Introduction to femtosecond laser spectroscopy and ultrafast dynamics at surfaces

Transcription

1 IMPRS Block Course Chemistry and Physics of Surfaces Introduction to femtosecond laser spectroscopy and ultrafast dynamics at surfaces Martin Wolf Department of Physics, Freie Universität Berlin Principle: Stroboscopic investigation of motion and structural changes structure dynamics kinetics E. Muybrigde 1887 Goal: Microscopic understanding of elementary processes at surfaces Application of femtosecond laser spectroscopy

2 Outline Introduction Why femtosecond laser pulses? Example: photochemistry of vision Generation of femtosecond laser pulses Laser basics and mode locking Femtochemistry Vibrational wave packet dynamics Surface femtochemistry Electron and phonon mediated pathways hot e - E F e- transfer substrate E adsorbate LUMO HOMO Non-linear optics as a probe of surface dynamics SFG Vibrational sum-frequency generation spectroscopy

3 Why femtosecond spectroscopy? Time scales of chemical reactions: Temporal evolution from reactants to products: Dynamics of the transition state Typical timescale: fs Need femtosecond laser pulses for direct observation in time domain

4 Example: photochemistry of vision How do we see? Eye Rod cells: A single rod contains 10 8 rhodopsins (renewed every 10 days throughout a person s life) cis-trans isomerization hν Rhodopsin Retinal 11-cis all-trans

5 Hirarchy of time scales Optical absorption changes during reaction cycle ultrafast conformational changes de-protonation therrmal relaxation Study time-resolved changes of optical absorption spectrum

6 Pump-probe spectroscopy Basic concept to resolve processes on ultrafast time-scales signal pump probe reflection transmission t Time resolution is determined by pulse duration

7 Primary step of vision Spectral changes after excitation excited state absorption (t ~ 0) t = 200 fs probe α t ~ nm Primary step (cis-trans isomerization) of Retinal must occur within 200 fs! How can this process be so fast? Vibrational coherent wave packet motion

8 History of time resolution Nobel prize 1999 A.H. Zewail time resolution (s) Year Break through by development of pulsed laser sources

9 Development of short laser pulses pulse duration (s) mode locking Titanium Sapphire Year

10 Properties of ultrashort laser pulses Short pulse: Electrical field E(t) = ε(t) cos(ω 0 t + ϕ) with envelope ε(t) ε(t) ε(ω) time bandwidth product (transform limit) normal dispersion (e.g. glass)

11 Generation of fs-laser pulses A generic ultrafast laser consits of a broadband gain medium, a pulse-shortening device, and a resonator: Why?

12 Laser basics I LASER: Stimulated emission Titanium-sapphire laser Population inversion Example: Four level laser (Nd:YAG)

13 Laser basics II Resonator modes: Stability: g 2 = 1-d/R 2 g 1 g 2 = 1 g 1 = 1-d/R 1 longitudinal modes: R 1 R 2 d refractive index Fabry-Perot condition for a stable resonator: 0 g 1 g 2 1

14 Mode-locking Synchronization of laser modes: One mode: All modes: m Constant phase between modes Random phases Frequency domain: Temporal structure: t = 1/ ν

15 2 beam interference Mode-locking

16 non-linear optical Kerr effect Induced polarization in non-linear optical medium 1) self phase modulation Linear optics: n 0 = ε = ε 0 (1+4πΧ (1) ) Non-linear optics: Longitudinal and transversal Kerr effect 2) self focussing Kerr lens Kerr lens

17 Kerr lens mode-locking and dispersion Pulse broadening due to dispersion Example: Ti:Sa laser Prism compressor (GVD compensation)

18 Measurement of pulse duration interferrometric autocorrelation auto correlation I(τ) =

19 Outline Introduction Why femtosecond laser pulses? Example: photochemistry of vision Generation of femtosecond laser pulses Laser basics and mode locking Femtochemistry Vibrational wave packet dynamics Surface femtochemistry Electron and phonon mediated pathways Non-linear optics as a probe of surface dynamics hot e - E F e- transfer substrate E adsorbate LUMO HOMO SFG Vibrational sum-frequency generation spectroscopy

