Journal Club 24.08.2010

1. Journal Club 24.08.2010

2. Photoemission and the grand challenges • Energy Problem: Mankind needs ~ 20TW of energy • Solar energy, hydrogen fuel  fundamental processes involving electron motion • Proteins • 3D structure of macromolecules -> understanding electron dynamics • Emergent Phenomena • Phenomena which are not properties of individual components, but of the assembly of the components • -> strongly correlated electron systems, e.g. high T_c superconductors • Ultra-small • Imaging of single atoms • Ultra-fast • Femtochemistry, Attosecond Science / Spectroscopy Why is Photoemission useful? • Electronic, magnetic, chemical, mechanical, optical, thermal, and structural properties of matter depend on electron behaviour (and atom location)

3. What is Photoemission? • Fundamental process of light matter interaction • Discovered 1887 by Heinrich Hertz • Explained 1905 by quantum nature of light by Einstein • Wave particle duality of light

4. Photoemission in Surface Science • Spectroscopy of Bandstructure! • k-space-resolution by ARUPS

5. When does Photoemission start? • One of the very fundamental questions!! • Extreme importance in time-resolved spectroscopy of matter • Questions: • CW- quasi-CW - excitation -> no time-zero defined • Using light XUV pulses approaching the atomic unit of time (here <200as) • -> When does photoemission start??

6. Experiment: Characterise Complex electron wavepackets • Wigner function = phase space representation of wavepacket (also laser pulse) • Wigner function of two delayed & coherent wavepackets: • (W(t,w) is real valued! Negative areas!) 2p 2s

7. Wigner examples • Distinct spectrum, overlap in time • Example: Two-colour fields • Overlapping spectrum, time-delay • Example: SI, SPIDER

8. Experimental method here:Attosecond streaking

9. Experimental method here:Attosecond streaking / FROG-CRAB • Single XUV attosecond pulse creates coherent 2s and 2p electron wavepackets • These get streaked by a strong (I~TW/cm^2) IR field • Momentum transfer by A(t) === phasemodulationin IR field • Analysis of the CRAB-trace with a PCGPA algorithm retrieves the spectral phase and hence group delay between 2s and 2p components

10. Can FROG-CRAB retrieve a delay? • The two electron wavepackets (2s, 2p) are separated by more than their bandwidth & streaking field photon energy (30eV == 18(!!) 750nm photons) • Interesting pulse characterisation problem. Not intuitively clear that this works! 2p Fringes = coherence information 750nm == 1.65 eV 2s XUV pulse duration > delay

11. Can FROG-CRAB retrieve a delay? 20 as 200 as 1000 as Overlap lost => no fringes => no directly accessible coherence information

12. Recap: FROG vs. SPIDER • FROG = spectrographic time-domain method • SPIDER = interferometric spectral domain method • FROG • SPIDER • FROG • SPIDER • FROG • (Also: CRAB!) • SPIDER • SEA-CAR-SPIDER Keusters, D., Tan, H., O'Shea, P., Zeek, E., Trebino, R., Warren, W. S., et al. (2003). Relative-phase ambiguities in measurements of ultrashort pulses with well-separated multiple frequency components. J. Opt. Soc. Am. B, 20(10), 2226-2237. OSA. Retrieved from http://josab.osa.org/abstract.cfm?URI=josab-20-10-2226.

13. Can FROG-CRAB retrieve a delay? • Some quick CRAB simulations… Group delay: 20 as For delays in this paper it works. For 1000 as delay PCGPA fails!!

14. Experimental setup

15. Experimental setup – some notes • 3.3 fs, 750 nm driving pulses. • -> 100 fold XUV photon flux increase (>10^11 photons/sec) • To separate 2s and 2p electrons, need to reduce the emitted XUV bandwidth! • Mirror: “lanthanum/molybdenum multilayer XUV” not Mo/Si • Metal foil: “a 150-nm-thick metallic foil” -> which material?? • -> bandpass filter centered at 106 eV, 14 eV FWHM bandwidth • -> XUV pulse duration < 200 as. • TOF electrostatic lens optimised for electrons 40 to 70 eV • From different paper: Electrons released with an initial momentum that is nearly parallel to the polarization direction of the NIR streaking field are collected by a time-of-flight (TOF) electron spectrometer that has a resolution of ~0.5 eV in the 30-100 eV range. An electrostatic lens is used to widen the angle over which electrons are collected to approximately ±20 degrees. This results into a five-fold increase in the count rate within the spectral bandwidth of the attosecond pulse. (typically the spectrometer collects ~500 counts/s without the electrostatic lens and some 2500-3000 counts/swith the lens activated).

16. Some FORTRAN code

17. Results:

18. Results: Only subset of CRAB scans with less than 3% satellite pulse content used

19. Some questions • Why are the wavepackets from 2p ans 2s differently chirped?? • Implications for FROG-CRAB? • It is normally assumed that the XUV-photon wavepacket gets mapped 1:1 to an electron wavepacket • They say: “We reproducibly observed a difference between the energy sweeps of the 2s and 2p wave packets on the order of several thousand attoseconds squared.” … “however, that the retrieved energy sweep, in contrast to the average group delay, is sensitive to the electrostatic lens. Hence, the energy sweeps cannot be reliably inferred from the current measurements”

20. References • The article in science: http://www.sciencemag.org/cgi/content/abstract/328/5986/1658 • Supporting online material: http://www.sciencemag.org/cgi/content/full/sci;328/5986/1658/DC1 Further Reading: • Perspectives: When Does Photoemission Begin? DOI: 10.1126/science.1191842 • Delayed Time Zero in Photoemission: New Record in Time Measurement Accuracy http://www.sciencedaily.com/releases/2010/06/100630110910.htm • Electrons Are Late Starters: Contrary to Previous Assumptions, Electrons Are Catapulted out of an Atom During Photoemission With a Delay http://www.sciencedaily.com/releases/2010/06/100630110910.htm Next Journal club?

21. A Supercomputer