Setup for in situ surface investigations of the liquid/glass transition with (coherent) x rays

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Setup for in situ surface investigations of the

liquid/glass transition with (coherent) x rays

Tilo Seydel, Anders Madsen, Michael Sprung, Metin Tolan, Gerhard Grübel,

Werner Press

To cite this version:

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liquid/glass transition with ( oherent) x-rays Tilo Seydel (1)() , Anders Madsen (2) , Mi hael Sprung (3) , Metin Tolan (3) , Gerhard Grubel (2)

, and Werner Press (1)

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Institut Laue-Langevin , BP 156, F-38042 Grenoble (Fran e)

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ESRF , BP 220, F-38043 Grenoble (Fran e)

(3)

Experimentelle Physik 1, Universitat Dortmund

Otto-Hahn-Str. 4, D-44221 Dortmund (Germany)

()

Corresponding author. Ele troni address: seydelill.fr

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Adedi atedsetup omprisinganeÆ ient ryogeni devi eforthe

in-situpreparationof largesurfa eareas ofprototypi al organi glass

for-mersina widetemperaturerange (170{340K) ispresented. Thissetup

providesthene essarytemperatureandvibrationalstabilityforsurfa e

x-rayandneutrons atteringexperimentsin ludingtheextremely

sensi-tive te hniqueofx-rayphoton orrelationspe tros opy(XPCS).XPCS

is an emerging method whi h is made possible by the high oherent

photon uxprodu edby3rdgenerationsyn hrotrons. Wedemonstrate

that mi ros opi motion at the surfa e an be studied in a dire t way

in the liquid and super ooled state using XPCS. In addition, we have

used a CCD-dete tor to re ord 2-dimensional images of stati spe kle

patterns formingon surfa es intheglassy state.

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Glasses are hara terized by the stru tural disorder known from liquids but

also by mi ros opi dynami al pro esses very far from those of a liquid [1℄.

The transition from a liquid to a glass is hara terized by an exponential

in- reaseofthevis osity. Littleisknown abouttheglasstransitionatthesurfa e

and there has been mu h debate about possible dieren es to the bulk

tran-sition. The rms roughness and long-range orrelations of a liquidsurfa e are

essentiallydetermined by thermallya tivated apillarywaves [2,3, 4,5℄. The

ommon hydrodynami theory predi ts that the high vis osity near the glass

transition auses apillarywavesatthesurfa etobeoverdamped. Thesewaves

are expe ted tograduallyfreeze due toanexponential in rease inthe

hara -teristi time 0

oftheirmotion[2,6℄. Onlyre ently,experimentaltestsofthese

predi tions have been a hieved by x-ray s attering onsurfa es of simpleglass

formers with bulk properties [7, 8℄. In this arti le we report onthe ne essary

instrumental pre-requisites for these experiments and give a full s ope of

ex-amples. Wemainlyfo usonthete hni alaspe ts,whilethephysi alquestions

to be resolved and underlying theory are dis ussed elsewhere, the referen es

being provided inthe ourse of this text.

Dira tionpatterns generated with light froma oherent sour e an show

a graininess known as spe kle. The spe kle pattern is aused by phase shifts

of thein identplanewaveintrodu edduringthe dira tionpro ess bya

mod-ulated spatial arrangement of the sample [9, 10℄. Sin e the advent of high

brillian esyn hrotron x-ray sour es ithas been possible to observe spe kle in

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spe kle pattern is related to the dynami s of the sample and x-ray photon

orrelation spe tros opy (XPCS) has forinstan e beenused toinvestigatethe

omplex dynami s of olloidalsuspensions [14, 15, 16℄. Thesestudies apply a

transmission geometry and hen e are sensitive to bulk dynami s. The XPCS

te hnique is losely related to dynami light s attering [17, 18, 19℄, but with

the advantage that the smaller wavelength of x-rays allows a ess to larger

momentumtransfers (q-ve tors) and permits toinvestigate opaque materials.

