chapter II
ExperimentalDetailsof the Molecular Beam, Liquid Surfaces,
andSpectroscopicDetection

2.1

Introduction
Experimental
studiesof gas-liquidscattering
dynamicsinvolvea molecular

beamthat strikesa refreshedliquid surfacewhere translational
andinternalstate

distributionsof the scatteredgas are measuredwith a pb-saltdiodelaser
spectrometer.Throughoutthis thesis,importantdetailsof thespecificexperimental
conditionsare listedin eachchapterto help understandthe measurements.
.l.ocomplementthesedescriptions,additionalinformationof thecomponentsare outlined
and discussedin this chapterfor the generalexperimentalsetupthat is illustratedin
Figure2.1. The objectiveto understandthe energytransfer
dynamicsinvolves
measuringthe changebetweeninitial and final statesof the CO2projectile.

Basedonthis scheme'the scatteringexperimentsnecessitate
a thoroughcharacterizationofthemolecularbeamand liquid surfaceto quantify the initial stateof the system.

These

preliminarystudiesalso are usedto show that the final distributions
reflectnascentcollisionconditionsbetweenthe gasand liquid surface.
In thesestudies,skimmedmorecurarbeamsare emproyedin
controlthe incidentenergy,(ii) reducethe angularspread,and (iii)

an effort to (i)Iimit thetemperatureof the internalstatedistribution.l Theseprojectilesare scatteredfromlow vaporpressureliquids in a chamberunderhigh vacuumwhere
the flux ismeasuredwith a high resolutioninfraredlaser. The liquids are prepared
via methods

25
InSb detectors
HeNelaser

noisesubtraction
rnultipass,"n.,p..o,.,r.flcontroll.ed/r\{ll }-} lock-rnrclcrcrrcc

llIJl'eserYolr

photodiode'\\\cl.,scd-ltxrp
hcatcrichillcr

refercnccgasccllpul)lpconfircaletalon

lock-inr r arnplificrs.rrrl)rrIrurs

l i - e c l t t c t t cdvi a g n o s t i c s

usedto detectCOz
Figure2.1
Schematicof the Pb-saltdiodelaserspectrometer
experiments.
in thegas-liquid
scattering

thatensurethe liquidtargetis cleanandfreshlyrenewed.'The laserdetectsthe
COzl-2 cm from the surface,which imposesan importantconditionon the
scatteredthemeanfreepathfbr COzin
choiceof incidentflux andliquidsurface.Specifically,
the chambershouldbe at leastan orderof magnitudelargerthanthe surface-to-laserreflectonly the dynamicsof the gasthe measured
distributions
distanceto guaranteepressures,
limitsthe incidentflux, background
liquid interaction.Sucha requirementof the liquid. The versatilityof high resolutiondiodelaserenables
andvaporpressure

26
us to characterizethe translation,rotation.vibration,and M;-statesof the scatteredco2. Importantaspectsaboutthe laserdiode spectrometer,
molecularbeam,and
liquid surfacepreparationare coveredto illustratethe initial characterizationof thesystem. In addition,the computer-controlledinterfaceand dataanalysisare describedin detail.

2.2

Laser Detection Scheme

2.2.1 Pb-saltDiode Laser
As illustratedin Figure2.1,the infraredspectrometeris built arounda tunable
singlemode Pb-saltdiode laser. The diode generateslight that is tunablearound), :4.2 pm,lvhich is usedto detectincidentand scatteredCOz throughdirect Beer'sLaw
absorption.Importantfeaturesof the diode includesinglemode lasing,low
amplitudenoise,and narrow line width (Avlur..= 20 MHz) to accuratelydeterminetheinternalstatepopulationsof the Dopplerbroadenedabsorptionprofiles. Detailsof thelaserphysicsare extensivelycoveredin expertreviewsand texts,while the specificoperatingcharacteristics
have beenpresentedin previousJILA theses.l,aBasedon
the collectionof previouswork, this sectionprovidesa brief overviewof the diodelaser,as well as specificsabout its use in the gas-liquidscatteringexperiments.
The diode laseris an optical oscillatorthat incorporatesboth the gain medium
and optical cavity insidea semiconductorr.vithcleavedends. The constructionof thelaseris basedon leadsaltsthat are dopedwith impuritiesfrom groupsIII and V on
the periodictable. Figure2.2 illustratesthe physicaland chemicalcomponentsof the27

Figure2.2

Pb-saltdiodelaser.
Heterojunction

heterojunctionlaser,which has beenconstructedfiom molecularbeamepitaxy(MBE) to preciselycontrolthe thicknessof the differentlayersin the diode.r In
wherethe electronic
general,the Pb-saltdiode behavesas a typical semiconductor
structureincludesvalance(VB) and conductionbands(CB) for the electrons(e ).s
Electronswithin thesebandsare distributedbasedon energy by a Boltzmannf-actor
that incorporatesa temperature.At 300 K, the energydifl-erencebetweenthe VB and
CB resultsin a distributionwhere the vast majority of electronsresidein the valanceband. Theseelectronsmay be excited into the conductionband if the semiconductor
absorbsa photon with an energygreaterthan the band gap, (Egup: Ecu - EvB),whichleadsto a positive hole (h-) in the valanceband. The physicalprocessis illustratedin
Figure 2.3ato show a transition,for which energyand momentum(ft) are conserved,
i.e. a vertical with respectto fr. Once the e--h- pair is created,dynamics in thesemiconductormay lead to recombinationthrough either radiativeor nonradiative
pathways.

28a) Typicalsemiconductor
bandstucrure
conduction
band

E(k)

E.u
\AA >
Euu

valenceo electrono holeb) Population
inversion

E(k)conduction
band

Ecu

\ A >
Euo

\A A>
t*">

v : (Evn-E('R)/hvalencendFigure 2.3
Electronicstructureof a semiconductor.(a) Energy versus
momentum for a typical semiconductorthat absorbsa photon to promotean electronfrom the valanceto conductionband. (b) Populationinversionleadsto radiative
recombinationto generatelasing in the diode.

29
To utilize thesepropertiesfor lasing,the Pb-saltdiode is grown with an active
regionthat is definedby thep-n junction. Figure2.2 showsthat the pb saltsin thediode have beendopedwith eitherTl (p-type)or Bi (n-type). Theseimpuritiesleadto
crystalsitesthat eitherhaveone too many (n) or too f.* (p) electronsto bond withthe semiconductorlattice. The extraelectronfrom the n-typeimpurity easily reaches
the conductionbandthroughthermalexcitation,at which point it may diffuse andrecombinewith a hole in a p-type impurity. As a result,positiveand negativecharges
developas the electronsand holesdiffuseacrossthejunction. The processstops
when the voltagedrop acrossthis depletionregion is largeenoughto preventfurther
diffusion of e- and h*. To generatea populationinversion,an externalvoltageis
appliedacrossthep-n junctionto counterthe inherentpotentialdifferenceacrossthe
depletionregion. once the diode is forward biasedin this way, electronsflow
throughthe conductionbandto createthe necessarypopulationinversion. Theseelectronsmay recombinewith holesin the valancebandto emit a photonas they areforced into the samephysicalregionof the diode. The processis illustratedin Figure
2.3b for a generalpopulationinversionin the semiconductor.Constantcurrent
injectioninto the activeregionmaintainsthe populationinversionthat leadto
continuous(cw) lasing.
In addition to the gain medium, the optical cavity of the laser is formed bycleavingthe endsof the semiconductorto utilize the properliesof Fresnelreflection.
The index of refraction in the diodes is = 5 at l. = 4.2 pm, which leads to cavity
mirrors that are = 44o/oreflective. As illustratedin Figure 2.2,the physical lengthofthe activeregion is : 250 pm, rvhichconespondsto a free spectralrange(FSR) of =4

30
cm-' for the cavity. The diode begins lasing rvhen the round trip gain in the cavity
exceeds the losses, rvhich include photons that are lost to reabsorptionby thesemiconductor,transmissionthrough the cleavedends,or pathrvaysoLrtof the active
region.

In addition, lasing competes with nonradiative pathways that involve

recombinationat lattice defect sites and Auger processes.Sustainedlasingrequires
increasingthe injectioncurrentto the point rvherethe radiativerecombinationleadsto
stimulatedemission in the cavity. The specific currentsto reach lasing thresholddependupon the temperatureof the diode. For example,the thresholdrisesfrom I I 0
mA at 8l K to 203 mA at ll I K for the Pb-saltdiode usedin the COz experiments.
In general,the thresholdcurrent increaseswith temperaturebecauselossesdue toAuger recombinationprocesses
tend to increasewith temperatureas well.
2.2.2 Laser Tuning
The laserusedin the COz scatteringstudiesis part of a spectroscopic
systemdevelopedby a companyonceknown as Laserphotonics,which is now Laser
Components.The laseris a heterojunctiondiode (model L5622-2100,serialno. g705) mountedon a cold finger insidea main liquid N2-cooleddewar(L5736, position#3), which is fed by an auxiliaryreservoir(L5737) to provideextendedperiods
befweenrefills. The frequencyof light dependson the bandgap of thesemiconductor,
which changeswith temperatureat arateof 34 cm-l4(. This featureof the diode laserrequiresa controllerthat providesa dynamicrangeto accessmany
temperaturesand long time stabilityfor scanningpurposes.The temperatureinsidethe dewar is regulatedthroughresistiveheatingtape(13 O) that is firmly securedto
the baseof the main reservoir.Trvo silicon diodesare also aftachedto this cold finser

3l
to provide a feedbackreferenceto the temperaturecontroller(Laserphotonics,
SP5720). The voltagedrop acrossthe diodesis measuredand comparedto the set
point voltageon the controller. The actualvoltagescorrespondto a particulartemperature
througha calibrationcurvelistedin Table2.1. Servoloop electronics
steadythe temperatureto within 0.1 mK so that the drift over the courseof a scan(=I m i n )i sn e g l i g i b l e .
In additionto stability,the controllerprovidesenoughcurrentto

resistively

heatthe diode to = 180 K. The basetemperatureis determinedby
the qualityofvacuumin the insulatingwalls of the dewar,rvhich is periodicallyevacuatedto
removecondensedgasthat leaksinto the chamber. Insteadof a restoration
to

room

temperature,a partialrvarmingroutineis fypically employedto help
maintainthe
mapped-outmode structureof the laser. To do this, the vacuumportis connectedto a
LN2 trappeddiflusion pump,at which point the temperaturecontroller
is set to bring
the dewarto 180 K (Ta"*,",:0.6900v) to herprereasegasessuch

as nitrogen,oxygen,argonand coz. After - l-3 hoursof pumping,the valvesare closed,
the heateris
turnedoff, and the dewar is allowedto cool backto LN2 temperatures.
Basetemperatures
typically reach80-81 K with this procedure.
The injectioncurrentalso determinesthe frequencyof the diode laser.

The

processis basedon the responseofthe index ofrefraction
to the local chargedensity
in the activeregionof the semiconductor.The index of refraction
increaseswithcurrent,which effectivelylengthensthe lasercavity. As a result,the diodemay becontinuouslytunedwith a modulatedcurrentat a rateof = 2 MHzJpA.
The controller
is a commercialunit from ILX Lightrvave(LDx-3620), which is designed
to be an)L

Table 2.1 Diode laserdervartemperaturecalibration
VoltageT (K)VoltageT (K)VoltageT (K)0.979770

0.935789

0.8972104

0.964377

0.93320.96200.959778
79

0.89210.8871

80
81
82
83
84
85

92
93

0 . 8 8 r2
0.87700.87190.8669

106
108
I l0

0.95740.95500.95270.95030.94190.9455
0.94310.9406
0.93820.93070.92810.9256

90
9l

tt2
| t4
I 16

0.86180.8567
0 . 8 3r 2
0.80540.77950.3664

I IB
t20
130
140
150
298

86

Bl
88

0.92300.92040.91780.91520.91250.90990.90720.9022

94
95
96
97
98
99
100
102

ultra low noisecurrentsupply. This is importantbecauseany currentnoiseon thesourcecorresponds
to additionalline width broadeningbascdon the tuning rateof thelaser. For example,the specificationsfrom ILX claim that the currentnoiseshouldbe<2ltA RMS in a l0 kHz bandwidth,which would correspondto an additional4-5
MHz of broadeningbeyondthe inherentline rvidth of the laser. Thesenoisesources
are carefully analyzedwith simplediagnosticsthat involve passingthe outputof thecurrentcontrollerthrougha l0 O resistorto generatea voltage,which is sentto aspectrumanalyzer. Adaptershave been made to attachat various points along thecablelayoutto help identify any additionalsourcesof noise. A samplenoise
spectrumis plotted in Figure 2.4 to assessthe differencebefweenthe high and low
bandwidth settings.In addition,detailedanalysisshowsthat the noiseamplitude

aa
JJ

a l
N

r / r 1

CJ
a

tll

I

lo'r
lirll banclnidtli
Iotr baucllidth

(J

l0l

l0r

101

l0i

frequenq (HzlFigure2.4

Spectralnoisedensityof dioderasercurrentcontroller.

overallfrequencies
is2-3 timeslowerforthe200mA versus500mA setting.Sucha resultclearlyindicatesthemodulation
circuitrycontributes
to thenoiseon the
output.Additionalfeaturesin the spectrumincludea peakat 100kHz that
corresponds
to the vibrationalfrequencyof a quartzoscillatorusedin the A/D forthe
digitaldisplay.Theseeffectshavebeenminimizedthroughan externalmodification
wheretheoscillatoris isolatedfrom the circuitboardwith an on-off switch. Suchafeatureonceagainillustrates
the limitationsof the ILX design.Currenteffortsin the
JILA electronics
shophaveproducedcurrentsources
with noisefloorsoneto threeordersof magnitudelowerthanthe bestILX drivers.

r

34
The procedureto scanthe diode laseracrossa rovibrationaltransitioninvolves
severalsteps. First, the temperatureis set and allorvedto
stabilize,at which point thecurrentis tunedto the generalvicinity of the transition.
once thehas beenidentifiedon the scopetraceof the referencegas

absorptionfeature

cell, the currentis slowly

decreasedso that the startingpoint of the scanis shifted500-1000
MHz belorvthe
centerlinetransitionfrequency.The dataacquisitionsoftware
providesan output
voltageto the currentcontrollerthat is incrementallyincreasedtoscanthe laserafter

every gas pulse' This voltageoutputfirst passesthroughthe groundloop breakerand
low passfilter that is illustratedin Figure2.5 with the

ILX currentcontrollerset to the

DC MoD position. Furtherdetailsof this routineare outlinedin the comments
within the sourcecode in of the scanningprograrnlisted'l'hein AppendixD.

diode lasercovers= 90%oof the frequenciesbctween2293

and2375 cm-lwith light powersthat rangefrom : 0.r - 2 pw depending
on tne remperatureandinjectioncurrent. Variouscombinationsof temperaturesand currentsare usedto
accessthe majority of thesefrequencies.Generally,thelasercovers= I cm-r in asinglelongitudinalmode beforethe gain curve shifis andthe frequencyjumps ahead

by one FSR of the cavity. Sampletuning curvesare plottedin Figure2.5 to show thefrequencycoverageas a function of current for trvo temperatures.
Spectralmaps are
generatedevery time the diode is temperaturecycled,
which tendsto restructurethe
semiconductorband structureby small amounts.The characterization
involves
sweepingthe laserwith a saw-toothwave from a function generator
so that signals
from the referencegas cell and etalonare visible on an

oscilloscope.Transitionsare^a35
output to

inputfromcomputerD/A

LDX-3620
externalmodulation

260
2-t0
220
200
(.)
:-i

O
CJ

I r{0

/

l(r0

l+0

- *'] Kl
T,r,,,,re120r.::l!

