CHAPTER 2
Experiment

2.1

Introduction
A variety of experimentsare performedfor this thesis. For the studiesof

clusterformationdynamicsand largeamplitudemotion in weakly boundcomplexes,
the speciesof interestare formedin a pulsedslit supersonicexpansionand probedvia
direct absorptionof a tunablePb-saltdiode laser. Both CO2 complexesin the vj
asymmetricstretchregion (4.3 ltm) and H2O complexesin the v2 bendregion (6.2 1tm)
are investigatedusing high resolutionspectroscopytechniques. For the studiesof
rotationalalignmentin a supersonic
expansion,CO2 is expandedin eithera pinholeor
slit pulsedsupersonic
expanslon,and the alignmentis detectedvia the polarization
dependentdirect absorptionof a tunablePb-saltdiode laserin the vj asymmetricstretch
region. Finally, for the studyof CO2sublimationdynamicsfrom bulk CO2,thin films
are grown in vacuumat77 K, warmedto sometemperaturefrom 90 to I 15 K, and thenascenttranslational,
vibrational,and rotationaldistributionsof the sublimingspecies
are measuredvia direct absorptionin the Vj aslmmetric stretchregion. In addition,
polarizationmodulationtechniquesare usedto probe for rotationalalignmentof thesublimingCO2.
Although particularsfor eachexperimentare briefly coveredin eachchapter,eachexperimenthaselementsthat warrantfurther description. Clearly the heartof all
the experimentsperformedhereinis the infrareddiodelaserspectrometershownin Fig.
2.1, which will be discussedin detail. Then additionsand modificationsunique roparticularstudieswill be covered.suchas gashandlingand supersonic
expansions.
the

I7

signal detector

40 cm modifieclHerriott cell
( I 0-20 passes)
rcference
detector
50/50 beam
splitterdiagnostic
dctcctors
conlbcal Fabry-Pcrot

rel-erenccgas

LNr cooled

collimating
opticsandspatialfilter

P b - s a l td i o d e l a s c r

t:

\

NWI
\-"p.*,l<--.--->
t
Y

ln-_

wl
\Fr5i:'-

Fig. 2.1: Infrared diode laser spectrometer. This is the general form of the
spec.trometer;
modificationsand additionsfor the variousexperimentsare discussedin
detail below.

18

detectionof alignmentvia the polarizationmodulationtechnique,and the cryogenic
growth and sublimationof COz thin films. Finally, dataacquisitionand analysisareconsidered.

2.2 Pb-salt diode laser
Pb-saltdiode lasersprovide a relativelyinexpensiveand efficient way togeneratetunablelaserlight over the 3 to 30 pm wavelengthrange. In additionto being
easyto use,the frequencyof theselaserscan be rapidly modulatedat ratesof up to a
GHz, making them ideal for the detectionof cw absorptions,suchas the sublimingflux
from a thin film. The narrowline width (S 15 MHz) is suitedfor Dopplerlimited highresolutionspectroscopy,
and the intrinsicallylow amplitudenoise(< t0-3on a 200 prs
time scale)leadsto a routine detectionsensitivityof l}u l^lHz.
b o o k s l - 3a n d r e v i e w s 4 ' o5 n n e a r( L : 3

There are several

p m ) a n dm i d ( 3 S L : 3 0 , u m ) i n f r a r e ddiodes,and an overviewof the principlesof diode laseroperationfrom thesetextsis
presentedin the next section. Then the actualoperationof the diode is considered.

2.2.1 Principlesof operation
The elementsof a laserfor a Pb-saltdiode, i.e., an optical gain medium andfeed-backcavity, are providedby a piece of semiconductorwith cleavedends. As
shown in Fig. 2.2, the Fermi level for a semiconductorlies midway betweenthe
valenceand conductionbands. The band gap (Egrp)is suchthat the majority of theelectronsdo not haveenoughthermalenergyto accessthe conductionband. However,a photonwith /zv , Esopcan excitean electronto the conductionband,leavingbehinda
hole. The oppositeprocesscan occur, namely, combinationof an electronin the
conductionbandwith a hole,to producea photonwith frequencyv =Egadh. Given a

