cnr infm wuta, frascati, 8 th october 2008 vuv (photoemission) spectroscopy k.c. prince, sincrotrone...
TRANSCRIPT
CNR
INFM
WUTA, Frascati, 8th October 2008
VUV (photoemission) spectroscopy
K.C. Prince,Sincrotrone Trieste, Trieste, Italy
Gas phase.1. Doubly excited states of helium.2. Biomolecules.3. Dichroism4. Two colours
Surfaces and solids.1. Classical application: band mapping2. Resonant photoemission from thin film catalysts, CeO2/Cu3. Adenine/Cu(110)
The future: the Fermi free electron laser light source.
CNR
INFM
WUTA, Frascati, 8th October 2008
Experimental
Gas Phase photoemission beamline, Elettra.VG 220i electron energy analyser.
Our undulator
Our beamline
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WUTA, Frascati, 8th October 2008
Doubly excited states of helium are two electron states in which neither electron is in a 1s orbital:
Naively – nln’l’ n, n’>1Lowest energy series: 2snl, 2pnl, i.e. the states below the second IP, N=2.
For Helium, they are in the energy range 60-80 eV.
Why study them? - The simplest three body problem in atomic physics, - the simplest system where correlation is important,-a benchmark system, etc.
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WUTA, Frascati, 8th October 2008
Doubly excited states were sought (and one state found [1,2]) as early as 1930. After this, there was a pause.
[1] P. G. Kruger, Phys. Rev. 36, 855 (1930).[2] R. P. Madden and K. Codling, Phys. Rev. Lett. 10, 516 (1963); Astrophys. J. 141, 364 (1965). [3] M. Domke et al, Phys. Rev. A 52, 1424 (1996).
The doubly excited states of helium: a brief history
Madden and Codling in the early ’60s then used synchrotron radiation to measure two of the 3 predicted series [2].Fano and Cooper provided the theory, and there was another pause (for experimentalists) while theoreticians thought hard.
1990s: third generation synchrotron light sources threw some light on the matter. Domke et al: observed all three 1Po series below N=2, plus many more below higher N.
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WUTA, Frascati, 8th October 2008
Recent results:- partial VUV fluorescence yield spectra gives a different view of the doubly excited states compared with ion yield [4, 5]- the ion yield does not give the true cross-section for these states [5, 6] - spin-orbit coupling is important, even for He [7]- with photons, you can excite not only 1P states, but also triplets 3P, 3D [8]- He offers a window on “quantum chaology” [9]- Lifetime measurements of the fluorescence tell us about correlation [10]- Stark effects are also interesting (more later) [11, 12]
[4] M. K. Odling-Smee et al, Phys. Rev. Lett. 84, 2598 (2000).[5] Jan-Erik Rubensson et al, Phys. Rev. Lett. 83, 947 (1999).[6] K.C. Prince et al, Phys. Rev. A. 68, 044701 (2003).[7] Thomas Ward Gorczyca et al, Phys. Rev. Lett. 85, 1202 (2000). [8] F. Penent et al, Phys. Rev. Lett. 86, 2758 (2001). [9] R. Püttner et al, Phys. Rev. Lett. 86, 3747 (2001).[10] J. Lambourne et al, Phys. Rev. Lett. 90, 153004 (2003)[11] J. R. Harries et al, Phys. Rev. Lett. 90, 133002 (2003).[12] X.M. Tong and C. D. Lin, Phys. Rev. Lett. 92, 223003 (2004)
The doubly excited states of helium: recent history
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WUTA, Frascati, 8th October 2008
1.6x10-9
1.4
1.2Ion
yiel
d (A
)
64.1564.1464.1364.1264.1164.10Photon energy (eV)
1500
1000
500
Intensity (counts)
Ion yield (upper curve, left scale) and fluorescent UV photon yield (lower curve, right scale) at the (2,-13), (2,14) resonances.
The difference between VUV fluorescence and ion yield.
The long lived state decays by fluorescence, while the shorter lived state decays by autoionization (ion yield). So that state is stronger in partial fluorescence yield.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Cro
ss s
ectio
n (M
bar
n)
64.1564.1464.1364.1264.11
Photon energy (eV)
7x10-10
6
5
4
Ion yield (am
pere)
2,14
2,-13Cross section (left axis)
Ion yield (right axis)
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WUTA, Frascati, 8th October 2008
Some recent work: the Stark effect for doubly excited
states.
