1. 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:

2. 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, (2005)
Schintke et al. Phys. Rev.
Lett. 87, (2001)

3. 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 x x
Nb x x
Maldonado et al. J. Appl. Phys. 107,
(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

4. 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

5. 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) −1.27
MgO/Mo(100) −1.74
MgO/Al(100) −1.46
BaO/Ag(100) −2.20
BaO/Pd(100) −3.17
BaO/Au(100) −2.80
Prada et al. PRB 78, (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

6. 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

7. 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

8. 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

9. 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.

10. 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

11. 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!

12. 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

13. 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

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 14

15. 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