100
l-l l02l l 5

:.110

1.125

l-1.102-1.15 2-l-+ofiequeno lcrn-l;
Figure 2.5
Spectralcoverageof diode laser. A simpledifferentialamplifier isusedto electronicallyisolatethe currentcontroller from the computerinterface.

recordedasa functionof T(V) andi for rotationalstatesin theground(0000)
anddegenerate
excitedstate(01r"/b;vibrationalmanifold.Tablesof temperatures,
currents,
andtransitions
areusedto find thecorrectfrequencies
in the incidentbeam
characterizati
on and scattering experiments.6,7
As thediodelaserscansacrossDopplerbroadened
absorption
profilesintheexperiment,
the line shapes
arebestcharacterized
by a convolutionof Gaussian
and

-a

36

a.t

106

N
'r)

N
(.)
a

O
()
(.)

10.'

I0r

lOr

lOr

lOr

fiequency(Hz)Figure2.6

Frequencynoiseof pb-saltdiode laser.

Lorentziancomponentswithin a Voigt function. The Gaussian
fiaction corresponds
to a velocitydistributionwithin rhe gas sample,which is typicallyBoltzmannand defined by the temperatureof the sample.

Maxwell_

Additional line shapebroadeningis due to the width of the laser(Av1u."= 20 MHz),,
which
from a leastsquaresdeconvolutionof the two components.

A simple test to confirm

thesevalues involves a frequency-to-amplitudetransducerthat
noise on the laserinto amplitudenoise. To do this, the laser

are extracted

convertsfiequency

is parkedon the side of aco2 absorption,at which point peak-to-peakamplitudenoiseis recordedfor the

given laserpower. The laseris then tuned off-resonanceto determinethe baseline
noise level' which is subtractedfrom the first measurement
in quadrature.This

-a

J /

amplitudenoise(in absorbance)
is convertedto frequencynoise
throughthe
the side of the coz absorption
rine shape.rvhichprovidesa

sropeatdirect rerationshipbetweenabsorbanceand frequency.Thesevarues
reflect a broad-bandcharacterizationof the noise. To herpunderstandthe freqLrencycomponents,thetransducersignalsare also recorded
by a spectru
m analyzertorevealthe noise
density
as a functionof frequency'The
noisespectraldensity
forthe coz laseris illustrated
in Figure2.6.

2.2.3 Detcctors
Infiaredright detectorsin the

spectrometerare buirt fiom semiconductormateriars
simirarto the constructionof the raserdiode. As depicted
in Figure2.3a,the detectorerementabsorbsincident
radiationrvherephotonsr.vith ,hv Erunexcite
electronsfrorn the varanceto
the conductionband. This process
is typica,ycharacterized
by a quantumyield, rvhichis thenumberof electron-holepairs
createdper photon' Detectorsmade
rvith materiarssuchas InSb
and HgcdTe havequantum
yields that typically exceed
90%' Basedon the propertiesof the semiconductor,
thesedevicesare combinedwith erectronic
amprifiersin one of two ways.
The first type isphotoconductive(PC) in which
the excitedelectronin the conduction
banddecreases
the impedanceof the semiconductor.
The secondcrassof detectors
is photovortaic(PV)' which is basedon ap-ndiodejunction wherephotons
exciteerectronsfrom thevB to the cB in the depretionregionof the diode. Theseerectrons
are thenacceleratedby the bias voltage tocreatea photocurrentthroughthe detectorelement.
Both types of detectorsprovide
an extremelysensitiveway tomonitor laserpower for

38a ) l n S bI ) l r o t o v o l t a li )ce t c c t o r .

h) MC 'TP h o ro co n cl rr clirI),ectcclorl r. Y_l-!tl,r"r:-- --J l------Figure2.7 Erectronic
amprifiercircuitsfor InSbphotovortaic
andMCT
photoconductive
detectors.

^a39
the direct absorptionexperiments.As a result,eachdetector
is fully characterizedtodeterminesourcesof noisethat may inhibit optimarperformance.
The laserdetectionschemeincludessplittingthe light onto

two detectors.The

first path involvesmultiple passesthroughthe vacuumchamber
to measurethe
densityof Coz in the scatteredflux, while the secondfraction
is passeddirectly ontoa referencedetector. In this configuration,the signalandreferencedetectorsare InSb

photovoltaics(lnfraredAssociates,IS-0.5),for which
the elementshave beenmatchedin an effort to generateidenticalamplitudeandfrequencyresponses
withincachdetectorcircuit. The semiconductoris mountedinwith sapphirewindows and a 30" field-of-view(Fov)a liquid N2 dewarthat is built

cold shieldto reduceshot

noisefronl incidentblackbodyradiation. The element
itself issquarewith an areaof

0.5 x 0.5 mm2,which leadsto an inherentcapacitanceof

20 pF acrossthe InSb

material' 1'hephotocurrentfrom the InSb passesinto an

amplifier circuit that is

directlymountedto the back of the dewar in a side sealed
compartment.The
schematicis printedin Figure2.7ato illustratethe two-stage
amplification,which isbasedon an initial transimpedanceof 39.4 ko. The photocurrentand transimpedance
leadto a voltagedrop acrossthe first stageof the amplifier,where v:rR. Thesecondstageof the amplifier providesadjustablegain froml_30.2thatis controlleda

potentiometerin the circuit. The low power fromthe diode laserrequiresthemaximum amplification,so the potentiometerhasbeen
set so the gain is 30.2. In
addition,a buffer is includedto measurethe voltagedirectly
after

the transimpedance

amplifier' The gain of the secondstagemay be calculated
directly fiom the ratio of
the two voltageoutputson the circuit.

40

6.1

N

G-eoe
4-IS
G-90e
5.IS

fiequency(kHz)
Figure2.8

currentnoiseof signalandrefbrence
InSbdetectors.

Thesensitivityof thesedetectors
is basedon fundamental
noiselimitationsofthe individualcomponents
in thecircuit. Thecontributions
include(i) detector
elementnoise,(ii) Johnson
noisefromthetransimpedence
resistor.
(iii) amprifier
noise,and(iv) shotnoisefrom backgroundblackbodyradiation.Theseincoherent
sourcesaddtogetherin quadrature
to limit the overallperformanceofthedetector,

which is characterizedwith an FFT spectrumanalyzer.Theplot in Figure
2.g showsthenoisedensityof bothdetectors,
wherethemeasurement
invorrespassinsthe

-

-/

4l
outputvoltagedirectly into the analyzer.The baseline
currentnoiseabove I kHz is =0.9 pAlHztt2for both detectors,which correspondsto

a voltagenoiseof 36.2

nY,-rlHzt/2acrossthe 39.4ke transimpedence.To better
understandthe
contributionsto this noiselevel,\\,ecan investigatethemagnitudeof eachcomponent
listed in the first part of this paragraph.First, the intrinsic
noiseon the detector
elementis not providedby the manufacturer,so the valueisestimatedfrom the noise

measurements
at the end of the analysis. Second,Johnsonnoisefrom
Y,,,,rlllzt/2: (4kBTR)tn,which leadsto 25.7 ny,^,/Hzt/2

the resistoris

for the 39.4ko resistorat

room temperature.Third, the amplifier (AD743N) includes
both currentand voltage
noisesources,which are specifiedto be 6.g fAr^r/Hzt/2
and2.9 nyr^r/Hzt/2,
respectively.The currentnoiseaddsan additionar0.2g
nv,-r/Hzll2

acrossthetransimpedence
resistor'i.e. completelynegligiblewhen the sum ofthesesourcesare
addedin quadrature.
The final componentis shot noisefrom the flux of photons
due to room
temperaturebackgroundblackbodyradiation. Shot noise
contributionsare governed
by i,*,[Llzl/2: (2qioc)t/', which requiresan estimateforthe photocurrentfrom theblackbody photons. The flux density (photons/s/mr)*uy

be integratedover

wavelengthsthat rangefrom the visible to 5.5 pm, whichis the spectralregionwhere
the InSb detectorrespondsto light. Sucha calculation
revealsthat the incidentfluxof photonsis 2.6 x l0l6 photonsislcm2,rvhichcoresponds
to 6.6 x l0r3 photons/sfor
a detectorareaof 0.5 * 0.5 mm2. As previousrymentioned,
the Fov cord shierdis
designedto significantly reducethis incident flux by only
exposingthe detectorto
room temperatureblackbodythrougha 30" solid angle.

-

The FOV decreases
the flux

l

42

()
a

V)

C)
C.)

()

().0I

c-9094-IS--+-

G-9095-ISl0l(x)I( XX)

F'requency(kHz)
Figure2.9

Frequency
responseof InSbdetectors.

by a factorof 0.0334to yield 2.21 x l012photons/s,which corespondsto 0.35 pA of
photocurrentif the quantumyield is assumedto be nearunity. Shot noisefrom thisbackgroundcurrent is 0.34 pA,^,lHztt2,which leadsto 13.3 ny,^JHztt2.
contributionsfrom shotnoise,Johnsonnoise,and the amplifierresultin 29.1
nYrn,rlHzt/2,
which can be usedto estimatethe noise on the detectorelement. As aresult,we estimatethe detectornoiseto be 2l.l nV,.n/Hzll2,which is a factor of two
betterthan the previous InSb detectorsused in earlier versionsof the Pb-saltdiodelaser spectrometer.

43
The detectorsrespondto changesin right revers
on a time scarethat
correspondto the finite bandwidthcreatedby thecapacitanceof thep-n junctionancl
the transimpedence
in the arnplifier. The bandwidthis measuredwith
a red LED tharis controlledby a squarewave from a functiongenerator.
Theserwo componentsare
testedwith a silicon photodiode(FND-100) rvith abandwidthof 700kHz ro ensure
the frequencyresponseof the testingequipmentis

adequatefor the InSb

characterization'The plot in Figure 2.9 showsthe
relativeresponseof eachdetector
at a seriesof frequenciesthat extendout to 500 kHz.

From this plot, the bandwidthof

thesedetectorsis definedby the -3dB point, whichfalls at 150kHz for the InSb
circuit.
Infraredlight in the frequencydiagnosticarm of
the spectrometeris detectedwith Hgcdre PC detectors,which are usedto record
the transmissionfringesthroughthe etalonand room temperatureabsorptionprofilein the rerbrencegas ceil. The

MCT elementis 1.0 x r.0 mm2and is mountedbehinda 30" Fov cordshierdin aliquid N2 cooleddewar. Thesesemiconductors
are sensitiveto light withwavelengthsthat reach 12.5 pm. Signalsare processecl
withprintedin Figure2.7b, which showsthe output

the amplifier circuit

vortagesare AC coupred.The

voltagesare sentto lock-in amplifierswherethe
signalsare

heterodynedagainsta

referencesignal. In general,the MCT elements
are an orderof magnitudelesssensitivecomparedto the InSb materials.Theseparticular
detectorsare worsedue topreviousdamageor othergeneralmishandling.
Despitetheselimitations,laser
powers that correspondto a fraction of pW on

each detectorare visible on the MCT

44detectors.
Thesensitivityof theelement+ amplifieris significantly
improvedby thelock-inamplificationscheme.
2.2.4 Spectrometer
other than the laserand detectors,thespectrometerconsistsof additional

optical componentsthat are usedto measure
the scatteredcoz frux and frequencyofthe laser' The entirespectrometeris built rvithin a purge box built fromt/q,,sheetsofplexiglassthat is fiiled with dry N2
to removeatmosphericcoz fiom thepath of thelaserbeam. The infrastructureof thebox incrudesaluminum posts,hinges,
and crasps
to keep the lids closedduring experiments.
Airtight sealsare generatedwith weather
strippingand erectricartapearongthe
searingsurfaces.poryflo tubing fromof nitrogenis passedinto the purge box

a rA tankto eriminatethe presenceof coz. In generar,l-2 full tanksare usedfor an incident
beam+ scafteringexpcriment. .l.he
remainingpartsof the spectrometerare constructedfrom

stockand custommirrors and lenses.

Stock lensesare made from eitherBaF2
or caF2,whire the mirrors are gordof chromiumwith Zeroduror fusedsilica
Janos,Edmundoptics, or cvl.

on a basesubstrates.Theseopticsare available
from

Furtherdetailsaboutthe custompieces
are provided

throughoutthe descriptionin the text.

