1. Quantum Efficiency Dependence on the Incidence Light Angle in Copper Photocathodes: Vectorial Photoelectric Effect
Emanuele Pedersoli
Università Cattolica del Sacro Cuore
Sede di Brescia
Dipartimento di Matematica e Fisica
ELPHOS Lab

2. Aims of the work
Study of photocathodes for the production of short electrons bunches through irradiation with femtosecond laser pulses e- hn

3. Ultrafast X-Ray Facility
Aims of the work
Electrons bunches can be accelerated in femtosecond pulsed X ray sources
Collaboration with the
LUX project
Linac/Laser-Based
Ultrafast X-Ray Facility
Lawrence Berkeley
National Laboratory

4. Features of copper Uniformity with the copper injector Long lifetime
Fast photoemission response
s 10-13s
Small quantum efficiency
W. E. Spicer et al., Modern theory and applications of photocathodes, 1993

5. Fourth harmonics hn = 6.28 eV
Photoemission from states up to eV under the Fermi energy
Direct photoemission also with the sample work enhanced by an imperfect cleanliness
Laser exposure contribution to the sample cleanliness Ef Ev
F = 4.65 eV
Occupied states
1.63 eV
Tempo
Photoemission signal
Contamination
M. Afif et al., Applied Surface Science (1996)

6. Quantum efficiency determination
QE = n/f = K . I/A
n = photoemitted electrons
f = incident photons
I = sample electric current
A = ∫Vdt = photomultiplier output
K = 31 Vs/A

7. Interferential filter
Experimental Setup
Ultrahigh vacuum chamber
Oscilloscope
Picoammeter
Optical flange
Mirror 2
Mirror 4
Mirror 3
Beam splitter
Mirror 1
Photomultiplier
with amplifier
Time of flight spectrometer
Interferential filter
l/2
Polarizer
LASER
Mirror
Pinhole
Second harmonics generation
Computer
Fourth harmonics generation
Dispersion prism
Mirror
Pulse width: 150 fs Repetition rate: 1 kHz
Average Power: 0.6 W Wavelength: 790 nm

8. Quantum efficiency measurements
Measurements of I and A
varying the light intensity
Linear fit of I = (QE/K).A (r > 0.99)
QE from the angular coefficient
QE from the mean on I/A values

9. Quantum efficiency Cu polycrystal Normal incidence QEmin = 0.80´10-4
QEmax = 1.17´10-4

10. Quantum efficiency Cu polycrystal Incidence 30° p polarization
QEmin = 1.98´10-4
QEmax = 3.25´10-4

11. Quantum efficiency Cu(111) single crystal Incidence 30° p polarization
QEfit = 2.57´10-4
QEmed = 2.58´10-4

12. Dependence on incidence angle
Cu polycrystal Single crystal Cu(111)
Agreement with the vectorial photoelectric effect
R. M. Broudy, Physical Review B 1, 3430 (1970)
J. P. Girardeau Montaut et al., Applied Physics Letters 63(5), 699 (1993)

13. Vectorial photoelectric effect R. M
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970)
Consider a light beam impinging on the sample with an angle q
The vectors representing it can be decomposed as shown in figure Ep E q
Vacuum k
Sample Ep
Ep^ Es Es
Ep|| kt kt

14. Vectorial photoelectric effect R. M
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970)
Call e|| the absorbed energy due to the Es and Ep|| components, parallel to the surface
Call e^ the absorbed energy due to the Ep^ component, perpendicular to the surface Es Ep
Ep^
Ep|| kt k q
Vacuum
Sample E

15. QE(q) = a[e||(q) + re^(q)]
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970)
If we suppose the photocurrent to be simply proportional to the absorbed light energy, but with two different efficiencies for e|| and e^, the quantum efficiency can then be written as
QE(q) = a[e||(q) + re^(q)] Es Ep
Ep^
Ep|| kt k q
Vacuum
Sample E

16. Vectorial photoelectric effect R. M
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970)
Decomposing with respect to the light polarization
e||(q) = ep||(q) + es(q)
e^(q) = ep^(q) Es Ep
Ep^
Ep|| kt k q
Vacuum
Sample E

17. Vectorial photoelectric effect R. M
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970) Es Ep
Ep^
Ep|| kt k q
Vacuum
Sample E

18. Vectorial photoelectric effect R. M
Vectorial photoelectric effect R. M. Broudy, Physical Review B 1, 3430 (1970)

19. Evidence of vectorial photoelectric effect on Copper E. Pedersoli et al., Applied Physics Letters 87, (2005)
Cu polycrystal Single crystal Cu(111)
Experimental data are well fitted by the just shown equations

20. Possible explanations
Symmetry does not affect this effect, because the single crystal and the polycrystal show the same behavior
Also roughness can be excluded: samples with different surfaces present the same phenomenon

21. Possible explanations
At the interface, the electromagnetic field perpendicular to the surface spatially varies on a scale of ∼ 1 Å
The dipole approximation is no longer applicable in this region
An additional term appears that enhances photoemission
P. J. Feibelman, Phys. Rev. B 12, 1319 (1975) P. J. Feibelman, Phys. Rev. Lett. 34, 1092 (1975)
H. J. Levinson, E. W. Plummer, and P. J. Feibelman, Phys. Rev. Lett. 43, 952 (1979)

22. Conclusions
Quantum efficiency of copper depends on the light incidence angle in a way that can not be explained by Fresnel absorption only
Vectorial photoelectric effect: light with electric field perpendicular to the sample surface is more efficient in producing photoemission
This can be due to a spatial variation of the field at the surface: the dipole approximation is not valid

23. ELPHOS Lab Fulvio Parmigiani Gabriele Ferrini Stefania Pagliara
Claudio Giannetti
Gianluca Galimberti
Università Cattolica del Sacro Cuore
Sede di Brescia
Dipartimento di Matematica e Fisica