When x-rays are in ident on a surfa e below the riti al angle for total

external re e tion the s attered intensity is solely oming from the atoms or

mole ulesinthenear-surfa eregion. S atteringof oherentx-raysin identona

surfa eundergrazingangles anthereforebeusedinadire tandunambiguous

way to measure temperature dependent hanges in the surfa e dynami s of

a super ooled liquid or glass. In general, the orrelation fun tion measured

by XPCS is related to the so- alled intermediate s attering fun tion S(q;t)

[20℄. With the lateral length and time s ales urrently a essible, surfa e

XPCS an thus be appliedtomeasure apillary wave u tuationswith ahigh

resolution in waveve tor and time [7, 21, 22℄. On highly vis ous liquids, the

ratio of the vis osity and the surfa e tension an be determined from XPCS

on mi rometer length s ales [23℄. Although on these length s ales XPCS and

visiblelightphoton orrelationspe tros opy(PCS)still ompete,theadventof

x-raysour eswithhigherbrillian ewillmakeshorterlengths alesa essibleto

surfa eXPCS.Thesurfa esensitivitybelowthe riti alangleandtheposibility

of x-raystopenetrate opaquewindows of ryostatedsampleenvironmentsand

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experiments[20,24℄mayoerafuture perspe tivetoa hieveeven largerwave

ve tor transfers.

In addition to the emerging te hniques using x-rays with a high degree

oftransverse oheren e,thewell-establisheds atteringexperimentsmeasuring

the re e tivity and diuse s attering of "in oherent" x-rays from a surfa e

may be applied. When the x-rays are in ident well above the riti al angle,

the resulting re e ted intensity allows for infering on the rms roughness of a

surfa e [4, 5, 25℄. The o-spe ular diusely s attered intensity then ontains

informationonthelateral orrelationsofthesurfa e[26℄. Su hameasurement

may therefore be used to determine the roughness of a liquid in-situ while it

is super ooled and undergoing a glass transition[8℄. Hen e, it may be tested

if the long-range orrelations whi h are hara teristi for apillary waveson a

liquidsurfa e remainun hangedor are altered with the rising vis osity.

Forthese typesof experimentslarge surfa es of prototypi alglass forming

substan es have to be prepared and ooled homogeneously through the glass

transition. The samples have to be suÆ iently deep to be free of substrate

ee ts, and rystallization has to be ir umvented. Furthermore, the setup

has toprovidethe possibilitytostabilizeany desiredtemperatureoverseveral

hours to better than 0:02K. Our sample environment fullls these

require-ments, and in the following se tion 2 we will des ribe it in detail. Se tion

3 ontains te hni al notes on the temperature-dependent surfa e

hara teri-zation by (in oherent) x-ray re e tivity and diuse s attering measurements.

In se tion 4, the experimental setup and examples of x-ray photon

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withaCCDdete tor willbedis ussed indetailinse tion5. Abrief on lusion

is presented inse tion 6.

2 The sample environment

For the in-situ experiments des ribed in this arti le a dedi ated sample

en-vironment has been developed [27℄. The samples with ma ros opi thi kness

are prepared in a trough of 140mmdiameterand approximately4mm depth

inside analuminum sample hamber (Fig.1). The large diameterof the

sam-ple ensures that the illuminated surfa e is not ae ted by a menis us of the

liquidat the borders of the sample ontainer and thata large footprintof the

in ident beam under grazingangles ts on the sample. The trough islled to

approximately0.1mmabove itsupper rim,and a menis us is reated. This is

possibledue tothe nite surfa e tensionof the liquid. Therefore, the in ident

beamdoesnothittheupperrimofthetroughevenatzeroin identangle. The

sample hamber and the trough within are ma hined out of a single pie e of

aluminumtoa hieve agoodthermal ondu tivity and toallowforathorough

leaning. The sample hamberis overed with a Viton-sealedtop lidthat an

beremoved forthe leaningofthe trough. The llingisdone throughaninlet

devi e to redu ethe exposure of the sample tothe outsideairto a minimum.

After lling the trough with the liquid, the sample hamber is eva uated

to approx. 10mbar inorder toredu e the ba kground s atteringfrom the gas

and then hermeti allysealed. Thepressure insidethe sample hamber an be

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the measured ba kgroundsignalinre e tivityexperimentssin e itisnot

pos-sible to ontrolthis temperature-dependent parameter. The ba kground level

isimportantinx-rayre e tivitymeasurementsfrombulkliquidsurfa essin e

isotropi bulk s atteringdominates the s attering for largewave-ve tor

trans-fers [28℄ and isotropi bulk s attering annot be distinguished from isotropi

ba kground s attering arisingfrom the remainingvapor. In the ase of

re e -tivity measurements the surfa e signal has to be determined by subtra ting

the separately measured ba kground. However, in the ase of s attering

ex-periments with the angle of in iden e being always smaller than the riti al

angle, the ba kground s attering fromthe bulkis negle tible.