Theiilustrationin Figure2.r showsthewaysin rvhich
the raserbeamis sprit
within the spectrometer
to measure
the scattered
flux asa functionof frequency.
Beforethebeamis sprit,therightis carefury
cotimatedwith an off_axis
parabora(oAP) (Janos,
48037-305,d = 3", f =r.5") positioned
directryin frontof theoutput'Thecartoonin Figure2.10shows
theoAP on anx yz

raser

translation
stagethatis

fixedto an adapterplatefor coarsealignmenton the lasertable. This plate has/q,,

-/

45

-

lns

f:20 cmof}:axis
parabolaspatialfilterpellicle
beamsplitter

f-5cm

locating
plate

- . ,
lns

Figure 2'10 Lasercollimationand telescopeconfiguration.
The diode laserlight isfirst collimatedwith an off-axis parabola,after which-it passes
through a 4: I reducing
telescope.A pinhole apertureis usedto spatiallyfilter
the beam. The IR beam isoverlappedwith a HeNe beam for alignmentthroughout
the rest of the spectrometer.

thru holespositionedin a patternthatallowstheoAP to be positioned
anywherewith
respectto the laser.To coarselylocatethe oAP in frontof
the laser,a HeNebeamis
aligned4" off the lasertable,andthenpassedontothe oAp toward
thedewar
window' TheoAP is translated
intoplaceso thatthe HeNespotfocusesdownto the40front of the diode. At this point, an IR detectoris placednearthe

OAp alongthe pathof the HeNe. The diode laseris turnedon, and rvith a little luck, thethe presenceof IR light. The positionof the translationstageis

detectorindicates

optimizedto focus

the IR light directlyonto the detectorelement.Beamcollimationcontinues
byrepositioningthe detectora few inchesfartheraway from the OAp along
thedesiredpropagationpath. Once againthe positionof the OAP is optimized
to refocusthe
light onto the detector.This schemeis iteratedthrougha seriesof distanceswherethe
goal is to focus the light onto a detectorthat is an infinite

distanceaway. At eachstep'the beamdiameteris measuredfor severalpointsalong the propagation
pathtohelp determinethe positionof the waist. Once the detectorreaches
the end of thetable,the outputof the laseris consideredto be well-collimatedwith
typicalbeam

diametersof 1.0- 1.2cm.
To help managethe laserbeamthroughoutthe spectrometer,
the Iight

ispassedthrougha seriesof opticsthat reducethe beamdiameterby a factorof 4.
reducingtelescopeshown in Figure2.10 is built from two BaFz
lenseswith the

The

first

lens(d: 50.8 cm, f :20 cm) fixed so that the centeris 4', abovethe
table. The
secondlens(d :25.4 cm, f : 5 cm) is mountedto an x y z stagethat positioned=is
25 cm away. The stageis usedto fine tune the positionof the second
lensso that theoutput is onceagainwell-collimated.The reducingtelescopealso includes
a pinholespatialfilter positionedat the beamrvaste.The pinhole is an adjustable
iristhat isalso mountedto an x y z stage,which enablesthe apertureto be precisely
positioned
to spatially filter the beam. The pinhole reshapesthe transverse
mode structureof thediode,which nominally involvesobliquebeamprofilesas illustrated
in Fieure2.2.

47
The apertureprovidesa profile that resembles
TEMooto help minimize noisedue roclipping on other optics in the laserpath.

while spatiallyfiltering remainsfeasible,

the challengesof rvorkingwith a pinhole include
furtherreductionof the low power
output and optical feedbackto the lasercavity.
be reachedbeforethe final alignmentduring

A balancebetweentheseissuesmust

an experiment.

After the telescope,the IR light is overlappedwith a HeNe tracerbeamto help
passthe invisible beamthroughthe remaining
partsof the spectrometer.The overrap
involvespassingthe red and IR light througha pelliclebeamsplitter(EdmundNT3g478)that transmits/reflects
g0/20 atx:4.2 pm
and 92/g atx:633nm.

Thesetwo

fractionsof the infraredare usedto detect
the scatteredco2 flux and determinethefrequencythroughoutthe scan. After the
initiar arignmentwith

the overrapped

beams'AR-coatedgennaniumrvindowsare
usedattenuatethe HeNe light along

bothbranchesin the spectrometer.
The schemeto detectscatteredcoz involves
passingthe first fractionof thebeam(80%) througha soft focusingrens(f :50 cm), after which the right is sprit

againwith a caFz beamsplitter(50/50).As illustrated
in Figurez.ll,thefirst half ispassedthroughcaFz rvindowson the vacuum
chamberinto

a modified Herriot cell.8

The multipassis createdwith two sphericargord-coatedmirrors (2,, diameter,R :20'0 cm) that are glued to JILA mirror

mountswith epoxyat a distanceslightly
lessthan 40 cm apart. The soft focusing lens (i)reducesthe beam diameterto prevent

unintentionalclipping on the first mirror in
the chamber,and (ii) pracesthe focusofthe first passnearthe centerof the cell.

The initial setuprequiresthe first passto

in the upper right corner of the far mirror.

L

land

The mirrors are adjustedso that the beam

^/

48

ffi
rl

ffi
o
lllourlt

s-z

Figure2'll
ModifiedHerriotcellconfiguration
is usedto passthelaserlightmultipletimes(: l4 passes)
overthe liquidsurf.ace.

is ref'lectedbackto a spotthat landson the closemirrorjust

aboveandto the

Ieftof the incomingbeam' Next,this mirroris adjusted
to passthethird spotbackto
the fbr mirrorso that it fallsjust belowandto the leftof thepreviousspot. At thispoint,only the far mirroris adjusted
to walk theremainingspotsontotheopposite
mirror in a parabolicpatternwherethe spotsizegrows
andthenshrinkswith thetotal
numberof passes.The lastpassrefocuses
the light undertheclosemirrorto help

minimizeclippingasthe beamleavesthe chamber.The
numberof passes
dependsonthe radialseparationof thetwo sphericalmirrors. Morespotsmay be fit on the

mirrorsasthe separation
approaches
40 cm. After the beamleavesthechamber,
anothersoft focusinglens(f : 20 cm) is usedto herp
colrimatethe light. At theend,
the beamis

-a

49

t0'3D

I-,*- t r3o

tO'3D

iili

#i$
lilt

ilr:

trii

d
a

i

t
I

Figure 2.12 Noisesubtraction
circuit.

-a50
directedonto an InSb detectorthrougha fastfocusinglens(d:12.7

cm, f : 2.54 cm)

that maximizesthe intensityof the signal.
The secondhalf of the laserbeam is passeddirectly onto a matched
InSb
detectorthat is usedas a referencesignal in a noisesubtraction
scJreme.Voltages
from the signaland referenceInSb detectorsare sentdirectly into

servoloop

subtraction
electronics,
which are illustratedin Figure2.12 for the versionusedintheseexperiments.The circuit scalesthe voltageon the reference
detectorto

match

the valueon the signalInSb, at which point the two voltages
are subtractedto
eliminatecommon modenoise. The scalingfactor is determinedby anwith a time constantthat is appropriatefor the durationof theintegrator

scatteringsignal. The

subtractioncircuit alsorequiresthat the DC offsetbe set to zero,
which

is

accomplishedwith a voltagesurnmingjunction controlledby
trim potentiometersonthe front face of the panel. The final setuprequiresfine adjustmentof the common
mode rejectionration (CMRR) knob to effectivelysubtractnoiseon the laserlight.
The detectionschemeroutinelyyield absorptionsensitivities = *of 2 10-6Hz't/2 for
0.3 pW of laserpower.

2.2.5 Frequency Diagnostics
As the laseris tunedacrossan absorptionprofile, diagnosticcomponents
inthe spectrometerare usedto determinethe frequencyat each pointin the scan. A

referencegascell providesabsolutefrequencies,
while transmissionfringesfrom a
confocalFabry-Perotcavity are usedto determinethe relativevaluesat eachdata
point' As depictedin Figure2.1, the lock-in detectionscheme
involveschoppingthe
IR + 11sNl.beamsbeforethe light is sentto eitherdiagnostic
element. A germaniumI51
window reflectsnearryail of the HeNeright onto a siriconphotodiode,wherethe

signal is usedas a rererenceforthe rock-in
amprifiers. The choppedIR right isdivided by a 50/50 caF2 beamsplitterinto

hvo partsthat are sentto the gas celr andetalon' Transmittedlaserpower in eachdiagnostic
component--ismonitoredwith an
MCT detectoras a functionof frequencythroughout
the scan. Vortagesfrom thedetectorsare sentto rock-inamprifiers(Stanford
ResearchSystems,SR5r0) that
rectifuthesesignalsby heterodyningthem

againstthe frequencyreferencefrom thesimultaneouslychoppedHeNe beam. The
outputsare filteredto remove

the2f

component'which resultsin a DC output
that is recordedthroughoutthe scan. At
ttre
end of the scan,the laseris brockedand thedataacquisitionsoftwarerecordsan

additional50 pointsto measureany detector
offsetvoltages.
The referencegascell consistsof threeglass
cylindersconnecte
d to a250 mL
glassbulb' Eachtube is sealedon eitherside withsapphirewindows and black wax.

which allow the entiresystemto be evacuatedand firted to a known pressure..fhecylindersvary in length(1.99 cm, 5.82 cm,
and 17.7cm) so that absorptionintensities
may be easilyadjustedthroughoutthe experiment.
In general,pressurein the cell islimited to = 100 mTorr of coz so that pressure
broadeningis a negligiblecontribution
to line shapeprofile. The shortestcelr is used
for most of the groundstate(00,0)transitions'while the longestpath lengthis employedfor vibrationalhotbandprofiles.
The triple cell is mountedby restingthe bulb
acts as a pivot in the ring so each of the threelaser beam in front of an MCT detector.

on top of a fixed ring stand. The butb
cells may be rotatedinto the path of the

Two wires connectedto the longestcell

usedto switch the ceils in a way simirarto
that

are

of a puppeteer.Thesewires are/52
passedthrougha hole in the sideof the purge box, rvherethen the threecells may beinterchangedunderpurgeconditions.RecordedCOz absorptionprofilesare fit to a
Voigt line shapethat fixes the Gaussiancomponentto a room tempe.rature
Doppler
width. In this scheme,the extractedparametersincludethe Lorentziancomponent
and the centerlinetransitionfrequency,rvhich is usedto definethe absolutefrequency
in eachscan. Internalstatepopulationsare calculatedfrom the areaunderthe curves
with an analysisroutinethat includesnormalizingmagnitudeof the absorbanceto thelengthof the particulargascell in placeduring the scan. A standardBoltzmann
analysisensuresthat the populationsin the 0000and 0lr0 vibrationalstatesare wellfit by room temperaturerotationaland vibrationaldistributions.
Transmissionfringesfrom an etalonare recordedduring the scanto determine
the frequencyof eachdatapoint in the absorptionprofile. The etalonis constructedfrom a 27.5 cm long quartztube(a 2.54 cm) with a 2.54 cm long piezo tube gluedto
one end. The cavity designinvolvestwo CaF2plano-convexmirrors that eachhavea
30 cm radiusof curvature. one mirror is fixed directly to the pZT tube withnitrocelluloseadhesive,while the othermirror is carefullypositionedon the opposite
end with a mirror mount attachedto an x y z translationstage.A HeNe beamis used
to set the secondmirror at the confocaldistancein the following way. The tube isplacedin the beampath so that the cavity axis is parallelto the HeNe beam,butslightly off center(displacedradially by = 0.5 cm) to help generatea bow-tie
configurationwithin the cavity. The secondmirror is coarselyalignedinto the flared
end of the quartztube. The PZT is scannedwith a high voltage saw toothedwave toallow multiple modesof the HeNe to transmitthroughthe cavity. Light is monitored

53
on a Si photodiodeas the secondmirror is accuratelypositionedwith the translation
stage. Fri4gessharpenas the cavity lengthreachesthe confocaldistance,which
ultimatelyleadsto a measuredfinesseof l8 for the HeNe beam. The mirrorsusedforthe currentcavity are 9l %oandg6oh reflectiveat 632.8 nm. which corresponds
toatheoreticalfinesseof 23. At x : 4.2 pm, the finesseis a differentstory basedonreflectivityvaluesof 65Yoand46o/oto yield a theoreticalfinesseof 2.6 for the bowtie
configuration.Unfortunately,the actualtransmissionfringesappearto yield afinesseof0.l - 0.5. Sucha discrepancy
hasnot beenfully resolved,but may be due to theline width of the Iaser,angledependenceof the coating,or perhapssomethingelse.
ln any event,thesefringesstill provideadequatesignalsto linearizethe frequency
axis in the scans.The free spectralrange(FSR) of the cavity is measuredby counting
the numberof fringesbefweentwo closelyspacecl
co2 transitions.values aremeasuredat a seriesof frequenciesin the CO2 spectrum,wherethe averageis250.52
MHz and standarddeviationof five measurements
is l. 12 MIfz. Frequency
interpolationbetweenthe transmissionfringesis coveredin the dataanalysis
discussion
in Section2.6.

2.3

Molecular Beam

2.3.1 Overvierv
The incidentgasmoleculesare preparedin a molecularbeamwherethe
supersonicexpansionprovidesa narrow distributionof velocitiesaroundan averagethat is controlledby the componentsin the gasmixture. The molecularjet
supersonicallycoolsCO2 into the lowestfew rotationalstates.Extensivedetailsof

54pulsedskimmervalve

Po,To,Po
pinholeaperture(d:500 prm)p --(d\'
r-l -

Po

\r)

|

Figure2.13 Incidentcoz molecules
arepreparedin a pinholesupersonic
expansionthatis skimmedto generate
a molecularbeam.

the theorybehindmolecularbeamshave beenpresented
elsewhere.r.e
The primary
focus of the current sectionis to provide a brief overview ofthe generaloperating

principlesthat determinethe optimal conditionsfor the scatteringexperiments.

