M ax-Born-In stitut

Femtosecond Laser Spectroscopy of C60

Nieuwegein, The Netherlands August 21, 2001

Eleanor Campbell, Göteborg University & Chalmers, SwedenR.D. Levine, Fritz Haber Center, Hebrew University

M. Boyle, K. Hoffmann, R. Stoian, C.P.Schulz & I.V. Hertel

Max-Born-Institut, Mark Boyle 21.08.01 2

Outline

1.) Observed Rydberg Structure in photo-electron spectra

•What do we learn about the Rydberg states from photoelectron spectroscopy?

•Excitation and ionization process

2.) Femtosecond Pulse Shaping on C60

•interested in impulsively exciting vibrational modes

Max-Born-Institut, Mark Boyle 21.08.01 3

Experimental TOF Apparatus

Electron TOF

e- Ion+

C60-Oven

Double µ-Metal Shielding

Wiley-McLaren Reflectron TOF

x

y

z

Max-Born-Institut, Mark Boyle 21.08.01 4

Experimental Variable Parameters

•Intensity - 1011-1013 W/cm2

•Wavelength- 800nm, 400nm, 660nm

•Bandwidth Limited Pulse Duration

•Chirp

•Polarization

Max-Born-Institut, Mark Boyle 21.08.01 5

Intensity Dependence of Photoelectron Spectra= 800 nm, = 1.5 ps

elec

tron

yie

ld /

lo

g u

nits

1.5x1012 W/cm2

e - -energy / eV

100

101

102

103

104

105

106

100

101

102

103

104

105

100

101

102

103

104

05 10 15 20

100

101

102

103

104

1 2

0.5

30

60

90

10

20

30

1

2

3

0 0,5 1,5

1 elec

tron

yie

ld *

104 /

arb.

uni

ts

1.1x1012 W/cm2

2.2x1012 W/cm2

3.6x1012 W/cm2

0

0

0

0

Max-Born-Institut, Mark Boyle 21.08.01 6

Wavelength Dependence of Binding Energy

0 1 2 3

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

-0,15

Binding Energy [eV]

No

rmal

ized

Sig

nal

800nm , 1.5 ps 660nm, 120 fs 400nm, 2.1 ps

***assuming 1-photon ionization

IP

Max-Born-Institut, Mark Boyle 21.08.01 7

Bandwidth Limited Pulse Duration Electron Spectra Comparison E*=0.441

0,0 0,5 1,0 1,5 2,0

E ~ 85meV ~ 30fs

Energy[eV]

E ~10meV ~180 fs

E~20meV ~100 fs

Rydberg spectra seen for pulsedurations as short as 25 fs

Indicates very fast population process

meas = laser decay laser

radiative decay << laser

Lifetimes of Rydberg states are400 fs or longer

Max-Born-Institut, Mark Boyle 21.08.01 8

Photoelectron Spectra for two chirps

0,6 0,8 1,0 1,2 1,4

negative chirp (blue leads red) positive chirp (red leads blue)

Kinetic Photoelectron Energy [eV]

Shift=energy bandwidth of laser

IP

Max-Born-Institut, Mark Boyle 21.08.01 9

Effect of different polarization

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

Kinetic Energy [eV]

Counts

[arb

. U

nits

]

e- TOF

e- TOFElectrons emitteddirectly within pulse duration

Max-Born-Institut, Mark Boyle 21.08.01 10

Experimental Summary

•Intensity - R.S. emerges from background in low intensity pulses

•Wavelength - indicate a non-resonant excitation process

•Pulse duration - indicates a very fast process

•Chirp - indicates electron emitted within one pulse duration

•Polarization - indicates the electrons are emitted directly

Max-Born-Institut, Mark Boyle 21.08.01 11

Modeling of Rydberg Series in C60

4 8 12

-0.5

-1.0

-4-8

r [a.u.]

-12

-1.5

0

•Solved the Schrödinger Equation for the bound energies of C60

•assuming a simple two particle system•the potential as shown below

Resultant BE values were inagreement with literaturevalues of

Puska and Nieminen, Phys. Rev. A 47, 1181(1993)

Max-Born-Institut, Mark Boyle 21.08.01 12

Calculated Rydberg Series

2 4 6 8 10 12 14 160,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

ni

ns

npnd

ng nfnh

nj

[Bin

ding

Ene

rgy

/ eV

]-1

/2

n

Max-Born-Institut, Mark Boyle 21.08.01 13

0

2

4

6

8

10

12

14

0 1 2 3 4 5

l = 7l = 5l = 3

(Binding Energy / eV)-1/2

n

Solid points from CalculationOpen points from fitting of exp.