20 Real-time probing of chemical reactions

21 Real-time probing of chemical reactions II Example: photodissociation Clocking of chemical reactions Analysis of repulsive excited state PES

22 Real-time probing of chemical reactions III reaction product Coherent wave packet motion transition state

23 Outline Introduction Why femtosecond laser pulses? Example: photochemistry of vision Generation of femtosecond laser pulses Laser basics and mode locking Femtochemistry Vibrational wave packet dynamics Surface femtochemistry Electron and phonon mediated pathways hot e - E F e- transfer substrate E adsorbate LUMO HOMO Non-linear optics as a probe of surface dynamics SFG Vibrational sum-frequency generation spectroscopy

24 Laser induced surface dynamics Question: How does laser excitation induce surface reactions? Thermal activation Phonon driven process (in electronic ground state) Electronic excitations and charge transfer Chemical bonding implies electronic coupling between adsorbate and substrate k B T E Electron stimulated desorption and surface photochemistry hot e - LUMO Vibrational relaxation at metals by electron-hole pair excitation E F e- transfer Energy transfer by coupling between electronic and nuclear degrees of freedom substrate adsorbate HOMO

25 fs-laser-induced chemistry at surfaces FirstShotYield Flux Example: CO oxidation versus desorption on Ru(001) single crystal surface fs-induced reaction pathways CO Oxidation Co-adsorption of O+CO on Ru(001) (UHV conditions) Flux [arb. units] E trans = 1590 K CO desorption E trans = 640 K 2 CO + O CO ads hν ads 2 CO + O CO + CO 2 ads ads + kt Time-of-Flight (µs) New reaction pathway is switched on upon fs excitation

26 metal Optical excitation of metal surfaces Mechanism and timescales of energy transfer after optical excitation fs-excitation direct absorption ~100 fs reaction adsorbate vibrations T ads ~0.1-1 ps >1 ps metal electrons T el ~1 ps phonons T ph Photoabsorption in a metal substrate creates a transient non-equilibrium electron distribution Energy transfer to phonons and adsorbate vibrational excitation Primary step: Electron thermalization and cooling

27 Electron thermalization dynamics in metals probed with 2PPE and the two-temperature model Electron dynamics in metals following optical excitation

28 The two-temperature model Model assumes two heat baths for electrons and phonons: C el >> C ph Nearly thermal (Fermi-Dirac) distribution for electrons Coupled diffusion equation for T el and T ph Pd

29 Time-resolved two-photon photoemission How to probe electron thermalization and cooling? energy- and time-resolution: E kin hν 2 e - TOF UV probe pulse e - pulse duration: 65 fs E F E - E E vac F = E hν 1 kin Φ + Φ hν 2 pump pulse momentum resolution: hν ϕ e - Electron thermalization dynamics: k = sin ϕ 2mEkin/ h e-e scattering hot electron cascades

30 Experimental setup Femtosecond time-resolved two-photon photoemission spectroscopy µj

31 Electron thermalization in gold Pump fluence: 120 µj/cm 2 Method: time-resolved photoemission with hν probe > Φ 300 µj/cm 2 Fermi-Dirac distribution W. S. Fann, R. Storz, H.W.K. Tom, J. Bokor, Electron thermalization in gold, Phys. Rev. B 46, (1992) Electron thermailization occurs faster with increasing excitation density

32 Time-resolved 2PPE from Ru(001) UV probe 800 nm pump t / fs 40 µj/cm e - Pump-probe scheme: 2PPE intensity µj/cm 2 probing electron dynamics around E Fv 1) Electron thermalization: e-e scattering µj/cm 2 Ru(001) 2) Relaxation by e-ph scattering ) Energy transport in the bulk E - E F (ev) 1.5 Lisowski et al., Appl. Phys. A 78, 165 (2004)

