synchrotron radiation core level photoemission...
TRANSCRIPT
Synchrotron Radiation Core Level Photoemission Spectroscopy
R. CiminoINFN-LNF, Frascati
• Generalities: photoemission; experimental apparatus• Core Levels : - position - width - shape - intensity• Examples: - Clean semiconductor's surfaces
- Interface formations: chemical reactions and electronic properties
• Conclusion and open problems
Roma, 14-10-2005 Roberto Cimino-LNF
Photoemission The experimental discovery of the photoelectric effect (Hertz 1887) prepared the ground for Einstein's formulation, in 1905, of the photoelectric effect.
Photoelectron spectroscopies are all based on the physical fact that, when illuminated by photons, matters may produce a photocurrent.
The analysis of photoelectrons require UHV and photoemission, as a spectroscopic technique, was developed only after the fifties.
Roma, 14-10-2005 Roberto Cimino-LNF
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Photoemission from a solid with a DOS.
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Experimental Apparatus :Hemispherical Analyzer
UHP
U+HS
U-HS
UL
SAMPLE
UHV
Preamplifier
UC
Channeltron
hν
collecting lens
counting stage.
dispersive electron-optic
ESCA-XPS
Hv= 14
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Hv= 14
ESCA-XPS
ESCA-XPS
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ESCA-XPS
Roma, 14-10-2005 Roberto Cimino-LNF
What do we measure?Our photoemission spectrometer measure the Kinetic Energy (EK) of the outgoing electrons.
This is related to the Core level binding energy (EB) by:
EB = hν EK - e Φanalyser
Where : hν is the photon energy� e Φanalyser is the analyser Work function
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
The Kinetic energy of what we measure is referenced to the analyser Work function.No matter what is the Work function of the sample, photoelectrons from its Fermi level will always appear, for a given hν at the same kinetic energy.
Changing the Work function of a surface, does not shifts the energy (Ek) observed by the spectrometer.
Binding Energy
The binding energy is defined as the total energy difference between the final core-hole state, with N-1 electrons, and the unperturbed initial N electron state:
EB = ET(N-1) - ET(N)
Roma, 14-10-2005 Roberto Cimino-LNF
Binding Energy
Roma, 14-10-2005 Roberto Cimino-LNF
Let us first assume that, when ejecting one electron from an N-electron system, the (N-1) remaining electrons are unaffected by the photo-event. In this approximation, (Koopmans' theorem) the CL binding energy is opposite in magnitude to the one-electron energy of the orbital.
EB = �lWithin this approximation, it is possible to
identify, by measuring EK, the atomic constituents of our material.
Binding energy shiftsRather than its absolute energy position, we analyse the differences in energy of the core levels of a given element in different environments, ∆EB.We can schematically assume that ∆ EB is formed by two separate contributions: - one keeps track of the differences in the
ground state ET(N); - the other considers the differences in the core-hole final state ET(N-1).
∆ EB = ∆ init + ∆ fin
Roma, 14-10-2005 Roberto Cimino-LNF
Initial state effects∆init = ∆ conf + ∆ charge + ∆ Madelung
∆conf represents the change in core level BE due to a change in the valence electronic configuration of the considered atoms (i.e. going from free atoms to surfaces or solid state).∆ charge considers the effects associated with a modification in the valence charge density surrounding an atom when this atom is included in a chemical bond (i.e. semiconductor surfaces reconstruction).∆ Madelung counts for the effects of charge density variation occurring at atoms surrounding the emitter.
Roma, 14-10-2005 Roberto Cimino-LNF
Initial state effects∆init = ∆ conf + ∆ charge + ∆ Madelung
In case of semiconductors, "band bending", can also be considered as an initial state effect.
Note that such initial state shifts (also called "chemical shifts") are roughly similar for all the core levels of the same atoms and do not depends on l.
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Roma, 14-10-2005 Roberto Cimino-LNFHollingher and Himpsel, Phys. Rev. B (1993)
Chemical shift Si 2p and Oxygen
SiSi
Si
SiSi∆EB=0 eV
OSi
SiSi
Si∆EB=0.9 eV
OO
SiSi
Si∆EB~1.8 eV
Chemical shift Si 2p + Oxigen
OO
OSi
Si
∆EB=2.6 eV
OO
OSi
O∆EB=3.5 eV SiO2
Hollingher and Himpsel, Phys. Rev. B (1993)
Roma, 14-10-2005 Roberto Cimino-LNF
Final State effects• This term is taken to be zero at the lowest
level of the fully independent electron approximation.
