ChapterV- ExperimentalMethodsofTime-ResolvedSpectroscopy

1. Generationof femtosecondpulses2. Femtosecondpump-probe experiments3. Generationofattosecond pulses4. Attosecond time-resolvedexperiments

The hierarchy of time scales

1 millisec.10-3 s

1 microsec.10-6 s

1 nanosec.10-9 s

1 picosec.10-12 s

1 femtosec.10-15 s

1 attosec.10-18 s

vibrationsrotations electronmotionproteinmotions proton transfer

ΔE=1eV:T=h/ΔE=4.12fs

spontaneousIRemission

Fastestenzyme-catalyzedreactions

lifetimeofelectronically-excitedstates

FastestIVRHigh-frequency

vibrations

Fastestchemicalprocesses (e.g.e-transfer)

A.H.Zewail,J.Phys.Chem.A,104,5660(2000)

A.H.Zewail,NobelLecture,1999

Generationofshortlaserpulses

Chirped-pulseamplificationKerr-lensmodelocking

2.7 fsλ =800nm

Q-switching:accesstonanoseconds

Q-switch

gain

medium

1) high losses,lowQ-factor,nolasing2) Q-factorsuddenly increasedà powerbuildup3) powerreachessaturationà outputpowerdecays

www.rp-photonics.com

1F. J. McClung and R. W. Hellwarth Giant optical pulsations from ruby,Journal of Applied Physics, Band 33, 1962, 828–829.

Mode-locking:accesstofemtoseconds

L Trt

modulator

gain

medium

Manysynchronizedmodesmakeupashortpulse.

Superposition ofNmodes:www.rp-photonics.com

Themodulatorintroduceslosstocw (narrow-band)operationandfavorsmode-lockedoperation.

Experimentalrealizationofmode-locking

e.g.Kerr-lensmode-locking:

• Kerreffect:n=n0+n2I• Shortpulseà highintensityà self-focusing• Anapertureinthecavityfavorsmode-locked

operationovercontinuous-waveoperation:

Pulseduration~10fs,Fourier limit8fs

Chirped-pulseamplification

http://en.wikipedia.org

FromnJtomJpulsesandbeyond...kJfemtosecondlasersarenowbeingbuilt:www.eli-laser.eu

Peakpowers:up to1PW=1015 W(50J/50fs)

Pump-probe:averyoldideaAugustToepler (1836-1912): Schlierenphotographie1864

1.lightflashgeneratessoundwave2.lightflashrecordsimage

explosion of 10 mg AgNO3,

3cm above ground

© H. Kleine

TheopticaldelaylineAbrahamandLemoine 1899:

Δt

2 Δz = c Δt

object

High-speedphotography

inventor: Harold Edgerton (1903-1990)

Time resolution = duration of light flash > 1 µs

Femtosecondpump-probeexperimentsThefastestdetectorshavesub-nanosecond responsetimeà fastereventscannotbemeasuredelectronically.

Time-resolvedlaser-inducedfluorescence

λpump =310nmλprobe=580-620nm

Δtoven

NaI beamvacuumchamber

Measuresthelaser-induced fluorescencesignal asafunction ofthepump-probe delay

λprobe =589nm

λprobe =612nmNa

(...)3s

(...)3p

589nm

Na-I

620nmLIF

M.Rosker,T.RoseandA.Zewail,Chem.Phys.Lett.146,175(1988)

LIF

Time-resolvedlaser-inducedfluorescence

A.H.Zewail,J.Phys.Chem.A,104,5660(2000)

Time-resolvedtransientabsorption

www.stfc.ac.uk

D.Polli etal.,Nature 467,440(2010)

Measurestheabsorptionspectrumoftheexcitedmolecules asafunctionofthepump-probedelay

Time-resolvedphotoelectronspectroscopyMeasuresthekinetic-energydistributionofphotoelectrons asafunctionofthepump-probedelay

Lochbrunner etal.,J.Chem.Phys.114,2519(2001)

proton transfer

internalconversion

Time-resolvedstrong-fieldionization

TSO =h/ΔESO :42.7fs (Ne+),23.2fs (Ar+)

Firstpulse:800nm,8fs,5x1014 W/cm2 ionizesNeà Ne+Secondpulse:800nm,8fs,5x1014 W/cm2 ionizesNe+àNe2+

H.J.Wörner andP.B.Corkum, J.Phys.B44,041001(2011)A.Fleischeretal.,Phys.Rev.Lett.107,093004(2011)

Measuresthestrong-fieldionizationyield asafunctionofthepump-probedelay