33 Extended heat bath model Goal: Understanding the role of non-thermalized electrons Extension of two temperature model Appl. Phys. A 78, 165 (2004) 2TM Extended model including ballistic transport 8 electron energy [J/cm 2 ] time delay (fs) 600 transient exzess energy of electrons 2TM 900 2TM underestimates rate of energy flow into the bulk efficient ballistic transport of non-thermal electrons however, peak energy is described reasonable well by both models

34 Femtochemistry at metal surfaces Mechanism and timescales of energy transfer after optical excitation fs-excitation metal ~0.1-1 ps electrons T el direct absorption ~1 ps ~100 fs adsorbate vibrations T ads >1 ps phonons T ph reaction Temperature (K) 0 T T electron phonon 1 2 Time (ps) 120 fs 50 mj/cm 2 Anisimov, et al. Sov. Phys. JETP 39, 375 (1974) substrate-mediated excitation mechanism dominates transient non-equilibrium between electrons and phonons: T el >> T ph new reaction mechanism? by non-thermal activation by separation of time scales of energy flow

35 Reaction mechanisms Phonon-mediated Electron-mediated DIMET - Desorption Inducded by Multiple Electronic Transitions E adsorbate E k B T ph substrate hot e - e - transfer τ ~1-10 fs excited state phonons E des E F E des ground state reaction coordinate substrate reaction coordinate How to distinguish?

36 DIMET mechanism Desorption induced by multiple electronic transistions NO/Pd(111) see also Brandbyge et al., PRB 52, 6042 (1995)

37 Surface femtochemistry Systems NO and O 2 /Pd CO/Cu CO/Pt O 2 /Pt CO/NiO CO + O 2 /Pt CO + O/Ru, H+H/Ru Misewich, Loy, Heinz Tom, Prybyla Ho Mazur Stephenson, Richter, Cavanagh Bonn, Wolf, Ertl Zacharias, Al-Shamery Domen Experimental Investigations Yield Fluence dependence Translational energy Vibrational energy Rotational energy Ultrafast dynamics Influence of laser pulse duration Influence of photon energy Dependence on adsorption site Dependence on coverage Competitive reaction pathways Isotope effects

38 Example I: Desorption and oxidation of CO on Ru(001) Oxidation of CO impossible under equlilibrium conditions First Shot Yield Flux (1x2) - O/Ru(001) + CO saturation 0 50 E trans = CO 2 : 1590 K CO: 640 K Time-of-Flight (µs) 200 Y(CO)~F 3.1 Y(CO 2 )~F CO + O CO ads hν ads 2 CO + O CO + CO 2 ads ads 0 x Absorbed Fluence (yield weighted ) [J/m 2 ] New reaction pathway upon fs excitation

39 Example II: Recombinative desorption of H 2 on Ru(001) H 2 formation induced by fs laser excitation of H/Ru(001) <E kin>/ 2kB H 2 : (2110 ± 400) K H 2 QMS flux [a.u.] H H Full monolayer of H/Ru(001) D : (1720 ± 290) K 2 Ru(001) flight time [µs] (x 10) 60 0 Yield (H 2 ) Yield (D 2 ) = 10 ± 2.4 High translational temperature of desorbing species Pronounced isotope effects in H 2 /D 2 yield

40 Temperature profiles and 2-pulse correlation Goal: Discerning electron and phonon driven mechanisms delay QMS pulse 1 pulse 2 CO O C O CO surface temperature [K] T el T ph reaction yield [a.u.] FWHM ~ 10 ps FWHM ~ 1 ps Ru(001) phonon-mediated: slow response electron-mediated: fast response time [ps] pulse-pulse delay [ps] Two-pulse correlation measurement of the reaction yield allows to distinguish between the two reaction mechanisms!

