Transcript CNR INFM VUV (photoemission) spectroscopy K.C. Prince,

VUV (photoemission) spectroscopy
INFM
CNR
K.C. Prince,
Sincrotrone Trieste, Trieste, Italy
Gas phase.
1. Doubly excited states of helium.
2. Biomolecules.
3. Dichroism
4. Two colours
Surfaces and solids.
1. Classical application: band mapping
2. Resonant photoemission from thin film catalysts, CeO2/Cu
3. Adenine/Cu(110)
The future: the Fermi free electron laser light source.
WUTA, Frascati, 8th October 2008
INFM
Experimental
Our beamline
Gas Phase photoemission beamline, Elettra.
VG 220i electron energy analyser.
Our undulator
WUTA, Frascati, 8th October 2008
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Doubly excited states of helium are two electron states in
which neither electron is in a 1s orbital:
Naively – nln’l’
n, n’>1
Lowest 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.
WUTA, Frascati, 8th October 2008
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The doubly excited states of helium:
a brief history
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Doubly excited states were sought (and one state found [1,2]) as early as
1930. After this, there was a pause.
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.
[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).
WUTA, Frascati, 8th October 2008
The doubly excited states of helium:
recent history
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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)
WUTA, Frascati, 8th October 2008
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The difference between VUV fluorescence
and ion yield.
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7x10
1.8
2,14
1.6
2,-13
Cross section (left axis)
6
1.4
1.2
5
1.0
Ion yield (right axis)
0.8
-10
Ion yield (ampere)
Cross section (Mbarn)
2.0
4
0.6
64.11
64.12
64.13
64.14
64.15
Photon energy (eV)
-9
1.4
1500
1.2
1000
500
64.10
64.11
64.12
64.13
64.14
Photon energy (eV)
WUTA, Frascati, 8th October 2008
64.15
Intensity (counts)
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.
1.6x10
Ion yield (A)
Ion yield (upper curve, left scale) and fluorescent
UV photon yield (lower curve, right scale) at the
(2,-13), (2,14) resonances.
Some recent work:
the Stark effect for doubly excited
states.
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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.
WUTA, Frascati, 8th October 2008
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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.)
WUTA, Frascati, 8th October 2008
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Experimental
+
Purpose built apparatus: parallel plate condenser, gap 5 mm.
Cost approx. 50 cents.
WUTA, Frascati, 8th October 2008
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Total VUV photon yield.
2000
1500
(2,-15),
(2,16)
(2,06)
Electric field F || P, photon polarization.
(2,07)
(2,-16),
(2,17)
(2,-1n) and (2,1n) states lose intensity.
(2,0n) states gain intensity (maybe an
artefact) a broad shoulder develops
at higher energy for higher n.
F=3.5 kV/cm
F=0.5 kV/cm
1000
500
0
F=0
65.00
65.20
Photon energy (eV)
Fields are very low!
65.40
2500
Intensity (arb. units)
Intensity (arb. units)
2500
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F=3.5 kV/cm
2000
F=0.5 kV/cm
1500
1000
500
0
F=0
(2,-15), (2,16)
64.90
(2,06)
(2,-16), (2,17)
65.00
Photon energy (eV)
WUTA, Frascati, 8th October 2008
65.10
Static field F perpendicular to polarization P
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3
Quantum defect (1)
=-0.435±0.005.
Matches 1Pe series.
Fluorescence yield (arb. units)
20x10
+
(2,16)
1
1
1 1
2 2 7 kV/cm
15
6 kV/cm
5 kV/cm
10
5
4 kV/cm
3 kV/cm
F=0
0
65.00
65.20
A new series is observed, labelled 1. Photon energy (eV)
65.40
Another new broad series labelled 2 is observed (as in other geometry.)
Big increases in intensity at high n.
WUTA, Frascati, 8th October 2008
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How do we explain all this?
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First, there are selection rules.
Parallel geometry, ΔM=0
perpendicular 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.
WUTA, Frascati, 8th October 2008
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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.
WUTA, Frascati, 8th October 2008
Conclusions
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VUV spectroscopy is a powerful method for investigating the fluorescence
decay dynamics of He doubly excited states.
The wave functions in the excited state can be probed in detail
and 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.
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.
WUTA, Frascati, 8th October 2008
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Biomolecules: interaction with UV radiation CNR
Interest: damage by ionizing radiation
astrobiology: synthesis and destruction of pre-biotic molecules
in space (particularly Lyman α and He I)
conformation and dynamics of this class of molecules
mass spectrometry
There exists a large body of electron impact ionization:
why use UV radiation?
Because it is more specific – dipole selection rules apply.
