Vibrational spectroscopy of clusters with a Free Electron Laser

Transcription

1 Vibrational spectroscopy of clusters with a Free Electron Laser André Fielicke Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin Abteilung Molekülphysik

2 Now: Free Electron Laser for Infrared experiments (F Nieuwegein, NL

3 FHI-FEL (under construction) to user lab far-ir FEL mid-ir FEL B A linear accelerator C D F first light end of 2011

4 Vibrational spectroscopy of clusters with a Free Electron Laser Action spectroscopy Multiple photon absorption Free Electron Lasers (how it works) Examples of cluster spectroscopy

5 Infrared spectroscopy of metal cluster complexes Structure of bare metal clusters internal cluster modes < 500 cm -1 (0.06 ev) Exploring the cluster s surface chemistry ligand modes cm -1 ( ev)

6 IR spectroscopy of clusters in molecular beams Direct measurement of absorption Not sensitive enough (low particle density) Not species specific (cluster distribution) More sensitive and selective: Mass spectrometric detection of absorption via Action Spectroscopy Changes of the charge state (ionization) Changes of particle mass (dissociation) An intense and tunable IR source is needed for the excitation

7 Cluster dissociation; Depletion spectroscopy i) Ions hν X n +* fragments X n + X n + MS hν geometry Lambert Beer absorption law I I o = exp(-σφ) H. Haberland, Clusters of Atoms and Molecules, Springer-Verlag, Berlin, 1995.

8 IR spectra of clusters via messenger technique AB + M (AB + M)* AB + + M chromophore spectator Example: IR photodissociation of mass selected V x O y+ He Absorption spectrum is obtained from the depletion of the He complex intensity vs. Laser frequency mass selection cluster distribution 14 K ion trap 2nd mass filter for selecting fragments Laser parent ion depletion K.R. Asmis, et al., J. Chem. Phys. 2004, 120, 6461.

9 IR photon energy vs. bond dissociation energies fingerprint region ν(m-m) ν(mo) ν(c=o) ν(x-h) Chemisorption energies: Binding energies in transition metal clusters Physisorption energies 1-3 ev 3-6 ev <0.2 ev Photodissociation of most systems requires absorption of multiple IR photons 1 ev = 96.5 kj mol -1

10 Resonant multiple photon absorption E E hν hν hν hν hν d E vib harmonic oscillator 1 = hν ( v + 2) anharmonic oscillator E Cluster with many vibrational modes hν IVR d hν hν hν E vib = hνe hx +L d e ( v ν e + ( v 1 2 ) ) 2 resonant absorption fast intramolecular vibrational redistribution (IVR), t IVR << 1 ns absorption of the next photon etc.

11 IR sources for Multiple photon excitation fingerprint region ν(m-m) ν(mo) ν(c=o) ν(x-h) CO 2 laser ns pulsed DFM, OPO/OPA Free Electron Laser

12 Infrared multiple photon dissociation spectroscopy Later: Knickelbein, Rayner, Walther CO 2 laser: line tunable around 10 µm (1000±100 cm -1 ) Today IR-OPO, DFM: down to ~16 µm (600 cm -1 ) but: rel. low power

13 General properties of Free Electron Lasers accelerator based light source intense, high brightness (usually) wavelength tunable monochromatic pulsed or continuous (less common)

14 IR-FELs worldwide (not complete) FELIX (NL) n.c MeV μm Mol. spectroscopy FELBE (D) s.c. cw MeV μm Solid state CLIO (F) n.c MeV μm Solid state, gas phase NijFEL (NL) n.c MeV μm High magn. field UCSB (US) El.stat. 2-6 MeV μm Solid state JLAB (US) s.c. cw MeV μm High power Vanderbilt (US) n.c MeV 2 9 μm Surgery FEL-TUS (JP) n.c MeV MeV 5 16 μm μm Clusters, isotope separation KU-FEL (JP) n.c. 25 MeV 4 13 μm Solid state, biochem. ifel (JP) n.c MeV μm Solid state BFEL (China) n.c MeV 5 25 μm Mat. science FHI-FEL n.c MeV μm Mol. & cluster Spec., surface science

