Core! What a Scorcher!

1. Core! What a Scorcher! Inner Shell Processes in Molecules P.A. Hatherly University of Reading

2. Outline • Core ionisation phenomena in molecules • Electronic and fragmentation processes • Techniques • Soft x-ray sources, data collection • Example results • State-selective experiments • Electron dynamics

3. M Auger Electron M L L M Energy L Photo-Electron K K K KLL Auger Decay Shake-Up Post-Collision Interaction Core Ionisation Phenomena • Electronic Processes

4. O 1s S 2p C 1s O C S Core Ionisation Phenomena • Fragmentation Processes • Photon energies correspond to inner shell ionisation energies of atoms • Energy localisation can occur • e.g., OCS - Three edges

5. Site Specific Ionisation - CO2 C 1s Photoelectron 2+ Auger electron ? Major Minor + + + +

6. Soft X-Ray Sources • Synchrotron Radiation • SRS: 5U.1, MPW6.1 • MAX II: I411

7. Synchrotron Radiation • SRS, Daresbury

8. Soft X-Ray Sources • Multipole wigglersand undulators

9. MPW 6.1 PHOENIX • XUV/Soft x-ray beamline on multipole wiggler 6 • Joint Reading/UMIST/Daresbury project • Large photon energy range • (40 - 350 eV) • High flux and resolving power • ~1013 - 1014 ph/s/100mA/0.1%BW • ~10,000 resolving power • First results June 2001

10. 1.2E+15 MPW6, 7.5 mrad fan 1.0E+15 8.0E+14 6.0E+14 5U, full aperture, tuning envelope 4.0E+14 2.0E+14 SRS bending magnet, 7.5 mrad fan 0.0E+00 0 50 100 150 200 250 300 350 400 450 500 Photon energy, eV MPW6.1 PHOENIX • Performance - Wiggler Output

11. MPW6.1 PHOENIX • Performance - Beamline MPW6 5D

12. MPW 6.1 - PHOENIX • Carbon contamination

13. Data Collection • Single-Particle Detection • Ion drift tubes, threshold and energetic electron analysers • Multi-Particle Detection • Coincidence techniques • Threshold Electron-Ion Coincidence • Auger Electron-Ion Coincidence

14. Threshold Electron Detection • Detect electrons with < 10meV kinetic energy • Tune photons to exactly the energy required • State selectivity • Only one initial state is selected • In conventional PES, many states are excited

15. Auger Electron Detection • Detect electrons with characteristic energies • For C 1s ionised molecules, typically ~250 eV • Auger spectrum independent of photon energy • Intensities may vary • Selects final state

16. Ion Detection • Wiley and McLaren configurationwhere:A and B are constants dependent on the geometry,m and q are the mass and charge of the ion, V is the potential applied to the field-free region and;where Pll is the component of momentum parallel to the drift tube axis +40 V 1 cm -40 V 1 cm 5.5 cm -97.5 V 1 cm 1 cm MCPs

17. t1 t2 Coincidence Studies Electron signal Start Electron detector Stop TDC +V1 SR Gating electronics -V1 Electron rate -V2 Ion rate Counter Ion drift tube Ion signal electrons t1 t2 ions

18. Example Results • Triatomic Molecules • OCS • Threshold electron - ion studies • CO2 • Auger electron - ion studies • Satellite threshold electron structure

19. OCS - Site-Specific Ionisation Strong OCS2+ at S 2p edges Absent at C1s, but returning at O 1s Variation in yields of C+

20. CO2 - Auger Electron - Ion Low Auger energy, high fragmentationHigh Auger energy,low fragmentation Low Auger energy, energetic O+High Auger energy,low energy O+

21. CO2 - Auger Electron-Ion

22. CO2 Shake-up Satellites • Shake-up satellites in carbon dioxide • Core ionisation above threshold • sufficient energy to excite a second electron • Auger decay fills core hole • electron dynamics • timescales of decay

23. 0.015 * Threshold electron yield p C 1s resonance Total ion yield 0.010 C 1s Threshold (a) 0.015 TPE yield (arb.units) Shake-up satellites 0.005 Double excitation 0.010 0.000 Total ion yield (arb. units) (b) 0.005 * s C 1s resonance 0.000 285 290 295 300 305 310 315 320 Photon Energy (eV) CO2 - C 1s Satellite TPES

24. CO2 - C 1s Satellites TPES S4 S2 S3 S1 S’ S0

25. CO2 - C 1s Satellites TPES • S0 - S4 seen in photoelectron studies • S’ is a new feature, only seen in TPES • Origin? • Split from either S4 or S1? • New transition? • Absolute cross sections obtained • previously only down to 5 eV above threshold

26. CO2 - C 1s Satellites TPES • Electron dynamics • Identify satellites with known states of molecular ions • Infer fast versus slow core hole decays via “Frozen Core” approach • relative timescales of threshold electron escape, shake-up and Auger decay

27. Electron Dynamics • Fast processes match CO2+ • photoelectron escape and shake-up on the sametimescale as Auger decay • Inner core looks like C

28. Electron Dynamics • Slow processes match NO2+ • photoelectron escapes beforeAuger decay • Inner core looks like N

29. Conclusions • Core processes in molecules provide insight into energy localisation and transport • Site-specific processes • State-to-state studies via threshold and Auger electron coincidence experiments • Study of electron dynamics via satellite states

30. Conclusions • Many new possibilities opening in the future • Study of transient species • Application to organic systems • New sources permitting new science • FELs - multiphoton processes in the XUV/SXR • Attosecond (10-18s) lasers allowing direct probes of electron dynamics

31. With Thanks to: • Research Students • Dan Collins, Barry Fisher, Mark Thomas • International Colleagues • Marek Stankiewicz (Poland), Jaume Rius i Riu (Spain, Sweden), Peter Erman and Elisabeth Kallne (Sweden) • Technical Support • Mick Millard • CLRC staff at Daresbury • Especially Frances Quinn (MPW6.1 Project Manager) • EPSRC • Funding for beamline construction and research programme