• More in general, we will have:
∆final = ∆ relaxation = ∆ intra + ∆ extra
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∆final = ∆ relaxation = ∆ intra + ∆ extra∆ intra considers the effect of intra-atomic relaxation; it is always present; it is large (in absolute value) but, since it is an atomic phenomena, does not play a major role in binding energy shifts.
∆ extra takes into account the variation of the surroundings of the excited atom due to the rearrangements of the most external charges to screen the core-hole. Such term is fundamental to understand core level shifts in metallic systems, where conduction electrons strongly relax to screen a core-hole.
• In semiconductors with high dielectric costant (εSi=12) one assumes that the core-hole is fully screened ===> ∆ extra =0
• In metals this approximation lose its validity.
Final State effects
Roma, 14-10-2005 Roberto Cimino-LNF
The width of a core level line
( σ = σAnalyser2 +σmono
2 )
• The width of a core level line in photoemission is the convolution of different contributions:
• Broadening caused by the limited resolution of the experimental apparatus.
• electron-phonon broadening• Core level finite life-time.• Structural disorder and/or Inhomogeneities
Roma, 14-10-2005 Roberto Cimino-LNF
Broadening induced by electron-phonon
couplingFranck-Condon principle:shows how the crystal lattice oscillations at T>0 can affect the width of a core level line. Small bond length variations around its equilibrium value can, in fact, result in appreciable changes of the transition energy [final state - initial state] due to the different number of phonons excited in the final state.
Roma, 14-10-2005 Roberto Cimino-LNF
Broadening induced by the finite core hole life-time.
Heisenberg uncertainty principle states that the smallest is the decay time of a core hole the biggest will be the uncertainity of its energy position (hence the biggest its line width). This broadening have a Lorentzian shape:
I(E) = I(E0).�L/[(E-E0)2 + �L2 ]
where: I(E) is the intensity at energy EE0 is the energy at the peak centre.�L is the broadening directly related to the decay time of the core hole. that is:
�L =1/2 FWHM = h/� = 6.58 . 10-16 /� eV, where: FWHM is the full width half maximum of the peak and �is the hole life time.
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How does a core-hole decay ?
E f
K.E.
B.E.
hv
a)
E f
hv
b)
E f
c)
a) photoemission from a core levelb) decay of the core-hole by fluorescence ( Z 4)c) core hole Auger decay ( Z 2)
Roma, 14-10-2005 Roberto Cimino-LNF
How does a core-hole decay ?
In addition, for L1 edges, we have together with the Auger process (i.e. LMM) the so called Coster-Kronigtransitions (one of the two holes in the final state has the same quantum number of the initial core-hole: i.e L1L2M) and the Super-Coster-Kronig (both final state holes have the same quantum number of the initial core-hole ; i. e. L1L2L3). Such extra processes will quicken the decay process increasing accordingly the line broadening
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Structural disorderIn an amorphous solid, the static fluctuations in valence charges brought about by bond-length and bond-angle variations causes a homogeneous broadening of core level lines.[as shown by L. Ley et al. PRL (82) in case of c-Si compared to a-Si 2p]
AmorphousSi
CristallineSi
Core level line intensity:It is proportional to the number of atoms which are present in the material and at its surface.
1. Reflects the high surface sensitivity of thetechnique.
2. It depends on the photoemitted electron atomicsubshell photoionization cross section.
3. It is connected to the structure of the analysed solid, if it is a single crystal, causing the so called "photoelectron diffraction".
P(E,z) ~ exp[-z λ(E)]
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
C. l. intensity: surface sensitivity λ(E) is the meen free path
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Cross section effects
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Photoelectron diffraction
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Roma, 14-10-2005 Roberto Cimino-LNF
In case of GaAs(110) clean surface
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The GaAs(110) clean surface
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The GaAs(110) clean surface Ga 3d Core Level Line:
1)We expect two components: a 3d5/2 and a 3d3/2 line splitted in energy by the spin-orbit interaction and with a relative intensity roughly proportional to their occupation probability (for a 3d orbital:(2j(3/2)+1)/(2j(5/2)+1) =0.66).
∆s.o.
Roma, 14-10-2005 Roberto Cimino-LNF
The GaAs(110) clean surface Ga 3d Core Level Line:
2) We expect the presence of a doublet photoemitted from surface atoms. (Similar to the one emitted by volume atoms but centered at a slightly different energy position and with different intensity. In simple terms, we expect the Ga 3d surface peak at higher BE (SCLS >0) due to the "missing" of an As, which, in the bond, results negatively charged. A similar reasoning suggests for the As 3d peak a negative SCLS).
