WUTA08 Laboratori Nazionali di Frascati Frascati, 8-10 October 2008

1. Probing free metallic and carbon clusters with VUV photons. P. Piseri1,2,3 (piseri@fisica.unimi.it), G. Bongiorno1,2,3, T. Mazza1,2,3 , L. Ravagnan1,2,3, M. Amati1,2,3, M. Devetta1,2, C. Lenardi2,3,4, and P. Milani1,2,3,M. Coreno5,6, M. De Simone6, P. Rudolf 7, F. Evangelista7 1 Dipartimento di Fisica, Università degli Studi di Milano. 2 CIMAINA, Università degli Studi di Milano. 3 CNR-INFM 4 Dipartimento di Farmacologia, Università degli Studi di Milano. 5 CNR-IMIP, Area della ricerca di Roma 1 6 Laboratorio Nazionale TASC INFM-CNR 7 University of Gröningen, The Nederlands Laboratorio Getti Molecolari e Materiali Nanocristallini - LGM Director: P. Milani (pmilani@mi.infn.it) WUTA08 Laboratori Nazionali di Frascati Frascati, 8-10 October 2008

2. Outline Core-level techniques became available for free-clusters with 3rd generation SR light sources • The CESyRa experience • Possibilities offered by the experimental setup • Perspectives with next generation sources

3. Simple hydrocarbons ethane ethylene acetylene Carbon sp3 sp2 sp

4. sp3 • Hard • Semiconductor ta-C Isolated carbon cluster with less than 30 atoms exist as purely sp carbon chains (carbynes) Disordered phases: mixture of hybridizations • Soft • Conductive a-C sp2 sp R.O. Jones, J. Chem. Phys.110, 5189 (1999) Carbon Diamond Crystal structures ? Graphene

5. N  32 atoms N > 32 atoms Increasing the number of C atoms per cluster Chains and rings (sp) Fullerenes and onions (sp2, sp3) N both even and odd N only even Pulsed Laser Vaporization source High power density: annealing of the clusters up to their ground state structure E.A. Rohlfinget al. J. Chem. Phys.81, 3322 (1984) Carbon clusters mass spectra

6. 1 mm Pulsed Microplasma Cluster Source (PMCS) developed at Laboratorio Getti Molecolari e Materiali Nanocristallini, Department of Physics, University of Milano (Italy) E. Barborini, P. Piseri, P.Milani, J. Phys. D, Appl. Phys.32, L105 (1999) H. Vahedi-Tafreshi, et al. Journal of Nanoscience and Nanotechnology6, 1140 (2006) anode insulating valve flange ceramic body pulsed valve graphite nozzle thermalization cavity rotating cathode

7. A-mode B-mode Both odd and even clusters are detected ~40% of the whole mass distribution consists in odd clusters Cluster size (atoms) The PMCS produces non-fullerenic clusters. Leaving the fullerene road… Residence time (s) M. Bogana et al. NJP 7, 81 (2005)

8. Gas-Phase Nanoparticle deposition, or: Cluster Beam Deposition (CBD) source e >> 1 Fragmentation e << 1 Memory effect Low Energy Cluster Beam Deposition (LECBD) or Supersonic Cluster Beam Deposition (SCBD)

9. ex situ T=300 K (RT) in situ T=300 K (RT) C band !! Raman spectroscopy of ns-C films D+G band First observation of a clear signature of sp bonds in a system of pure carbon !! sp-chains are destroyed by oxygen: in situ measurements are mandatory L. Ravagnan et al. PRL 89, 285506 (2002)

10. ex situ T=300 K (RT) in situ T=300 K (RT) in situ T=150 K Raman spectroscopy of ns-C films D+G band Also the substrate temperature plays a crucial role! L. Ravagnan et al. PRL 89, 285506 (2002) L. Ravagnan et al. PRL 98, 216103 (2007)

11. sp-rich a-C Carbon sp3 • Hard • Semiconductor ta-C Disordered phases: mixture of hybridizations • Soft • Conductive a-C sp2 sp Ternary phase diagram of the amorphous pure carbon system.

12. Auger electron Gaseous acetyleneandethylenehave * resonances at 285.9 eV and 284.7 eV respectively. A.P. Hitchcocket al.J. El. Spec.10, 317 (1977) Beyond Raman: NEXAFS spectroscopy * resonances are fingerprints of the specific molecular bonds

13. in situ T=300 K (RT) We know from Raman that by heating the sample we induce the decay of the sp chains and a partial reordering of the sp2 matrix. CESYRA: in situ NEXAFS of ns-C films C.S. Casari et al.Phys. Rev. B69, 75422 (2004)

14. Normalization in situ T=300 K (RT) in situ T=350 K CESYRA: in situ NEXAFS of ns-C films The spectra evolves both in the * and * region.

15. Difference (pre-edge) Normalization sp2 284.7 eV in situ T=300 K (RT) in situ T=350 K 285.9 eV sp CESYRA: in situ NEXAFS of ns-C films We observe thedecay of *(CC)and the increase of the *(CC): NEXAFS spectroscopy is capable of distinguishing between sp and sp2 in a system of pure carbon!!

