ar mixtures

ar mixtures

Journal of Molecutar Structure. 45 (1978) 0 ElsevierScientific Publishing Company, PICOSECOND DEPHASING J.LANGELAAR, Laboratory Nieuwe 389-394 Ams...

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Journal of Molecutar Structure. 45 (1978) 0 ElsevierScientific Publishing Company,

PICOSECOND

DEPHASING

J.LANGELAAR, Laboratory Nieuwe

389-394 Amsterdam

OF COHERENTLY

H.M.M.HESP,

EXCITED

D.BEBELAAR

for Physical

Prinsengracht

389

- Printed in The Netherlands

Chemistry,

VIBRATIONS

and J.D.W.VAN University

126, Amsterdam,

IN LIQUID

N,/Ar

MIXTURES

VOORST

of Amsterdam

The Netherlands.

ABSTRACT The dephasing been measured Raman

time of the coherently

by a single

high power

excited

laser pulse

molecular

vibration

experiment

of N2 has

via stimulated

scattering.

In mixtures can be changed the influence excited

by varying

dependent

linewidth

measurements.

of the surrounding

on the dephasing

Resonant

molecules 2 In this way N

of vibrationally

out that the dephasing

of Ar and varies

pure Ar. The measured

to the dephasing,

of the excited

of Ar in the mixture.

It turns

upon the mole fraction

does not contribute perturbations

fraction

interactions

can be studied.

N2 to 35 ps in almost with Raman

Ar the surroundings

the mole

of intermolecular

N2 molecules

strongly

N2 with

of liquid

dephasing

time

is

from 90 ps in pure

times are in good agreement

V-V transfer

but the dephasing

between

N2 molecules

is initiated

by stochastic

molecules.

INTRODUCTION Direct obtained phase

information

be measured In order

directly

mixtures

0 and 0.96.

vibrational

litterature,

which

in pure argon.

time.

stimulated

the influence

Argon

the dephasing

as solvent

allows

calculations

a comparison

This quantity

liquids

Raman

scattering

vibration

transfer.

and argon

of the dynamics

(ref.

1).

on the vibrational of N2

was varied

that it cannot

by energy

It can

experiments

environment

of Ar in the mixtures

of nitrogen

the loss of

vibrations.

time of the molecular

molecules

can be

measures

of the molecular

has the advantage

of the nitrogen

dynamics

in simple

of the molecular

with Ar. The mole fraction

excitation

molecular

dynamics

the excitation

in picosecond

we have measured

in liquid

dephasing

set up during

to investigate

dephasing,

detailed

the molecular

from the vibrational

coherence

between

about

trap the Moreover

are present

in pure nitrogen

in the

with

that

390

EXPERIMENTAL The liquid mixtures were prepared by successive condensation of nitrogen and argon gas of highest purity in a calibrated sample cell. For the coherent excitation of the molecular vibrations via stimulated Raman scattering (SRS), the second harmonic (0.5mJ) of a mode-locked neodymium glass laser was used. The fundamental (1 mJ> was used as a probe pulse. Values for the pulse duration of 6-8 ps were obrained from two photon fluorescence measurements. The same pulse duration is observed from the anti-Stokes Raman signal measured for cyclohexane (Fig. 1) and ethanol

from

dephasing time (0. 26 ps) is much shorter

which it is known (ref. 1) that the than

the laser pulse. The decay of the

Raman signal therefore follows the laser pulse, which in turn gives the time resolution limit of the picosecond SRS method.

-5

0

5

10

TD(PS) 15

Fig. 1. Anti-Stokes Raman signal for cyclohexane as a function of the delay time. A schematic diagram of the apparatus is depicted in figure 2. The equipment is similar to the experimental system used by Kaiser, Laubereau and coworkers (ref. 1) A single pulse is selected from the front of a pulse train of the mode-locked Nd3+-glass laser operating in TEM

mode. Two etalons in the oscillator cavity assure 00 the Dulse to be nearly transform limited. After amplification and frequency doubling, the infrared'and the green pulse are separated and their intensities are monitored by means of photodiodes. The infrared pulse travels through a variable delay system before it probes the phase coherence of the molecular vibrations generating a

391

KDP

M-L. f

I

f’

\

i\

ee

L n

V.D.