The Stark effect: the response of a quantum system to an external electric field – energy shifts and splitting of magnetic sub-levels.
An early triumph of quantum mechanics.
Fields up to 84 kV/cm; new propensity rule.
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WUTA, Frascati, 8th October 2008
3 ways of looking at the Stark effect
Classical electrostatics: charge moves from one side of atom to other; gives an approximate value of Stark shift.
+ -
Mixing of atomic orbitals picture: explains asymmetric charge distribution.
+ + =+ -
Stark operator mixes states of opposite parity.Nearby states of even parity are mixed.Photoabsorption sees only 1Po states; there are “dark” S, Pe, D, F, etc. states.(Triplet states allowed by spin-orbit coupling, recently observed.)
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WUTA, Frascati, 8th October 2008
Purpose built apparatus: parallel plate condenser, gap 5 mm. Cost approx. 50 cents.
+
Experimental
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WUTA, Frascati, 8th October 2008
Electric field F || P, photon polarization.
Total VUV photon yield.
2500
2000
1500
1000
500
0
Inte
nsity
(arb
. uni
ts)
65.4065.2065.00Photon energy (eV)
(2,06)(2,-15),(2,16)
(2,07)
(2,-16),(2,17)
F=0
F=0.5 kV/cm
F=3.5 kV/cm
2500
2000
1500
1000
500
0
Inte
nsity
(arb
. uni
ts)
65.1065.0064.90Photon energy (eV)
(2,06)(2,-15), (2,16) (2,-16), (2,17)
F=0
F=0.5 kV/cm
F=3.5 kV/cm
(2,-1n) and (2,1n) states lose intensity.(2,0n) states gain intensity (maybe anartefact) a broad shoulder develops at higher energy for higher n.
Fields are very low!
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WUTA, Frascati, 8th October 2008
Static field F perpendicular to polarization P
20x103
15
10
5
0
Flu
ores
cenc
e yi
eld
(arb
. un
its)
65.4065.2065.00
Photon energy (eV)
(2,16)+
1 1 1 12 2
F=0
3 kV/cm
4 kV/cm
5 kV/cm
6 kV/cm
7 kV/cm
A new series is observed, labelled 1.Another new broad series labelled 2 is observed (as in other geometry.)Big increases in intensity at high n.
Quantum defect (1)=-0.435±0.005.Matches 1Pe series.
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How do we explain all this?
First, there are selection rules. Parallel geometry, ΔM=0perpendicular geometry, ΔM=±1.
M= zero for ground and S states, so in the perpendicular geometry there is no mixing of S states.
Then, theory confirms the series “1” is 1Pe.The broad features “2” are due to a pair of 1De states.In the parallel geometry, mixing occurs with 1Se states.
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Quantitative modelling of energies,5 kV/cm, F perpendicular to P.
First order perturbation theory – A. Mihelič and M. Žitnik, Ljubljana.
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WUTA, Frascati, 8th October 2008
VUV spectroscopy is a powerful method for investigating the fluorescencedecay dynamics of He doubly excited states.
The wave functions in the excited state can be probed in detailand agreement with experiment is satisfactory.
Stark effects on these states can be seen at moderate fields (< 1 kV/cm).
A new series of the He doubly excited states observed, the 1Pe series.Indications of other series observed, 1Se and 1De.
Conclusions
V. Feyer, M. Coreno,CNR-IMIP, Montelibretti (Rome), Italy, and INSTM, Trieste, Italy,
R. Richter,Sincrotrone Trieste, Trieste, Italy,
M. de Simone, A. Kivimäki, INFM-TASC,Trieste, Italy,and INSTM, Trieste, Italy,
A. Mihelič and M. Žitnik,J. Stefan Institute, 1000 Ljubljana, Slovenia K.C. Prince et al, Phys. Rev. Lett. 96 (2006) 093001.
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WUTA, Frascati, 8th October 2008
Biomolecules: interaction with UV radiation
Interest: damage by ionizing radiationastrobiology: synthesis and destruction of pre-biotic molecules
in space (particularly Lyman α and He I)conformation and dynamics of this class of moleculesmass spectrometry
There exists a large body of electron impact ionization: why use UV radiation?Because it is more specific – dipole selection rules apply.