T9O electronoconductionband

Fermrtevet-].-\.^l_-|o o o o o o o o o ovalence banda) semiconductor

stimulated
emissionholeooooaooooo\-4.-r\>b a n dg a poooooooooo
sponlaneousemissionb) semiconductor
withpopulation
inversion

Fig. 2.2: Semiconductor
as an optical gain medium. If a population inversion can beformed, electron/holepairs can be stimulated to recombine, releasing a photon of
frequencyv =Erar/h.

populationinversionof holesand electrons,electronscan be stimulatedto recombinewith holesby a photon,creatingan opticalgainmedium.
To createa populationinversionin a semiconductor,
a p-n diodejunction6isused,as shown in Fig. 2.3. The semiconductor
is dopedwith an impurity which
typically hasone too many or too few valenceelectronsto bond completelywith thesemiconductor
lattice. This createseitheran electronthat is very easyto removefrom
the impurity (known as an n-type impurity). or a "hole" for a neighboringlatticeelectronto occupy (known as a p-type impurity). Becauseit takes relatively little
energy comparedto the semiconductorband gap to excite one of these impurity
electronsto the conductionband,or to move valencebandelectronsinto theseholes,
the impurity electronsand holescan diffuse throughthe lattice from thermalexcitation
alone. Also, this movesthe Fermi level so that it is just below the conductionbandfor
an n-type material,andjust abovethe valenceband for a p-type. As shown in Fig.
2.3a,whena junction is formedbetweenan n and p-typematerial,holesand electrons
diffuse into thejunction and recombine.When this occurs,the initially neurralimpurity
gainsa charge. This processcontinuesuntil the net chargesacrossthejunction from20a)p-typedopant

depletionregionl
activearea

5 t o 1 0p m

p-typeI oeotetion)

I reqion"n-typep-nLUncuon

end viewof a homostructure
diode

b)

=l

p-rypeFermilevel

=n"o
{

\-

Y*
holes

forward bias

n-rype\regionwithbothelectronsand holes

distance

Fig.2.3: a) ,P-njunction of a homostructure
diode laser. Forwardbiasingof thejunction providesthe electron/holepairsnecessary
to createa populationinvelsionin
the semiconductor.Electricalcontactto provide a forward bias is madedirectly above
the activearea,and at the baseof the diode. b) Fermi diagramsfor a p-n junction. Fora dopedsemiconductor,
thermalenergyplaceselectronsin the conductionband andholes in the valence b.and. Applying a forward bias to the junction brings theelectron/holepairs into the sameregion,wherethey can recombineto form photons.

thermal recombinationcreatea large enoughpotential such that furlher electronsand
holes do not possessenough energy to diffuse into the junction. This sets up a
depletionregion with a voltagedrop acrossit.