These

ideasare followed by a descriptionof the specificpulsedvalve
configurationsand
methodsthat are employedto characterizethe incident beam.
The molecularbeam is generatedwith pinhole aperturein a pulsed

valve thatis illustratedin Figure 2.13. In general,a supersonicjet requires
the flow of gas froma high (Pe)to low (P-) pressureregion, which generatesan

expansionthat

adiabaticallycools the atoms and morecules. Supersonicflow
is reachedwhen (i) themean free path of the gas in the nozzle is much less than the pinhole

diameter (d), and55(ii) the pressuredifferencebetweenthe tr.voregionsis largerthan a factorof two.
Thesetwo requirements
ensurethatcollisionswith the walls of the pinholeandbackgroundgas in the chamberare negligible. Under thesecircumstances,thepressure,temperature,and numberdensityof moleculesin the expansionare
determinedby the fype of gas in the mixture and the initial conditionsin thestagnationregion. For exarnple,the numberdensityfor a monatomicgasalong the
expansionaxis (r) is,
p
Po

/

r t 2

- , ( d ll - t .
\r)

(2.r)

where p6 is the numberdensitl,in the stagnationregion andC = 0.143. The
expressionfor an expansionof a polyatomicgas follows the sameform, but the valueof C changesslightly basedon the differentheatcapacitiesof the particulargas. The
terminalvelocity of the gas in thejet is definedby the temperaturein the valve andcomponentsof the mixture. Dependingon the desiredincidentenergy,CO2 is mixed
with hydrogen,helium, neon,or argonbeforebeingpassedthrougha room
temperaturevalve. The terminalvelocityfor moleculesin a jet is,(2.2)
wherew is the averagemolecularweight of the mixture,y is the ratio of the heat
capacities(CplC"),and T is the temperaturein the stagnationregion. Velocitiesof thegasesin the molecularbeamare characterized
for the variousmixturesin thefollowing section.
With theserelationshipsin mind, the scafteringexperimentsaredesignedto
ensurethat the measuredCO2distributionsexclusivelyreflectnascentinteractions

56
with the Iiquid' Simpleback-of-the-envelope
calculations
help illustratethe
importantcriteriafor the initial operatingconditions.
First, thenumberdensityat thesurfaceand terminalvelocityare combinedto determine
the incidentflux of coz per
unit area'which we rimit to : 0.1 monorayer(ML)per gas purseto herpguaranteethe

singleinteractionconditions.Fractionalsurface
coverageis calculatedfrom the flux
and lengthof the typicargaspurse,which are combined
and comparedto the typicalnumberof exposedsurfacesites(= 4* l0r5 sites/cm2).
Typicaroperatingconditions
involve COz in a hydrogencarriergas (ps : 100Torr) wherethe pinhole(A 400 pm1
is positionedI I cm from the surface.The total

numberdensityat the surface(= s x1012#lcm3land the terminarverocity(r.42 x r0scmls)resurtin a flux of= r * r0r8#/cm2ls' Typical 300 prsgaspulsesleadto adepositionof = 3 x lOramolecules/cm2_

This value reflectsthe toral flux of CO2* Hz,

which is kept below 0.1 ML in the

expcriment.
Another importantaspectof the molecularbeam

involvesthe positionof theMach disk with respectto the pinhole-to-surface
distance.The disk is a shockwave
createdin the regionwherethe densitiesof the gas
inthe expansionand backgroundapproachsimilar values. As thejet beginsto equilibratewith the backgroundgas,thebulk velocitydropsbelow the local speedof

sound,at which point the molecularbeambeginsto breakdown. An empiricalrelationshipforthe distanceof the Machdisk of a pinholeexpansionis given by( p \ " 'r = 0 . 6 7 . ld2 l

\ P ]
\

€ - l

(2.3)

57

10,000Torr
Baratron->ICO,gas tank

vacuumX

I

caffier gas

1,000Torr
BaratronX'--L
regulator

pulsedvalve

250mL
calibrated'
volumnFigure 2'14 Schematicof the gas manifbldthat is used
to mix and deliverCo: tothe pulsedvalve.

Typicalexperiments
involvestagnation
andbackgroundpressuresof po: r00 Torr
andPo,: l-3 x l0-sTorr,respectively,
with a 400pm pinhoreaperture,which
estimatethe Machdisk to be= 50- I 00 cm from the pinhole.
Importantly,
this value
is an orderof magnitudelargerthanthedistancefrom the pinhole
to surfbceto makecertainthatthe molecularbeamremainsstablefor the scatteringexperiment.
2.3.2 Gas Manifold
Thegasfor the molecularbeamis deliveredto thepulsedvalve
throughamanifoldthat is illustratedin Figure2.14. Theprumbingof
the systemis configured
to mix two or moregases.andthendeliverthemixtureto thepulsed
valvein a

58
controlledfashion. The manifold is constructedfrom copper,
brass,and polyflo
tubing,which are isolatedat varioussectionswith stainlesssteelballvalves.The

lines may be evacuatedrvith a srnallmechanicalpump that is connected
throughanotherisolatingball valve. Gas mixturesare madeand storedina standard5 gallon
propanetank, rvhich is connectedto the manifold u,ith aninternalsafetyvalve. Thesevalvesare includedin most propanetanksto limit the flow throughthe valve.
Importantly,the input pressuremust reacha certainthreshold
beforethe valve
and releasesa small squeal. Mixtures are madeby filling the

opens

tank with coz and aseedinggas,wherethe partialpressureof eachcomponentis
measuredwith a 10,000
Torr capacitancemanometerpressuregauge(MKS Baratron
mixing tank- The generalprocedureinvolvesfirst filling an

722A)at the inlet of thc

evacuatedtank with 100-

200 Torr of coz from a standardlA tank that is connectedto the rnanifold. Next, thebuffer gas is addedto the propanetank in spurtsto turbulently
mix the contentsof the
tank' This processis continueduntil the pressurein the tankof l}Yo co2, i.e. a total pressureof 1000-2000Tom. The

corresponds
to a mixture

final stepis the easiest

sinceit only involveswaiting overnightfor the gasesto thoroughlymix.
After a mixture hasbeenmade,the gas is passedto the pulsed
varvethroughthe remainingportionof the manifold. To preciselycontrol
the pressurein the valve,
the Coz mixture flows througha singlestagestainlesssteel
regulator(MathesonTriGas, Inc, model 3494) that drops the pressurefrom above atmosphere =to 100 Torr.
The pressureis monitoredby a secondmanometer(IvIKSBaratron

72zA),which is

ratedto measurevaluesbefween0-1000 Ton. The output
of both pressuregaugesis
displayedby a digital voltmeter where the voltage is directly proportional
to the'.il\gL--b59
pressureon a 0-10V scale. An additionalfeature6f this sectionof the manifold
is acalibratedglassbulb with an isolatingvalve. The bulb is attachedto the systernwith
a standardl0 mm glasso-ring sealand metalcramp. The known volume (262.g mL)of this bulb plusthe gasmanifold(109.6mL) enablesus to determinethe absolute
flux throughthe pinholeon the pulsedvalve. Polyflo tubing is usedto connectthe
outputof the regulatorwith the valve throughan Ultra-torr feedthroughon the sideofthe main vacuumchamber.

2.3.3 PulsedValvc and Skimmer
The gas-liquidscatteringstudiesrevolvearounda molecularbeamthat is
generatedwith a piezo-controlled
pulsedvalve. Two versionsof this pZT valve havebeenused,both of which havebeenbuilt in the JILA machineshop. The olderof thetwo valvesis usedin the gas-liquidscatteringchamber,while the newervalve resides
in the gas+ SAMs experiment.Generalfeaturesof both valvesare basedon thedesignsby Prochand Trickl,r0where small modificationshave beenincludedto
streamlinethe initial setupand improveoverallperformance.The setupprocedure
has beenoutlinedin detail within previousexperimentalreports;therefore,a basic
descriptionof the valvesis includedto providedetailsaboutusageand performance
characteristics.Schematicsare illustratedin Figure2.15 to help describethe essential
featuresof both valves. In addition,the high voltagedriversfor both valvesare
illustratedin Figures2.16 and2.17.
In general,the valve is constructedin two parts to provide accessto theinteriorcomponentsthat are normally sealedfrom the vacuumchamberrvith a seriesof o-rings. The front part of the valve housesthe piezo disk (PhysikInstruments,p-

60

high
voltageTi nutDelrinadapterpiezodisk

Ti.Plunger
pinholefaceplate wlth o-rlng

Figure 2.15 Cross-sectional
schematicof piezoelectric-controlled
pulsedbasedon
the initial desisnsof Prochand Trickl.

286.23)that opensand closesthe valve. The PZT elementsare designeclto buckle atthe centerwhen a high voltageis appliedto the crystal,which then providesa way totranslatea plunger away from the pinhole apertureto createa pulseof gas. As shownin the schematic,two Delrin adaptersare mountedto the centerof the hole in the
middle of the circular PZT disk. Thesetwo piecesare threadedtogetherandtightenedto the disk with a home-built brassspanner.The inside Delrin adapteris
also threadedso that the titanium plunger may be mountedto centerof the pZT.
Once thesethree partsare attachedto the disk, the PZT is bolted securelyto the front
frame of the valve. The pinhole apertureface plate is mountedto the front face of thevalve with four screwsthat compressan o-ring seal. The generalsealingmechanism
involves translatingthe plunger away from the pinhole to allow gas flow out of thevalve. To createa vacuum seal,a small o-ring is attachedto the tip of the plunger so

-.IY

6l

I
c

2

:i

'l

=
-1
6

3 i -

34

rl('

Figure 2.16 Pulsedhigh voltagedriverfor the PZT valveusedin thegas-liquid
scatteringchamber.

-::-{---

62

t

E=

t
r B d

=
e

ilr

j'llr

fir

.

:El ':!

Itll.i ,||

;

* r

.

<
a

3

',

6

3

a

t,..

'--

l

.:

Eb=

l

?:
&

r. j 6&
ER

L.--1
. ;

I
= r

-

-;fr;i
-JF-

r 'J

J

I

1'

ll

I

q'
+

l "

L

r

5

3

* -

- i ; a4

3

a - l
..
I

!

i

.+.-r'-lF.
( i ) I
; e

t'-

"f

, O

lL'

l <
..1.J
r o

a

l'D

r l
-

l

a ( o
=

=

d

Figure 2.17 Pulsedhighvoltagedriverfor the PZT valveusedin the gas-SAMs
scatterins
chamber.

63that the o-ring sits flush on the apertureface plate when
its neutralposition. The plungeris positionedin

the pZT elementis relaxedto

the Delrin adaptersso that a slightresistanceis felt to indicatethat the o-ring has madecontactwithplungeris then carefuilyrockedinto position by a

the face plate. The

titanium nut.

At this point, the valve is testedto ensurethe plunger
appropriatelysealsthe
valve. The procedurerequiresan adapterflange(Figure
directly to the side of the gas-liquidvacuumchamber.
fixed to the flangewherean o-ring sealsthe faceto2.rg) that is mounted

The front part of-thevalve isthe chamberso that the pinholeis

directedthroughan openinginto vacuum. Before
the chamberis evacuated,amicrophoneis mountedon a rod and passedthroughan Ultra-torrfitting on theadapterflange. The microphoneis positionedin
front ofmay then be movedto translatethe microphonealong

the pinholewherethe rod

the molecularbeamexpansion

axis' once setup'the chamberis evacuatedto provide

an initial test of the plunger

seal' If the valve leaks,then the titanium bolt is loosened
so that the plungermay be
adjustedto createa proper seal. To characterizethe
gas pulse,the pZT elementis
connectedto a high voltagedriver that provides
control ofthe durationand delayof

the gas pulse. The initial setupinvolvesan external
TTL triggerthat is supplied
eitherfrom a functiongeneratoror from a computer-controiled
digital_to_anarog
output' with the initial voltageto the PZT setlow,

a trim potentiometeron the valve

driver is slowly adjustedto increasethe magnitudeof the pulsedvoltageto the point
wherethe valve opensand gas flows throughthe pinhore.

The pZT erementtypicaily

translates= 100 pm at the maximum voltage(V,u*: _1000V),
which then requires
the driver to outputsomethingcomparabreto adequateryopenthe varve. Typicar

-/

64

faceof

PZT
valve

Figure 2.18 A custom-builtflangeis usedto mount the PZT valve to the gas-liquidchamberfor initial setupand characterization.A microphoneis usedto monitor thegas pulseas the valve and driver are optimized.

voltagesthat are requiredto open the valve range fiom -300 to -400 V in both of theinitial setups.Once the valve is pulsing.the microphonesignalsare monitored
througha home built amplifier illustratedin Figure2.19. The openingvoltage,pulsed
width, and delay are setto optimizethe time profile of the gas pulse. In general,thedriver voltage is setjust above the point where the amplitudeof the microphonesignal remainsconstant. Additionally, the width is set to = 300 - 350 trs to ensurethe
valve fully openswith every pulse.
Once the initial valve setupis complete,the front face is removedfiom thetest flange and reassembledwith the other half of the valve. The remainingsteps
include connectionsto the gas manifold and high voltagepower supply. on theliquid chambervalve, a gas line from the manifold is attachedthroueh an Ultra-torr

65

microphone

il--distancetrme J<_
gaspulse+I,__t__i {I It r .o

; r-1_--.'
I

i T i

I

V

MICROPHONE

Figure 2.19 Schematicof thetime-of-flightexperiments
usedto measure
thecenterlinevelocityof thegaspurse.The micropt
onesignat-ir'pro..rr"dby thegiven
amplifiercircuit.

connection
on the backsideof thevalvebody. This feedthroughlimitsthetotalpressure
thatcanbe passedinto the valvesincethe sealing
connection
is simplya
frictionfit' Negativevoltageis suppliedto the pZT

discthrougha MHV connector.

wherethebodyof thevarveprovidesthereference
to ground.The newervarve
designincorporates
a VCR connector
to deriverthegas,anda singreisolatedpin to-/66
transferthe high vortage. once again,the bodyof the varve

is groundedto provide

the potentialdifferencefor the PZT. Both valves
are mountedthroughtapped-holeson the backside' Experimentshave beenset up

in variousconfigurationswherethe

valve bodiesare positionedin front of a skimmer

in a specificscattering

configuration.
Thejet expansionis skimmedto reducethe angulardivergenceof the beam sothat out-of-planescatteringmay be detected.
The skimmers(Beam Dynamics,Inc.)
are constructedfrom nickel with an ultra-thinaperture
edgethat is designedto reduce
interferenceeffectsin the molecularbeam. Skimmersin the initialnormal incidence

studiesare 5'0 rnm in diameterand mountedon plateapositioned2.5 cm downstream
along the expansionaxis. As spacebecamecramped
in glancingincident
experiments,a smaller3.0 mm diameterversionis usedin awith the skimmerorifice 1.5cm away from the pinhole.

simirarconfiguration

In both cases,the skimmer isfirst placedbetweenaluminumand Teflon adaptersto proviileadditionalstructural
support' Thesepiecesare then mountedto a platethat is boltedto the scaffoldingthat
supportsand positionsthe pulsedvalve with respect
tothe liquid surface.A laser

pointer is usedto align the centerof the aperture
with the pinholealongthe expansion
axis beforethe finar bolts are tightenedto fix theskimmerin place. To do this, theIaseris mountedin a machinedadapterthat fixes
theoutput in the samepositionas

the pinhole,wherethen this adaptermay be attachedin placeof the pulsedvalve.
Geometrically,the skimmerconfigurationrestricts
the angurardivergenceof themolecularbeamto + 6o.

