SR Spectroscopieson Solids, Surfaces, and Interfaces

Giancarlo Panaccione

Lesson IV:From the source to the sample:

The Beamline

1) Handbook on Synchrotron Radiation, ed., E.-E. Koch, North-Holland Publishing Company; (North Holland, Amsterdam, 1983)

2) www.lightsource.org

3) The future of Synchrotron Radiation, Y. Petroff, Journ. El. Spec. and Rel. Phenomena,156-158 (2007) 10.

4) J. Als-Nielsen and D. McMorrow, Elements of Modern X-ray Physics(Wiley, New York, 2001).

5) X-Ray Data BookletSection 2.1 Kwang-Je Kim Characteristics of Synchrotron Radiation

Useful References

S

S

e-

Circular Polarization

hν

S

S

e- hν

Linear Polarization (Vertical)

Phase Shift 0 = linear horizPhase Shift ½ λu = linear vertPhase Shift ¼ λu= circular

APE beamline

Light source ---- Undulator

e- hν

Linear Polarization (Horizontal)

APPLE II-type undulatorsAPE beamline undulators: Characteristics

Switch Time < 10 secNO ELLIPTICAL POL

NO POL ANGLE

New devices: Figure 8 undulator

APE beamline undulators: Zig-zag configuration

Two undulators are placed in the same straight section.2 mrad zig-zag angle. Closed loop.Separation of beams 40 mm a 20 metersNo pinholes, high thermal load

24 meters from the undulatorFirst mirror

APE beamline

The Shapal® target Gap HE=40 mmGap LE=80 mm

Gap HE=40 mmGap LE=60 mm

Gap HE=40 mmGap LE=30 mm

(Horizontally polarized light)

Measured Expected• HE beam @ Gap 40 mm 15 mm 12 mm• LE beam @ Gap 80 mm 15 mm 12 mm• LE beam @ Gap 60 mm 22 mm 25 mm

APE beamline undulators: Reality

• Low Energy Polarization: Horizontal → Vertical• High Energy Polarization: Vertical

• 2 mRad angular divergence between beams → Distance between the beams = 34 mm

APE beamline undulators: Reality

(From Horizontal to Vertical polarized light)

120.63100.4880.7817125

1230.51930.43620.773660

ε1 (eV)B0 (T)ε1 (eV) B0 (T)ε1 (eV)B0 (T)

Vertical Polarization

Circular PolarizationHorizontalPolarization

Npperiod (mm)

MeasuredEU 6.0 HEEU 12.5 LE

130.60100.4880.7717125

1230.51940.42590.783660

ε1 (eV)B0 (T)ε1 (eV) B0 (T)ε1 (eV)B0 (T)

Vertical Polarization

Circular PolarizationHorizontalPolarization

Npperiod (mm)

DesignR. Walker, B. Diviacco(ELETTRA)

APE beamline undulators: Parameters at minimum gap

APE beamline EU 12.5: Quasi-periodic

Spectra computed from ideal (black) and measured (red) field

in linear and circular polarization

APE beamline EU 12.5: Quasi-periodic

2x106

1

0

Inte

nsity

(a.

u.)

80604020Kinetic energy (eV)

2.2 X 106 counts

1.2 X 104 counts

1.0

0.5

0Inte

nsity

(a.

u.)

20.7520.70Kinetic energy (eV)

FE width 45 meVT~120 K

Ag(100) Valence band extended spectra at hν=25 eVthe kinetic energy region spans

the second and third order energy range, showing 1% overall contribution.

APE beamline EU 12.5: Quasi-periodicity and ARPESSuppression of higher order intensity

APE - EU 12.5: quasi-harmonicsafter monochromator: data vs. calculation

Entrance Slits or Slitless?The importance of the source stability!