Roma, 14-10-2005 Roberto Cimino-LNF
∆scls
The GaAs(110) clean surface Ga 3d Core Level Line:
3) Each of these lines will have alorentzian broadening (which, given the high angular momentum and the low binding energy of the 3d level, can be expected to be small) convoluted with a gaussian(∆Eexp=80meV).
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Roma, 14-10-2005 Roberto Cimino-LNF
The GaAs(110) clean surface Ga 3d Core Level Line: Experimental data
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
The GaAs(110) clean surface As 3d Core Level Line: Experimental data
The GaAs(110) clean surface Ga 3db and As 3d Core Level Line: FIT
One can FIT the experimental data to obtain a quantitative estimate of the relevant physical quantities.
Ga 3d and As 3d in GaAs(110)
Roma, 14-10-2005 Roberto Cimino-LNF
Eb
(eV)SCLS(eV)
∆b
(eV)∆s
(eV)Γb
(eV)Γs
(eV)BR ∆s-o
(eV)Is/Ib
Ga 3d
18.67 0.28 0.17 0.25 0.09 0.08 0.65 0.45 0.61
As 3d
40.45 -0.40 0.24 0.27 0.09 0.09 0.70 0.70 0.63
± 0.02 0.01 0.05 0.05 0.05 0.05 0.03 0.01 0.05
Roma, 14-10-2005 Roberto Cimino-LNF
Given the well resolved surface component: check surface sensitivity
ab
Surface sensitivity
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(Dependence of the relative intensity Is / Iv on the photon energy)
Photoemission spectra from Ga 3d and As 3d core level from GaAs(110).[From: Eastman et. al. PRL 45, 656 (1980)]
a
b
Surface sensitivity We can than confirm the "universal" dependence of the photoelectron mean free path versus kinetic energies.
In first approximation, the ratio between the emission intensity of the volume atoms and the surface atoms is given by:
where: d is the distance between two lattice planes iz the distance between the atom emitting the p.el and the surface.λ is the mean free path
than: λ = d [ln (Is/Ib + 1)]-1.being d = (21/2/4) a0 for the (110) surface, with a0, the GaAs lattice constant, equal to 5.65 Å, we obtain a mean free path :
λ ~ 4 Å.
Roma, 14-10-2005 Roberto Cimino-LNF
Core Level Lines FIT
WARNING:
Often such fitting procedures are overrated, especially when it is not possible to individuate by eye-inspection or by means of a sound physical prediction, the different components which shouldform the signal.
It is than fundamental, before proceeding to a CL fit, to reduce at minimum the free parameters and to individuate the various components in order to obtain physically significant information.
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Wrong Core Level Lines FIT
Huttel et al Surf. Rev. and Lett. (1995)
Metal-semiconductor interfaces
Due to the high surface sensitivity, core level photoemission is one of the most powerful technique to study metal semiconductors interfaces. We will see how it is possible to gain information on:
a) Chemical reactions occurring during the early stages of interface formation;b) Growth morphology; c) Electronic properties of the system.
Roma, 14-10-2005 Roberto Cimino-LNF
Some examples:Chemistry and morphology at
Fe/GaAs(110):a reactive interface.
a) - Exchange reaction b) Interface chemistryc) Importance of experimental resolution
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Fe/GaAs(110):
ReactedComponents
Free metallic Ga
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Fe/GaAs(110):
Same exp. at low
resolutionBy: Ruckman et al
PRB 1986.
Some examples:Chemistry and morphology at In/GaP (110) and
In/GaAs(110):two non-reactive interfaces.
a) Morphology versus Temperatureb) Cross section effectsc) Chemistry at the interface
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
In/GaP (110):
What do we learn from The Core level analysis As a function of Temperature:Different growing mode!
Interface Growing Modes:
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In/GaP (110):
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TIF
From the attenuation curves
of the CL reacted/Surface component we can understand
the growing mode!
L.T. R.T.
Cross section Vs. Photon energy
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Cross section Vs. Photon energy
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Playing around with such cross section effects, we can better understand the chemistry at the
interface.
Bulk Ga 3d Interfacial
Ga 3d
Metallic In 4d
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In/GaAs(110)
In/GaAs(110) similar to Ag/GaAs(110): a look at the Valence Band gives…
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Metal-semicondutor interfaces: Schottky Barrier formation
(Macroscopic property of great techonological interest)
What is a Schottky barrier ?A metal-semiconductor interface is called a Schottky barrier in honor of W. Schottky who first described the rectifying property of such system in 1938.From then on a lot of work has been done on the subject. Up to now a unique and complete theory capable of explaining all the experimental findings is still lacking.