67
2.3.4 Incident Beam Characterization
Propertiesof the supersonicjet are characterized
to determinethetranslational,angular'and internalstatedistributions. ThepzT
valve isstablesourcethat showedno significantdeviationsafter the
initialincidenta remarkably

setup.

Translationaland angulardistributionsare analyzedrvitha severalanalyticaltoolsthat include(i) a simplemicrophonetime-of-fliglitanalysis,and (ii) high resolution
Dopplerimetryanalysisof the absorptionprofiles. In termsof the internalstates,
populationsare recordedfor a seriesofrotational
statesin both the ground(0000)andbendexcited(01r0)vibrationalmanifolds.Brief detailsof theseresultshavebeen
presentedthroughouteachchapter.Therefore.we
cover the basisofthe techniquesand measurements
in more detailto list the methodsthat

leadto a systematiccharacterizationof the incident COr.

2.3.5 TranslationalDistributions
velocities of coz in the pinhoreexpansionspanthree
directions(x y ,)where
x andy are the samebasedon the azimuthalsymmetryof the aperture.Two
independenttechniquesare usedto characterizethedistributions
both (i) along,and(ii) perpendicularto the expansionaxis. To help illustratethesetechniques,a sampletransientabsorptionprofile is plottedin Figure2.20 for
R(0) at the centerlinetransitionfrequency(v6)to show the generartemporarprofireof the purse. These
pulsesare also monitoredwith the microphonewhere
the transientsignalsare nearly
the same' In a simpletime-of-flightexperiment,the microphone
is attachedto a rod
so that the positionof the sensoris adjustablethroughan

ultra-torr feedthroughon

the oppositesideof the vacuumchamber.As illustrated
in Figure2.19.time sisnals

68

R(o){\
0.15
()
O
-O

fr tr)l\(- 350ps

a
o0i

500

1000

1500

100(l

t i m e( p s )Figure2.20 centerline(ve)absorption
profilefbr J - 0 in theincidentbeam.

are recordedfbr a seriesof distanceswhere the microphoneis moved away from thepinholealongthe expansionaxis. The easiesttimesto measureare thosealongthefront edge of the FWHM becausetheslope is the steepest.The best-fit line to asimpledistanceversLls
time analysisreflectsthe averagevelocityof the gas in themolecularbeam. Sampleplotsare illustratedin Figure2.21 for mixturesof CO2 withargon' neon-70,helium, and hydrogen.Values reportedin Chapter4 show excellent
agreementbetweenthesemeasuredvaluesand thosethat are calculatedftom therelationshipin equation2.2. For example,the measuredvelocity for the hydrogene x p a n s i o ni s l . 4 l ( 9 ) ' 1 0 5c m / s .w h e r e a st h e c a l c u l a t e n
d u m b e ri s 1 . 4 2 . 1 0 5c m / s .

69

(r()

I

-ltt

C)
q)

U)

-o 20

H.
^ H e
.
Ne-70
.
-\r'
-l()

(r()

u u r e( p r s )Figure2.2l
CO: velocitiesare measuredwith the microphonein a ToF experiment
fbr gas mixturesthat inclr.rde
Hz, F{e.Ne-70.and Ar.

Similaragreementis found betweenthe othermeasuredand calculated
beam
velocities.
To characterize
velocitiesalongthe x andy direction,Doppler-broadened
absorptionprofiles are recordedand analyzedfbr a seriesof experimental
configurations. The frequencyat which CO2 absorbsdependson

the projectionof its

velocity along the laserpropagationdirection. Thesedetuningfrequencies(v_vo)
reflect the Doppler shift associatedwith transration.whereV - V ^v = V^ l. (

v
L

(2.4)

70

SampleIncidentFlux - P(1)l0'%CO"in H"

o-+

^Y

[-ruskiuunccl
profrle:1
0(
5)N I H z
fvr,(198K)
Sut'tirce
teurpenrture
lr'r, - l-ll ltlHz

II

Skinunedprotile
f\'r, : 6lt 2l NIFIz

o2
:l

o
(.) 0 0

a

-o.2
-J{x)

- 2 ( x)02(X)-+()()\ ' - \ ' , ,( M H z )Figure2.22 Sampleincidentbeamabsorption
profile.

The simplestsetupinvolvespassingthe laserthroughthe molecularbeamso that the
expansionaxis and laserpropagationdirection are orthogonal. Sampleabsorption
profiles are illustratedin Figure 2.22 for an unskimmedand skimmed supersonic
expansionfor a laserscanof = | GHz. The profiles are modeledwith a Voigt line
shapein voigtb.x,which is a convolution of Gaussianand Lorentizianfunctions. The
Gaussiancomponentcharacterizesthe translationaldistributionand the small
Lorentzianfactor accountsfor the residualline width of the Pb-saltdiode laser(Avro,..

7l

laser
multipass

g*
r.,00

;;
iJ
(r
+

\r' rrlc->j2

6(t(2) j. . *-**__/;"1

MIl,

.

o

4

8

<i

.

i

r

v'

30"
r

(r-

l

ri

:..r
t

+| -

I
(,)
C)

1

i
\r' l l ) r : --> r _1-

I i

6l(2)

r

|

',

lv]Itz tir-**.f=:r.e*'

i,

tll_

_

_t

t*uastv

I ljg( l),t 1t rwrt'_

qr-,*d^-.--.+r

Itr- 45"
l-r

li

(r)

(r

,

+

-r
{,

|

<-

\r'+

n

I lls

c

.55(2)
Mllz--r'

228(2)MI Iz

Vl(t 60o
IJ()+\r'l n c*

,)

44(2)
Mllz

0

v lllc,/'
I-+(X)-2(X) 0-r-V2tttr(l)
MIIz

200 +(x)V - V,,(MHz)Figure 2.23 Incident beam absorptionprofiles measuredin the gro,.,: 0oconfigurationfor 0'n.: 0o- 60o. Line shapeanalysisis usedto extractfull width-halfmaximum (FwHM) and peak frequencyshifts for each configuration.

72= 20 MHz). The extractedFWHM valuesshorvthat the skimmer reducesthe angulardivergencefrom a half angleof = 20.3"to 5.5o. Importantly,the Dopplerbroadening
is smallerthan a typical room temperaturer,vidthof = 129MHz, which allow us toinvestigateenergytransferinto translationaldistributionsof scatteredCOz.
The Dopplerimetryanalysisis also usedto characterize
the centerlinevelocityof the expansionin a seriesof experimentsthat complementthe TOF studies.From
the cartoonillustrationin Figure2.23,themolecularbeamis moved into the (ptor",:0oconfigurationso that the expansionaxis intersectsthe laserbeamat a fixed incident
angle(0;".). As 0;n.extendsbeyond0", the absorptionprofilessplit into two peaks
reflectingthe back-and-forthnatureof the lasermultipass.In theseconfigurations,the centerlineabsorptionfrequenciesare proportionalto the projectionofthe averagemolecularbeamvelocity,while the GaussianFWHM reflectsa convolutionof theangularandvelocityspreadin the beam. Profileshavebeenfit to a modelthat
extractsthe centerlinefrequencyand FWHM for each J-state,which are plotted in
Figure2.24. Both valuesshow a statedependence
that most likely reflectsvelocity
slip effectsof CO2 in the expansion.The centerlinefrequenciesof eachstateareconvertedto velocity,at which point the fractionalpopulationper J-stateis usedto
calculateda weightedaveragevelocity. Thesevaluesreflectthe projectionof theterminalCO2speedin thejet. The spreadin velocitiesis obtainedfrom a simple
Gaussiandeconvolutionof the FWHM that accountsfor the + 5.5ospreadin themolecularjet. Extractedvaluesfor Av/v- are0.105,0.108,and 0.093for 06.:30o,
45o,and 60o,respectivelywith an averageof 0.102(8).

73a) lncidentCO..'P eakFrequencies
j jtt
.l{,o
I5tt
N

loo

z

I

t"l^-

= li{)

.

l(|()
rtt '
t,

+()

l0

l(l

J

il

l

b ) lncidentCO: [ . w H M.ll t()=(il|lr'- -li

H

= 1 r

-r

litt
N

lil( )

4

I rtt
I
lll()
I

io|^:1^'

.trt

-lt I

.l
Figure2.24 Dopplerimetry
analysisof the incidentbeamline shapesextracts(a)peakfrequencies
and(b) FWHM asa functionof J-statefor eachincidentangle.

74
2.3.6 Internal StatePopulations
The internalstatedistributiondependson the collisionconditionsduringthe
expansion.In general,higher pressuresand largerpinholeaperturesleadto colder
distributions
sincethe numberof collisionsscaleslinearlywith bothparameters.
While many studiesin the group haveexploitedtheseconditionsto generateclusters,
the scatteringexperimentsrequireCO2monomerin the molecularbeamso that themeasureddynamicsreflectthe nascentinteractionsbetweenindividualmoleculesthat
collide with a Iiquid surface.To testthe clusteringconditionsfor a given mixture,Jstateabsorptionprofilesare recordedfor a seriesof valve pressures
to determinethe
regimewhereclusteringis prevalent.The profilesare integratedover all Doppler
detuningfrequenciesto generatea column integrateddensity(A",r). Typicalclusteringtestsincludepopulationsin J = 0 - 8 sincemore than 90% of the total
populationis cooledinto thesestatesin the supersonicexpansion.Column densities
for eachJ-stateare plottedversusstagnationpressurein Figure2.25 for a seriesof
mixturesthat includeco2 with (a) hydrogen,(b) helium, (c) neon-70,and (d) argon.
ln addition,the sum of eachcolumn densityis plottedto showthe regimewherethe
signalgrowth is linearwith pressure.In eachcase,the curvesare linearuntil= 150
Torr, at which point the monomersignalstartsto decreaseas clustersform in thejet.
Optimal operatingpressures
are typically set at 100 Torr in the scatteringexperiments
to ensurethe dynamicsonly involve collisionsbetweenCO2monomerand the liquidsurface.
The general characterizationof the internal statedistribution involves astandardBoltzmannanalysisof the populationsin the 0000and 0ll0 vibrationala) Hydrogcnb ) Hcliurn

l:,,
-

\

lr{r

\

=

s

,

,

2

i

-lr I

=
l r

1,,

- .{,,

l"
' r F
rr

i!

1,,'

I i,,

1,",

1,rrr

lni

c) Neon-70' t H lSr ('tir [!

d ) /Ar rr 5c n\ rnr r,i

1 r'-'

).

l,'

2

1

-|il'rS t a l r r i tito r rI t r t s s t r r c( ' l ' o r lt

r *'

i

'r rrr

,Stl!.:rtirliort[\'csstrlc t'lirr r I

:.

irr

I,,t.rl
I t,
I
l

t,'

l
l

t , ,

./

l , l

t

\

11 ,*
l

i , r

l r i rl i , rl , r ,

S t i r ! - ! r t l r l r oI 'tttc s s t t l ct ' l i r r r I

rr

ir,

l,r,

l\,'

-lr

S t i t : r t t r t t i o It t t ' c s s t t l e( ' l ' r r r r)'

Figure 2.25 Growth curvesof monomerCOz determinethe effectsof clusteringin
the molecularbeam. Column integrateddensities(A".1)are measuredas a functionof
J-stateand pressurein the stagnationvolumeof the pulsedvalve.

manifblds. Absorption profiles are recordedfbr a seriesof J-stateswithin each
ribrationalstate. Detailsof thesemeasurements
are coveredin Section2.6. which
explainsthe generaldetectionscheme. To generatethe populationper quantumstate,
the absorptionprofiles are integratedto generateAu.1.To properly accountfor the
Honl-Londonline strengthfactors(S1),the column densitiesare dividedby either
J /(2J+I ) for P-branchor (J+ l )l(2J+l) for R-branchtransitions. Thesevaluesare now
directly proportionalto the productof the C02 number densityand absorptionpath

76

+
)

II

c..l

o

F

0
-2
-l
-(l

-8

-lo
2txI

+ ( X)(r(X)

8 ( x)(cm-l)
E,,.,,,
Figure2.26 J-statedistributionfor CO: in the incidentmolecularbeam.
Populations
fbr ground(00"0)andbend-excited(0 I I0) vibrational
aremeasured
states.

length. l'he column densitiesf,orrotationalstatesin both vibrational manifblds areplotted againstthe rotationalenergy in a standardBoltzmannanalysis. Sample
distributionsare plotted in Figure 2.26 for COz in Hz to illustratethe supersoniccooling in the expansion. The best fit line to the lowest rotationalstatesshow that therotationaltemperatureis T,ot= l5 - 20 K. The fractionaldifferencebetweenthe two
vibrational statesprovidesan estimateto the vibrationaltemperature,Tvib= 170 K.
While the majority of the populationcools into the lower rotationalstates,theBoltzmann plot also revealsa small fraction of the incidentCOz remainsin higher J

77states.This nonthermaldistributionreflectsthe inefficientcooling of COz out ofhighrotationalstatesin a pinholeexpansion. In scatteringexperimentsthat involve
passingthe incidentmoleculesthroughthe laserbeam,the initial rotationalstate
populationsare measuredfor a seriesof J-statesso that thesepopulationscan thenbesubtractedfrom the valuesof the scatteredCO:. At normal incidence,a comparisonof scatteredto incidentflux populationsshorvsthat the ratio is betweenl0 and 50 lor
rotationalstatesgreaterthanJ: 10. In theseexperiments,
valuesforthe incident
column densitiesare subtractedfrom the scatteredflux beforethe populationsare
analyzedwith the fwo-temperature
Boltzmannmodcl. In otherscattering
configurationswith Oin.) 0o,the incidentbeamtypically missesthe laserand no
subtractionis needed.Theseconditionswere testedin every experimentso that theappropriatevaluescould be subtractedifnecessarv.