WUTA, Frascati, 8th October 2008
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Photofragmentation with laboratory sources of UV
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glycine
proline
5
5
21.2 eV
4
21.2 eV
4
16.67 eV
3
2
16.67 eV
3
11.62 eV
2
10.0 eV
1
11.62 eV
10.0 eV
1
parent
ion
x10
9.56 eV
0
Intensity (arb. units)
0
20
40
60
9.56 eV
0
0
3
16.67 eV
2
11.62 eV
1
10.0 eV
0
40
60
80
m/z
100
60
80
100
120
Aliphatic amino acids and
Pro show strong
fragmentation, even close
to threshold.
Mostly loss of HCOO.
21.2 eV
20
40
80
4
0
20
120
140
leucine
(Our work and Lago et al,
Chem. Phys. 307(2004) 9).
WUTA, Frascati, 8th October 2008
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Methionine is different:
- loss of COOH is not the main channel
-at low energies, the parent ion is the dominant peak
Why?
CNR
6
C4H10N
parent ion
+
C4H10NCOOH
+
5
21.2 eV
4
16.67 eV
3
11.62 eV
2
10.0 eV
1
9.56 eV
0
0
20
40
60
80
O. Plekan et al,
Chem. Phys. 334 (2007) 53–63.
WUTA, Frascati, 8th October 2008
100
120
140
160
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UV photoionization removes an electron from
a valence orbital.
He I
5000
The HOMO of methionine has
S lone pair character. The S
atom accommodates the charge
without fragmenting.
Ar I
Kr I Xe I
nS
methionine
4000
Intensity (arb. units)
The Highest Occupied Molecular
Orbital of aliphatic amino acids
has nitrogen lone pair character.
Ne I
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nO
OO
3000
proline
2000
nN
1000
glycine
0
20.0
17.5
15.0
12.5
Binding energy (eV)
WUTA, Frascati, 8th October 2008
10.0
7.5
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Amino acids with aromatic groups.
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Phenylalanine
Tryptophan
Tyrosine
Tryptophan mass spectrum
5
c)
4
Intensity (arb. units)
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.
3
21.2 eV
16.67 eV
11.62 eV
2
1
10.0 eV
8.43 eV
0
0
50
100
150
m/z
WUTA, Frascati, 8th October 2008
200
250
Next step – coincidences.
INFM
500
any ion
2a"
+
D
+
E.g. Methanol,
CD3OH.
400
+
CD3 OD
+
1a"
4a'
+
DCO COD
+
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5a'
3a'
D2COH
2a'
+
D3COH
counts
300
200
100
0
26
24
22
20
18
16
14
12
10
electron binding energy in eV
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.
Work done at Spring-8, in collaboration with R. Richter, K. Ueda, G. Pruemper.
WUTA, Frascati, 8th October 2008
What is the conformational (folding) structure of a
free bio molecule?
Dilute species, biomolecules
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CNR
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).
“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.)
CD spectrum and secondary
structure of proteins.
WUTA, Frascati, 8th October 2008
Recently natural CD of small chiral
molecules investigated. Optimal conditions:
CD signal=[I(left)-I(right)]/I~0.04.
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
CD (arb. units) Intensity (arb. units) CD (arb. units) Intensity (arb. units)
Dichroism
Fig. 1
1.2
R(+)
1.0
a)
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0.8
0.6
0.4
0.2
0.0
0.06
R(+)
0.04
b)
0.02
0.00
-0.02
1.2
c)
1.0
S(-)
0.8
0.6
0.4
0.2
0.0
0.02
d)
0.00
-0.02
-0.04
S(-)
-0.06
4
5
6
7
8
9 10 11 12 13
Kinetic Energy (eV)
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)
WUTA, Frascati, 8th October 2008
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Proline, valence band spectrum
100x10
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-3
A_state
D parameter
80
60
40
20
0
500x10
6
3
8
10
12
14
Kinetic energy (eV)
16
40
B_state
D parameter
Signal, left
Signal, right
difference, x10
400
0
-20
200
-40x10
-3
6
100
8 10 12 14
Kinetic energy (eV)
16
0
40
-100
6
7
8
Kinetic energy
Dichroism from 0 to 4%.
9
10
D parameter
Intensity (arb. units)
300
20
0
-20
-40x10
WUTA, Frascati, 8th October 2008
C_state
20
-3
6
8 10 12 14
Kinetic energy (eV)
16
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Intensity (arb. units)
6
5
Valence band photoemission.
He I.
The two highest MOs are due
to two pairs of conformers.