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WUTA, Frascati, 8th October 2008
5
4
3
2
1
0
806040200
21.2 eV
16.67 eV
11.62 eV
10.0 eV
9.56 eV
parentionx10
Photofragmentation with laboratory sources of UV
glycine
5
4
3
2
1
0
120100806040200
21.2 eV
16.67 eV
11.62 eV
10.0 eV
9.56 eV
proline
4
3
2
1
0
Inte
nsity
(a
rb. u
nits
)
140120100806040200m/z
21.2 eV
16.67 eV
11.62 eV
10.0 eV
leucine
Aliphatic amino acids and Pro show strong fragmentation, even close to threshold.Mostly loss of HCOO.(Our work and Lago et al, Chem. Phys. 307(2004) 9).
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WUTA, Frascati, 8th October 2008
O. Plekan et al, Chem. Phys. 334 (2007) 53–63.
6
5
4
3
2
1
0
160140120100806040200
21.2 eV
16.67 eV
11.62 eV
10.0 eV
9.56 eV
parent ion
C4H10NCOOH+C4H10N
+
Methionine is different:- loss of COOH is not the main channel-at low energies, the parent ion is the dominant peakWhy?
CNR
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WUTA, Frascati, 8th October 2008
5000
4000
3000
2000
1000
0
Inte
nsi
ty (
arb
. un
its)
20.0 17.5 15.0 12.5 10.0 7.5
Binding energy (eV)
glycine
proline
methionine
nN
nO
OO
nS
Ar IHe I Ne I Kr I Xe I
UV photoionization removes an electron from a valence orbital.
The Highest Occupied Molecular Orbital of aliphatic amino acids has nitrogen lone pair character.
The HOMO of methionine has S lone pair character. The S atom accommodates the charge without fragmenting.
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WUTA, Frascati, 8th October 2008
Amino acids with aromatic groups.
Tyrosine
5
4
3
2
1
0
Inte
nsity
(ar
b. u
nits
)
250200150100500m/z
21.2 eV
16.67 eV
11.62 eV
10.0 eV
8.43 eV
c)
Phenylalanine Tryptophan
For all three amino acids, there is a significant parent ion signal at low photon energy.Fragmentation pattern differs: aromatic ring breaks off from rest of molecule.HOMOs have π character.
Tryptophan mass spectrum
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WUTA, Frascati, 8th October 2008
For amino acids we measured all ions produced by ionization of several orbitals.If we measure ions and photoelectrons in coincidence, we observe the fragments due to the ionization of a specific orbital.
E.g. Methanol, CD3OH.
Next step – coincidences.500
400
300
200
100
0
cou
nts
26 24 22 20 18 16 14 12 10
electron binding energy in eV
any ion
D+
CD3+ OD
+
DCO+ COD
+
D2COH+
D3COH+
2a"
5a'1a"4a'
3a'2a'
Work done at Spring-8, in collaboration with R. Richter, K. Ueda, G. Pruemper.
CNR
INFM
WUTA, Frascati, 8th October 2008
Dilute species, biomolecules
What is the conformational (folding) structure of a free bio molecule?
Circular dichroism (CD) in the near UV is a standard tool for secondary structure determination/control of large molecules. [I(left)-I(right)]/I~0.001-0.0001.
+ Sample in solvent, structure is “true” structure.- Sample in solvent, wavelength range limited to 180-250 nm circa.- low info content (few peaks).
CD spectrum and secondary structure of proteins.
“Conventional” CD is best for: monitoring conformational changes due to a perturbation, quality control etc. Less good for absolute structure.
Being extended to 120 nm (Daresbury-> Australian synchrotron, Brookhaven etc.)
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WUTA, Frascati, 8th October 2008
Recently natural CD of small chiral molecules investigated. Optimal conditions: CD signal=[I(left)-I(right)]/I~0.04.
Dichroism
Can we use VUV spectroscopy (and later Fermi) to obtain structural (folding) information?
Higher energy-> no windows-> molecules in vacuum.
Free molecules-> range extended above the IP-> potentially larger info content
-0.02
0.00
0.02
0.04
0.06
b)R(+)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Fig. 1
Inte
nsity
(ar
b. u
nits
)
a)
CD
(ar
b. u
nits
)C
D (
arb.
uni
ts)
Inte
nsity
(ar
b. u
nits
)
R(+)
4 5 6 7 8 9 10 11 12 13
-0.06
-0.04
-0.02
0.00
0.02
Kinetic Energy (eV)
d)
S(-)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
c)S(-)
S. Turchini, N. Zema, G. Contini, G. Alberti, M. Alagia, S. Stranges, G. Fronzoni, M. Stener, P. Decleva, and T. Prosperi, Phys. Rev. A 70, 014502 (2004)
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WUTA, Frascati, 8th October 2008
500x103
400
300
200
100
0
-100
Inte
nsity
(ar
b. u
nits
)
109876Kinetic energy
Signal, left Signal, right difference, x10
Proline, valence band spectrum
Dichroism from 0 to 4%.