2.4

Liquid Surface

2.4.1 Liquid Chamber
The bulk of experimentsinvolve scatteringCOz off liquid surfacesunderhighvacuumconditions. Surfacesare generatedfrom methodsfirst developedby
Lednovichand Fenn,who useda glasswheel that srowly rotates(= 0.5 - | Hz)
througha reservoirof liquid to preparea liquid surface.A razorbladescrapesaway
the top liquid layerto reveala renewedliquid surfacefor the scatteringexperiment.
The pulsedvalve and wheel assemblyare housedwithin an 60 liter aluminumchamberthat is constructedas a cubewith interchangeable
aluminumand plexiglass
flanges. The chamberis evacuatedrvith a 6" diffusionpump (vHS-6, 1000l/s),

78

0tu..,: 90" ctlnflguration$tnr*.: 0" configr'rration

-{>+
.l'

0,,',;--4,,

-M
{t

a

\

T

I

l / /

G .\ \ l l / . ' 'Figure2.27 Experimentalconfigurationsto measurescatteredCO2 over the 2nhemisphericalscatteringvolume.

which is separated
fiom the cubewith a gatevalve and liquid N2 cold trap. The pumpis backedby an EdwardsE2M30 rotary vane mechanicalpump with plumbing that
includes3" rubbertubing and a 3" gatevalve to isolatethe diffusionpump if
necessary.The tubing is attachedto a block of concretesubmergedin sand fbr
additionalvibrational isolation. Thermocouplepressuregaugesare mounteddirectly
to the chamberand inlet of the roughingpump to monitor the pressurewhile thechamberis roughedout from atmosphere.In addition,an ion gaugemountedto the

top flangeto monitorthe systempressure
whenthe diffusionpumpis on. The base

79

ffi
r.-\
S;

F
a)
\--l

Figure 2.28 Front view of the lasermultipass.molecularbeam,and liquid surf'acein the <pror",.:90oconfiguration.

pressureof the systemis 2.0 x l0-oTorr. The diffusionpump is usedin all scatterins
studiesto ensurethe measurementreflect nascentdistributions. Typical operating
conditionsincludepulsedvalve repetitionratesof l I - 17 Hz with a stagnationpressureof 100 Torr. In this environment,the averagepressurein the chamber
reaches= 3 x l0-s Torr, which correspondsto a mean fiee path of = 500 cm for CO2.
Within the chamber,the pulsedvalve and liquid surfaceassemblyare
mountedon a scaffoldingstagethat is illustratedin Figures2.27 - 2.32. The stage

80

Figure2.29 Sideviewof thelasermultipass,molecularbeam,andIiquidsurfbceintho{prascr:
90oconfiguration.

providesthe flexibility to measurescatteredCOz in the two configurationsillustrated: 0o and 90'. In general,the laserbeam
in Figure 2.27, i.e. plaser
path remains
stationaryin each experimentalconfiguration,which then requiresa ceftain degreeofmobility for the gas sourceand wheel componentsto cover the full rangeof incident

8l

Figure2'30 F'rontviewof thelasermultipass,molecularbeam,andliquidsurf.ace
in theq1o...:0oconfiguration.

andfinal scattering
angles.The infrastructure
is basedon two

verticalpoststhatare

mountedon railsfixedto theoppositeedgesof thechamber.
Thesepostsnot only
providethe structuralsupportfor thepulsedvalveand
wheelassembly,
but also
definethe planeof the surfacealongthe fronttwo faces.The
rail

systemisconstructedfrom two platesr.vitha combinationof thru holesthatenablethepoststo
transfate
towardor awayfrom the laserbeamin /a" increments.
This flexibility

isusedto definethe final scattering
angrefor a particurarexperimentar
configuration.

-l

82

Figure2.31 Sideviewof thelasermultipass,molecularbeam,andliquidsurfacein
thetpluser:
0oconfiguration.

83
For both {p16,.,
coflfi$urations.the postsarethat is illustratedin Figure2.32- The tool

initially positionedwith an alignmenttool

is a plexigrassprotractorthat has been

attachedto an aluminum"T", rvhichcan
be clampedat any heightto the front fbcesof the two posts' The opaquetransparencyof the protractorindicatesthe positionofthe centerof the HeNe beamin the multipass
rvith respectto the planeof the sLrrface.
To setupan experimentin the: 90" configuration,elaserthe protractoris

moved into placeby adjustingthe posts(forward

or backward)and tee (up or dorvn)

so that the HeNe beamis arignedrvith markings
on the prexigrassat the desired0,.u,
angle' For most experiments,the centerof the multipassis fixed2.0-2.2 cm fromorigin of the protractor,which is usedto

thedefinethe point wherethe molecularbeam

expansionaxis intersectsthe surface.The
time-of:flight

experimentsin chapter 9

requirelongersurface-to-raser
distances,wherethe physicalconstraintof thecomponentsinsidethe chamberpracesan
upperlimit of = g

cm. once the protractor

is situated,the postsand rails are securely
fixed to the chamber,anclthe positionsofthe two postsare checkedto ensurethe

laserbeamis parailelto the planeof thesurface' with the protractortool in place,the rnolecularbeamis mountedon a,,u,,shapedboom that is attachedto the two main
posts yvith/q,. rocatingpins on either
side' As illustratedin Figure2.29,the boom

pivots arounda line that is fixed in theplaneof the liquid surface' This configuration
constrainsthe distancebetweenthe
pinholeand surfaceat I I cm for each
incidentangle.

The setupcontinueswith thehelp of a laser-alignment
tool that is mountedin placeof the pulsed
valve. The
locatingpins are moved up or down so thelighttravelsthroughthe origin alongthe

desired0;n".once the incidentpathhas
beenestabrished,
the boom is fixed in prace

-a

84

-'*'/ '.,.
I_._vA,\.0 '" *" ':9 0 "Figure 2.32 Initial setupof the pulsedvalve,skimmer,and glasswheel involvesanalignmentlaserand plexiglassprotractor.

with an assortmentof bracketsand side posts. In many of the configurations,thelocating pins and adaptersmust be removedso as not to clip the infrared laserbeamin the multipass.Once this path hasbeencleared,the valve and skimmerplatearereattachedto the boom beforethe experiment.
Setupfor experimentsin the elaser:0o configurationstartwith the HeNe spotnear 0r.u1: 0o on the alignmentprotractor. The laser is positioned= I cm from thesurface,where the compactconfigurationis necessaryfor glancing incidentangles.
As illustratedin Figures2.30 and 2.3l,the sameU-shapedboom is usedto mount the-85pulsedvalve off two additionalrails that have beenattachedto the main posts.
These
additionalrails may be translatedup and dorvnso that the molecularbeamexpansion
axis intersectswith the centerof the lasermultipass.The laseralignmenttool

is onceagainusedto help set the heightof the valve r.r,ithrespectto the IR laser. The
locatingpins placedso that the pulsedvalve rotatesarounda point on the planeofthesurface.The boom is locked in placeat the desiredincidentanglewith brackets
attachedto variouspart of the scaffolding. The bottomarm of the boom is replacedwith a shortenedversionthat allorvsthe liquid wheel reservoirto be fixed inthecorrectposition. In this setup,the skimmerplate is reconfiguredto minimize thephysicalinterferencewith the laserin the rnultipass.
Once the valve has beenpositionedfor eithergrur.,configuration,partsof theliquid wheelassemblyare addedto the chamber,wherethe locationis onceagain
determinedwith the protractoralignmenttool in placeof the glasswheel. The
assemblydesigninvolvesa glasswheel that is fixed at its centerto an axel,whichismountedparallelto the floor of the chamberwith two ball bearingson a verticalpost.
The bearingsslide up and down the post to help positionthe axel at the correctheight.
In addition,the post is mountedto the chamberwith a bracketthat allows translation
left, right, forward,or backward. The axel (and subsequently
the wheel)is positioned
by first markingthe spot wherethe laseralignmenttool hits the aluminumtee;

i.e. theorigin. The heightof the axel is first set so that the molecularbeamstrikesthe glass
wheel on a spot that is = half way from the edgeof the wheel to its center. Then,
front faceof the axel is positionedflush againstthe back of the alignmenttee.

theAt thispoint, the bracketsare tightenedand the protractoralignmenttool is removed.

--A

86
The final stepsinvolve the additionof the liquid reservoir
and glasswheel.
The reservoiris a coppersleeve,which is placedinsidea

holderfixed to the back of

the two superstructure
posts. In the temperaturestudies,thea temperature-controlled
sleeveas illustratedin Figure2.1.

reservoiris placedinside

Thecoppertubing in thesleeveis connectedto a Laudaclosed-loopcirculatorwith polyflo

tubing. Once thereservoiris secure,the wheel is boltedto the axel so thatthe front

surfaceis nowalignedin the planedefinedby the two posts. A razorbladeis attachedto a holderthat fixed to the front edgeof the reservoir.once the wheel

is setup,a gearon the far

side axel is attachedto drive systemthat is designedto rotate
the wheel throughthe
reservoir' The gear is coupledto two additionalgears;(i)the first is directly attached

to a 30 VDC motor througha ferrofluidicfeedthrough,and (ii)
add tensionto the rubberdrive belt that connectsall three.
the externalside of the flange,rvherea seriesof gearsand

the secondis usedto

The motor is mountedto

bicyclechainprovidethenecessarymechanicalflexibility. The motor is connectedto a power
supplythrougha
voltagedivider that providescoursecontrolof the rotational
speed.
2.4.2 Liquid Surface and Characterization
The primary reasonfor the riquid wheerassembryis to expose
a renewed
Iiquid surfacefor the incidentmolecularbeam. The ways inwhich thesesurfaces

have been characterizedin previousstudiesinvolves neutronand Ar* scatteringto

show that the liquid is relativelyclean. In additionto thesetechniques,simplesurface
tensionmeasurements
are an easyand reliablemethodto characterizetheimpurities
on the liquid. The generaltechniqueinvolvesthe wilhelmyplatemethodthat

extractsthe surfacetensionfrom the measuredforce required
to pull4

a paperplate out-a87of the liquid. Equipmentfor thesetypesof experiments
has beendeveropedby
companiesIike NIMA that specializein Langmuir-Blodgetr(LB)

troughsfor precise

surfacepressuremeasurements.
The honorsgeneralchemistrycourseat CU includes
a lab that exploressurfacetensionwith experimentsthat involvetroughsand sensors.
which havebeenbuirt by the machineshop incristol chemistry. Fortunately,theshop built an extrasetupthat was then carefullymoved

to 82l 7 in JlLAto help

characterize
the liquid surfaces.The equipmentincludesa

LB trough and sensorthat

are computercontrolledthrougha serialconnection
with NIMA interfaceelectronics.
Detaileduserinstructionshave beenprovided,so the generaroperatingprocedureis
simply outlinedin the following few paragraphs.
The surfacetensionmeasurements
are madewitha sensorthat is designedto

measureforce on a lever arm connectedto an electricmotor.
adjustthe currentto the motor to keepthe positionofServoloop electronics

the leverarm constantwith

increasingor decreasingforces. The key to themeasurement
is that currentthroughthe motor is directly proportionalto the force on

the leverarm. The NIMA interface

convertsthis currentto a voltagethat can be readthrougha serialconnectionon acomputer' Basedon this scheme,the sensoris
plate by the surface.To do this, the sensoris

usedto measurethe force exertedon afirst calibratedwith a seriesof known

weightsthat rangefrom r - 100grams. once caribrated,a creanpaperprate(r0 mm
wide, 0.25 mm thick) is attachedto the leverarm,

at which point it is dippedinto theliquid to the point wherethe paperis thoroughrywetted. The forceson the prateincludegravity and surfacetensionpuiling down
andGravitationalforces remain constantthroughoutthebuoyancypushingup.

measurement,which simply leads

i-L

Wr

11-

---/

88

u

.t

{.)
,1)
<)

.t

l i t tt
tllllc

(s)

(rt l

(J

()
a,

5rr
Ji t

jl

a
C)

Iir

()
(J

l1r
!

a

l ( r lr

IStr

T" (K)
Figure 2.33 Surfacetensionof PFPEis measured
asa functionof temperature.

89
to a constantvortageoffsetthat may be zeroedby the computerprogram. The
competitionbetweenbuoyancyand surface
tensionare monitoredas atime as the plateis puiledfrom the riquid.

functionof

once thepaperis out of the riquid,theonly force is surfacetensionon the perimeterof the pratesincethe buoyancyterm
dropsto zero' The force is recordedas plateisfinally puiled free of the liquid, where

the differenceis equarto2 x width x thicknessx

surfacetension. Therefore,thedimensionsof the plate leaddirectly to thesurfacetension.
surfacetensionare iilustratedin Figure2.33(top) for pFpE

Sampletime tracesof theat z9gK. The dottedlines correspondto averagedifferencebetween
stepsthat result from the repeatedmeasurementswith the sameprate. The temperaturedependenceof the surface
tensionis illustratedin Figure2.33 (bottom),wherethe dataalso includesthe value
from the Krytox specificationsheetfrom

Dupont. In generar,an ariquotof riquidistransferredto a cleanPetri dish for analysis
becausedirectaccessto the reservoirisatechnicalchallenge.Measurements
have beenmadebeforeand after many
experimentsto ensurethe setupguarantees
a freshlyrenewedand cleanliquidsurface.

