrance slit ( mm, curved or straight), resolution Hemisperical energy analyzer Radius of ideal trajectory 100mm Energy dispersion Electron detector 2 Multichannel plates (MCP) Phosphor screen CCD camera Lens system Slit carrousel

31 Multi channel plate MCP Electron current ist multiplied by V MCP =1800V by factor 10 6 and hits phosphor screen by V ph = 3800 V Light flashes are detected by CCD camera ->PC

32 Massive parallel detection in angular mode of the electron lens Energy is scanned by sweeping lensvoltages Analyser accepts for each energy angles of + 5 Matrix (E i,ϑ j ) i,j ~ 100 x 100 Camera Phosphor screen channelplate Radiation from UV source z-y plane: energy dispersion x-y plane: angular dispersion

33 View on channelplate

34 Slit carrousel Voltage feedthrough Voltage feedthrough View into analyzer from detector side (channelplate removed)

35 Sample preparation and transfer Grow sample (HTC) Characterize (Laue, suscept., EDAX, Glue on sample holder (2 component epoxy)

36 Sample preparation and transfer (HTc, layered crystal Insert to load lock chamber, attach scotch tape on sample Pump to UHV (overnight) Cleave in vacuum by Scotch tape Transfer to cryo manipulator Start measurement

37 Measurement: start He lamp Pressure o.k. (5x10-10 mbar)? He inlet has pressure? Turbo pump on He lamp is on? Turbo pump on He lamp is on? Open dose valve to some 10 3 mbar High voltage on (~200 V) Current filament 1 Amp Press ignition button (6KV pulse) Let it operate 10 min for cleaning Open valve to main chamber

38 Controllers anode current Lakeshore temperature controller Filament current Anode voltage Ignition button AML ionization Gauge controller Turbo pump controllers

39 Measurement: Transfer sample to manipulator Adjust sample to sample center by adjustment lasers Set analyzer voltages, kinetic Energy Optimize to maximum photoemission signal intensity in spatial mode Start measurements in angular mode

40 Other UV Sources: Gas discharge lamp LH 10/35 Arc discharge Radiation from Positive column of cathode fall (170 V for He) 200 mm Quarz capillary With 1.1 mm bore And two ground joints For two pumping stages Windowless! Ignition by 5KV In operation 650V He I (21.22eV) at 2-5 mbar Max. 5x10 12 Photons/s He II (40.81 ev) at 0.5mbar adjustment He-gas Inlet flange Fan Anode Discharge Capillary Cathode 1. Pump stage 2. Pump stage Bellows Beam Guidance capilary

41 Main lines of different gases HeIα, HeIβ, ( 1 P(1s 1 np 1 )-> 1 S(1s 2 )) Main broadening mechanism: Doppler broadening HeI: E 1-4 mev, depending on pressure From dirty gas

42 He lamp complete pumping system D,E valves KF He gas purifyer (Zeolith) DV dose valve PS differential pumping stage SF sorption pump TMP Turbo pump M manometer DSP rotary pump

43 Scienta Omicron VUV5000 He plasma, generated with the ECR (Electron Cyclotron Resonance) technique, HF-waveguide high intensity, narrow bandwidth extreme UV source. He I (21 ev and 23 ev) and He II (41eV). 1 mev bandwidth The flux density per unit wavelength interval comparable to undulator beam lines. The photon flux through a 2 mm aperture about 500 times higher than that from conventional discharge VUV sources. + monochromator

44 Electron analyzer essentials Side view Spherical analyzer Energy dispersion plane Energy resolution E Det energy resolution of half sphere E pass pass energy = energy on set path r o set path radius α incoming electron angle

45 Fringe field correction by Jost plates At entrance slit of hemisphere Equipotential Lines without correction Equipotential Lines with correction Effect of Jost Plates: undercorrection optimal correction overcorrection K. Jost, J. Phys. E: Sci. Instrum., 12, 1001 (1979)

46 Electron lens, raytracing Calculation of electron trajectory in DC electric field Lens: cylinder coordinates Only beams close to axis Equation of motion / 2 + )/ 4 r =0 (( φ = φ (z) electrostatic potential on axis from arbitrary geometry by finite difference method (program Simion ) => r(z) by Runge Kutta method An angular resolving mode for four-element electrostatic lenses T. Böker, A. Müller, J. Brügmann, J. Wichert, W. Frentrup, C. Janowitz, R. Manzke Journal of Electron Spectroscopy and Related Phenomena 87 (1998)

47 4-element electrostatic lens sideview

48 What resolution do we have to expect? The α term from Abbe Helmholtz (Conservation of phase space) α α 0 Mobj/Mimag = magnification Example Mobj/Mimag=: 1 Epass 0.5 ev Ekin 9eV α 0 =1 = rad (emission angle of electrons from sample, defined by grounded orifice at entrance of lens) s=1mm 2r o = 130mm E = ev = 6 mev

49 2r o = 400mm SCIENTA lens, imaging mode: Electrons from different positions on sample get magnified on channelplate: microscopy

50 SCIENTA lens, angular mode: Electrons at different angles from sample are imaged on different positions on the channelplate => E Kin (θ) => E B (k) (momentum resolved bandstructure)

51 Some further remarks on perfomance UV lamp operation: Use purified (6.0) grade helium No leaks in lamp or He line Use monochromator or thin (10-50 μm) transmission foils to suppress unwanted lines and/or ions from discharge (plasma) Small focus on sample Analyzer construction: raytracing Shield magnetic fields by μ-metal (chamber, analyzer) Compensate fringe fields Use Aluminum with graphite coating for all elements = equal work function Avoid charging (insulators, spikes, not grounded elements) Use ripple free power supply (<1meV)

52 Thank you

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