2I

Another way to representthe p-n junction is with a Fermi-diagram,shown in
Fig. 2.3b. The processof thermalrecombinationleadingto the depletionregion isequivalentto the n and p-typematerialscomingto chemicalequilibrium. This occurs
when the Fermi level is continuousacrossthe junction. As can be seen,the voltage
formed acrossthe depletionreglon ls an energy"step" in the Fermi diagramthat keeps
electronsin the conductionband and holes in the valenceband from occupyingthe
sameregion of spacein the semiconductor.If, however,an externalvoltageis applied
to the p-n junction,i.e.,it is forwardbiased,the voltageacrossthe depletionregioncan
be overcome,and electronsand holes are forced into the same region of thesemiconductor.They can then recombine,emitting a photon. The externalbiasing
continuallyinjectsmore electronsand holes into this "active region," providing the
populationinversionnecessary
for lasing.
The simplestdiodelasersare basedupon this p-n junction for the opticalgain
medium,and areknown as homojunction,or homostructure
lasers.The 6.2 pm diode(LaserPhotonics,model L5621-1600,serialnumber5061-07)usedto probethe v2
bend of H2O in the presentstudy is a homostructure
diode,and a detail of the active
area is shown in Fig. 2.3a. In order to provide the other ingredientnecessaryfor
lasing, i.e., optical feedback,the ends of the diode are cleaved. Since the index ofrefractionfor Pb-saltsemiconductors
is = 5 at 6 pm, this providesa mirror with = 44Vo
reflectance.The cavity thus formedis approximately250 pm long, and is definedto be
5 to 10,umwide by etchingtwo groovesthroughthe homojunction.The thicknessof
the activeregioncan vary over a few tenthsof a pm, and is not as well definedsincethejunction is formedby diffusion of an impurity throughthe semiconductor.
At this point it is worthwhileto discussthe actualsemiconductormaterialsthe
diodes are made from. It is important to rememberthat the p-n junction simply
provides a meansto createa population inversion, and that the material the diode is22basedupon must be an intrinsic semiconductor.Additionally, it must a direct band gap
semiconductor,such that transitionsfrom the maximum of the valenceband to theminimum of the conductionbanddo not rnvolvea changein the momentumvectork of
the electron. In the nearinfrared,the binary semiconductorsformed from elementsin
columnsIII and V of the periodic table satisfythis criterion,while for wavelengths
longer than 3 pm the binary complexesSnTe, PbTe, PbS, PbSe,and SnSe are all
directbandgap semiconductors.
Moreover,thesecomplexesall form a cubic rock salt
structure.so it is possibleto grow ternary complexessuch as Pb11_";Sn"Teor
PbSn11-";Se"that havebandgapsintermediate
to the binarycomplexes.In this way theband gap can be "tuned"by varyingthe composition.For particularcompositionsof
Pblr-";SnrTeand Pb11-r;Sn"Se
the band gap actuallygoesto zero,so in principleit ispossibleto make semiconductorlasersfor arbitrarily long wavelengths,though inpracticewavelengthsonly out to 34 1tmhave been observed.l The homostructure
diode usedin the H2O studiesis basedupon the ternarysemiconductorPbSll-.r)Se,t,wherex = 0.1.
The ability to vary the bandgap of thesesemiconductors
by forming ternaryor
evenquaternarycomplexesleadsto a refinementof the diodelaseroptical gain mediumthat enhancesthe efficiencyand improvesthe modequality of the beam. The second
diode laserusedin the 4.3 1tmregion to study the v3 symmetricstretchof CO2 is aheterostructurelaser(LaserPhotonics.model L5622-2100,serial number87-05). The
compositionof its activeareais Pb11-".;EurSe1l_.,.;Te_r,
wherex andy are = 0.01 Thislaseris grown usingmolecularbeamepitaxy(MBE), so the thicknessand dimensionsof the different semiconductorlayerscan be preciselydetermined.A schematicof theheterostructure
laseris shown in Fig. 2.4a. Notice that the activelayer is undoped,and
is surroundedaboveand below by an n and p-type layer. Thesecladding layers haveslightly differentcompositionsthan the activearea,and thushavea largerbandgap; the

LJ

b)

al

PbTecappinglayer

Pblr-"1Eu*SerTelr-yy: Bi
secondcladdinglayer.
p-tvpePb11-u;EuuSe11_4Te,
undoped active layer

lumf

-i
, |

Pblt-yyEurSerTelr-y;: Tl
firstciaddinglayer

---1

I

acltve-____arean-typeelectrons

w Y\-.\-r>|=

ffiwft-end view of a heterostruclure
diodeFig. 2.4: a) Structureof a Pb-saltheterostructure
diode laser. By growing these
diodesvia MBE, the activelayeris smalland well defined. Electricalcontacris madedirectlyabovethe activeregionand at the baseof the diode. The n or p-typeimpurity
fo.t,.lglt layer is listed after the colon. The substrateand cappinglayersare also dopedwith Tl and Bi, respectively.b) Fermi diagramof a forward-biasedheterostructure
diode. Becausethe p and n-typecladdinglayershavelargerbandgapsthanthe active
layer,the electron/holepairs areconfinedto a well definedregionfor recombination.

