1

Controlling vibrational wave packets in condensed phase:

Halogens in rare gas solids

Markus Gühr, Heide Ibrahim, Nikolaus SchwentnerInstitut für Experimentalphysik

Freie Universität Berlin

FRISNO 8, 21.02.2005 M. Gühr 2

MotivationHalogen Molecule Rare Gas Crystal

• Spectroscopic properties• Ultrafast experiments done• Long lasting coherent wavepackets

• Simple host (fcc)• Chemical inert• High ionization energy• Phonon dispersion

+Br Br

Ar

FRISNO 8, 21.02.2005 M. Gühr 3

Motivation

• Double substitutional site• No rotation• Ultrafast vibrational relaxation and decoherence

FRISNO 8, 21.02.2005 M. Gühr 4

Outline

I: Method, RKR Potential and energy relaxation

II: Vibrational dephasing using the focusing schemeWave packet control, revivals and electronicdephasing

III: Coherent phonons of the rare gas crystal

FRISNO 8, 21.02.2005 M. Gühr 5

Part IPotentials and energy relaxation

FRISNO 8, 21.02.2005 M. Gühr 6

Method: Pump-probe spectroscopy

0.20 0.25 0.30 0.35 0.40 0.45 0.50024

16

18

20

50

55

E

B

X

E [1

03 cm

-1]

RBr-Br [nm]

Pum

p

Pro

beLIF

∆t

0.0 0.5 1.0 1.5 2.0

560 nm

Pump =Probe = 345 nm

Inte

nsity

[arb

. uni

ts]

∆t [ps]

T

Br2:ArFree Br2

FRISNO 8, 21.02.2005 M. Gühr 7

Method: Pump-probe spectroscopy

0.20 0.25 0.30 0.35 0.40 0.45 0.50024

16

18

20

50

55

E

B

X

E [1

03 cm

-1]

RBr-Br [nm]

Pro

be

0.0 0.5 1.0 1.5 2.0

560 nm

520 nm

500 nm

Pump =Probe = 345 nm

Inte

nsity

[arb

. uni

ts]

∆t [ps]

T

Br2:Ar

FRISNO 8, 21.02.2005 M. Gühr 8

RKR Potential

0.20 0.25 0.30 0.35 0.40 0.45 0.50024

16

18

20

50

55

E

B

X

E [1

03 cm

-1]

RBr-Br [nm]

0.0 0.5 1.0 1.5 2.0

560 nm

520 nm

500 nm

Pump =Probe = 345 nm

Inte

nsity

[arb

. uni

ts]

∆t [ps]

T

Br2:Ar

Br2:Ar

FRISNO 8, 21.02.2005 M. Gühr 9

Part IIDispersion, dephasing

and wave packet control

FRISNO 8, 21.02.2005 M. Gühr 10

Dispersion of wave packets

Anharmonic: WP broadens due to dispersion

FRISNO 8, 21.02.2005 M. Gühr 11

Dispersion and Focusingno chirp

• chirp parameter β‘• anharmonicity ωexe• osc.frequency ν

Topt=-β‘ν2/(4πωexe)

neg. chirp

Classical MorsePeriod!

Kohler et al., Phys. Rev. Lett. 74 3360-3363 (1995)

FRISNO 8, 21.02.2005 M. Gühr 12

Focusing Experiment

• Compensate dispersive background• Remaining background is dephasive• vibrational dephasing time 3 ps

0 1 2 3 4

SIMULATION

Inte

nsity

∆t [ps]

β'=0 fs cm β'= -1.76 fs cm

0 1 2 3 4

Experiment

∆t [ps]

Dephasing

Br Br

Ar

Energy RelaxationDispersionDephasing

Gühr, Ibrahim, Schwentner, PCCP 6, 5353-5361 (2004)

FRISNO 8, 21.02.2005 M. Gühr 13

Revivals of a vibrational wave packet

Trev = 2π/ωexe = 21.08 ps for B of Br2

0 2 4 6 8 10

Trev/6

TT

Trev/2Trev/4

∆t [ps]I. Sh. Averbukh et al., Sov. Phys. Usp., 34, 572 (1991)

