Dissipation and Coherence:Halogens in Rare Gas Solids

• Signatures of Dissipation in Pump-Probe Spectra

• Dissipation of Energy in Excited Halogens

• Dispersion and „Decoherence“: Classical vs. Quantum Effects

• New Experiments with Phase-Locked Pulses

M. Bargheer, M. Gühr, P. Dietrich, M. Fushitani, T. Kiljunen and N. Schwentner

Institut für Experimentalphysik

Diatomics in Solid Rare Gas

• I2 in Kr

• Fcc lattice, closest packing

• Adiabatic dynamics including dissipation are well described in classical simulations

• strong coupling to „solvent“

• some similarities to gas phase

P u m p = P ro b e = 4 9 0 nm 5 0 0 nm

E 0

R

t0ji

ji

tt

EE

d t

d E

R elaxation R ate :

0.0

0 .2

0 .4

0 .6

0 .8

1 .0

norm

.flu

ores

cenc

e

0 1 2 3 4

t / ps

Energy loss from oscillations

Bargheer et al., PCCP 4, 75 (2002)

Energy loss from signal envelope

probe =

480 nm

490 nm

500 nm

510 nm

520 nm

530 nm

540 nm

fluor

esce

nce

sign

al/a

.u.

pum p = 500 nm

m

E

E0

t0 t

Sig

nal

ji

ji

tt

EE

d t

d E

R elaxation R ate :

Vibrational relaxation ofI2/Kr

-3000 -2000 -1000 0 10001

10

100

1000

10000

-3000 0 3000 6000

1

10

100

1000

10000

dE

/dt

/ cm

-1/p

s

Eexcess

/ cm-1

dissociation limit

dissociation limit

dE

/dt / cm

-1/ps

Eexcess

/ cm-1

I2/Kr

Vibrational relaxation ofI2/Kr, ClF/Ar and Cl2/Ar

-3000 -2000 -1000 0 10001

10

100

1000

10000

-3000 0 3000 6000

1

10

100

1000

10000

and ClF/Ar 100 % loss in one collision for I

2/Kr

dE

/dt

/ cm

-1/p

s

Eexcess

/ cm-1

dissociation limit

dissociation limit

dE

/dt / cm

-1/ps

Eexcess

/ cm-1

I2/Kr

ClF/Ar Cl

2/Ar

Collisions Cause Coherence?

0 1 2

LIF

/ a.

u.

t / ps

Collision of I2 with surrounding Kr

•Width of wavepacket: 500 cm-1

•Energy loss in collision: 1500 cm -1

•Collision populates new vibrational levels coherently!

• well defined timining of collision

• well defined scattering geometry!

R

E 0

Mechanisms of „Dephasing“

• Decoherence due to collisions with solvent (pure dephasing T2´)

• Population decay by vibrational relaxation (and non-adiabatic couplings) (relaxation time T1)

• Dispersion due to anharmonicity (dispersion time Tdisp)

0.0 0.5 1.0 1.5 2.0

E = 600 cm-1

E = 200 cm-1

E = 50 cm-1

Ove

rlap

with

pro

be w

indo

w

t / psR

Ene

rgy

Dispersion: Classical and Quantum Effects

• Compensation of dispersion by negative chirp of excitation pulse

• Classical!

0 5 10 15 20

calc

. sig

nal

t / ps

• Rephasing of wavepacket after dispersion

• Rephasing time Trep = 1/exe

(after dispersion of the packet)

• discrete vibrational levels needed

0 1 2 3 4

calc

. sig

nal

t / ps

Dispersion in Morse-Potential (Classical)

•Morse-potential:

•Frequency:

200 exp1)( rrDrV

01 DE

emorse

eexm 2

R

ExTee

morsedisp

2

T1

T2

T = n(T1 - T2)

•Dispersion-time:(wave packet width T > 1/2Tmorse)

Ene

rgy

Dispersion of I2-Wave-Packets

17.5 18.0 18.5 19.0 19.5 20.00.0

0.2

0.4

0.6

0.8

1.0

1.2

570 560 550 540 530 520 510 500

FWHM = 785 cm-1 (10 states)

FWHM = 285 cm-1 ( 4 states)

VibrationalLevels

sign

al /

a.u

.

excitation pulse / 1000 cm-1

[nm]

Tdisp= 2 ps

Tdisp= 5 ps

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

sim

ula

ted

sig

na

l

t / ps

10 states 4 states

0.0

0.2

0.4

0.6

0.8

1.0

1.2

me

asu

red

sig

na

l FWHM = 785 cm-1 (10 states)

FWHM = 285 cm-1 ( 4 states)

eeee

morsedisp xNExT

2

12

If N = number of excited vibrational levels:

Dispersion of ClF-Wave-Packets

Tdisp< 1 pseeee

morsedisp xNExT

2

12

3 4 5 6 7

Ene

rgy

/cm

-1

R / a0

C l F+ -

B 30

0 1 2 3

Ar3

t / ps

Experiments with Phase-Locked Pulses

Generation of Pulse-Pairs: ConstructiveInterference

DestructiveInterference

Scherer et al., J. Chem. Phys. 95, 1487 (1991)

Piezo

Piezo tunes phase by movingdistance /2

Explanation in Frequency Domain

• Pulse-Pairs in frequency domain yield spectral interferences, if pulses overlap.• Frequency resolution of monochromator broadens pulses.

freq u en c y

inte

nsit

y

• Observed signal: Fluorescence, i.e. integrate from -∞ to +∞• Vibrational states act as a monochromator => interference

constructivedestructive

nointerference

Phase-Locked Pulses in the Presence of Dissipation: Proposed Experiment: Cl2 in Ar

• Phonon side-bands increase for higher vibrational levels• Excitation of zero-phonon lines => oscillation of free molecule?

Cl2 / Ar

• Excitation of phonon sidebands => dissipative dynamics

• Coherent control of dissipative vs. free wave packet motion

constructivedestructive

nointerference

Summary

R

E 0

-3000 -2000 -1000 0 10001

10

100

1000

10000

-3000 0 3000 6000

1

10

100

1000

10000

and ClF/Ar 100 % loss in one collision for I

2/Kr

dE

/dt

/ cm

-1/p

s

Eexcess

/ cm-1

dissociation limit

dissociation limit

dE

/dt / cm

-1/ps

Eexcess

/ cm-1

I2/Kr

ClF/Ar Cl

2/Ar

• Signatures of relaxation

• Energy loss of halogens in Rg

•Collisions cause coherence

• Dispersion in anharmonic potentials

• Experiments with phase-locked pulse pairs

0 1 2 3 4 50.00.2

0.40.6

0.81.0

sim

ulat

ed s

igna

lt / ps

10 states 4 states