Fermi diagramfor the diode underforward bias is in Fig. 2.4b. With a heterostructure,
the electronsand holes are effectively confined to a well defined region forrecombination.In addition,the claddinglayershave a different index of refraction,
leadingto a well definedwave guidefor the radiation.
To startthe lasing in a diode,it is necessaryfor the round trip cavity gain inphotonsdue to stimulatedemissionto be greaterthan the losses.Photonsare lost due
to reabsorptionby the semiconductor,movementout of the activeregion,or the - 56Vo
transmissionat the cleavedends. In addition,electron/holepairs injectedinto the active
areacan recombinenonradiatively.This type of loss must be compensatedby a greater
injection current. Thereare two main nonradiativeloss mechanisms:the first is due toelectron/holerecombinationat lattice defect sites,and the secondis due to Auger
processes,which is the dominant loss mechanismfor Pb-saltdiodes^l The Auger
processinvolvesexcitinga secondelectronin the conductionbandto a higherenergy

24level during the primary recombination.or evenexcitationof a lower energyelectronin
the valencebandto a hole. This processusuallyinvolve a changein the momentumkof the electronsand is often phononassisted.The Auger processtendsto becomemore
importantfor smallerband gapsand higher temperatures.Therefore,it is a far greaterloss mechanismfor the Pb-saltdiodesthan the nearinfrareddiodes.and is the reason
that the Pb-saltdiodesmustbe cooledbelow 120 K beforelasingis observed.This isachievedin the presentstudy by mounting the diodesin a vacuum sealeddewar inthermalcontactwith a liquid N2 reservoir(LaserPhotonics,model L5136).
When the injectioncurrentis largeenoughthat radiativerecombinationleadsto
an optical field that can reproduceitself via stimulatedemissionfor eachround trip
throughthe cavity, the lasingthresholdis reached.For the diodesusedin the present
study,the thresholdcurrentrisesfrom approximately50 mA at 80 K, to over 200 mA
for temperaturesin excessof 110K. The principlereasonfor this rise is the increasing
importanceof the Auger processat higher temperatures,which leads to greater
nonradiativeloss of electron/holepairs. Althoughthesecurrentsmay not seemlarge,
when divided by the actualactiveareaof the diode they translateinto currentsof 20
kA./cm2,highlightingthe densityof electronftrole
pairs neededfor lasing.
When considering the lasing properties of the different diodes, theheterostructure
laserstend to be preferableto the homostructurelasersfor several
reasons. First, the spatialmode quality of the laser beam is improved by the welldefinedactiveregion. In the presentstudy,it is much more difficult to collimateand
focus the beam from the homostructurelaser than from the heterostructurelaser.
Second,concentrationof the electronftrolerecombinationto the activeregion enhances
the efficiency. Thresholdcurrentsfor lasing are lower, and the heterostructurelasers
thereforeoperateat higher temperatures.Lastly, the heterostructure
laserstend to havebetterfrequencymode structure,often lasing in a singlemode (>95Vopower in one

frequencymode) over a wide range of currents and temperatures.However, thehomostructurelasers are easier to produce, since the p-n junction is formed bydiffusion, which is a simplermanufacturingprocessthan MBE. Lasersfor frequencies
lessthan = 1600cm-l are only avatlableas homostructuresfrom LaserPhotonics,theonly makerof Pb-saltdiodesin the U.S. This is likely to changein the next few years,
though,as the demandincreasesfor diodesthat operateat highertemperatures.

2.2.2 Frequencytuningand noisecharacteristics
There are two main ways to tune the frequencyof the Pb-saltdiode lasers.
Coarsetuning is achievedby changingthe temperatureof the diode. For the Pb-salt
semiconductors,
the bandgap increaseswith temperatureat a rate of 3 - 4 cm-l/K. It
shouldbe notedthat this is an intrinsicpropertyof the semiconductormaterial,and is
not due to the p-n junction. Also, this is oppositethan for nearinfrareddiodes; theband gap of the III-V semiconductors
decreoseswith increasingtemperature.This
dependenceof the band gap on temperaturecan be exploited to tune the diode indiscretestepsover nearly 100cm-1. To do this,the diodesare first cooledto liquid N2
temperaturesin a dewar,and thenresistrvelyheatedover the rangeof 80 to 120K. At
temperaturesgreaterthan 120 K, the Auger processesdominatethe electron/hole
recombination,and lasingis not observed.
To achievethe level of frequency stability necessaryfor Doppler limited
spectroscopy,
frequencydrifts of the diode shouldbe lessthan = 10 MHz over a few
seconds.Therefore,the temperatureservomust be able to achievea set point from 80
to 120 K, and be stableto 0.1 mK on a time scaleof seconds.The temperatureservo(Laser Photonics,model SP5720)uses the voltage drop acrossa silicon diode to
determinethe temperature,and a 13 f) heaterattachedto a plate in contactwith thediodesas well as the liquid N2 reseruoirvia a cold finger to control the temperature.