100x10-3
80
60
40
20
0
D p
aram
eter
1614121086Kinetic energy (eV)
A_state
-40x10-3
-20
0
20
40
D p
aram
eter
1614121086Kinetic energy (eV)
B_state
-40x10-3
-20
0
20
40
D p
aram
eter
1614121086Kinetic energy (eV)
C_state
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WUTA, Frascati, 8th October 2008
Inte
nsity
(ar
b. u
nits
)
11.0 10.0 9.0 8.0
Binding energy (eV)
0
1
2
3
4
5
6
0.44
0.42
0.40
0.38
I(8.
9)/I(
9.5)
440435430425420415410405
Temperature (K)
3.80
3.75
3.70
3.65
ln (
I(8.
9)/I(
9.5)
)
2.45x10-3
2.402.352.302.25
Temperature (1/K)
Valence band photoemission.He I.The two highest MOs are due to two pairs of conformers.
By measuring spectra as a function of temperature, can extract free energy difference, 6-9 kJ/mol.
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WUTA, Frascati, 8th October 2008
Surfaces and solids
Photoemission from O/Ag(110).
One of the most important applications of VUV photoemission spectroscopy has been band mapping. It will continue to be a standard method.
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WUTA, Frascati, 8th October 2008
Z.-X. Shen and co-workers are prolific users of low energy photons and Angle Resolved UPS.Huge output of results on oxide materials.
ARUPS-> direct access to valence band structure.
What science will be done?
The valence band: gaps in high Tc superconductors, transport properties, bonding, etc.Oxides and related materials have large unit cells-> high momentum resolution required-> high angular resolution at low electron energy.
K.M. Shen et al, PRL
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Microscopy: Pb/Au/Si(111)
SPELEEM photoemission microscope, (now closed, upgraded to Nanospectroscopy). Sample: Au/Si(111) + 5 ML Pb. The Au induces layer by layer growth.Area: about 1 micron diameter.Time: one photon energy, one kinetic energy, all angles: about 60 s.One photon energy, all valence kinetic energies, all angles: tens of min.
Valence band.
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WUTA, Frascati, 8th October 2008
Inverse model catalyst: CeO2/Cu
Many catalysts consist of metal particles supported on oxides.Oxide maintains dispersion, is a reservoir for oxygen, participant in SMSI (strong metal support interaction), catalyst, etc.
Difficult to prepare single crystal oxides, easy to prepare metal crystals.->Prepare oxides on metals.
HR-TEM image of (a) Cu loaded ceria powder(b) elemental mapping of the area; white - cerium, red – copper.
F. Šutara, V. Matolín et al, Thin solid Films, in press.
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We can grow defect-free, well-ordered epitaxial CeO2 layers on Cu(111)
LEED of CeO2/Cu(111), E = 98 eV, (a) discontinuous, (b) 2.5 ML, (c) 5 ML.Arrows mark substrate spots.
Can we check for point defects (oxygen vacancies) with resonant photoemission?
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WUTA, Frascati, 8th October 2008
Inte
nsity
, a
rb. u
nits
8 6 4 2 0BE(eV)
115eV 122eV 124.5eV
Ep=
O 2p
Ce 5d, 6s, 4f
Ce3+
Ce0
Ce0
Ce4+
Resonant spectra show primarily Ce4+.
Conclusion: film is epitaxial (LEED) and has low point defect density.
F. Šutara et al, Thin Solid Films, 516 (2008) 6120.
Configurations CeO2, Ce4+, (4d105p6) 4f0, resonates at hv=124.5 eVCe2O3, Ce3+, (4d105p6) 4f1, resonates at hv=122 eVCe metal, (4d105p6) 4f15d16s2, resonates at hv=122 eV
Resonant process:Ce 4d104fn 4d94fn+1 4d10fn
interferes with direct valence photoemission.
Resonant photoemission
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WUTA, Frascati, 8th October 2008
A DNA base on a metal surface: a prototypical bio/metal interface.Has been studied by STM, vibrational spectroscopy, theory.