2.4.3 SAM chamber
Furtherscatteringexperimentshave been
prannedto exproreotherrealmsof
gas-surface
interactions.The first seriesof experiments
involvesurfacesof self-

assembledmonolayers(SAMs) that are generatedfrom
alkanethiols and cleangold
substrates.To help ensurethe cleanlinessof a sample,a vacuumchamberhas been
built with turbomorecular
pumpsthat aredesignedto eriminatethe presenceoff90

Figure2.34 Gas* SAMsscatteringchamber
includesa differentially-pumped
sourcevolumewithinthemainscatterins
chamber.

oils. A cartooncross-section
backgroundof thechamberis illustrated
in Figure2.34
Thechamberis basedon a modifiedsix-waycross
to showtheessential
components.
(Nor-Cal)that is constructedfrom l0" tubesof stainlesssteel.The modifications
includeadditionallengthon two armsandsideportsfor differentialpumping. The
to a baseplateon an aluminumsupportframe,which standson
chamberis attached
the groundwith rubberfeetfor vibrationisolation.The mainturbopump(Pfeiffer
TPH l20l) is alsoattachedto the bottomof the baseplatethroughan ISO 200 o-ring

TED

'-

91
seal. This pump is backedby a secondEdr,vards
E2M30 rotary vanepump throughstainlesssteelbellows. The chamberis divided into two parts,sourceand scattering,by an insertconstructedin the JILA machineshop. The sourceregioncontainsthe
pulsedvalve and skimmer,while the scatteringchamberhousesa sampleholderanc
multipassoptics. Fully assembled,
the chamberreachesa basepressureof 2 x l0-s
Torr with both pumpsoperating.Furtherdescriptionsof the componentsare outlinedin the following paragraphs.
The chamberdesignincorporates
a differentialpumping schemeto reduceto
the backgroundgasin the scatteringchamber. The pulsedvalve is mountedin one
arm of the six-way cross,which is then partitionedfrom the remainingpart of thechamberthroughthe differentialinsertillustratedin Figure2.35. The insertis
constructedfrom two main rings that are connectedby four supportrails. Theserails
also includetappedt/q-20thru holesto mount variouscomponentsinsidethis regionof the chamber.The two rings are assembledbeforebeing positionedin the mainchamber.Once inside,eachring hasfour threadedholes for 5116-18
grub screwsthat
are usedto centerand securethe ring insidethe tube. once thesescrewsare
extended,the pressureon the outsidewall locksthe differentialinsertinto place. To
sealthe insert from the rest of the chamber,a thick o-ring is wedged betweenthe
outerwall of the chamberand a bevelededgeof the inner aluminumring. The inset
in Figure2.35 illustratesthe seal,wherea secondaryouter ring is boltedto the main
insertto compressthe o-ring befweenthe fwo surfaces. This secondaryring also hasa seriesof threadedholes for additional bolts that may be usedto close the larger gaps
aroundthe ring. once setup,the o-ring sealis checkedfirst with light to visibly

92

Figure 2.35 Differential insertfor the sourcechamberincludesscaffoldingto
mount the pulsedvalve and f'lexibleskimmer plate. The insert is sealedfrom thescatteringchamberwith an oversizedo-ring and compressionring, which isillustratedin the bottominset.

L-_---:I

93
observeany gapsthat would leadto leaksbefweenthe sourceand scatterins
chambers.
The two other featuresin the sourcechamberincludemountsfor the pulse
valve and skimmer. Initial studieshave involveda secondpzT valve,which ismountedto the bracketsillustratedin Figure 2.35. Thesebracketstranslateverticallv
to increasethe rangeof incidentanglesin the scatteringexperiment.Additional
bracketshavebeenmachinedto securelymount an Even-Lavievalve if shortergas
pulsesare required. The plate at the end of the insertis attachedto the inner ring withan o-ring seal. A hole in this platemay eitherbe coveredwith a skimmeror left open
for directthroughputfor the gas from the skimmer. The skimmeris mountedon twoplatesthat provideverticaland horizontaltranslationto matchthe mobility of thePZT valve. Theseplatesare clampedtogetherand sealedby o-rings. 'I'hesource
regionof the chamberis pumpedthroughone or two sideportsthat have beenrveldedto the sidearm of the six-way cross. An Edwardsturbo pump (EXT 250) is attached
to one of the ports,rvhich is then backedby an EdwardE2M1g rotary vanemechanicalpump. A secondport providesaccessfor a secondpump if greaterspeeds
are required. This sectionof the crossis sealedwith a ISO 250 aluminumflangewith
throughputsfor gas, power, thermocoupleand ion gauges,and a vent valve that is
attachedto a tank of N2.
The scatteringchamberincludesa scaffolding structurethat is usedto mountthe sample. Detailsof this constructionare depictedin Figure2.36,which showsthe
sampleholder and support structurethat is attachedto the internal frame. The
sampleholderis designedto supporta l" x 3" gold slide,wherethe entiremount may

94

Figure 2.36 Surfacesamplesare mountedon a holder in the oppositearm of thesix-way crosssourcechamber.

95

be rotatedin l5o incrementsto changein the incidentscatteringangles. The entire
samplemay be translatedwith respectto the laserbeamto changein final scattering
angle. The laseris passedinto the chamberthroughCaFzwindows onto gold mirrors
in a multipassconfiguration.The mirrors are mountedon an adapterwith a setscrew
so that no glue introducedto the chamber. Theseadaptersfit in a standard1,,
Newport Supremaopticsmount that is attachedto the stainlesssteelside flanges.
The actuatorsfor thesemountsare controlledthroughUltra-torr feedthroughs
thathave beenweldedto the stainlesssteelside flanses.

2.5

Polarization Modulation

2.5.1 Experimentalsetup
The basiccomponentsof the spectrometerare kept in placefbr thestereodynamic
studiesin Chaptersl0 and I l. Severalmodificationsand additional
componentsare necessaryto controlthe polarizationof the laserIight. First,the50/50 beamsplitterthat normally dividesthe tight betweensignaland retbrenceInSb
detectorsis replacedwith a gold mirror so that all light is passedinto the chamber.
The reasoninvolvesthe additionallossesto the laserpower that resultfrom the
Rochonpolarizer(Karl Lambrecht,MFRVS) and photoelasticmodulator(Hinds
International,PEM-80 ZS). The positionsof thesecomponentsare illustratedin
Figure2.37. The polarizerspatiallyseparates
the two polarizationswith MgF2

96

InSb
detector

Rochonmultipass
1T

Pb-salt_/\_
fiequency /l 11

wheel

skimmerpulsedvalve

scrapsre$eryorr;til;;;',.,fulli_JAJL
Figure 2.37 Stereodynamics
are measuredwith polarizationmodulationof the laser
light and lock-in detection.

crystalsthat redirectone of the linear componentsaway fiom the initial laser
propagationvector. With the Polarizationaxis set perpendicularto the lasertable.thetransmittedlight correspondsto :8l o/oof the total power, which indicatesthe lightfiom the diode laseris ellipticallypolarizedto a degree.As a test,a secondRochon
polarizer is used in a cross-configuration,
which then attenuatesmore than 99.8% of
the light. Additional lossesincludereflectionfrom the MgF2 (n: 1.35at )": 4.2pm)
and PEM, which is constructedfrom a ZnSe crystal(n:2.43 at ), : 4.2pm). The
lossesfrom reflectionand initial polarizer lead to =43%otransmissionof the initial
power. Since the diode output is = | pW or less,the secondbeam splitter is omitted
so that higher laserpower may be usedto detectthe alignmentand orientation
sisnals.

97

Rochonpolarizer

PEM

Rochonpolarizer

InSb

PIIM
Setting
I

0.8

l

_ 5 0

(n
(n

.+

t

{l()

a

-'-- l(x)_c,/ i ( ) +
F

2(X)
.l(x)
.l_50

+(x)
-

(r(X)

2f reference
signalFigure 2.38 A photoelastic
modulator(PEM) is usedto rotatethe polarization
fromparallel(ll) to perpendicular(J-)or fiom right(RCp)to left circular(LCp). The
amplitudeof the PEM oscillationis optimizedby passingthe rotatinglight througha
secondcross-configured
Rochonpolarizer.A reference
signalfrom the pEM
controlleris usedin a standardlock-indetectionscheme.

98
The initial setupinvolvesfirst overlappingthe IR and HeNe beamthroughtwo irisesthat spanthe width of the lasertable. The beamsare then alignedinto themultipassto establishthe detectionaxis for the scatteringconfigurations.Two
additionalirisesare placedin front of the chamberto mark the laserpath into thechamber.Next, the Rochonpolarizerand PEM are mountedin betweenthe first twoirisesthat were usedto overlapthe beams. The polarizeris fixed in placeso that thepolarizationvector for the transmittedlight is perpendicularto the lasertable. Next,
the PEM is mountedso the beampassesthroughthe centerof the ZnSe crystal. The
PEM is designedto rotatethe polarizationvectorthroughstressinducedbirefringencealongone of two crystalaxes. Trvo piezoelectricbarsare gluedto oppositeedgesof
the eight-sidedcrystal. Thesebarsdefinethe primary axis of the crystal,which isthen alignedat 45'with respectto the polarizationvector from the Rochoncrystal.
Thesetwo componentstend to shift the beampath slightly so that the initialalignmentof the Herriot cell multipassis lost. To reestablishthe definedpath
throughthe chamber,turning mirrors are usedto realignthe HeNe and IR beams
throughthe two irisesthat were placedat the entranceto the multipassoptics.
The piezoelectrictransducersin the PEM are driven with a 37.5kHz
sinusoidalvoltagefrom a high voltagecontroller. When the crystaloscillates,thematerialis eitherstretchedor compressedalongthe principleaxis,which leadsto
birefringencein this direction. when the light is passedthroughthe pEM, thebirefringencecausesa phaseshift of the electricfield componentalongthe principle
axis. The magnitudeof this effect is controlledby the amplitudeof the crystal
oscillation. As the driving voltageincreasesfrom zero,the polarizationof light is99..lF''.-,e.->*-rF---+--l----+
T-I

I

P9

t o
O Q

Figure2.39 Polarization
signaldemodulator.

r00
rotatedfrom linearto eilipticar. At somepoint,the phaseshiftreaches90o,which

createsoscillatingcircularly polarizedlight at 37.5 kHz.
Eventually,the phaseshift
reachesI 80' to generatethe orthogonallinearstateat twice
the driving

frequency,i.e.

75 kHz.
The amplitudeof the PEM is set by passingthe rotatinglight
Rochonpolarizer'which may be configuredin eithera

througha second

crossor parallelconfiguration.

The setupis illustratedin Figure2.38,alongrvith sampletime

tracesfiom a parallel-

configurationcalibration. The plot shorvsthat the transmission
throughpolarizeris 100%with the pEM driver off. As the pEM
is turned
wave is observedas the oscillationamplitudedrivesthe

the2don, a75 kHz sinebirefringencein the crystal.

The rnagnitudeof the sinewave time traceincreases
withthe voltage,which is varied

by a dial on the PEM controller. The maximum amplitudeof this signalcorresponds
to linearpolarizedlight with the orlhogonalpolarization.
As the voltageincreases
pastthis value,the phasebetweenthe two polarizations
shiftsbeyond 1g0.,whichshowsup as an additional75 kHz componentin the time
trace.

Experimentswith

oscillatinglinearpolarizationsinvolvedfixing the PEM dialat 400. Calibrationof
the oscillatingcircularpolarizationsinvolvespassingthe

light througha MgF2

Babinet-Soleilcompensator(Karl Lambrecht,BSMF-13-2)beforethe 2nuRochon
polarizer' The BS compensatoractsas a Yt-waveplate
to turn circular into linear
polarization.The transmittedlight onceagainoscillates
at 75kHz,but

the maximum

signalcorresponds
to circularpolarizations.For theseexperiments,the dialreadins

was fixed at 163on the pEM controller.

-l

r0l

I 800

160"
I.+00
1200
()

I (X)O

()
a

800
600
--G-

-100

+-

20"
0"
1

6

lF - 7r Skllz
lli - 7l SIJ{zl F - 16 l) kHz
rtr.
16 9 LHz

I

I

a
()

I0

phasedial
Figure2.40 Phasecalibrationcurves.

Table 2.2 Phasedemodulatorcalibration.

Diall F ( 7 3 . 8k l l z )2 F ( 7 3 . 8k H z )l F ( 3 6 . ek H z )2F (36.9kHz)

0 .0 0r.00
2.00
3 .0 0
4 .0 0
5 .0 0
6 .0 0
7.00
8.00
9 .0 0
10.00

6.650

5.32"

1.06'

0.80"

3 0 .8 4 "
5 7 .9 5 "80.06"
l I 1.65"
138.23'160.56"I8 . 6 1 "15.95"

10.63"

36.69'

28.44'

18.61'

54.76"

42.53'

27.65'

74.70'

58.48"

37.22'

95.70'

73.10"

47.85"| 16.96"89.32"

59.01'

I33.98'102.34'71.24"

I 50.98'

119.62"82.41"

162.15"134.24"93.04"

146.20'98.35'

102-/'

T.ikllz_--]: ) kllztrr
ntr.ttlrtlatrorr

absorption

higlrpassdernodulate

lor.vpasstinre .--_>

Figure 2.41

Sampleabsorption
and demodulated
sisnals.

