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Title: A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes

Abstract

Even though theoretical estimates predict response times for the photo emission process of electrons from a negative electron affinity GaAs photo emitter in excess of hundreds of picoseconds, recent measurements found electron bunch durations of 40 ps or less. This work presents precise measurements of picosecond electron bunches from a negative affinity bulk GaAs photo cathode and develops a model which explains the measured bunch durations as well as the observed bunch shapes. The bunch shape turns out to be independent from the quantum efficiency of the photo emitter.

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA (US)
Sponsoring Org.:
USDOE Office of Energy Research (ER) (US)
OSTI Identifier:
755358
Report Number(s):
DOE/ER/40150-1320; JLAB-ACP-98-04
TRN: US0002432
DOE Contract Number:
AC05-84ER40150
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 86; Journal Issue: 4; Conference: Conference title not supplied, No location, No date; Other Information: PBD: 27 Oct 1998
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; BEAM INJECTION; ELECTRON EMISSION; PHOTOCATHODES; GALLIUM ARSENIDES; ELECTRON SOURCES; MATHEMATICAL MODELS; QUANTUM EFFICIENCY

Citation Formats

P. Hartmann, J. Bermuth, D. v. Harrach, J. Hoffmann, S. Kobis, E. Reichert, K. Aulenbacher, J. Schuler, and M. Steigerwald. A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes. United States: N. p., 1998. Web.
P. Hartmann, J. Bermuth, D. v. Harrach, J. Hoffmann, S. Kobis, E. Reichert, K. Aulenbacher, J. Schuler, & M. Steigerwald. A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes. United States.
P. Hartmann, J. Bermuth, D. v. Harrach, J. Hoffmann, S. Kobis, E. Reichert, K. Aulenbacher, J. Schuler, and M. Steigerwald. Tue . "A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes". United States. doi:. https://www.osti.gov/servlets/purl/755358.
@article{osti_755358,
title = {A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes},
author = {P. Hartmann and J. Bermuth and D. v. Harrach and J. Hoffmann and S. Kobis and E. Reichert and K. Aulenbacher and J. Schuler and M. Steigerwald},
abstractNote = {Even though theoretical estimates predict response times for the photo emission process of electrons from a negative electron affinity GaAs photo emitter in excess of hundreds of picoseconds, recent measurements found electron bunch durations of 40 ps or less. This work presents precise measurements of picosecond electron bunches from a negative affinity bulk GaAs photo cathode and develops a model which explains the measured bunch durations as well as the observed bunch shapes. The bunch shape turns out to be independent from the quantum efficiency of the photo emitter.},
doi = {},
journal = {Journal of Applied Physics},
number = 4,
volume = 86,
place = {United States},
year = {Tue Oct 27 00:00:00 EST 1998},
month = {Tue Oct 27 00:00:00 EST 1998}
}
  • The transverse momentum of photoelectrons released from a negative electron affinity GaAs cathode is small compared with other thermonic or field emission electron sources. A low photoelectron transverse momentum in vacuum promises highly focused, low-energy beams useful for numerous applications. A simplified theory for electron emission from GaAs predicts much lower electron transverse momentum than those previously measured experimentally. To address this theory{endash}experiment mismatch, Monte Carlo based calculations were compared with experimental data. We checked the possibility of there being electron scattering in the Cs,O layer; however, none of the scattering checked (isotropically distributed, cosine distributed, and Rutherford scattering) properlymore » fit the experimental results. The assumption of conservation of the parallel component of the crystal momentum {ital k} during the emission is mainly believed to be responsible for the calculation-theory disagreement. The best simulation-experiment fit was obtained through a relaxation of the conditions imposed on the transverse momentum inside and outside of the semiconductor when an ideal interface is considered. We obtained the best results by assuming that the effective mass of the electron inside GaAs is equal to the effective mass of the electron in vacuum. Two independent experiments confirmed that in both cases, this {ital same}-{ital mass} approximation gives the best fit. The {ital physical} meaning of this is not clear, but it seems to be related to the amorphous nature of the Cs,O layer. We conclude that the way to get lower electron transverse energy spread cathodes is to study alternative activation methods and new materials with smaller effective electron masses. {copyright} {ital 1996 American Institute of Physics.}« less
  • No abstract prepared.
  • At the pulsed electron gun testfacility at MAMI we have measured electron bunches from GaAs samples with different layer thicknesses. We observe an increasing dc-polarization with decreasing layer thickness. From the pulse measurements this behaviour may be explained in terms of bunchlength and spin relaxation time.
  • Energy distribution of the photoelectrons from InP(100) photocathodes are investigated with a photon energy range from 0.62eV to 2.76eV. When the photon energy is less than 1.8eV, only electrons emitted from the Gamma valley are observed in the energy distribution curves (EDC). At higher photon energies, electrons from the L valley are observed. The angular dependence of the electron energy distributions of InP and GaAs photocathodes are studied and compared. The electrons emitted from the L valley have a larger angular spread than the ones from the Gamma valley due to the larger effective mass of the L valley minimum.