15 There are FELs for all possible wavelengths! Microwaves 10 mm 10-4 ev X-rays 0.1 nm ev

16 Most recent family members: X-ray FELs FLASH, DESY (Hamburg, Germany), SC Linac, nm XFEL, nm, first light planned for 2014 LCLS, Stanford (USA), SC Linac, nm (0.8-8 kev) working since April 2009

17 The Laser Light Amplification by Stimulated Emission of Radiation Energy source resonator gain medium

18 Processes in a light field (2-level system) Rates for induced processes depend on the spectral energy density ρ(ν) stimulated absorption stim. emission spontan. emission Amplification

19 Free Electron Laser (FEL) Medium and energy source: High energy (relativistic) electron beam Interaction of the light field with the medium? Amplification mechanism

20 Motion of electrons in a periodic magnetic field λ U B NSNSNSNSNS S N S N S N S N S N laboratory system internal system Oscillation frequency f = λ c U Hertz Oscillator radiating with f

21 Relativistic electron beams v e c 1. radiation characteristics at rest relativistic Δϑ ( ) 2 v e, z 1 c

22 Relativistic electron beams v e c 2. Lorentz contraction λ U λ U λ U = γ γ = E m c kin 2 = 1 1 ( ) v 2 0 e, z c 2 m c 0.5 MeV 40 MeV beam γ 80 0

23 Relativistic electrons in the undulator I γ λ = λ U U λ γ λ λ + = λ ,, obs c v c v obs z e z e Oscillation (=radiation) frequency γ λ = c f U Corresponding wavelength (internal, moving system) Wavelength in the laboratory system is detected with a strong Doppler shift γ λ = γ γ λ = λ U U

24 Relativistic electrons in the undulator II But: electrons do not move straight along the z-axis in the undulator λ = λ γ U z 1 2γ z = λ 2γ U 2 z Wiggling motion reduces speed in forward (z) direction Effective γ is reduced γ z < γ = E kin m c 2 0 Reduction depends on the deflection and z 2 thereby the magnetic field strength B (1 + K ) γ = γ ( K B) Spontaneously emitted radiation from electrons in a periodic magnetic field λ = λ 2γ ( 2 K ) U z

25 Setting the wavelength λ = λ 2γ ( 2 K ) U z K: typically FHIFEL: E kin = 50 MeV mid-ir Undulator: λ U =0.04 m Wavelength for K=1?

26 Setting the wavelength λ = λ 2γ ( 2 K ) U z K: typically FHIFEL: E kin = 50 MeV mid-ir Undulator, λ U =0.04 m Wavelength for K=1? λ 2 ( ) ( 1+ 1 ) 2 50 MeV 0.5 MeV 4µm (0.3 ev) 0.04 m 2 Wavelength and small signal gain

27 Setting the wavelength λ = λ 2γ ( 2 K ) U z K: typically BESSY II: E kin =1.7 GeV Undulator UE56 λ 2 ( ) ( 1+ 1 ) GeV 0.5 MeV 5nm (250 ev) m 2

28 Ways for tuning the wavelength Beam energy E kin Changing accelerator parameters and electron optics λ = λ 2γ ( 2 K ) U z magnet field B Adjusting the undulator gap d d

29 Interaction of the electrons with the light field Initial situation: incoherent spontaneously emitted radiation captured in the resonator Ponderomotive force acts on the electrons depending on relative phase Acceleration or slowing down

30 Interaction of the electrons with the light field Microbunching of the electron packets through momentum transfer between light field and electrons Result: electrons move in phase Radiations from single bunches can interfere constructively

31 Oszillator FEL Stored light field increases with time New electron bunches traverse the undulator More efficient bunching enhances energy transfer into the light field exponential rise until saturation

32 X-rays can not be stored No effective mirror materials Amplification in a single pass along the undulator Self amplified stimulated emission (SASE)< FLASH, DESY (Hamburg, Germany)

33 A train of short light pulses Electron beam determines temporal characteristics of emitted light: Short pulses of a few fs ps length Transform limited bandwidth (FELIX: %) Repetition within a macropulse (also pulsed cw possible) Pulse scheme of FELIX