41 Experimental setup Ti:sapphire oscillator + amplifier 4 mj, 110 fs, Hz, 800 nm Feulner cup detector 80 mm O / = 33 mm / QMS H 2 /DCO 2 2 equally intense pulses QMS ionisation volume H 2 UHV Ru(001) CCD pulse profile diagnostics Ru(001)

42 2-pulse correlation measurements 800 nm 120 fs t Goal: discriminate between electron and phonon mediated mechanism CO 2 2-pulse correlation: Temperature (K) O C T el O 0 T T electron phonon 1 2 Time (ps) Tph CO 120 fs 50 mj/cm 2 Ru first shot yield (a.u.) First Shot Yield FWHM=20 ps FWHM=3 ps CO x24 CO pulse-pulse delay (ps) Desorption : phonon-mediated Oxidation: hot electron-mediated

43 DFT calculations C. Stampfl, M. Scheffler, FHI Berlin O unoccupied anti-bonding orbital ~1.7eV above EF Ru(001) increasing Tel leads to weakening of Ru-O-Bond Hot electron-induced activation of the O-Ru bond

44 Mechanism for CO oxidation Hot electron mediated vibrational excitation of the O-Ru bond CO + O g g hot electrons E Fermi hot electrons E Ru(001) 1.7eV from DFT anti-bonding O-level ad V = 2 V = 1 O + CO ad ad CO reaction coordinate CO + O ad 2,g E ~1.8 ev a g 4.7 ev 0.3 ev Non-adiabatic energy transfer from electronic excitations to adsorbate coordinate M. Bonn et al, Science 285, 1042 (1999)

45 Discussion Reaction pathways for thermal versus femtosecond excitation E thermal excitation O* O C O C O kt O 0.8 ev 1.8 ev O C fs-laser excitation 1.8 ev O O C O * O C 1.0 ev e - O reaction coordinate Under thermal excitation the CO has desorbed before oxygen is activated Seperation of time scales opens new reaction pathway

46 Electronic friction model Electronic friction: Energy transfer by coupling between adsorbate vibration and metal electrons Coupled What is heat electronic baths: T el friction?, T ph, T ads Example: Change of electric -1-1 resistivity -1 in a thin Coupling metal film rates: uponτ el Xe, τph adsorption, τ el-ph. hot electrons E Fermi hot electrons E adsorbate level τ el T ads τph -V Xe -E a/kbt Rate ~ e ads metal Coupling rate η 1 1 = ~ τ el M metal T el τ el-ph ~1 ML T ph heat diffusion Two-Temperature Model Struck et al. PRL 77, 4576 (1996) ; Brandbyge et al.prb 52, 6042 (1995).

47 2PC of laser-induced H 2 formation metal Ultrafast response indicates coupling to hot electron transient Coupled heat baths: T el, T ph, T ads Coupling rates: τ el, τph, τ el-ph. firstshotyield[a.u.] H 2 exp. data model FWHM = 1.1 ps τ el T el T ads τ el-ph -E a/kbt Rate ~ e ads τph T ph heat diffusion Two-Temperature Model pulse-pulse delay [ps] Brandbyge et al., PRB 52, 6042 (1995) Electronic friction model yields: E a = 1.35 ev and τ el = 180 fs for H 2 (τ el = 360 fs for D 2 )

48 Isotope effect in recombinative desorption H 2 formation induced by fs laser excitation of H/Ru(001) <E kin>/ 2kB H 2 : (2110 ± 400) K H 2 QMS flux [a.u.] H H Full monolayer of H/Ru(001) D : (1720 ± 290) K 2 Ru(001) (x 10) 0 Isotope effect flight time [µs] 60 Yield (H 2 ) Yield (D 2 ) = 10 ± 2.4 Pronounced isotope effects in H 2 /D 2 yield

49 Isotope effect and electronic friction model Origin: - mass-dependent distance traversed on excited PES - lighter adsorbate starts moving more rapidly Electronic friction: Energy transfer by coupling between adsorbate vibration and metal electrons hot electrons hot electrons E adsorbate level potential energy r electronically excited PES E Fermi metal ground state of substrate-adsorbate-complex 1 M Coupling rate η = ~ 1 τ el Ru-H/D distance Brandbyge et al., PRB 52, 6042 (1995)