A va uum (p < 10 5

mbar) is maintained in an outer aluminum ell

sur-rounding the sample hamberfor thermal insulation. Forthis purpose, a

tur-bomole ular pump is onne ted to the outer ellvia a exible metal tube. It

turned out that even spe kle imaging measurements are not hampered by

vi-brationswhilethe pumpisrunning. High-frequen y vibrationsare apparently

ee tively redu ed by the large mass (approximately 15 kg) of the ell (i.e.

the sample hamberplus the outer ell) and itsrigid mounting onthe

dira -tometer. An a tivevibration dampingsystem isalsonot ne essary, sin e

low-frequen y vibrations whi h may be indu ed from outsidedo not ae t highly

vis ous liquids. Su h problems may, however, o ur for low vis osity liquids

su h as water.

The x-ray beam penetrates both the inner and outer ell walls through

Kapton foil windows (thi kness approx. 25m per layer). For the XPCS

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This redu es the number of Kapton foil windows in the beam, whi h might

ae t the degreeof oheren e of the radiation.

In order to re ord the temperature of the sample two erami s- oated

platinum resistor (Pt100) sensors are dipped into the sample approximately

100mmapart fromea h otheron opposite sides of the beam footprint.

The ooling of the sample is a omplished with a onstant ow of liquid

nitrogen evaporating in a heat ex hanger underneath the sample hamber.

The heat ex hanger has the shape of a hollow diskof 10mm thi kness and a

diametermat hing that of the sample hamber. The brass disk has an empty

volumeofapproximately53 m 3

fortheevaporationof theliquidnitrogen. Its

fabri ationrequires the so- alledhard solderingof its omponentstoensure a

suÆ ient thermal stress resistan e. The disk stands with its bottom fa e on

smallte onpads inthe outeraluminum elltoallowfor aneva uated volume

for thermal insulation. The liquid nitrogen is fed through a narrow (5 mm

outer diameter) brass inlet tube from outside the outer ell, and the exhaust

is a omplished by an equivalent outlet tube. The brass tubes are in onta t

withthe bottomwalloftheouter ellsolelyviaVitonva uumseals inorderto

minimizethe thermal loss. The onne tionbetween the brass inlet and outlet

and, respe tiviely, the nitrogendewar and exhaust gas pump is a omplished

by exible Te on tubes.

The sample hamber is atta hed to the heat ex hanger using a bayonet

xingensuringagoodthermal onta ttothediskatbothitstopfa eandside.

The thermal onta t isfurther enhan ed by an adequateheat sink ompound

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ow ontroller(adjustable owrate0to30l/min.). The liquidnitrogendewar

is kept atnearly ambientpressure and a membrane pump in the exhaust gas

tubegeneratesthe ow. Theheatingof thesampleisa hievedwithanele tri

resistorsandwi hedintoKaptonfoil(Min oProdu ts,Minneapolis,MN,USA)

underneath the heat ex hanger. The resistor is onne ted to a Lakeshore

temperature ontroller allowing for a maximum heating power of 45 W. In

addition, the top lid of the sample hamber an be independently heated. A

temperature stability better than 0:02K is a hieved in a temperature range

from 170K to 340K. A third Pt100 sensor is used to separately monitor the

temperatureoftheheatex hanger. Thegoodstabilityofthetemperatureover

along timeisinparti ularrequiredforthe XPCSexperiments,asinstabilities

may introdu eparasiti time orrelationsin the measurements.

The nitrogen ow rate for a ooling rate of about 1.5 K/min. amounts to

approximately20 l/min. of gaseous nitrogen(measured at roomtemperature

and ambient pressure) in the exhaust gas stream. Only taking into a ount

the heat ex hange due to the evaporation of the nitrogen, a heat transfer of

about 510 3

J/min. an be estimated. The total mass of the sample

ham-ber (i.e. the inner ell alone) is nearly 1:5kg. Assuming the heat apa ity

Al

900J/kgK (at T = 300K) of aluminum to des ribe the total heat

a-pa ity of the hamber, about 210 3

J/min. is therefore ex hanged. Thus,

the oolingdevi e ishighlyeÆ ientunderthe diÆ ultgeometri alrestri tions

(largesamplesurfa e). ThesamplemaybeeasilyheldatT = 90 Æ

Cformore

than 24h, whi h has proven to be parti ularly useful for neutron s attering

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X-ray re e tivity measurements using the sample hamber des ribed in the

previous se tion have already been published in Ref.[8℄. Here we add some

omments onthe te hni al aspe ts for performing su ha type of experiment.