Roma, 14-10-2005 Roberto Cimino-LNF
Schottky Barrier formationWork function model :
Φ Bn =S (Φ m −X s ) S = 1 in the Schottky model
where :
Φ Bn = SB Hight for an n-type semiconductor
Φ m = Metal work function
Xs =Semiconductor electron affinity.
Different models have been proposed. Among them:
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Schottky Barrier formationModel based on interface states (defects or MIGS)
(Bardeen derived model)
ΦBn =S (Φm−X s ) - ∆
where:
∆ = interface dipole
Roma, 14-10-2005 Roberto Cimino-LNF
Measuring a Schottky Barrier using Photoemission:
What is a band bending experiment? In most photoemission spectrometers the B.E. is referenced to the sample Fermi Energy.
During the formation of a Schottky barrier we will observe band bending, that is a rigid shift of the measured B.E. compared with flat band emission.(Assuming the very often valid condition: Photoelectron escape depth << Band bending depth λ<<W )
EB1 -EB2 = Band bending contribution to the induced barrier height.
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Measuring a Schottky Barrier using Photoemission:
Measuring a Schottky Barrier using Photoemission:
Roma, 14-10-2005 Roberto Cimino-LNF
In practice the substrate core level spectra are measured versus metal coverage. The determination of a rigid core level shift will give us the band bending of our system.
Measuring a Schottky Barrier using Photoemission:
In the previous case we have (M. Prietsch et al. Europh. Lett. 6, 451 (1988)) :
Roma, 14-10-2005 Roberto Cimino-LNF
Measuring a Schottky Barrier using Photoemission:
The general observation is that a Schottky barrier is well established after metallic coverages as small as one Monolayer.
Hence the importance of studying SB at the very early stage of interface formation in order to gain an understanding of the mechanisms controlling the Fermi level pinning.Photoemission, due to its surface sensitivity, is considered as a optimal tool for those experiments.
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Photoemission can now benefit from:
Higher photon flux
Better energy resolution
Better Instrumentation
BUT:
Flux and T- an interesting historical case:
•Between 1988 and 1990, using high flux beamlines and Low temperature manipulators, photoemission studies performed at <60 °K seemed to give new insight on the role played by the doping and by the temperature itself in determining the coverage-dependent Fermi level movement at the metal-semiconductor interface.
•Those data suggested that Fermi energy movements are controlled by the dynamic-coupling between atom induced states and substrate states. This coupling seems to have a strong dependence on the bulk doping of the semiconductor and on the temperature. [Aldao et al. P. R.B.39, 12977 (1989), Vitomirov et al. P.R. B 40, 3483 (1989)].
Roma, 14-10-2005 Roberto Cimino-LNF
Flux and T- an interesting historical case:
Roma, 14-10-2005 Roberto Cimino-LNF
Also on GaP:Photoemission studies performed by depositing different metal on GaP(110) (a larger gap semiconductor than GaAs) seem to indicate that this system reveals an almost ideal Schottky-like behavior. This is in contrast with the GaAs case, where no dependence on the metal work function was found for the Schottky barrier height. [Chiaradia et al. J.V.S.T. B5 1075 (1987) and B7, 195 (1989)]
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
GaP
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Also on GaP:
Roma, 14-10-2005 Roberto Cimino-LNF
Than in 1990: Ag/n,p-GaP(110)
M. Alonso, R. Cimino K. Horn Phy. Rev. Lett. 64 1947 (90)
Than in 1990:
Ag/n,p-GaP(110)
M. Alonso, R. Cimino K. Horn Phy. Rev.Lett. 64 1947 (90)
Roma, 14-10-2005 Roberto Cimino-LNF
looking also at the V.B.
Surface Photovoltage effect
The possibility that photoemission data may be affected by photon-induced electron-hole pair creation and transport processes leading to a non-equilibrium charge distribution, has been largely overlooked in this studies, in spite of observations that such effects may occur on clean as well as on metal-covered semiconductor surfaces [Demuth et al. P.R.L. 56, 1408 (1986)].