Results from Calculation and fitting

l=5

l=5 l=7 l=9l=3l=1

Max-Born-Institut, Mark Boyle 21.08.01 14

Excitation of Rydberg Series in C60

Resolved Rydberg Structure has been observed with Laser Photoelectron Spectroscopy of C60.

Electrons populate the Rydberg states with a four photon process, and are then single photon emitted within the same pulse.

Results of calculations using a simple two particlemodel show excellent agreement to experimentalspectra.

Max-Born-Institut, Mark Boyle 21.08.01 15

Next Experimental Steps

1.) Two color pump probe

4.) C70, NC59, ...

2.) angular distribution of Rydberg electrons

3.) cold C60 source

Max-Born-Institut, Mark Boyle 21.08.01 16

Comparison to C70

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

+0,15

C60

, 1500 fs, 1*1013 W/cm2

C70

, 1300 fs, 2*1013 W/cm2

Kinetic Energy [eV]

Nor

mal

ized

Cou

nts

Max-Born-Institut, Mark Boyle 21.08.01 17

Femtosecond Pulse shaping and C60

Idea: To excite C60 using trains of pulses with frequency equal to the vibrational frequencies. (Impulsive Excitation)

The two frequencies of highest interest are the two energetically lowest Raman active frequencies.

•Ag(1) with energy 496 cm-1 and oscillation period 67 fs

•Hg(1) with energy 271 cm-1 and oscillation period 123 fs

Max-Born-Institut, Mark Boyle 21.08.01 18

Motivation for Excitation

Diameter of C60 versus time following a 12 fs laser pulse witha fluence of 0,44 J/cm2 at T=300K

B. Torralva and Roland E. AllenProceeding of 24th InternationalConference on the Physics of Semiconductors

Dia

met

er (

Å)

For impulsive excitation to occur,

Periods of oscillation:

67fs, 123 fsOur pulse duration: 30fs

2 )(

-1Rduration pulse

Max-Born-Institut, Mark Boyle 21.08.01 19

Schematic of Pulse Shaping Apparatus

f fff

LiquidCrystal

Modulator

Input waveform O

utpu

t wav

efor

m

•Fourier synthesis spectral modulation

•Diverse Applications: optical fiber communications, coherent quantum control

micro machining

•Usable with pulse durations from picoseconds to below 10 fs

Max-Born-Institut, Mark Boyle 21.08.01 20

How the crystals change the pulse shape

Frequency Response

Time Domain: eout(t)=dt´h(t-t ´)ein(t ´)

Frequency Domain: Eout(w)=H(w)Ein(w)

h(t)Impulse Response

ein(t) eout(t)

H()Ein() Eout(w)

Max-Born-Institut, Mark Boyle 21.08.01 21

Examples of Shaped Pulses

~ 1000 fs

~ 500 fs

~ 250 fs

Double Triple

-1000 -500 0 500 1000

Inte

nsity [arb

. units]

Femtoseconds

-1000 -500 0 500 1000

Inte

nsity

[arb

. units

]

Femtoseconds

•Pump-probe measurements simplified

Max-Born-Institut, Mark Boyle 21.08.01 22

Ion Results from shaped pulses

0,0 1,0x1013

2,0x1013

3,0x1013

4,0x1013

0

400

800

1200

Double Pulse with 500 fs separation 1000 fs separation

C

60

+ Cou

nts

Intensity [W/cm2]

Memory of Signal

Max-Born-Institut, Mark Boyle 21.08.01 23

Schematic of Optimization Experiment

O scilla tor M odula tor A m plifie r E xperim ent

M easurem ent

A daptiveA lgorithm

C ontro l o f S haper

C om puter

Feedback S igna l

I.e. SHG

I.e. Autocorrelation

Oscillator Modulator Amplifier Experiment

Measurement

AdaptiveAlgorithm

Control of Shaper

Computer

Feedback S ignal

I.e. SHG

Max-Born-Institut, Mark Boyle 21.08.01 24

Optimization with C60

Optimize certain fragmentation or ionization patternsor discriminate between competing paths.

1 10 100

101

102

103

104

105

106

107

C60

+

C60

2+

apparent frag. onset

=800 nmh=1,55 eV

Ion

Yie

ld /

arb.

units

Laser Intensity/ 1013

W/cm2

Regions of interest

•low energy- primarily ionization, C2 loss

•mid-energy-fragmentation and ionization

•high energy-bimodal fragmentation pattern

Pulse shape gives additional information

Max-Born-Institut, Mark Boyle 21.08.01 25

ConclusionsIntroduction of Pulse Shaping and use on C60

1.) Impulsive Excitation-creating pulse trains of varying separation to excite C60

2.) Using optimization feedback control to learn information

about C60