Study of negative electron affinity photocathodes Student: Brian Henderson Advisors: Ivan Bazarov and Yulin Li November 10, 2018November 10, 2018 REU 2009 CLASSE

Beyond Einstein… GaAs He told us that for photoelectrons KE = hn - f True for metals, but generally not true for semiconductors Took over 50 years before a comprehensive understanding of photoemission process was obtained Still, no one can predict quantitatively what energy distributions will come out from a given photocathode KE << hn - f November 10, 2018November 10, 2018 REU 2009 CLASSE

Why we care? Beam Brightness: Flux/((source area)*(solid angle)) ‘Cold beams’ – ones with negligible transverse momentum spread A number of large scale accelerator projects depend on photoinjectors, in which the electrons are produced from photocathodes The transverse energy distribution of the photoelectrons sets the upper limit on beam brightness achievable from these devices The longitudinal energy distribution provides insights into the physics of photoemission November 10, 2018November 10, 2018 REU 2009 CLASSE

1) Design 2D spectrometer APL 78, 2721 (2001) E/B = const a Provides simultaneous measurement of the transverse and longitudinal energy spectra of photoelectrons Brian will come up with a practical design & specifications and understand limitations to the resolution of the device November 10, 2018November 10, 2018 REU 2009 CLASSE

Negative Electron Affinity after Cs before Cs Monolayer (or few) of Cesium atoms deposited on GaAs crystal: strong dipole field allows photoelectrons to escape Cs alone is not stable on the surface, instead use CsF or Cs2O November 10, 2018November 10, 2018 REU 2009 CLASSE

2) Making a GaAs Cathode Get a GaAs cathode atomically clean Grow an active electron emitting layer by dosing GaAs cathode w/Cs and NF3 (thus Yo-Yo method) Achieved QE > 10%, but dark lifetime not good enough. Poor lifetime seems to be due to some sort of surface reaction Yo-Yo Activation November 10, 2018November 10, 2018 REU 2009 CLASSE

What’s on the Cathode Surface ? NH3 Thermal Desorption Spectroscopy SR Photoelectron Spectroscopy APL 92, 241107 (2008) It is commonly believed that the Cs-NF3 activated layer consists CsF, but evidences indicated unstable bonded nitrogen on the surface Is it better to use other F-carrying molecules : XeF2 to substitute NF3 November 10, 2018November 10, 2018 REU 2009 CLASSE