For liquid surfa es, a parti ular experimental diÆ ulty in measuring the

re e tivity arises from the ne essity to keep the sample surfa e horizontal.

The in ident beam has therefore to be in lined within a large angular range

and with a pre ision better than one mrad. Using an x-ray tube sour e, this

may be a hieved by rotating the sour e around the sample or, in the ase of

a rotating anode, rather by making use of the large divergen e of the

emit-ted radiation through the rotation of a pair of slits around the anode [27℄.

Re ently, liquid surfa e dira tometers with x-ray tube sour es have be ome

ommer ially available (D8 by Bruker-AXS), and additionally a number of

individually designed instruments exists at rotating anodes [27℄. However, in

order to determine the roughnesses of omparatively smooth liquid surfa es

withhighpre ision, thehigh uxofasyn hrotronradiationsour eisrequired,

sin e the measured re e tivity deviates fromthe Fresnelre e tivity of an

ide-allysmoothsurfa eonlyatlargeq z

. Onlyafewdira tometersatsyn hrotrons

worldwideareoptimizedforthispurpose,oneofthembeingtheHarvard/BNL

liquidsurfa edira tometerattheNationalSy hrotronLightSour ebeamline

X22BatBrookhavenNationalLaboratory(seee.g. [4℄andreferen estherein).

The rms roughness measured from a free surfa e of a deep liquiddepends

onthe experimentalresolution througha logarithmi term whenmeasured by

x-ray re e tivity [25℄. A detaileddis ussion of resolution ee ts in surfa e

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resolution than at dira tometersusing syn hrotron radiation. This is due to

simple me hani al reasons, be ause fewer movements are required to tilt the

beam at tube sour es. For instan e, at the liquid surfa e dira tometer in

the Physi s Department atthe University of Kiel(Germany) using arotating

anodex-raysour e,anangularresolutionofapproximately0.35mradhasbeen

a hieved by the spe ial design of the instrument [27℄. To illustrate this, an

example of a measurement at this instrument is shown in the gure 2. The

diuse s attering from a gly erol surfa e at room temperature around the

spe ular peak is shown in a log-log plot as a fun tion of the lateral wave

ve tor transfer q x at a xed q z 0:07 A 1

(symbols in the gure). Open

symbolsdenotedatataken atq x

<0afterthetransformationq x

! jq x

j. The

agreementbetween the data denoted by open and lled symbols,respe tively,

isthusanindi ationofthequaltiyofthe alignmentofthedira tometer. The

width of the spe ular peak at q x

= 0 is essentially given by the experimental

resolution. The resolution at q z

0:07 A

1

has been estimated from the

dire tbeamproleofthe instrumenta ordingtoq x = 1 2 f q z ,where f isthe

dete tor take-oangle(dashed lineinthe gure). The solidlineindi atesat

to the apillary wavesmodel[5℄.

4 X-ray photon orrelation spe tros opy

Whether the apillary wave modes are propagating, overdamped or frozen-in

as a fun tion of temperature and waveve tor annot be distinguished by the

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dynami s ina dire t way [30℄.

Usingthe setup des ribed inse tion2,wehaveperformedXPCS

measure-mentsongly erolanddierentpolymersurfa esattheID10TrokaAbeamline

of the ESRF with 8keV radiation (wavelength =1:548

A) originatingfrom

threeinsertiondevi es. Theexperimentongly erolhasbeenreportedindetail

in Ref.[7℄. In this se tion we add some te hni al aspe ts. To tilt the in ident

beamdown fromthe storageringplane,boththe Si(111)mono hromatorand

a Platinum mirror whi h was lo ated downstream from the mono hromator

were aligned toprovide adequate angularosets (see Fig.3). The angle of

in- iden e onthesamplewaskept onstantatabout0.08 Æ

wellbelowthe riti al

angle for total re e tion. The envanes ent wave penetrates in this ase only

the topmost80

A. The alignment of the in ident angle wasdone by making

use of the relative rotating movement of the platinum mirror, the absolute

mirror tosample and sampleto dete tor distan es, the relative sampleheight

movementand theliquidsamplesurfa ewhi h an beassumed tobeperfe tly

horizontal.