50 Fluence dependence: isotope ratio and first shot yield isotope ratio Y(H 2 )/Y(D 2 ) experimental data electronic friction model first shot yield [a.u.] 0 10 <F>~75J/m laser shot # 40 H 2 counts [a.u.] isotope ratio Electronic friction model* simultanously describes: 2-pulse correlation fluence depedence of yield and isotope ratio exp. data model power law fit Y(H 2 ) ~ <F> 2.8 and Y(D 2 ) ~ <F> 3.2 * Brandbyge et al., PRB 52, 6042 (1995)

51 Outline Introduction Why femtosecond laser pulses? Example: photochemistry of vision Generation of femtosecond laser pulses Laser basics and mode locking Femtochemistry Vibrational wave packet dynamics Surface femtochemistry Electron and phonon mediated pathways hot e - E F e- transfer substrate E adsorbate LUMO HOMO Non-linear optics as a probe of surface dynamics SFG SFG Vibrational sum-frequency generation spectroscopy

52 Real-time probing of vibrational dynamics Goal: Probe suface reactions directly in the time domain fs-excitation direct absorption reaction ~100 fs adsorbate vibrations T ads ~0.1-1 ps >1 ps metal electrons T el ~1 ps phonons T ph So far probed only the (desorbing) reaction products Use surface sensitive non-linear optics (SHG, SFG) to obtain a direct look inside the excited adlayer

53 Vibrational spectroscopy IR absorption spectroscopy (FTIR) C-O stretch O C sum-frequency generation (SFG) VIS 5 µm nm = 690 nm SFG Pot. Energy v=2 v=1 v=0 IR Pot. Energy v=2 v=1 v=0 IR C-O distance With background, IR detection optical linear technique C-O distance Background-free, visible detection -3 Sensitivity: 10 ML Short pulses: time resolution Surface specific

54 Vibrational SFG spectroscopy Principle of sum-frequency generation (SFG): VIS SFG ω SFG = ω IP + ω VIS Second order non-linear optical process P ω = χ (2) (2) SFG ω SFG E ω E IR ω VIS Pot. Energy v=2 v=1 v=0 IR C-O distance C-O stretch O C Surface sensitive method: SFG is symmetry forbidden in isotropic (bulk) media (dipole approximation) Time-resolved SFG as molecule specific probe of surface reactions

55 Broadband IR vibrational SFG spectroscopy Vibrational spectroscopy without tuning the IR frequency VIS ~5 cm -1 VIS IR VIS SFG IR VIS SFG Γ SFG Ru IR Γ n ~150 cm -1 IR Use spectrally broad fs- IR pulse with spectrally narrow VIS upconversion pulse L.J. Richter et al., Opt. Lett. 23, 1594 (1998). SFG intensity SFG spectrum ( 3x 3)-CO/Ru(001) K IR IR wavenumber [cm -1] SFG FWHM 9 cm SFG wavelength [nm] IR intensity

56 SFG experimental setup Ti:sapphire oscillator + amplifier 4 mj, 800 nm, 110 fs, Hz, OPG/OPA (TOPAS) µ m, µ J pump pulse pulse shaper FWHM 7 cm -1, 7 µ J spectrometer CCD camera SFG probe QMS narrow band VIS pulse tunable IR pulse multi-channel detection Ru(001)

57 Example water/ru(001) Sum-frequency generation (SFG) intrinsically surface sensitive SFG signal is enhanced when IR Photon is resonant with vibrational transition = Example: D2O covered and bare Ru(001) surface D2O multilayers: free OD resonance bare Ru: nonresonant background

58 Equivalence of spectral- and time-domain measurements VIS IR SFG 1) Time-resolved SFG: temporally short, spectrally broad IR + VIS-pulse 2) Frequency-resolved SFG: temporally long, spectrally narrow VIS-pulse SFG intensity FID I~e -2t/T2 0 1 IR field IR polarization VIS field time (ps) SFG intensity time (ps) SFG spectrum linewidth Γ~1/T IR frequency (cm -1 )

