1. WUTA08 Laboratori Nazionali di Frascati Frascati, 8-10 October 2008 Probing free metallic and carbon clusters with VUV photons. P. Piseri 1,2,3 (piseri@fisica.unimi.it), G. Bongiorno 1,2,3, T. Mazza 1,2,3, L. Ravagnan 1,2,3, M. Amati 1,2,3, M. Devetta 1,2, C. Lenardi 2,3,4, and P. Milani 1,2,3, M. Coreno 5,6, M. De Simone 6, P. Rudolf 7, F. Evangelista 7 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)

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

3. ethane ethyleneacetylene Simple hydrocarbons Carbon sp 3 spsp 2

4. Isolated carbon cluster with less than 30 atoms exist as purely sp carbon chains (carbynes) R.O. Jones, J. Chem. Phys. 110, 5189 (1999) Diamond Graphene sp 3 spsp 2 Crystal structures ? Carbon a-C ta-C Hard Semiconductor Soft Conductive Disordered phases: mixture of hybridizations

5. Chains and rings (sp) N  32 atoms E.A. Rohlfing et al. J. Chem. Phys. 81, 3322 (1984) Pulsed Laser Vaporization source Fullerenes and onions (sp 2, sp 3 ) N > 32 atoms Increasing the number of C atoms per cluster N both even and oddN only even High power density: annealing of the clusters up to their ground state structure Carbon clusters mass spectra

6. pulsed valve insulating valve flange anode rotating cathode thermalization cavity graphite nozzle ceramic body 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) 1 mm H. Vahedi-Tafreshi, et al. Journal of Nanoscience and Nanotechnology 6, 1140 (2006)

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

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

9. But how can we embed sp structures in a disordered phase of carbon? Linear carbon chains survive only as small isolated clusters: if they are deposited they cross-link undergoing to graphitization T. Wakabayashi et al. J. Phys. Chem. B 108, 3686 (2004) Deposition of linear chains in Ar-matrix (4 K): the chains remain isolated. When the matrix sublimate (by annealing to 43 K) the chains interact producing a highly exothermic reaction. sp-chains

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

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

12. sp 3 spsp 2 a-C ta-C Hard Semiconductor Soft Conductive Carbon Disordered phases: mixture of hybridizations Ternary phase diagram of the amorphous pure carbon system. sp-rich a-C

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

14. CESYRA: in situ NEXAFS of ns-C films C.S. Casari et al. Phys. Rev. B 69, 75422 (2004) 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 sp 2 matrix.

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

16. Difference (pre-edge) 284.7 eV 285.9 eV sp sp 2 CESYRA: in situ NEXAFS of ns-C films We observe the decay of  *(C  C) and the increase of the  *(C  C) : NEXAFS spectroscopy is capable of distinguishing between sp and sp 2 in a system of pure carbon!! in situ T=300 K (RT) in situ T=350 K Normalization

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

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

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

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

21. CESYRA 2 : PEY NEXAFS of isolated clusters Increasing delay time 15 - 21 ms 75 - 81 ms Small change in the  * region.

22. Increasing delay time 285.7 eV284.8 eV CESYRA 2 : PEY NEXAFS of isolated clusters 15 - 21 ms 75 - 81 ms

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

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

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

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

27. Events Time Structure Recording the full information 0 Delay from start (ms) 1.00.5 Delay from start (ms) t el,j t ion,k Actual He injection ~ 1 ms He injection trigger 350  s Discharge 60  s

28. Events detection efficiency Ion detector array light clusters heavy clusters electrons Photon beam Cluster beam Electron detector Cluster source Electron-ion coincidence count rate (s -1 ) Fraction of detected events Detector number PEPICO

29. Events detection efficiency Ion detector array light clusters heavy clusters electrons Photon beam Cluster beam Electron detector Cluster source Average charge-state Fraction of detected events Detector number PEPICO guessed electron- detector efficiency: 20% Charge-state

30. Average charge state does not evolve significantly ! Photon Energy (eV)Cluster residence time (ms) Fraction of detected events

31. Ion detector array x y z light clusters heavy clusters electrons Photon beam Cluster beam Electron detector Cluster source Determination of cluster velocity 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.

32. 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.  is proportional to the collision cross section and is given by Where  is 2/3 for a spherical shaped particle Bu. Wrenger and K. H. Meiwes-Broer, Rev. Sci. Instrum. 68 (5), May 1997, 2027

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

34. Charge state = 2 Charge state = 3 Charge state = 4 Charge state = 5 Charge state = 6 Charge state = 7 Charge state = 8 Determination of cluster v(m) distribution: deconvolution of t.o.f. spectra in different charge states according to the detection efficiency

35. Channel # 2 Channel # 3 Channel # 4 Channel # 5 Channel # 6 Channel # 7 2 nd condition: different charge state must arise from a same original mass distribution

36. Determination of cluster v(m) distribution: deconvolution of t.o.f. spectra in different charge states according to the detection efficiency

37. Beam kinematics: velocity vs residence time

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

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

40. Ion - Ion correlation intensity Maps of n th -order correlated ions intensity

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

42. Ion Ion coincidence spectra Space resolved

43. Fragmentation yield Space resolved x100

44.  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. Conclusions and outlook

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

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

47. XPS

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