4
3
2
1
0
10.0
9.0
Binding energy (eV)
8.0
0.44
I(8.9)/I(9.5)
3.80
ln (I(8.9)/I(9.5))
11.0
0.42
3.75
3.70
3.65
0.40
2.25
0.38
2.30
2.35
2.40
-3
2.45x10
Temperature (1/K)
405
410
415
420
425
430
Temperature (K)
435
440
By measuring spectra as a function of temperature,
can extract free energy difference, 6-9 kJ/mol.
WUTA, Frascati, 8th October 2008
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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.
WUTA, Frascati, 8th October 2008
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What science will be done?
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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.
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.
K.M. Shen et al, PRL
WUTA, Frascati, 8th October 2008
Microscopy: Pb/Au/Si(111)
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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.
WUTA, Frascati, 8th October 2008
Inverse model catalyst: CeO2/Cu
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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.
WUTA, Frascati, 8th October 2008
We can grow defect-free, well-ordered epitaxial
CeO2 layers on Cu(111)
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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?
WUTA, Frascati, 8th October 2008
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Resonant photoemission
Configurations
CeO2, Ce4+, (4d105p6) 4f0, resonates at hv=124.5 eV
Ce2O3, Ce3+, (4d105p6) 4f1, resonates at hv=122 eV
Ce metal, (4d105p6) 4f15d16s2, resonates at hv=122 eV
Ce
4+
Intensity, arb. units
Resonant process:
Ce 4d104fn  4d94fn+1  4d10fn
interferes with direct valence photoemission.
Ep=
115eV
122eV
124.5eV
CNR
Ce
0
Ce
3+
Ce
0
Ce 5d, 6s, 4f
O 2p
8
6
4
BE(eV)
2
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.
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0
DNA bases II: adsorption of
adenine, C5H5N5, on Cu(110)
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A DNA base on a metal surface: a prototypical bio/metal interface.
Has been studied by STM, vibrational spectroscopy, theory.
Adenine, C5N5H5
Q. Chen and N. V. Richardson, Nature Mat. 2 (2003) 324
Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219
D. J. Frankel, Q. Chen and N.V. Richardson, J. Chem. Phys. 124, 204704 (2006).
WUTA, Frascati, 8th October 2008
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)
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Theory agrees: bonding through amino group.
Molecule tilted up from surface, 18-26º.
Vibrational studies:
Q. Chen, D. J. Frankel, and N. V. Richardson, Langmuir 18 (2002) 3219
A. 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
Preuss et al, PRL 94, 236102 (2005)
WUTA, Frascati, 8th October 2008
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N 1s photoemission
CNR
0.6 ML
> 1 ML
Ordering
Desorb excess
Deposited
Gas phase
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.
WUTA, Frascati, 8th October 2008
NEXAFS of adenine:
Near Edge X-ray Absorption Fine Structure Spectroscopy
150x10
CNR
3
1.0x10
Grazing incidence
100
50
Intensity (arb. units)
Intensity (arb. units)
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
0


6

0.8
Grazing incidence
0.6
0.4
Normal incidence
Normal incidence
395
400
405
410
415
Photon energy (eV)
0.3 ML, annealed 430 K
420
395
400
405
410
Photon energy (eV)
415
1 ML, annealed 430 K
Fractional monolayer: molecule lying almost parallel to the surface.
Saturated monolayer: molecule(s) tilted
WUTA, Frascati, 8th October 2008
420
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Collaborators.
Oksana
CNR
The Gas Phase team at Elettra
O. Plekan, V. Feyer, R. Richter, M. Coreno, M. de Simone,
Materials Science (Czech) Beamline
T. Skala, V. Chab, F. Sutara, V. Matolin
Tomas
Vitaliy
Robert
Marcello
WUTA, Frascati, 8th October 2008
Monica
Free Electron Lasers.
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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 ps
50 Hz
GW power levels, 1014 photons/pulse
Start operation at the beginning of 2009
WUTA, Frascati, 8th October 2008
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If you can control the timing of light, you can learn new things…
CNR
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.
WUTA, Frascati, 8th October 2008
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Two photon double ionization
Physics is different for two photon (hn>39.5 eV)
and many photon (hn~ few eV) double ionization.
->control of relative field strengths.
WUTA, Frascati, 8th October 2008
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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.
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Our schematic setup.
WUTA, Frascati, 8th October 2008
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Setting up with Synchrotron Radiation:
two photon spectroscopy of neon
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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.
WUTA, Frascati, 8th October 2008
Where is Fermi being built?
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WUTA, Frascati, 8th October 2008
Brightness
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P ~ Ne2
FERMI@Elettra FEL
1010 Increase
ELETTRA
Storage Ring FEL
ELETTRA
WUTA, Frascati, 8th October 2008
P ~ Ne