ThewavelengthbandpassfromtheSi(111)mono hromatoris=10 4

yielding alongitudinal oheren e length l

= 2

=of about 1m. A pinhole

with a diameter of 12m was mounted 278mm upstream from the sample

in order to obtain a ollimated and (partially) oherent beam. The intensity

behind the pinhole amounts to approx. 210 9

photons/se ond. The typi al

transverse oheren e length at the pinhole is 10m. A guard slit in front of

the sample was used to suppress the Fraunhofer dira tion fromthe pinhole.

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100m was hosen in front of the dete tor. The adjustable size allowed to

balan e between a high spe kle ontrast (small aperture) and a high ux by

averagingoverseveral spe kles(largeaperture)[31℄. Thetime-auto orrelation

fun tions were re orded with adigital ALV5000/E orrelator.

The s atteredradiation wasdete ted under dierent exitangles f

within

the s attering plane. Hen e the lateral and perpendi ular omponents of the

waveve tor transfer, q x and q z , are given by q x =(2=)( os f os i ) and q z = (2=)(sin f +sin i ). From the q x

-value the respe tive lateral length

s ale x 0

that is probed may be obtained by x 0

= 2=q x

. The error in the

determination of x 0

is related to the error in the measurement of the ight

path lengths whi h is a few millimetersyielding a relative error on the order

of 10 3

.

Surfa e dynami s an be measured in a photon orrelation spe tros opy

experimentbyevaluatingthenormalizedtime-auto orrelationfun tionG()=

hI(t+)I(t)i t

=hI(t)i 2

t

where I(t) is the observed intensity in a dete tor [17,

18,19℄. Wefound thatourdata an bewelldes ribed bythe exponentialform

G()=g 0 exp ( = 0 )+1; (1) with 0g 0

1andatime onstant 0

. The onstantg 0

,alsodenotedas

on-trast, depends onthe degreeof oheren eof thein ident beamand the degree

of ensemble averaging for the in ident and s attered beam [31℄. Equation 1

des ribes the measured orrelation in the ase of overdamped apillarywaves

[6, 18℄. Very re ent further experimentsand al ulations have revealed that a

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In the gure 4 two examples of orrelation fun tions that have been

ob-tained from asurfa e of gly erol are given. In the rst example (Fig.4 (a)), a

parti ularly high ontrast C = (67:60:2)% has been a hieved. The lateral

length s ale probed was x 0

= 90m and the opening of the exit aperture in

front of the dete tor was set to 5m15m (v h). The temperature of

the gly erol sample was T = 265:7K. The t of Eq. (1) to the data (solid

line in Fig. 4)yields the relaxation rate 1 0 =(0:2530:01)ms 1 and hen e 0

= 3:95ms, and the ontrast quoted above. The total orrelation time for

this measurement was nearly 15 minutes and the average ount rate in the

dete tor approximately 100Hz. In the se ond example (Fig.4 (b)), a shorter

lateral length s ale x 0

= 5:6m was probed. The temperature of the gly erol

sample was T = 234:15K. The aperture in front of the dete tor was set to

50m300m (v h). Here, the t yields C =2:52%and 0

=106:38ms.

The total orrelation time for the se ond example was 20 minutes and the

average ount rate inthe dete tor approximately150Hz.

Themaximuma hievable ontrastispartlydeterminedbygeometri al

on-ditions [31℄, and it turns out that with a xed slit setting it de reases with

rising lateral wave ve tor transfer q x

. To measure a orrelation fun tion, a

minimum ount rate depending onthe time s ale of the u tuation to be

de-te ted and other fa tors su h asthe stability of the setup isrequired, thereby

limiting the a essible q x

-range. Up to now, the minimum length s ale that

an betested onour samples istherefore onthe order of one mi rometer. We

note that a orrelationfun tion fromour highlyvis ous liquidsamples an be

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Improvements of the storage ring urrent and the undulators willin rease

the available ux in ident on the sample in the future. Furthermore, a less

mono hromati in identbeammaybe onsidered,asaslightlyredu ed

longitu-dinal oheren e anbeallowed inthegrazingin iden egeometry. Inaddition,

the stability of our experimental setupmight stillbe further improved. Thus,

we envisage that shorter lateral length s ales willbe a hievable in the future.

