VIBRATIONAL ENERGY RELAXATION OF BENZENE DIMER STUDIED BY PICOSECOND TIME-RESOLVED INFRARED-ULTRAVIOLET PUMP-PROBE SPECTROSCOPY

R. KUSAKA and T. EBATAHiroshima University

38600 38800

(1+1’)REMPI

hole-burning(isomer1)

hole-burning(isomer2)

C6H6 601

Structure of benzene dimer

h(Top)

isomer 1 isomer 2

h = C6H6 , d = C6D6

h(Top)

h(Stem)

(C6H

6-C

6D6)

+C6D6 60

1

d(Top)

d(Stem)

Vibrational energy relaxation

Top

Stem

How fast is vibrational energy relaxation ?

IRS0

kIVRinitial

bath

IRS0

kTop

kSteminitial

bath

kStem kTopAre and different ?

Site-specified excitation

isomer 1 isomer 2

h(Stem)h(Top)

h(Stem)

h(Top)

d(Top)d(Stem)

h(Stem)

d(Top) h(Top)

d(Stem)

homodimer heterodimer

vibr

atio

nal e

nerg

y

vibr

atio

nal e

nerg

yBh(Stem) - Ah(Top)

Ah(Stem) + Bh(Top)

delocalized vibration localized vibration

We can investigate pure site-specified relaxation by using heterodimer.

isomer 1 isomer 2

h(Stem)

h(Top)

site-specified excitation

E

IR-UV pump-probe spectroscopy

IR

kIVR

S0

monomer

IR spectra in CH stretching region, obtained by ns laser system.

h(Stem)

h(Top)

UV

t

IP

S1

real time measurement ofpopulation decay

Experimental setup

Energy and time resolutions are 5 cm-1 and 12 ps.

(monitor hd +

mass)

Excitation scheme

h*(Stem)

h*(Top)

Two isomers can be distinguished

by choosing UV frequency.38600

h(Top)

h(Stem)

38700

(1+1’)REMPI

hole-burning

hole-burning

Electronic spectra obtained by ns laser system

C6H6 601

ps laser

ns laser

IR spectra of heterodimer

ns laser

Excitation scheme by picosecond pulses

Site-specified vibrational relaxation

1 2

15 ps95 ps670 ps

365003640036300

IR=3077 cm-1

UV wavenumber / cm-1

h*(Stem)

h*(Top)

1

2

12008004000

= 500 ps

1000600200

2

1 = 100 ps

The lifetimes are quite different, in spite of similar IR excitation energy.

h*(Stem)

h*(Top)

191 191

Transient UV spectra obtained by ps laser system

Why is the relaxation faster in Stem site ?

HH

HH

H

H

12008004000

= 500 ps

1000600200

2

1 = 100 ps

Coupling with inter molecular vibration Stem > Top

Reasonparallel

perpendicular

monomerh(Top)

Why is the relaxation faster in Stem site ?

Additional reasons

Vibrational frequencies of Stem benzene

are more red-shifted.

Symmetry of Stem benzene is lower.

are IR-inactive CH stretching

vibrations in monomer and Top benzene.

obtained by ns laser system

Relaxation process

IVRIR

S 0

initial

bath

UV t

IP

S1

UV

hd+

x 5

x 115 ps95 ps670 psIR off

delay (t) IR=3077 cm-1

broad transition from bath state

x 5

transition from initial level

VP

IRS 0

initialVP

case 1

case 2

Binding energy ~800 cm-1

Excitation energy 3077 cm-1<

Vibrational predissociation (VP) occurs.

Lifetimes of vibrational predissociation

10005000 1500

Time profile of the broad UV transition

IR=3077 cm-1

Intensitybroad(t)kStem, IVR

kStem, IVR – kStem, VP

{exp(–kStem, VP t) – exp(–kStem, IVR t)}

k Top, IVR

kTop, IVR – kTop, VP

{exp(–kTop, VP t) – exp(–kTop, IVR t)}

= A

+ B

(A / B = 1)

kIVR

IRS0

initial

bath

kVP

IVR / ps VP / ps

Stem

Top

100

500

decay rise

Stem

Top

Stem

Top

UVS

1

UV

hd+

= 1/k)

100 ps 500 ps500 ps 900 ps

500

900

Process and lifetimes of vibrational relaxation

HH

HH

H

HH

H

HH

H

HH

H

HH

H

HIVR

IVR

VP

VP

100 ps 500 ps

500 ps 900 ps

Stem

Top

IR=3077 cm-1

statistically redistributed

(E*)Stem = (E*)Top [density of states]

Coupling between initial and bath states is 5 times stronger in Stem than in Top.

initial bath dissociation

x 5 x 1.8

Summary

Vibrational energy relaxation of benzene dimer was studied,

in real time, by picosecond time-resolved pump-probe spectroscopy.

hdinitial hdbath h + d

IVR

IRS0

VPinitial

bath

dimer monomerS0

IVR / ps VP / ps

Stem

Top

100 500

500 900

Stem

TopBy using deuterated heterodimers, process and lifetimes

of pure Stem and Top relaxation were investigated.

RI-MP2(Full)/VDZ with BSSE correction.

global minimum

J. Phys. Chem. A. 2007, 111, 3446

J. Chem. Phys. 1999, 111, 572

monomer

monomer

C6H6-C6H6

40200

Population time evolution

60

Benzene monomer

IRS 0

IVR

UV

t

IP

S 1

D6h

CH

Process and rate of vibrational relaxation

160012008004000

[hdbath] =kIVR

kIVR - kVP{exp(-t/VP) - exp(-t/IVR)}

step function

IVR

IR

UV t

IP

S1

S 0

h UV

hd+

VPinitial

bathIVR = 110 ps

VP = 530 ps

1. larger clusters2. six-zero-one transition of Stem and Top3. mode dependence4. homodimer5. the reasons of kIVR