26This proportionaland integralservoloop hasa specifiedstabilityof + 0.001K over l0min, and it is found that the servois stableenoughto averagerepeatedsweepsover a
single 75 MHz full width half maximum (FWHM) absorptionprofile taken over a l0
secondintervalwithout any discerniblebroadeningof the line shape.
Fine tuningof the diodelaseris achievedby varyingthe current. This hastwo
effects: the first is a small amountof resistiveheatingin the activeareathat temperaturetunesthe band gap and changesthe index of refraction,which is also temperaturedependent.The secondeffect is a changein the index of refractionof the activearea
due to the changingchargedensity"It is the changingindex of refractionthat leadsto
continuoustuningof the diodefrequencyby effectivelychangingthe cavitylength; thisp r o c e s sw i l l b e c o n s i d e r e di n d e t a i l i n S e c . 2 . 2 . 5 . F o r t h e p r e s e n td i s c u s s i o n ,however,the typicalcurrenttuningrateof a Pb-saltdiodeis = I MHz/1tA. Therefbre,
the currentsupplyshouldbe variableand ableto supplyup to 500 mA (the maximum
currentfor mostMBE diodes),plus havecurrentnoiseof well under I pA in a l0 kHz
bandwidth(for fast frequencysweeps)to preventbroadeningof the laserlinewidth.
This is actually a seriousissuefor diode laser spectroscopy.For example,the best
currentsupply sold by Laser Photonicshas a statedcurrentnoise of 15

1tArms in alOkHz bandwidth-This leadsto additionalfrequencynoiseon the diodelaserof =l5

MHz rms' exceedingthe 10 to 20 MHz Doppler widths of most speciesin rhe
supersonicexpansion.Therefore,two currentsupplieshavebeenusedin the present
study. The first is a modificationof a circuit designedby the Hall labs,a,5
and thediagramis shown in Fig. 2.5. This supply can provide up to 300 mA, with currenr
noiseof 160nA rms in a 10 kHz bandwidth.More recently,a batterypowered
supplywas purchasedfrom ILX Lightwave (model LDX-3620), which can drivemA with a statedcurrentnoiseof 500 nA rms in the samebandwidth.

up to 500

27

,-i
a

C)
a'"!
Ft
,L<
(r

bo
a

11

Ch
F{

U)

L

tr
(n
O
.o

a
I

O
(-

\n
ol
fr

28
Even afterthe stabilityof the temperatureservoand currentsupplyis optimized,
diodefrequencynoiseis still observedon the diode laserlight. For the heterostructureused in the CO2 studies,the linewidth and line shapeof the frequency noise isdeterminedby measuringroom temperatureCO2 line shapesin a staticcell. The
pressurein the cell is - 0.1 torr, so pressurebroadeningis negligibleand the measured
line shapeshouldbe due to Dopplerbroadeningalone,i.e.. Gaussianwith a FWHM
consistentwith room temperature.The measuredline shapesare slightlybroaderthan
expected,however, and in a Voigt deconvolutionit is found that the additional
by a Lorentzianwith a FWHM of = l5 MHz. This
broadeningis best represented
frequencynoise is most likely not due to instrumentalfactors,but is intrinsic to thediode. Free running diode lasersoften exhibit linewidthson the order of 10 to 100
MHz. This has been attributed to small changesin the refractive index due tofluctuationin chargedensityin the activeregion.TThe intrinsicfrequencynoisecan
vary from diode to diode,and sometimesfrom mode to modefor the samediode. Each
diode must thereforebe testedwhen it is boughtto insurethat its intrinsic frequency
noiseis acceptable.
A relatively simple way to characterizethe frequencynoise of a diode is byparkingthe frequencyof the laseron the sideof an absorptionprofile or etalonfringe.
As shown in Fig. 2.6, this createsa frequencyto amplitudetransducer,and themeasuredamplitude noise can be related to the frequencynoise. In addition, an
approximatemeasureof the spectraldensityof the laserfrequencynoise can be madeusingthis samemethod. First,the laseris parkedon the sideof an absorptionprofile,and the amplitudenoiseis sentto a fast Fouriertransform(FFT) deviceto recordthe
spectraldensityof the noise. Sincethe measuredamplitudenoisespectrumcontains
noisedue to the transducedfrequencynoisep/us the amplitudenoiseof the laserlightand any additionalnoisedue to the detectora similar measurement
is madewith the29s c a n o v e r C O 2a b s o r p t i o n
t i m e t r a c eo f l a s e r
a m p l i t u d en o i s e w h e n
p a r k e do n s i d e o f
a b s o r p t i o np r o f i l e