59 IR-broadband SFG: Spectral and time-resolution Frequency domain: Time domain: delay VIS IR SFG VIS IR SFG CH 3 CN Au CH 3 CN Au SFG intensity (arb. units) C-N stretch 2100 SFG spectrum IR spectrum IR frequency (cm -1 ) SFG intensity (arb. units) x10 3 non-resonant response from Au surface IR-VIS delay (ps) C-N stretch 3 4 Experimental frequency resolution: better than 3 cm -1 Experimental time resolution: better than 150 fs M.Bonn et al. (Univ. Leiden)

60 Probing surface inhomogeneities with SFG SFG intensity (arb. units) IR Frequency (cm -1 ) C-H stretch Γ=8.0 cm -1 T 2 =0.64 ps SFG intensity (arb. units) 0 1 IR-VIS delay (ps) C-H stretch T 2 =0.61 ps 2 SFG intensity (arb. units) C-N stretch SFG intensity (arb. units) Γ=7.6 cm -1 homogeneous T 2 =0.68 ps ω=2.5 cm -1 inhomogeneous T 2 =1.65 ps T 2 =0.68 ps hom. C-N stretch ω=2.5 cm -1 ; T 2 =1.65 ps inhomogeneous Time-domain measurements are particularly sensitive to inhomogeneities in surface adsorption sites due to temporal separation of interferences.

61 Time-resolved SFG of CO/Ru during desorption Spectroscopic snapshots of the CO stretch mode SFG intensity (a.u.) T 0=340 K, F=55 J/m ps 3 ps 23 ps 2100 CO yield (a.u.) IR wavenumber (cm -1 ) CO/Ru(001) 10 0 shot number Pump-Probe delay ps 2.0 ps 1.5 ps 1.0 ps 0.5 ps 0.5 ps 3.0 ps 23.0 ps 68.0 ps ps optical probe Pronounced transient red shift, linewidth broadening and decrease of intensity Problems: Dipole-dipole coupling in the adlayer and small concentration of products M. Bonn et al, Phys. Rev. Lett 84, (2000) 4653

62 Time-resolved SFG of CO/Ru(0001) Calculation: - thermal excitation of frustrated translation - anharmonic coupling to CO stretch mode Low fluence: Good fit At high fluence: - excitation and coupling to higher frequency modes Strong redshift indicated that frustrated rotation is dominante

63 Real-time probing of surface reactions: 2PPE Snapshots of excited electronic states during evoltution of wavepacket Time-resolved 2PPE spectra Desorption of Cs/Cu(111) photoemission probe

64 Summary Introduction Why femtosecond laser pulses? Generation of femtosecond laser pulses Laser basics and mode locking Femtochemistry Vibrational wave packet dynamics Electron thermalization in metals Test of the two-temperature model 2PPE intensity 1 2PPE Intensity Ru t / fs Surface femtochemistry E - E F (ev) 1.5 Electron and phonon mediated pathways SFG Isotope effects and electronic friction model Vibrational sum-frequency generation spectroscopy

65 thanks to C. Gahl, U. Bovensiepen, J. Stähler, M. Lisowski, P. Loukakos, S. Wagner, C. Frischkorn, Freie Universität Berlin D. Denzler, S. Funk, C. Hess, and G. Ertl, Fritz-Haber-Institut Berlin S.Roke, M. Bonn, Leiden University, The Netherlands

66 Literature: H. L. Dai and W. Ho, Laser Spectroscopy and Photochemistry at Metal Surfaces World Scientific (1995). H. Petek and S. Ogawa, Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals Progress in Surface Science 56, (1998). H. Nienhaus Electronic excitations by chemical reactions at metal surfaces Surface Science Reports 45, 1-78 (2002). P.M. Echenique, R. Berndt, E.V. Chulkov, Th. Fauster and U. Höfer, Decay of electronic excitations at metal surfaces Surface Science Reports 52, (2004). M. Bonn, H. Ueba and M. Wolf Theory of sum-frequency generation spectroscopy of adsorbed molecules by density matrix method - Broadband vibrational SFG and applications J. Phys.: Condens. Matter 17, 201 (2005)