From (www.coe.berkeley.edu/AST/sxreuv)

78°

84°

1,9 m

0,5 m

2,9 m

M1

RM2a

M3

24 m

Plane Grating 700l/mm 10-25eV 150° 1200l/mm 20-45eV 150° 160l/mm 35-80eV 160°

Toroidal Mirror R = 13,46 m r = 0,477 m

Spherical Mirror R =115,3 m

R { +0° +15°

160°

150°

0,256 m

M2b

M2a R=61,65 m M2b R=104,6 m

4 m

0,730 m

5,9 m

0,25

m

8,05 m

APE – Low Energy Beamline – Optical layout

Side view

Top view

Pin

hole

Pref

ocus

ing

Entra

nce

slit

Mon

ochr

omat

or

Exit

slit

Def

lect

ion

mirr

or

Ver

tical

focu

sing

Hor

izon

tal f

ocus

ing

VFM

HFM

Prefocusing section: Adapt the source to the monochromator requirementsAdsorb the unwanted power radiation

Monochromator:Select the proper photon energy

Refocusing section: Adapt the spot shape at the necessity of the experiment

Courtesy , D. Cocco, ELETTRA, Trieste

Nanospectroscopy – X-ray PEEM

Pre-focusing MirrorSpherical Gratings: different for HE and LE

Same principle

Exit Slit

Re-focusing MirrorToroidal

Sample

Photon Source(Undulator)

Mirror

Plane Grating Monochromator (PGM) +Spherical Mirrors

APE beamline: Layouthttp://www.elettra.trieste.it/experiments/beamlines/

No entrance SlitsNo pinhole!

Schematic of the two entrance mirrors of the APE beamline.

Concrete block and granite support are used to guarantee mechanical stability

First Mirror – Power load and stability

Maximum slope error : 1 µrad RMS for both transverse and longitudinal profiles.

Maximum roughness : 3 Å RMS

280

Silicon blankCopper coolingblock

Mirrors are cooled to 40 K with He gas systemR. Dausman (Cryomech inc.)

Mirror temperature <100 K under power load (average 220 W on each mirror)

First Mirror – Power load and stability

360

320

280

240

200

160

120

8040

0

Verti

cal S

cale

(µm

)

450400350300250200150100500

Horizontal Scale (µm)

200

100

0

-30x10 -6

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

effic

ienc

y (%

)

280260240220200180160140120100806040200

Photon ene rgy (eV)

-10x10-6

-5

0

5

10

slope

(rad

)

0 10080604020x (mm)

Slope error of the internally cooled mirror

Slope error bendable mirror

Courtesy , D. Cocco, ELETTRA, Trieste

VUV and Soft X-ray mirrors: quality tests

VUV and Soft X-ray Monochromator: gratings

X-ray diffraction grating: Structure having a surface capable of reflecting x-rays, said surface being grooved so that in cross-section the profile alternates regularly between geometrically similar lines, in which the height of the alternation varies significantly over the area of the surface. The grating may be plane or curved; the variation in height may occur as a single step, a plurality of steps or may be continuous; the variation may occur either transverse to or parallel to the grooving.

mλ = 2d (sin α - sin β)

m = order of diffractiond = distance between grooveα, β = angles incidence,reflection

VUV and Soft X-ray Monochromator: gratings

N = order of diffractionDifferent reflection angleDefine dispersive planeDefine where slits are

Blaze is used to improve one reflection only and

depress the others

Blazed Diffraction GratingBy tilting the facets of the grating so the desired diffraction ordercoincides with the specular reflection from the facets, the grating efficiency can be increased.

Specular means angle of incidence equals angle

of reflection.Input beam

Efficient diffraction

Inefficient diffraction

Even though both diffracted beams satisfy the grating equation, one is more intense than the other.

d

How accurate is a diffraction-grating spectrometer (a grating followed by a lens)?Recall the grating dispersion: cos( )

m

m

d md aθλ θ

= 1 [ cos( )/ 2]m

fwd

λπ θ

=

2w1

f

Two similar colors illuminate the grating.

f

cos( )mm

mf fa

δθ δλθ

=

Two nearby wavelengths will be separated by:

12 2cos( ) [ cos( ) / 2]m m

m f fwa d

λδλθ π θ

= =Setting this distance equal to the focused-spot diameter:

4 am dλδλπ

= 4m Nλδλπ

= where N = # grating lines illuminated = d / a

dcos(θ

m )

or

cos( )mm

ma

δθ δλθ

=

Diffraction-grating/spectrometer resolution:simple formulas

2w0

2w1

ff

4m Nλδλπ

=

λ ≈ 600 nm m = 1 N = (50 mm) x (2400 lines/mm) = 120,000 lines

0.007 nmδλ⇒ ≈

For simple order-of-magnitude estimates, 4 / p ≈ 1:

1mN

δλλ

≈And the resolution depends only on the

wavelength, order,

and how many lines are illuminated!

