Photoreactions of Hybrid Photocathodes Browse EMSL Capabilities at: Wayne Hess Pacific Northwest National Laboratory Richland Washington, USA 99352 Photocathode Physics for Photoinjectors Workshop Cornell University, Ithaca New York, October 7-10, 2012 Funded by DOE Office of Science’s Office of Biological and Environmental Research Located in Richland, Washington Browse EMSL Capabilities at: www.emsl.pnl.gov

Outline *Excited state reactions of photocathode coatings: CsBr and KBr *Effects of metal on CsBr and KBr photophysics NaCl on Ag(100) MgO on Ag(100) Pivetta et al. Phys. Rev. B 72, 1154041 (2005) Schintke et al. Phys. Rev. Lett. 87, 276801 (2001)

Photoemission from Coated Metals Multilayer film of CsBr show greatly enhanced quantum efficiency Quantum Efficiency Enhancement at 4.8 eV Bare Metal Coated Factor Cu 5.0 x10-5 3.0 x10-3 50 Nb 6.4 x10-7 5.0 x10-4 800 Maldonado et al. J. Appl. Phys. 107, 013106 (2010); Microelectron. Eng. 86, 529 (2009) Enhancement process requires photoactivation Photoreaction of CsBr could be important! CsBr film 5 to 25 nm Metal substrate JR Maldonado et al. Microelectronic Engineering 86, (2009) 529 & references therein 3

Work Function and Activation of Cu:CsBr and KBr Activation Using 325 nm Laser (3.8 eV) Sample Work Function (eV) Bare Cu 4.3 Cu:CsBr (7 nm) 3.8 Cu:CsBr (Activated) 3.7 Cu:KBr (7 nm) 4.1 Cu:KBr (Activated) 4.0 4

Activation Mechanism & Photoreaction of CsBr on Cu Insulating film on surface significantly reduces work function! Does Cs metal accumulate on surface? VB EF EVBM Metal CsBr ECBM E0 DF CB F Calculated Work Function Reduction DF = DFCT + DFComp + DFSR F DF MgO/Ag(100) 2.96 −1.27 MgO/Mo(100) 2.15 −1.74 MgO/Al(100) 2.86 −1.46 BaO/Ag(100) 2.03 −2.20 BaO/Pd(100) 1.99 −3.17 BaO/Au(100) 2.33 −2.80 Prada et al. PRB 78, 235423 (2008) Also calculated for Au, Mo, Pd, and Pt Activation e- hn ~ 3.5 eV ~0.4 eV ~0.1 eV F center band + e- 5

Pump-probe experiment Laser Induced Reactions of Alkali Halides MCP Detector Time-of-Flight Mass Spec UV Excitation Ion Extraction Laser Ionization + Sample –V Pump-probe experiment Hyperthermal: Surface exciton mechanism Bulk mediated thermal mechanism Pump Probe  6

Bulk and Surface Reactions (1) Laser excitation of surface (1) Laser excitation of bulk (2) Creation of surface exciton (2) Creation of bulk exciton (3) Desorption of hyperthermal Br-atom (3) Exciton self trapping (4) Formation of F-H pair (5) Diffusion of H center to surface (6) Desorption of thermal Br-atom “Hyperthermal” “Thermal” Br Br e Br2– Cs+ Br– e- Cs+ Br– Br2– Br2– Br– Br– Br2– Br2– Cs+ Br- Cs+ e Cs+ Br– Cs+ Br– e- 7

What About Metal Atoms? Thermal Cs atom desorption observed by Mass Spec 1. Excite surface F center (~2 eV) 2. F* center neutralizes Cs ion 3. Neutral Cs atom desorbs thermally for layer-by-layer removal Cs+ Br– F hn Cs+ Br– F* Charge transfer Cs+ Br– Cs0 Neutral Cs atom desorption

What About Cs Atoms (cont) ? Diffusion of anion vacancies may lead to desorption of Cs atoms: 1. Excite bulk F center 2. F* center diffuses to surface 3. F* neutralizes surface Cs-atom and subsequent “thermal” desorption F center photoexcitation Cs+ Br– F hn Cs+ Br– F* F* center diffusion Cs+ Br– Surface reaction/desorption F* These mechanisms lead to layer-by-layer material removal and can maintain CsBr stoichiometry.

XPS of Cu:CsBr Before / After Photoactivation with 325 nm (3 XPS of Cu:CsBr Before / After Photoactivation with 325 nm (3.8 eV) Laser Preliminary Results: *Cs and Br peak intensities decrease *Cu peak intensities increase Cu0 and Cu+1 Cu+2 10

But…..The Cu Substrate can change the CsBr photophysics! KBr (7 nm) on LiF(100) CsBr (7 nm) on Cu(100) KBr on Cu(100) CsBr (9 nm) on KBr(100) The hyperthermal surface photoreaction is changed dramatically!

Impact of Metal Substrate on Photophysics Excitation of CsBr on Cu Br2– e– + – Br hn * Br - Metal – Alkali Halide Band Schematic Cs+ Vacuum Level CB Surface exciton hn= 7.9 eV hn=6.4 eV Fermi Level F Center VB Metal Alkali Halide Impact on thermal bulk reaction? To be continued…. 12

Summary and Acknowledgements Thin film coated metals have highly modified optical/chemical properties - QE is dramatically increased - Work function is reduced - Studies of photoreaction of CsBr and alkali halides on metals and QE enhancement mechanism ongoing Acknowledgements: Weidong He, Tim Droubay, Alan Joly Pacific Northwest National Lab Juan Maldonado Stanford University Kathy Harkay, Karoly Nemeth Argonne National Lab US Department of Energy, EMSL 13

Bulk or Surface Excitation of KBr Above band gap excitation Uncontrolled Br emission Surface specific excitation Only Hyperthermal halogen-atom emission Bulk exciton bands Absorption Absorption Band gap Animation … Selective excitation of surface atom at terraces Photon energy Energy (eV) 7 8 9 10 14

Bulk or Surface Excitation of KBr Above band gap excitation Uncontrolled Br emission Surface specific excitation Only Hyperthermal halogen-atom emission Bulk exciton bands Absorption Absorption Band gap Animation … Selective excitation of surface atom at terraces Photon energy Energy (eV) 7 8 9 10 15