34 Applications of Free Electron Lasers high intensity (ultra-) short pulses Weak absorbers (low density and/or low cross sections) Multiple photon processes Time resolution Relaxation processes To take snapshots

35 The Free Electron Laser for Infrared experiments (FELIX) FOM Institute for Plasma Physics Rijnhuizen, Nieuwegein, The Netherlands tunable between cm -1 (up to ~3700 cm -1 on 3rd harmonic) up to 100 mj per macropulse (10 10 W/cm 2 in a micropulse) bandwidth typically % of the central wavelength

36 Rh 5 CO + Rh 6 CO + FELIX 4.92 μm cluster source m/ z (amu) Nd:YAG laser reactive gas Reflectron time-of-flight mass spectrometer Ion Detector gas pulse metal rod IR Multiple Photon Dissociation (IR-MPD) Spectroscopy of neutral and charged clusters UV laser (F 2, 7.9 ev) FELIX beam

37 Probing the surface chemistry of the ligand character of CO binding dissociative molecular Surf. Sci. 603 (2009) 1427.

38 CO at Rh n+ : Size dependence of the binding site Observation of CO bound in 3-fold face capping (µ 3 ), 2-fold bridging (µ 2 ), and linear (µ 1 ) geometries CO binding depends on cluster size mainly µ 1 ligands Isomers, e.g. for n = 7 JACS 125 (2003) J. Phys. Chem. B 108 (2004)

39 Comparison of neutral, cationic and anionic Rh n CO +/- cations neutrals anions

40 Effect of charge: example of Rh 8 CO +/0/- p σ (6σ) p π (2π) p s s σ (5σ) p π (1π) p M C O M(σ) CO(5σ) σ donation p σ (4σ) s σ (3σ) s M C O M(δ) CO(2π) π back donation C CO O

41 Saturated Rhodium Cluster Carbonyl Cations Inferring structures from counting electrons 18 electron rule, Rh: 4d 8 5s 1 Saturation composition Rh(CO) 5 + Rh 2 (CO) 8 + Rh 3 (CO) 9 + Rh 4 (CO) 12 + Rh 5 (CO) 14 + Rh 6 (CO) 16 + CVEs 9 + 5*2-1 = 18 2*9 + 8*2-1 = 33 3*9 + 9*2-1 = 44 4*9 + 12*2-1 = 59 5*9 + 14*2-1 = 72 6*9 + 16*2-1 = 85 (18-3)*4 = 60 (18-3)*2+(18-2)*2 = 62 The Bonding Capabilities of Transition Metal Clusters J. W. Lauher, J. Am. Chem. Soc. 100 (1978) 5305

42 Saturated Rhodium Cluster Carbonyl Cations J. Am. Chem. Soc. 130 (2008) 2126

43 Rhodium cluster carbonyls: Cations vs. Neutrals cation neutral SAME DIFFERENT DIFFERENT No bridging CO in the cations of Rh 2 (CO) 8 and Rh 4 (CO) 12 Neutrals in Xe, Allian et al., Vib. Spec. 41 (2006) 101

44 Rhodium cluster carbonyls: Cations vs. Neutrals O C Rh x 10-3 Destabilization of µ 2 -CO upon ionization comes from removal of electron density out of orbitals with Rh-µ 2 -CO binding character

45 Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes IR absorption spectrum depletion spectrum resonant absorption Fragmentation of the complex cross section intensity (%) frequency (cm -1 ) wavelength (µm)

46 Obtaining the structural information rel. cross section exp. spectrum calculated IR spectra for different structures structure assigment frequency (cm -1 ) Au 7 Density Functional Theory Predict stable isomers (may include a global optimization) Compute infrared spectrum Effects of rare gas atoms? Accounting for anharmonicity, fluctional behavior via MD simulations

47 Analyzing the structure vs. understanding the IR spectra Experiment: Au 7 Kr at ~100 K i) Structural isomers ii) Effect of Kr atoms iii) Dynamics at 80 K L. Ghiringhelli, M. Scheffler FHI Theory Dep. (DFT+D) PBE+D