c
o

.=
E
a
-

l<-----___+ |(g

17 MHz725 MHze t a l o nt r a c efrequency

Fig.2.6: Frequencyto amplitude noise transducer.

laserfrequencynot on the sideof the absorptionprofile. If the absorptionprofile does
not attenuatea significantfractionof the light, this providesan approximatenoisefloor
that can be subtractedfrom the first spectrumto obtain a true measurementof the
f r e q u e n c yn o i s e s p e c t r u m . S i n c e i n c o h e r e n tn o i s e a d d s i n q u a d r a t u r e i,. € . ,/1- r l / lv,,,,= \vi + vr')' . the subtractionis carriedout accordingly; the resultingfrequency
noisespectraldensityfor the heterostructure
diodeis shown rnFis.2.l.
105
N

I

E
N

I
a
a

tt av 4

o
c
c
C)
f,

I
103
10

102

10 3
f r e q u e n c y( H z )

104

105

Fig.2.7: Spectraldensityof heterostructure
diodelaserfrequency
noise.

-

30

The spectraldensityof the frequencynoise providesa diagnosticto determine
whetherlow frequencyinstrumentalcontributionssuch as 60 Hz modulationof thecurrentsupply are responsiblefor a large fractionof the frequencvnoise. As can beseen,the noisedropssmoothlywith increasingfrequencyand it appearsthat additional
componentsdue to instrumentalfactorsare not present. This smoothdrop is quite
similar to measurements
made for AlGaAs diodes,awhere the noise peaksat low
frequenciesand steadilydecreases
for frequenciesout to 107Hz. It is then relatively
flat until - l0l0 Hz, at which point relaxationoscillationsbecomeimportant(seeref.
4). Due to detectorbandwidthlimitations,the noisespectrumhereis recordedonly out
to 50 kHz, but the drop off of the noisewith increasingfrequencyis consistentwith thenear IR diode measurements.In addition.the magnitudeof the frequencynoise isconsistentwith the measuredlinewidth of = l5 MHz. We alsomentionthat the diodelaserlight hasamplitudenoiseon it as well. This will be discussedin Sec.2.3 alone
with signalsubtraction
for the detectors.

2.2.3 Beamcollimation
As can be seenin Figs.2.3 and2.4,the activeareaof the diode is comparable
to or smallerthan the actualwavelengthof the light being emitted. Therefore,due todiffraction effects,the exiting beam will expandin roughly a 30" cone. Moreover,sincethe emitting areais rectangular,the beamis elliptical and canbe astigmatic.Also,
therecan be spatialmode structurein the activeregion that leadsto nodesacrossthe
beamprofile. Clearly the diode beamis not Gaussian,and it must be collimatedand
given a reasonablespatialprofile to be madeuseful.
The initially expandingbeamcanbe collimatedwith an off-axis parabolamirror
or a lens; an aluminum,diamond-turned90" off-axis parabolawith a focal lengthof
1.5" is usedin the currentexperiment.As shown in Fig. 2.8, the focal length of the

