Chapter7- ExperimentalMethodsofTime-ResolvedSpectroscopy

1. Generationoffemtosecondpulses2. Femtosecondpump-probeexperiments3. 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 electronmotionprotontransferproteinmotions

DE=1eV:T=h/DE=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

A.H.Zewail,NobelLecture,1999

Generationofshortlaserpulses

Chirped-pulseamplificationKerr-lensmodelocking

2.7 fsl =800nm

Q-switching:accesstonanoseconds

Q-switch

gain

med

ium

1) highlosses,lowQ-factor,nolasing2) Q-factorsuddenlyincreasedà powerbuildup3) powerreachessaturationà outputpowerdecays

www.rp-photonics.com

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

Mode-locking:accesstofemtoseconds

L Trt

mod

ulator

gain

med

ium

Manysynchronizedmodesmakeupashortpulse.

SuperpositionofNmodes:www.rp-photonics.com

Themodulatorintroduceslosstocw (narrow-band)operationandfavorsmode-lockedoperation.

Importanceofthespectralphase

Experimentalrealizationofmode-locking

e.g.Kerr-lensmode-locking:

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

operationovercontinuous-waveoperation:

Pulseduration~10fs,Fourierlimit8fs

Chirped-pulseamplification

http://en.wikipedia.org

FromnJtomJpulsesandbeyond...

ELI

©Johan Jarnestad/The Royal Swedish Academy of Sciences

Focu

sed

inte

nsity

(W/c

m2)

CPA

Towards ever higher intensities

Several methods were developed for emitting extremely powerful short laser pulses, but then development stopped – it was not possible to amplify the light pulses further without damaging the amplifying material.

The world’s first functioning laser was built by the American physicist Theodore Maiman.

Extreme Light Infrastructure (ELI) is a European project with three sites that will be completed in a few years.

Pump-probe:averyoldideaAugustToepler (1836-1912):Schlierenphotographie 1864

1.lightflashgeneratessoundwave2.lightflashrecordsimage

explosion of 10 mg AgNO3,

3cm above ground

© H. Kleine

TheopticaldelaylineAbrahamandLemoine 1899:

Dt

2 Dz = c Dt

object

High-speedphotography

inventor: Harold Edgerton (1903-1990)

Time resolution = duration of light flash > 1 µs

Femtosecondpump-probeexperimentsThefastestdetectorshavesub-nanosecondresponsetimeà fastereventscannotbemeasuredelectronically.

Time-resolvedlaser-inducedfluorescence

lpump =310nmlprobe=580-620nm

Dtoven

NaI beamvacuumchamber

Measuresthelaser-inducedfluorescencesignal asafunctionofthepump-probedelay

lprobe =589nm

lprobe =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-energydistributionofphotoelectronsasafunctionofthepump-probedelay

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

protontransfer

internalconversion

Time-resolvedstrong-fieldionization

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

Firstpulse:800nm,8fs,5x1014 W/cm2 ionizesNeà Ne+Secondpulse:800nm,8fs,5x1014 W/cm2 ionizesNe+à Ne2+

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

Measuresthestrong-fieldionizationyield asafunctionofthepump-probedelay