Evidence of vectorial photoelectric effect on copper Page: 2 of 9
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The advent of the 4th generation free electron lasers (FEL) sources [1-3] triggered several
important technical questions. A fundamental issue regards the photocathode material for
the laser-driven photoinjector devices, to obtain short electron bunches with high charge
density and low emittance. Metal photocathodes are good candidates, having a high re-
liability, long lifetime and a fast time response (1-10 fs). However, two major drawbacks
limit their usefulness, the small quantum efficiency (QE) and the high work function (b),
requiring light source in the ultraviolet (UV) for efficient linear photoemission.
In this Letter we study the experimental conditions to maximize the QE of Cu photocath-
odes using UV short laser pulses from the quadrupled output of an amplified Ti:Sapphire
laser. The QE for linear photoemission in the femtosecond regime is measured as a function
of the angle of incidence 0 in the angular range -55 < < +800, both in s and p polar-
izations. The maximum quantum efficiency Y ~ 4 x 10-4, obtained with p polarization at
O = 650, is four times the value at normal incidence.
The QE dependence on angle of incidence and light polarization is a long standing problem
[4-8] that largely remains to be understood. Our data are well fitted by a phenomenological
model [6] that keeps into account only light absorption, without any explanation in terms of
microscopic quantum physics. A justification of the phenomenological model based on the
calculations of the conductivity tensor for a jellium model is proposed.
The photoemission from a polycrystalline Cu sample and a Cu(111) single crystal is
investigated with 150 fs laser pulses with a photon energy of 6.28 eV, obtained by two
successive doubling processes of the Ti:Sapphire fundamental frequency (hv = 1.57 eV)
in 73-barium-borate (BBO) crystals. The second doubling process is obtained out phase-
matching in a thin (200 pm) BBO crystal. The fourth harmonic is selected by dispersing
the doubling crystals output with a MgF2 prism, with minimal temporal and pulse front tilt
distortions.
We do not use a more efficient third harmonic conversion to obtain linear photoemission
from Cu (3hv = 4.71 eV, b = 4.65 eV for polycrystalline Cu [9]) because of the onset of
multiphoton regime upon a work function increase due to sample contamination. Moreover,
an effective laser-induced oxide removal and contaminants chemical-bond breaking obtained
with UV short laser pulses [10, 11] improves with shorter wavelengths [10]. Working with a
6.28 eV photon energy should thus help to increase the duty time of machines based on Cu
photocathodes.2
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Pedersoli, E.; Banfi, F.; Ressel, B.; Pagliara, S.; Giannetti,C.; Galimberti, G. et al. Evidence of vectorial photoelectric effect on copper, article, May 27, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc787623/m1/2/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.