16. Interaction part Source part High voltage supply Beam diagnostic device (see inset) Time of flight mass spectrometer CESyRa apparatus layout Turbo 2000 l/s Turbo 500 l/s Turbo 300 l/s Turbo 300 l/s Turbo 500 l/s Beam dumping chamber and quartz monitor microbalance (not shown) cluster beams machine Quartz and steel gate valves (not shown) Cluster beam skimmer Cluster source Light entrance flange Mass flow controller Feedthrough of the deposition substrate for in-situ cluster assembled film analysis Gas cell and deflection stage chamber Beam diagnostic device chamber Source expansion chamber Interaction chamber

17. Normalization ns-C film in situ (RT) CESYRA: TEY NEXAFS of isolated clusters 285.6 eV TEY isolated clusters The * region is peaked at 285.6 eV: the cluster are predominantly made by sp carbon!

18. Normalization ns-C film in situ (RT) ns-C film in situ (350 K) CESYRA: TEY NEXAFS of isolated clusters 285.6 eV TEY isolated clusters The * region is peaked at 285.6 eV: the cluster are predominantly made by sp carbon!

19. First exiting clusters: short “annealing” Last exiting clusters: long “annealing” PEY: binning of the electron yield for intervals of delay times Pulsed source discharge CESYRA: PEY NEXAFS of isolated clusters Delay time: time elapsed between the discharge and the detection of the photo-electron. v ~ cost Residence time of the probed clusterin the source.

20. Small change in the * region. CESYRA2: PEY NEXAFS of isolated clusters 75 - 81 ms Increasing delay time 15 - 21 ms

21. 284.8 eV 285.7 eV CESYRA2: PEY NEXAFS of isolated clusters 75 - 81 ms Increasing delay time 15 - 21 ms

22. Electron Yield Spectra What kind of systems can we study?

23. Many different relaxation channels open up in free clusters. h’ + + e- e- h h e- h XAS on free clusters

24. Many different relaxation channels open up in free clusters. h’ + + e- e- h h e- + h’ h’ + e- e- e- + h h e- e- h XAS on free clusters Mixed clusters and cluster-molecule systems add more possibilities

25. Multiple Ion detectors Signal to TDC stop channels 1-7 (100 ms range, 80 ps resolution) PEPICO TOF setup PxPy…CO setup Ex-post reconstruction of electron-ion coincidence spectrum (100 µs range) by software computing the stop1-7-stop8 time differences TDC start signal from pulsed source discharge Cluster beam Photon beam Signal to TDC stop channel 8 (100 ms range , 80 ps resolution) Electron detector

26. Delay from start (ms) Events Time Structure Recording the full information Actual He injection ~ 1 ms Delay from start (ms) 0 0.5 1.0 tion,k tel,j He injection trigger 350 s Discharge 60 s

27. z x y Determination of cluster velocity Ion detector array Photon beam light clusters heavy clusters Cluster source Cluster beam electrons Electron detector • Cluster velocity is obtained dividing the detector position by the mean detected time of flight at different detection time; • Complete timing information allows residence-time resolved velocity measurements.

28. Beam kinematics The velocity of a particle with mass m, seeded in a He supersonic expansion can be modeled by: After k collisions with the He carrier gas. Bu. Wrenger and K. H. Meiwes-Broer, Rev. Sci. Instrum. 68 (5), May 1997, 2027  is proportional to the collision cross section and is given by Where  is 2/3 for a spherical shaped particle

29. Clusters have a fractal structure! Vapor Further growth steps Beam kinematics: velocity vs residence time A fitting parameter ~0.84 is found against =2/3=0.667 as expected for dense spherical particles Data fitting by varying: vHe, k, 

30. + h’ + + e- h’ + e- e- h e- e- h e- h’ h’ e- + h e- e- h + e- XAS on free clusters ? What relaxation channels in complex clusters ?

31. PEPICO Ion - Ion correlation spectra 1st order inter-arrival time distribution 2nd order inter-arrival time distribution 4th order inter-arrival time distribution n-Erlang distributions for the false coincidence background instead of exponential PInCO

32. Ion - Ion correlation intensity Maps of nth-order correlated ions intensity

33. (channel) = 0 (channel) = 1 (channel) = 2 (channel) = 3 (channel) = 0 (channel) = 1 (channel) = 2 (channel) = 3 Channels 1-4 Channels 4-7 Ions per bunch Ions per bunch Ions per bunch Space correlation • More channel correlations

34. Ion Ion coincidence spectra Space resolved

35. Fragmentation yield Space resolved x100

36. Conclusions and outlook • We have demonstrated the feasibility of X-ray absorption spectroscopy experiments on free carbon clusters transition metal clusters and oxide clusters • The experiment has been performed by coupling a supersonic cluster beam apparatus (based on a PMCS) with the Gas Phase beamline at Elettra • An “event reconstruction” approach is used to gain insight into the occurring relaxation channels • Improved TOF and position resolution are expected to bring better insight into the fragmentation process. • Independent structural determination of the free clusters is desirable for a validation of the aerodynamic acceleration model.

37. XPS bulk surface S. Peredkov, et al. Phys Rev B 75, 235407 (2007)

38. ∆Z = ±1 Source of uncertainty for R XPS S. Peredkov, et al. Phys Rev B 76, 081402(R) (2008) S. Peredkov, et al. Phys Rev B 75, 235407 (2007)

39. XPS

40. ACKNOWLEDGEMENTS: UniMI: People at LGM (Group leader Prof. Paolo Milani): Senior: Paolo Piseri, Cristina Lenardi, Post-doc: Tommaso Mazza, Gero Bongiorno, Luca Ravagnan, Matteo Amati Graduate/PhD-students: Michele Devetta, Flavio Della Foglia GasPhase: Marcello Coreno, Monica De Simone, Lorenzo Avaldi, Kevin Prince University of Gröningen (The Nederlands): Petra Rudolf, Fabrizio Evangelista