A lowerlimitfortheminimuma essible times aleisgivenby thetime

stru -ture indu ed by the ele tron bun hes in the storage ring. At the ESRF this

times aleisafewnanose ondsinuniformllingmode. Experimentsdete ting

motion on a time s ale of approximately 50 ns have already been performed

[32℄using a faster avalan he photodiode (APD) dete tor instead of the NaI.

Itappears thatinour XPCS experimentwiththe angleof in iden e ofthe

x-raybeambeingallwayssmallerthanthe riti alangleoftotalexternal

re e -tion, thereare only littlediÆ ulties arisingfrombeam damage. Bys attering

fromsurfa esofdeepliquids,damage ausedbysubstratephotoele tronsis

ir- umvented. Wehave hangedtheilluminatedspotonoursampleso asionally

and observed onlyslight hanges inthe maximum a hievable ontrast.

5 2-dimensional dete tion with a CCD

The dire t visualization of a spe kle pattern using a two-dimensional CCD

ameragivesadditionalinformationrelatedtothespa ialshapeofthespe kles

and their q-dependen e [31℄. Furthermore, the CCD experiment an provide

(17)

s ale that large is not suitable for XPCS with a point dete tor. By ontrast,

the CCD-experiment using a 2-dimensional dete tor is well-suited to study

time-averages, as we dis uss in this se tion. The CCD-dete tor an as well

be used for XPCS experiments to study slow dynami s, as has been shown

by Kim etal. [33℄ on thin polymer lms on sili on substrates. For a detailed

dis ussion of usinga CCD toperform XPCS we referto Lummaet al.[34℄.

The same beamlinesetup as des ribed in the previous se tionwas used to

dire t the mono hromati and partially oherent beam downwards to be

in i-dent on ahorizontalgly erol surfa e with an in iden e angle i

=0:08 o

. The

radiation s attered from the surfa e was dete ted using a dire t illumination

CCD amerafromPrin etonInstrumentswith apixelsize of22:5 22:5m 2

.

The CCD amera was pla ed instead of the NaI dete tor and dete tor

aper-ture des ribed in se tion 4 to re ord the two-dimensional o-spe ular diuse

s attering image from the surfa e. The two oordinates of the CCD image

thus orrespond to the lateral omponents (q x

;q y

) of the waveve tor transfer

within and perpendi ular to the s attering plane.

In the example presented here, the slowing down of surfa e dynami s is

dire tly visualizedby the CCDimages obtained attwodierenttemperatures

of a gly erol sample (Fig.5). In the gure, the intensities in the pixels of the

CCD amera along the in-plane diuse rod (i.e. along q x

with q y

0) are

given for (a) T = 34:3 o

C and (b) T = 80:0 o

C. Theseintensities have been

obtained byaddingtheindi ated numberofframes,i.e. onse utiveexposures

of the CCD. Ea h frame has been exposed for 50ms for the measurement

at T = 34:3 o

C, respe tively for 20ms for the measurement at 80:0 o

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CCD. Hen e, the total integration time to re ord the several hundred frames

persamplewasontheorderofonehour. Thedataarenormalizedtoequaltotal

intensity andshifted for larityalong the intensity axisinthe plot. Abovethe

plotted intensities along q x

, the orresponding two-dimensional CCD images

for the respe tive maximum numberof frames are shown. The rod-likeshape

of the s attered intensity [31℄ and the spe kle pattern inthe low-temperature

ase be omeapparent.

We note that only two orders of magnitude in intensity dieren e may

be dete ted at on e along the CCD that we have used. This is related to

the large number of harges reated at on e in a CCD pixelby a single x-ray

photon. Furthermore,thequantumeÆ ien yofasingleCCDpixelislimitedto

approximately30%. Inaddition,asigni antintensitywaspresentinthedark

images. Therefore,whenonlyafewframesareadded,boththeimagesfromthe

low-temperatureandthehigh-temperaturesampleshowstatisti alnoisewhi h

mightbemistakenasspe kle(Fig.5). However,whenafewhundredframesare

added,the datafromthe frozen-insamplemay be learlyidentied asastati

spe kle pattern, whereas summing the frames from the u tuating sample

resultsinasmoothimage. Thelow-temperaturesampleisthereforestati ona

times aleofatleastonehouroverwhi htheCCDframeshavebeenintegrated.