2” grating with 2400 lines/mm

Resolution:

Diffraction-grating/spectrometer resolution:simple formulas

VUV and Soft X-ray gratings: dimensions

Kohzu monochromator installed at the Advanced Photon Source (APS).

(Credit: Argonne National Laboratory)

Diffraction EfficiencyMany optical systems demand high optical throughput. For a holographic grating, 1st order diffraction efficiencies better than 95% are obtainable in either transmission or reflection. However, in order to achieve these high efficiencies in manufacturing, proper modeling tools as well as intuition and experience are required.

Stray Light Holographic gratings inherently offer lower stray light and structured noise than ruled gratings. However, less experienced holographers may not perform the steps necessary to reduce stray light for the most demanding systems.

DurabilityChoose gratings made of materials resistant to high temperatures and solvents. Replica gratings produced from thermal epoxies typically have low Tg (<80°C) and deform when exposed to harsh solvents. If durability is important, be sure to use cold formed gratings.

Coatings When coating a grating with enhanced aluminum, gold, silver, or multilayer dielectrics, care must be taken to avoid crazing, defects, voids, and artifacts. Coating recipes that work on conventional lenses and mirrors often must be redesigned to work on diffraction gratings and replicated optics.

Quality When quality is important, avoid committing to catalog stock gratings produced as 5th or 6th generation replicas.

VUV and Soft X-ray gratings: quality features

VUV and Soft X-ray gratings: quality tests

Mask etching procedure to obtainVLS Variable line spacing

Variable groove profile

Main characteristicsDouble track

Coating → ReflectivitySlope error 0.5 µrad RMS

< 0.5 µrad RMSSlope error

Pt (500 Å)Coating material

175 deg170 degDeviation angle

240 × 10 mm2120 × 10 mm2Useful area

0.550.55 to 0.6Duty cycle (groove/period)

50 ±5 Å60 to 70 ÅGroove depth

1000-2000 eV180-500 eVEnergy range

1800 mm-1

track 21800 mm-1

track 1Line Density

VUV and Soft X-ray gratings: quality tests

10

5

0

-5angl

e de

viat

ion

(arc

sec)

0.200.150.100.050.00-0.05encoder deviation (mum)

The monochromator is moved and the deviations with respect to average position for every point is computed. In the image the deviation of the optical Heidenhain is related to the deviation of the mirror effective angle.

Mirror has a deviation of about 8 arcsec, not related to encoder position.

Mechanical and encoder problem.

Gratings and monochromator: optics + mechanics

-1.0

-0.5

0.0

0.5an

gle

devi

atio

n (a

rcse

c)

0.100.00-0.10optical encoder deviation (mum)

Effective mirror angle reproducibility: ±0.5 arcsec ≅ 90 meV (at 450 eV)

Optical endcoder dead band: 0.1 µm (before it was more than 0.5 µm)

Test duration: 11 hours (1280 energy changes)

1) linear movement registration 2) proper positioning of laser of interferometric encoder 3) electrical shielding of Heidenhain electronic (it’s an old model very sensitive to pick-up electrical noise)

Gratings and monochromator: optics + mechanics

- (APPLE-II quasi-periodic undulator )125 mm period, 17 periods- Energy range:10-120 eV- Monochromator – variable line spacing PGM; ( gratings: 900 lines/mm, 1200 lines/mm, 1600 lines/mm), double track- Spot size ~100x50 µm2

- Open to users on 75% base1600 lines/mm1200 lines/mm, track 21200 lines/mm, track 1

APE – Low Energy Beamline – 10 -120 eV

Next Tuesday, 1st quasi-examChoose one energy range

Soft X-ray (till 1800 eV)XPS, XAS, XMCD

Representative beamline worldwideDefine your experiment

Describe CharacteristicsSource, optics, mono, experiments

1) Comment and questions2) Estimate costs

VUV (5-200 eV)ARPES

Hard X-ray (> 5 keV)Diffraction,Scattering,

other