48 IR Spectrum and Structure of Neutral Nb 8 neutral: ν=180 cm -1 anion: ν=165 cm -1 T. P. Marcy and D. G. Leopold, Int. J. Mass Spectr. 196, 653 (2000). Calc. spectrum (DFT) Sy m. a2 a1 b1 b2 a1 a2 b1 a2 a1 b2 a1 b2 b2 a1 b1 b1 a1 a1 Calc cm Scaled cm Int E

49 Magnetism in small rhodium clusters A.J. Cox et al. Phys. Rev. B 49 (1994) Cubic growth can explain magnetic properties Eight-center bonding through d orbitals Y.-C. Bae et al. Phys. Rev. B 72 (2005)

50 The (predicted) cubic structures of rhodium clusters Rh 8 cube, O h symmetry 1 IR active mode (t 1u )

51 Predicted isomers of Rh 8 + (basin hopping DFT) 1.0 Dan Harding, Tiff Walsh, U Warwick, UK Stuart Mackenzie, U Oxford, UK cube rel. energy (ev) 0.5 anticube anticube bicap-tp diamond-p bicap-oh bicap-tp 0 cube PBE bicap-oh PBE-hybrid J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77, (1996) 3865 J.P. Perdew, M. Ernzerhof, K. Burke, J. Chem. Phys. 105, (1996) 9982

52 Assignment of the structure of Rh ev ev ev b 2 0 e bicapped octahedral structure as identified also for other transition metals J. Chem. Phys. 132 (2010)

53 Structures of small transition metal clusters n = cation Nb neutral Rh cation Polytetrahedral packings

54 Cooling processes following IR multiple photon excitation IR multiple photon dissociation (IR-MPD) IR resonance enhanced multiple photon ionization (IR-MPD) For most systems: BDE < IP Exceptions: refractory materials metals (W, Mo, Nb ) oxides (MgO, Al 2 O 3, TiO 2 ) carbides (TiC, NbC) fullerenes C 60

55 IR Resonance Enhanced Multiple Photon Ionization (IR-REMPI) Bulk niobium carbide: metallic, interstitiell C in octahedral holes of Nb fcc lattice ionization energies 4 ev (67 photons@500 cm -1 ) binding energy per atom: > 6 ev Ionization of neutral NbC clusters with IR light of ~500 cm -1 (0.06 ev) NbC: cubic bulk structure (NaCl) geometric shell closings at Nb 14 C 13 (3x3x3 atoms) Nb 18 C 18 (3x3x4 atoms) Nb 24 C 24 (3x4x4 atoms) Nb 32 C 32 (4x4x4 atoms)

56 IR-REMPI spectra of niobium carbide clusters Bands are observed close to the surface phonon modes: S 4 Lucas mode at 640 cm -1 C atoms move within the surface plane S 2 Wallis mode at 490 cm -1 C atoms move perpendicular to the surface plane C. Oshima et al. Phys. Rev. Lett. 56 (1986) 240 Small clusters (upto Nb 9 C 9 ) show only bands around ~670 cm -1

57 From FELIX to FELICE 10 J 100 mj (1 %) FELIX: Intensity still insufficient for many multiple photon excitation experiments FELICE: Free Electron Laser for IntraCavity Experiments

58 FELICE: Free Electron Laser for IntraCavity Experiments wavelength range beam energy micropulse rep. rate micropulse energy macropulse energy µm MeV 1 GHz 1 mj some 10J

59 Electron detachment and ionization by IR multiple photon excitation (with V. Lapoutre, J. Bakker, FOM Rijnhuizen) Vibrationally mediated electron detachment from anionic clusters Ta 4 C - Ta 4 C (ca. 1-2 ev) Nb 13 IR-REMPI of neutral niobium clusters Ionization (IP=5.5 ev) Nb 13 Nb + 13 Dissociation (BDE<0.1 ev) Nb 13 Ar Nb 13 +Ar

60 Summary Techniques: Raman (matrix) Anion photoelectron spectroscopy Photodissociation, IR-MPD IR-REMPI Vibrational spectroscopy is a suitable tool for obtaining structural information for metal (compound) clusters, especially with complementary theoretical studies (DFT). Surface chemistry and binding geometries of adsorbed molecules can be identified.