2.5.2 Signal Processingand Analysis
Voltagesfrom the signalInSb detectorare split so that one fractionis sent
directlyto the computerA/D (PCI-61l0E, Channel0), while the other is demodulated
in the circuit outlinedin Figure2.39. The home-builtsignaldemodulatoris designed
as a standardlock-in that rectifiesthe signalvoltageby heterodyningit againsta
referencesignal from the PEM controller box. The correct phasebetweenthe signal
and referencevoltagesis set in the follorvingway. Light with oscillating

,

t

/

103

t0"
8-polelow passlJesselfilter

(.)
a

a
O

l0-r

l-

0.)

t0-:
c.)
l-

4-polehighpass
Bcssclfiltcr

l0-r
0 l

t 0

1 00

1 0 00(kHz)
frequency
Figure2.42 Bandwidth of the active high and low passBesselfllters in thepolarizationsignal demodulator.

polarizationsfiom the PEM is once again passedthrough a cross-configuredRochon
polarizerto generatea sine-waveat the correctfrequency. This signal is passedinto
the demodulatorcircuit, at which point the phasecontrol is usedto null the output.
The phaseis recorded,and then shifted 90ousing interpolatedvaluesfrom a known
calibration. Values for this curve are plotted in Figure 2.40 and listed in Table 2.2 forthe various f and 2f settings.
Output voltagesfrom the signal demodulatorare sentto Channel I of the A/D
to be storedin parallelwith the absorbancetraces. A simulatedtime tracesareplotted in Figure 2.41 to show the sequenceof signal manipulationthrough thesil\'il--

104
experiment.The absorptionsignalfor eachquantumstatetransitioncorrespondsto+ AL)12.This signal is filteredwith a 4-pole highthe value (Ancp+ Aysp)/2or (A11passBesselfilter to isolatethe modulatedcomponentsof the signal. The bandwidthof this filter is plottedin Figure2.42. Oncefiltered,the signalis rectifiedand thenpassedthroughan 8-polelorv passBesselfilter to removethe 2f component.The
resultingtime traceof the demodulatedsignal is proportionalto eitherAncp- A16por
Att- Ar. Appropriatecharacterizationof the circuit providesthe necessary
gain
factorsthat are usedto turn the raw voltagesinto differentialabsorbances.The data isanalyzedin a programcalledalignment.c.One additionalcornmentaboutthe signals
is well-explainedin Miles Weida'sthesis. Straybirefringencein the optical pathleadsto spurioussignalsthat can alter the magnitude,and eventhe sign of thedemodulatedsignal. Theseeffectsare taken into accountwithin the analysisprogram
sincethe absorbanceis recordedin parallelwith the demodulatedsignal.

2.6

Data collectionand analysis

2.6.1 Experimental Interface
The experimentis computer-controlled
throughtwo NationalInstrumentsdata
acquisitioncardsthat providean interfacebetweenhardwareand software. A Cbasedprogramcalledscan4.chasbeenwritten in the CVI programmingplatformto
run the experimentthroughthesefwo dataacquisitionboards.The sourcecode for
scan4.cis includedin AppendixD. The DAQ cardsare equippedwith timing
functions that are usedto control standardanalogand digital input/outputto the

- | I

l0s

6
p6
d

\

O

d

>

.
6

l

l
t

l

t
l

.--1.

{ l-

\ d o Ai" llFigure2.43 Remote-controlled
shutterelectronics.

- - i l , - -r06
experiment.The first DAQ card(PCI-61l0E)includesfour l2-bit analog-ro-digiral(A/D) convertersthat sampleinput voltages(-10 to +10 V) at a

maximum rateof 5

MHz over the four channels.one of theseA/Ds is usedto recordthe voltages
fromthe signalsubtractionelectronicsand. ifnecessary,a secondchannelrecords
voltages'l'hefrom the polarizationdemodulatorin the stereodynamics
studies.

boardalso

includestwo l6-bit analogoutputs,one of lvhich is usedto scanthe laser
after

a

transientabsorptionsignalhas beenrecordedat a particularfrequency.
In addition,aTTL voltagefrom a generalcounteron the boardis usedto trigger the pulsed
valve.
The secondDAQ board(pcl-6025E) is usedto performa varietyof additional
functionsin the experiment.The card haseight A/Ds that samplevoltages(_10+10 V) at200 kHz with l2-bit resolution.Thesechannelsaretousedto recordsignals

from the etalonand referencegascell detectors,DC powersfrom thesignaland

referenceInSb detectors,and stagnationpressurein the pulsedvalve.
one of theanalogotttputson the 60258card alsotriggersa remote-controlledlasershutterthat

isusedat the end of the scanto determinethe laserpower levelson eachdetectorfor theconversionto absorbance.The schematicfor the shuttercontrolleris printed
inFigure2.43
Beforeeachexperimentstarts,initial efforts includeIaseralignment,
surfacepreparation,and initiationof the pulsevalve. The laserbeamis aligned
throughthe
multipasswhile the vacuumis ventedto atmosphere.once the valve andliquidsurfacehave been setup,the flangesare replacedand the systemis roughedbyasecondmechanicalpump beforethe gatevalve to the diffusionpump is
opened.
the pressurecontinuesto drop,the multipassis realignedsincethe beam
tendstoAs

t07

6 Channel0

Adive SignalChannels
f Channel1
f Channel2

ti Plott^ Plott^ plot

Scan Parametersl- Channel3
f plotPointstoAverage:F-

PulseValveFlate:F1

S i g n a lC h a n n e G
l a i nS e t t i n g s- P C I 6 11 0
Channel0
C

o

m

p

u

t

Channel1eAdditionat
tTrt.oo

r

5

0

v

Acquisition
Delay[m$];[fi0Channel2f,hannel3
5 0 v v ' Il v

lTnorSt"pSireit"rlil :|ffiScanRateFactor

h rro

Frro

:F00

- PCI6025
SignalChannelGainSettings
Etalonl

RefGasCell 5ig lB Power Fef lF Power

v

l

v

l

v

l

v

l

Pressurev

S i g n a l & B a s e l i n eG a t e s
Baselinel

Baseline2 Signal

COZTransition
6ano

vl I

start :m-

:mo

J6oo

BranchllT- P(-tl/F(11t-enen
lpoo-

:fzro

:lroo

J

Figure 2.44

;F-

Data acquisitioninterfacepanelto selectscanparamcters.

move as the flangesf'lexand shift the mirrors insidethe vacuum. At this point, thepurgebox is sealedand flushedwith N: from a lA tank. As COz is slowly driven
from the spectrometer,the scanningprogram is startedto initialize the pulsedvalve.
Flux through the pinhole is calculatedfrom the measureddrop in pressureover timeas the valve after the regulatorin the gas manifold is closed. Thesevaluesare usedto
calculatean effective areaof the pinhole,which is then comparedto the actual
physicalarea. Close agreementbetweenthe two values is expectedwhen the valve isoperatingcorrectly. At this point. the liquid on the wheel is visually inspected

108

l.+
l.:

incidcn
J:O

t 0
C)
(J

0.8
0.6
0.1

a

0l

background
J :36

0.0
- 0 . 2 basclinc signal
1000

1500

time ( FNec)
Figure 2.45 Sampletime profilesfbr incident,scattered,and backgroundsignalsof
CO2 scatteringfiom PFPE liquid surtace.

through the plexiglassflange to make certainthe coverageand consistencyare
satisf'actorybefore startingthe experiment.
With sconL c running, the laseris tuned to a rovibrationaltransitionandparked= 500 - 1000 MHz below the centerlinefrequency. The user interface
includesthe Data Setupwindow illustratedin Figure2.44. The panelallows the user
to input various control parametersthat are usedto scanand recorddata. Most ofthesevaluesare recordedin the headerof eachfile as it is savedatthe end of thescan. The interfacerequiresthe userto choosebaselineand signal gatesthat are used
to integratethe scatteredCO: signal. Thesevaluesare simply usedto visually present

109

Table2.3 Header information for vscanda/e._r_rx
files.

C
0
I
2
3
A
+

6
7
B
9
l0
ll
12
t-1

t4
l5
l6
t7
t8
t9
20
21
22
a1
L-)

24
25
26

F77
I
2
J

4
5
6
7
8
9
l0
1l
t2
I J

14
l5
t6
17
l8
l9
20
2l
22
z)

24
25
26
27
28

Variable
max_zahl
maxspurrrPulseAve/B I Start/B I Length/SigStart/SigLength/B2StartlB2LengIhu/PulseRate
icounterstagpress
StoreHourStoreMinuteStoreTimeStoreDateiBandiBranchiJStatesCompGain
dOtherGain
RefWgau
WIor (refWlor (sig)
Wgau
Wgau
IntAbsl (sig)
IntAbsl (rel)Programscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cscan4.cvoigta.f
voigta.f
voigta.f
voigtb.f
voigtb.f
voigtb.f
voigtb.f

Notes
Numberof transmission
fringesNumberol traces
Pulsesaveragedper frequencystep
Startof baselineI gate
Length of baselineI gate
Startof signalgate
Length of signalgate
Startof baseline2 gate
Lengthof baseline2 gate
Repetitionrate for pulsedvalve
Data pointsper scan
Stagnationpressure
FIourofscan
MinLrteof scanTime of scan
Date of scan

I (oooo),
2 (otr"o),3 (ol'ro)P branch(-l), R branch(+l)
RotationalState
Softwareselectedgain
Othergain lactors
298 K Dopplerwidth (MHZ)
Lorentizianline width (ref peak)
Lorentizianline width (latesig peak)
SignalDoppler Width (ref Av1o,)
SignalDoppler Width (sig Av1o.)
Signalcolumn density(A*MHz)
Referencecolumn density(A+MHz)uHeadervariables28-39 are filled by othe progru*r, s.rcha, voigt2.f,vv3.f,and
other line shapefitting algorithms.

110
the data in real time as the laseris scannedover the transition.The programstoresthe entiretime profile (2 ms) at eachfrequencyso that integratedsignalsfrom various
time ,uvindows
may be analyzedafterrvard.Sampletime tracesare plottedin Figure
2.45 to show typical incidentbeam,scatteredco2, and backgroundsignalsat thecenterlinefrequency.The plot also includestypical rvindowsfor the signalgate,
which integratessomefractionof the rising edgeof the scatteredpulse. The scanning
programplots absorptionversusfrequencyin a paneldisplayedin Figure2.46,which
also showsthe etalonfringes,referencegasabsorption,and DC power on the InSb
detector.At the end ofthe scan,the beamis blockedand 50 pointsare recordedto
provideoff-setvalueson eachdetector.Data is then storedin a format listedin thesourcecode commentsin Appendix D, as well as Table 2.3. Absorptionprofilesare
measuredthreetimes eachfor the groundstateand hotbandtransitionsthat are
acccssible
with the laserdiode.

2.6.2 Analysis
Frequency-dependent
absorptionprofilesare generatedfrom the twodimensionaldataarray in a programcalledgate.c. Signaland baseline gatesare
chosenfrom initial analysisof the centerlinetime profilesfor a seriesof groundstatetransitions.In general,the shapeof the time profile dependson the specificrotational
statesincethe translationaldistributionsare sensitiveto the quantum-state.Signals
from high J-statesarrive earlier than thosefrom lorv J, which createssomeconfusion
about the appropriateintegrationwindorvs. Profiles for a seriesof J-statesare plottedto revealthe spreadin arrivaltimes,at rvhichpoint the time tracefrom a middle J-

lll

f

/
/
400-

./
200-wi'*,ia\r+trA.^44"\4,nrrM"]4w

|

l

\
l

/ ' ttf,.
IWW1n &,*^r+

0-r00-,
0

1024

8dl

J30

Sirufrov

naa*o [: m

v

H.rd€

t!2 i

Flt"t*-lFigure2.46 Samplefrequencytrace with scan4.cillustratesabsorptionprofiles fbr
COz in the scatteredflux and ref-erencecell. along with the transmission
fringesfrom
the etalon.

stateis selectedto representthe average. From the peak absorbance,the signal
integrationwindow is selectedto encompass2575Yoof the rising edgefor thistransition. Such arl hoc criteriahas been rigorously testedby integratingthe time
profiles in incrementalstepsof 100 psecto show the eff'ectslead to negligible
differencesin line shapesand relativepopulationdistributions. This routineprovides
a reasonableapproachto accountfor diff'erencesin each experimentalconfiguration.
The rising edge of the transientscatteredsignal is selectedto minimize thecontaminationby backgroundC02, which often lead to relatively large absorbancesignalsbecausethe laseris passedacrossthe width of the entire chambermultiple

G-"-

1t2
times in the Herriot cell. Thesefiles are savedas scanDATEb.x.rx,
where DATE is a4-digit rnonth/dayand xxx is a file listing from 001 to 999. In additionro these
profiles,signalsat the end of the time traceare integratedto show the resulting
Doppler profilesleadto room temperaturerotationaland translationaldistributions.
Thesefiles are savedas sconDATEa.xxr.
Once absorptionprofileshave beengenerated,
the analysiscontinuesby firstinterpolatingbetweenthe transmissionfringesto provideaccuratefrequencysteps
over the courseof the laserscan. Files are processedby a programcalledet.f,whichusesfree spectralrangeof the etaloncavity (FSR : 250.51MHz) in the interpolation.
Afterrvard,thesefiles are fit to a voigt line shapefunction,where "a" and "b" files
are processedby programsvoigta.fandvoigtb.f,respectively.Theseroutinesextract
Gaussianand Lorentzianwidths,and also calculatethe areaundereachprofile. Eachparameteris storedin the headerof the outputfile, which are listed in Table 2.3 forvscandoteb.xxx
files. The integratedareasare tabulatedas column integrated
densitiesfor eachtransition.Thesevaluesarethen fit to the two-temperature
Boltzmann model in a program called logalpha.f. Extractedvalues of a and T,ot(lS)
are then usedto refit the absorptionprofilesrvith a two-temperatureline shapein aprogram called voigt2.f. The final step in the Dopplerimetryanalysisrequiresmockline shapesthat are generatedfrom the sum of TD and IS components.Thesecurves
are calculatedin a scriptcalled2temp.pl,which are then fit to a singleGaussiancurvein gauss.f. Furtherdetailsof eachprogram are included in the commentsin Appendix
E.

113
Referencesfor Chapter II
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D. R. Miller, in Atomic anclMolecular BeamMethocls,editedby G. Scoles(OxfordUniversityPress,Nerv York, 1988),Vol. l, pp. 14.
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