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

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

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

Features of copper Uniformity with the copper injector Long lifetime Fast photoemission response tCu @ 10-15s tsemiconductor @ 10-13s Small quantum efficiency QECu @ 10-4 QEsemiconductore @ 10-1 W. E. Spicer et al., Modern theory and applications of photocathodes, 1993

Fourth harmonics hn = 6.28 eV Photoemission from states up to 1.63 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 96-98 (1996) 469-473

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

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

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

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

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

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

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)

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

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

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

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

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

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

Evidence of vectorial photoelectric effect on Copper E. Pedersoli et al., Applied Physics Letters 87, 081112 (2005) Cu polycrystal Single crystal Cu(111) Experimental data are well fitted by the just shown equations

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

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)

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

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