Roma, 14-10-2005 Roberto Cimino-LNF
Surface Photovoltage effect
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Ag/n,p-GaP(110)
Referring each Core level to its real Fermi level (when measurable):
M. Alonso, R. Cimino K. Horn Phy. Rev. Lett. 64 1947 (90)
SPV effect
metal
conduction band
valenceband
w
hω
J leak
J
J th
tu
JrecJ pc
EF
SPVV
Jph= Jth + Jtu + Jrec + Jleak
Jph= The photo-generated electron hole pairs are separated by the built in potential in the depletion layer giving raise to the photo-induced currentJth = Thermionic excitation of electrons over the barrierJtu = carriers tunneling through the barrierJrec= recombination of electron hole pairs in the deplationJleak=leakage either through the semiconductor or along conducting paths on the crystal sides.
Where:
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
A. Bauer, et al. Europhys lett (1990)M. H. Hecht Phys. Rev. B 41, 7918(1990) and J.Vac. Sci. Technol. B 8, 1018 (1990).
Roma, 14-10-2005 Roberto Cimino-LNF
Higher photon flux is welcome provided that one takes care of:
Charging effectsSurface photovoltageTemperature effectsNon-linear phenomena etc….
Those items will be essential if one consider future experiments using Free-Electron-Lasers in the x-ray region with peak brilliancevalues of 1034 (ph/sec·mrad2·0.1%bw) !!!!!
One more example: Line shape analysisand surface preparation.
Si 2p = ( Si 2p1/2 + Si 2p3/2 ) bulk +( Si 2p1/2 + Si 2p3/2 ) surface
Each doublet is formed by 2 spin-orbit splitted lorentzian lines convoluted with a gaussian (Phonon broadening & experimental resolution).
The number of surface peaks and their energy position depends on the surface reconstruction
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Si (111) 7x7
Karlson et al PRB (90)
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By a fitting procedure one can obtain information on the parameters defining the core level shape.
Si 2p bulkparameters:
From ref:
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Back to III-V: STM-STS evidences
A density of different surface defects of about 1011- 5 1012 atoms/cm2 is typically present on a III-V (110) cleaved surface.
Some of this defects are electrically charged inducing a local band bending which roughly extends over the doping dependent Debye length.
See:Ebert et al PRL 94 and Stroscio et al. PRL 87
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See the case of As Vacancies on GaAs(110) Lenghel et al PRL 94
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
What is the role played by any inhomogeneity of the surface in
broadening a core level line?• Defects, steps, etc, can:Change the atomic position around them
Create locally differently shifted components
At semiconductor surfaces, act as pinning centers
Form around them a region with non zero barrier hight
A)
B)
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The core level photoemission signal is calculated as the sum of a discrete contributions from different atoms at different distances x from the pinned region, shifted according to the particular band bending ratio
Roma, 14-10-2005 Roberto Cimino-LNF
How can we single out the effect on core level line shape of the presence of pinned regions on an otherwise unpinned surface?
By artificially flatten the bands with the help of Surface photovoltage effect.
Roma, 14-10-2005 Roberto Cimino-LNF
The existence of such barrier height inhomogeneity results in an extra broadening due to an averaging over differently pinned regions. This effect need to be taken into account in order to correctly analyze semiconductors core level line shapes. (Cimino et al. Europhys. Lett (95))
RT
LT
Roma, 14-10-2005 Roberto Cimino-LNF
Better resolution requires careful and exact knowledge of what we
are measuring.
Detailed sample preparation and characterisation is required !!!!
Roma, 14-10-2005 Roberto Cimino-LNF
One more instructive example
Multi atom resonant photoemission
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
Single atom resonant photoemission
Multi atom resonant photoemission
LATER A REVISION APPEARED:
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Roma, 14-10-2005 Roberto Cimino-LNF
A CAREFULL DETECTOR CALIBRATION GAVE:
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After atom B excitation: increase of backgroundOff resonance lower background
Roma, 14-10-2005 Roberto Cimino-LNF
Roma, 14-10-2005 Roberto Cimino-LNF
NEW AND HIGLY SOPHYSTICATED TECHNOLOGY IS NECESSARY BUT
REQUIRES CAREFULL KNOWLEDGE.
IT CAN NOT BE USED AS A IT CAN NOT BE USED AS A BLACK BOX!BLACK BOX!
THE SAME APPLYES TO NEW AND MORE SOPHISTICATED CALCULATIONS AND THEORIES.
Roma, 14-10-2005 Roberto Cimino-LNF
PHOTOEMISSION IS AN EXTREEMELY POWERFOOL
TOOL AND A WELL CONSOLIDATED TECHNIQUE.
STILL THERE ARE PIONIRING APPLICATIONS WHICH PRESERVES THE CHARM FOR DISCOVERY……... AND THE RISK OF UNSUCCESS……