This interpretation is further supported by the strong intensity modulation

along q x

at small q x

in the ase of the stati sample. Here, statisti al noise

would have the opposite ee t of strongly modulating at large q x

where the

absolute intensity is low.

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wafer servesasastati referen esample. When illuminatedthrough theusual

12m pinhole, a spe kle pattern appears in the diuse s attering (Fig.6(a)).

Wenotethatthewafermaynotbeasperfe tlyalignedastheliquidand

frozen-in gly erol surfa es presumably be ause itis slighlybent due to itslarge size.

Thesili onwaferhasalsobeenusedtoverifythatwhendeliberatelydestroying

the oheren eofthein identbeambyopeningthepinholeto100m,asmooth

s attering pattern results similar to the pattern from the u tuating sample

(Fig.6 (b)).

6 Outlook

We have presented an experimentaldevi e whi h allows for a wide variety of

in-situ surfa einvestigations withx-ray s atteringmethods. Inparti ular,the

devi e is adapted to the extremely sensitive XPCS experiments. The devi e

may as well be used for experiments with neutrons, sin e the thin (0.25m

ea h)Kapton foilwindows auseonly negligibleabsorptioneven for old

neu-trons. In addition, the experimental setup has already been shown to be

equallysuitableforgly erolandprototypi alpolymerglasses. Infuture

exper-iments,itmay bepossible totest predi tions relatedtothe surfa e dynami s,

vis osity and surfa e tension of polymers using XPCS inthe des ribed setup.

It turnsout fromthe re e tivity measurementsthat the super ooled

gly -erol surfa es are surprisingly smooth with an rms roughness of approx. 2 A

in the glassy state. The sample preparation auses no imperfe tions on the

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ir-providesproofof asurfa ethatis ompletelystati . The mainfo us onfuture

developments will be to a ess shorter lateral length s ales in XPCS. So far,

the limitshave ertainly not yet been rea hed.

A knowledgements

This work was supported by the 'Deuts he Fors hungsgemeins haft' (proje t

numbersPr325/9-1,2,3,Pr325/12-1andPr325/14-1). Themeasurementswith

oherentx-rayswere arriedoutattheTrokabeamlineID10AoftheEuropean

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[21℄ Fast density u tuations (e.g. elasti shear modes) are, however, beyond

the a essible time window.

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vis ositymaybeobtainedindependently.Inthe aseofoverdampedwaves

onveryvis ousliquids, onlythe ratioofthesequantities anbeobtained.

Sin e the apillary wavesatthe surfa eare aresult of u tuations inthe

bulkliquid,the vis osity asdeterminedfromXPCS isnot ne essarilythe

vis osity at the surfa e.

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incident

beam

liquid

nitrogen

intake

nitrogen

exhaust

Goniometer

outer cell with high vacuum p<1e-5 mbar

low pressure (approx. 10 mbar)

sample surface

Cooling/Heating

flange to high vacuum pump

140 mm

controller

membrane pump

inner chamber with

top

lids

mass flow

dewar

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111

00

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Figure 1: Sket h of the sample environment (side view, drawn to s ale)

on-sisting of an outer ell eva uated to approximately 10 5

mbar for thermal

in-sulationandaninner hamberwhi h ontainsthetroughforthe sample. Both

the inner hamber and outer ell have a ylindri al symmetry along the

ver-ti al axis. The inner hamber is mounted onthe heat ex hanger (see text for

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a fun tion of the lateral wave ve tor transfer in the s attering plane q x

while

q z

was kept onstant at q z

=0.07 A

1

(symbols). The data were taken at a

liquid surfa e dira tometer using a rotatinganode x-ray sour e. A onstant

ba kground has been subtra ted, and the data have been orre ted for the

variationofthe illuminatedanddete ted spotsizeonthesampleasafun tion

of in iden eand take-oangle. The dashedlineis anestimation ofthe

resolu-tion fun tion from the measured dire t beam prolea ording to q x = 1 2 f q z , where f