I I

LASER

-

1060 nm 530 nm 850 nm

--. . . ......

Fig.2 A schematic diagram of the experimental set up for the picosecond stimulated Raman scattering experiments. R= Raman cell. 1,2 and 3 = photodiodes, P q photomultiplier, l.=lens, VD = variable delay. M = monochromator,

coherent

anti-Stokes

intensity grating The

of the latter

monochromator linewidths

a 1.5 m Jobin double

ion laser.

matching

conditions.

attached

The

to a

.

(measured

signal

phase

by a GaAs photomulriplier

of the incoherently

scattered

Raman

with a holographic

resolution

was generated

signal

grating

were

measured

(2400 g/mm)

with

used

in

350,000). by the 514 run line of a single

limitations

were

a measured

bandwidth

frequency

CW argon

of 0.0015

nm(FWHM).

AND DISCUSSION

intensity

result

of a picosecond

of the coherently

exponentially

with

the time

The dephasing

time

is obtained

figure

at 850 nm under

is measured

The experimental

A typical The

pulse

Yvon monochromator

passage

The Raman

RESULTS

light

are the averageof

SRS experiment

scattered

interval

between

the green

from the exponential

5-10 individual

is shown

anti-Stokes

signal

and the

slope.

measurements.

in figure

Raman

3.

decreases

infrared

The data

pulse.

in this

TD(Ps)

I

200

-100

Fig.3. of N2

Semilog plot of the coherently scattered anti-Stokes q 0.64;Td = 35 ps versus the delay time. X Ar

Figure

4 shows the influence

time of the nitrogen experiments Linewidth The

is satisfactory measurement,

dephasing

time

of the argon concentration

vibrations.

for argon

however,

T has

The reproducibility

could

Raman signal

upon the dephasing

of the picosecond

SRS

concentrations

up to 65 mole percent.

he

whole

been calculated

lines which have a Lorentzian

stimulated

done

in

the

from the linewidth

concentration

range.

(FWHM) of the Raman

shape: T-l =ZacAb

It can be seen in figure means

4 that the results of both methods

that the Raman linewidth

processes

fit very well. This

by vibrational

dephasing

only. time

The energy relutation to be 1 second measured

in N2 is determined

(ref. 2). Thus energy

dephasing

the energy

in pure liquid

times.

degradation

However,

relaxation

resonant

of the molecular

the dephasing

rates does not decrease

when nitrogen

is diluted

with argon

nitrogen

has been established

will not contribute

V-V transfer

vibrations.

on dilution.

and indicates

can proceed

Our experiments On the contrary,

recently

to the faster

than

show that it is enhanced

that the contributions

from

393

100

80

l

';360

l

l0

zi Q 2 40

00 0 t

l&

20

0 0

0.5 ‘Ar

1.0

-

Fig.+. Dephasing time of the molecular vibration of N2 in liquid N_/Ar mixtures as a f?unction of the mole fraction of argon, XAr. l from coherent anti-Stokes Raman experiments o from isotropic spontaneous Stokes Raman linewidths experiments resonant

V-V transfer

processes

alone

cannot

explain

the vibrationa3. dephasing

time of nitrogen. Therefore

the experimental

perturbations

results

due to the surrounding

will be published

elsewhere

were anaLysed

in terms

molecules. The results

of stochastic

secular

of these analyses

(r&f. 3)-

ACKNOWLEDGEMENT The Investigations Chemica3. Research

for the Advancement

were

(S.O.N.)

supported

in part by the Netherlands

with financial

of Pure Research

Faundacion

aid from the Netherlands

(Z.W.0.).

For

Organization

394

REFERENCES 1 D.von der Linde, A.Laubereau and W-Kaiser, Phys.Rev.Lett.26,(1971)954-957. S.F.Fischer and A.Laubereau, Chem.Phys.Lett.35,(1975)6-12. A.Laubereau, Chem.Phys.Lett.27, 1974,600-602 2 W.F.Calaway and G.E.Ewing, Chem.Phys.Lett.30, 1975, 485-489. 3 H.M.M.Hesp, J.Langelaar, D.Bebelaar and J.D.W.van Voorst, to be published.