Q. Chen and N. V. Richardson, Nature Mat. 2 (2003) 324Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219D. J. Frankel, Q. Chen and N.V. Richardson, J. Chem. Phys. 124, 204704 (2006).
Adenine, C5N5H5
DNA bases II: adsorption of adenine, C5H5N5, on Cu(110)
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Experimental conclusions from the literature: the molecule is lying flat (Yamada et al) the molecule is tilted slightly (Chen et al) or the molecule is strongly tilted (McNutt et al)
Theory agrees: bonding through amino group.Molecule tilted up from surface, 18-26º.
Preuss et al, PRL 94, 236102 (2005)
Vibrational studies:Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219A. McNutt et al, Surf. Sci, 531 (2003) 131.T. Yamada et al, Surf. Sci. 561 (2004) 233–247.
Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219
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Bonding is very different at high and low coverage. N is involved in bonding.In particular, the two amino N atoms have unsaturated (imino)At low coverage: N 1s (398.7 eV) characteristic of π bonded N.
N 1s photoemission
Gas phase
Deposited
Desorb excess
Ordering
0.6 ML > 1 ML
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WUTA, Frascati, 8th October 2008
NEXAFS of adenine: Near Edge X-ray Absorption Fine Structure Spectroscopy
0.3 ML, annealed 430 K 1 ML, annealed 430 K
Fractional monolayer: molecule lying almost parallel to the surface.Saturated monolayer: molecule(s) tilted
1.0x106
0.8
0.6
0.4
Inte
nsity
(ar
b. u
nits
)
420415410405400395Photon energy (eV)
Grazing incidence
Normal incidence
150x103
100
50
0
Inte
nsity
(ar
b. u
nits
)
420415410405400395Photon energy (eV)
Normal incidence
Grazing incidence
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WUTA, Frascati, 8th October 2008
Collaborators.
The Gas Phase team at ElettraO. Plekan, V. Feyer, R. Richter, M. Coreno, M. de Simone,
Materials Science (Czech) BeamlineT. Skala, V. Chab, F. Sutara, V. Matolin
Oksana
Vitaliy
Robert
Monica
Marcello
Tomas
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WUTA, Frascati, 8th October 2008
Fermi specs.
A seeded free electron laser: -> a conventional laser bunches the electrons -> the bunches pass through an undulator -> the electrons in each bunch emit coherently -> intensity is proportional to square of number of electrons, and square of number of periods in undulator.
Fermi FEL 1: 12-30 eV; FEL 2: 30-126 eV.Pulses of 50 fs-1 ps50 Hz GW power levels, 1014 photons/pulseStart operation at the beginning of 2009
Free Electron Lasers.
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WUTA, Frascati, 8th October 2008
If you can control the timing of light, you can learn new things…
Eardweard Muybridge, 1878: do all 4 horse’s hooves leave the ground at once?A bet by Leland Stanford, wealthy ex-governor of California: early research at Palo Alto. Sub-second resolution.
Harold E. Edgerton, 1964. Microsecond strobe.
Or applied research, H. E. Edgerton, 1934.
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WUTA, Frascati, 8th October 2008
Two photon double ionization
Physics is different for two photon (h>39.5 eV)and many photon (h~ few eV) double ionization.->control of relative field strengths.
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WUTA, Frascati, 8th October 2008
FEL light is so intense that it ionizes all molecules in its path
-> ultradilute samples.Clusters and “flying proteins”.
Nanospray set-up for spectroscopy,with “unfree” lasers, T.R. Rizzo et al, PPCM, EPFL.
Our schematic setup.
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WUTA, Frascati, 8th October 2008
Setting up with Synchrotron Radiation: two photon spectroscopy of neon
Synchrotron light: 30-65 ps bunch length, 2 ns interval.Neon atoms excited with synchrotron light. Then excited by a laser in a second step to resonant ionizing states.Two photon transitions: Δl=0,2.Proof of principle of lifetime measurements on nanosecond time scale.
A. Moise et al, Nucl. Instrum. Methods A 588 (2008) 502.
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WUTA, Frascati, 8th October 2008
Where is Fermi being built?
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WUTA, Frascati, 8th October 2008
ELETTRA
FERMI@Elettra FEL
ELETTRAStorage Ring FEL
1010 Increase
P ~ Ne
P Ne2
Brightness