ν 3ν

Trev /6 at 3.5 psFrequency: 3ν

3.0 3.5 4.0T/3

Rutz et al., Chem. Phys. Lett., 257, 365-373 (1996)

FRISNO 8, 21.02.2005 M. Gühr 14

Revivals in Wigner plot

R

p

FC Area

Probe

FRISNO 8, 21.02.2005 M. Gühr 15

Control of Revival Structureno chirp pos. chirp neg. chirp

• chirp parameter β‘ determines the focus in time

Topt=-β‘ν2/(4πωexe) ωexe=1.64 cm-1

ν=118cm-1 (570 nm)

FRISNO 8, 21.02.2005 M. Gühr 16

Control of Revival Structure

0 2 4 6 8 10 12

Sign

al

∆ t [ps]

Si

gnal

Sign

al no chirp

negative chirp

positive chirp

FRISNO 8, 21.02.2005 M. Gühr 17

Positive Chirp Excitation Experiment

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Experiment (Br2:Ar) Simulation (freies Br2)

Inte

nsitä

t

∆t [ps]

TT/3

Four level coherence about 1.2 ps

Gühr, Ibrahim, Schwentner, PCCP 6, 5353-5361 (2004)

FRISNO 8, 21.02.2005 M. Gühr 18

Part IIICoherent Phonons

FRISNO 8, 21.02.2005 M. Gühr 19

Coherent host dynamics

0 1 2 3 4 5

340 fs 500 fs

normalized spectrum

Inte

nsity

[arb

. uni

ts]

∆t [ps]

Pump: 520 nmProbe: 348 nm

experiment average

b)

a)

Br2:Ar• Molecular period vanishes• New period of 500 fs (2 THz)• for different excitation (A, Bstate)

• No molecular period• Host property

Gühr, Schwentner, PCCP 7,760-767 (2005)

FRISNO 8, 21.02.2005 M. Gühr 20

Coherent host dynamics

0 1 2 3 4 5 6 7 8 9 10

A excitation

∆t [ps]

B excitation

650 fs

c)In

tens

ity [a

rb. u

nits

]

b)

a)

I2:Kr • For I2:Kr host oscillation650 fs (1.5 THz)

• Phase stability• No forced excitation • Impulsive excitation• Decoupling from molec. motion

Gühr, Bargheer, Schwentner, Phys. Rev. Lett. 91, 085504 (2003)

FRISNO 8, 21.02.2005 M. Gühr 21

Zone Boundary Phonon

• Host modulation has frequency of ZBP for Ar or Kr crystal• ZBP has vg=0

0.0 0.5 1.00.0

0.5

1.0

1.5

2.0

2.5

f [T

Hz]

36Ar

k

I2:Kr

Br2:40Ar

84Kr

FFT Power

FRISNO 8, 21.02.2005 M. Gühr 22

Phonon Excitation

3

1

X B or APump Pulse

FRISNO 8, 21.02.2005 M. Gühr 23

Phonon Detection

Covalent state

Internuclear distance

Ene

rgy

Ener

gy re

lax.

WP energydistribution

Prob

e se

nsiti

vity

Ewin

Rwin

0 1 2 3 4 5 6 7 8 ∆t

+ -

FRISNO 8, 21.02.2005 M. Gühr 24

Phonon Detection

Covalent state

Internuclear distance

Ene

rgy

Ener

gy re

lax.

WP energydistribution

Prob

e se

nsiti

vity

Ewin

Rwin

0 1 2 3 4 5 6 7 8 ∆t

+ -

FRISNO 8, 21.02.2005 M. Gühr 25

Summary• Focusing allowed to separate dispersion and dephasing

• Revivals can be controlled by positive chirp

• Coherent Phonons

0 1 2 3 4

Experiment

∆t [ps]

0 1 2 3 4 5 6 7 8 ∆t

FRISNO 8, 21.02.2005 M. Gühr 26

Thanks to:N. Schwentner and groupD. Tannor for a major part of the Matlab codeJ. Manz for TDQM lectureMr. Zimmermann (glassware)Sfb 450 Analysis and control of ultrafast photoinduced reactions