-

I

3I

b a s e p l a t ew i t h /r e f e r e n c ee d g e lmagneticbase

ill inchmicrometers-o- oo ff - a x i s- parabolad i o d e si ndewar

p o s i t i o n i n gO
pl a t e\Is pa t i aI

f i I t er
l a s e rb e a m
to experimentI

I

mirrorb e a mr e d u c e r
Fig. 2.8: Optics layout for diode laserbeamcollimationand reduction. The second
lens of the beamreducerand the spatialfilter are both on x, y, z translationstagesas
well for fine adiustment.

off-axis parabolais chosenso that it can be placedas closeas possibleto the exiting
diode beamto insurethat the resultingcollimatedbeamis as small as possible. The
spatialrestrictionsof the liquid N2 dewar,however,limit how closethe collimating
elementcan be placedto the diode,and the resultingcollimatedbeamhas a l/e electricfield radius,i.e.,a spotsize,of = I cm.
This initial collimation is one of the trickier aspectsof using Pb-saltdiodes,
particularly since there is no way to "see" the infrared radiation at 4 or 6 ttm.
Therefore,the stepsfor doing this arecoveredhere. The off-axisparabolais on an;r,y, z translationstage,which is mountedon a magneticbase. This was designedso thatthe optic is squareto the table and the front of the magneticbase. The dewarthat holds
the diodesis mountedon a baseplatethat hasa referenceedgein front of the diodes.
This referenceedge was also designedto be squareto the exiting diode beam.
Therefore,by sliding the edgeof the magneticbasealongthe referenceedgeof the baseplate,the optic is ensuredto be squareto the exiting laserbeam. This providescoarse
positioningin one dimension. A positioningplate that bolts into the opricstableprovidesthe secondconstraint,as shownin Fig. 2.8. Sliding the magneticbasealong
the referenceedgethensnugto this plateprovidesa kinematicsystemto placethe offaxis parabolain approximateposition.
Now that this elementis squareto the laser beam, the micrometerson thetranslationstagecan be usedto preciselypositionthe off-axis parabolasuchthat its
focal point is at the emittingareaof the diodelaser. To get an approximateposition,it
is usefulto expanda HeNe laserbeamto = 0.5 cm diameter,ensurethat it is 4" above
the table,then bring the HeNe beamalong the reversepath the diode laserbeamwill
take. In this way, the HeNe beamwill be focusedby the off-axis parabola. Using amagnifying glass,one can adjust the micrometerson the translationstageuntil the
HeNe beamis focusedon the approximatelocationof the emitting areaon the diode"
Next a suitableinfrareddetectoris placedabout 30 cm from the cliodelaser
alongthe path the HeNe beamtook, as shownin Fig. 2.9. A chopperwheel is placed
directly in front of the detectorapertureto chop the incoming IR light so that it can beviewedon an oscilloscope.When the diodelaseris turnedon, thereshouldalreadybe
light on the detector from this coarse alignment. If not, adjusting the x or y
micrometers(seeFig. 2.8) on the off-axis parabolashouldbring light on the detector.
Once thereis light on the detector,the z micromerercan be usedto focus thelight down on the detectorusing the off-axis parabolaalone, shown in Fig. 2.9.
Typically the x and,v positionshave to be optimized for every movementin the

z

aa
-1 -a

diodeandoff-axisparabolachopperdetector

measurespot size alongbeam usingiris

2.9

Procedurefor collimatingdiode laserbeamusingoff-axis parabola

direction. When as much light as possibleis focusedon the detectorelement,a 5 cm
focal lengthCaF2lensis insertedto ensurethat the entireIR beamis on the detector;
the off-axisparabolatypicallydoesnot focusthis non-Gaussian
beamto a spotsmaller
thanthe 1.0mm diameterdetectorelement. Oncethe lensis in place,a notecardcan beusedto ensurethat the IR beamis squarelyon all of the mirrors and lenses,and is not
clipping any element(someof the diodeshavedivergentbeamsthat overfill the off-axis
parabola;unfortunately,becauseof spacerestrictions,thereis nothing that can be done

34

aboutthis without buying a larger mirror with a longer focal length,leadingto an evenlargercollimatedbeam).
The size of the diode beamcan now be determinedby moving an iris along thebeam. If the iris is closed down so that 40Voof the power is transmitted,then the
diameterof the iris is equivalentto the lle electricfield radiusfor a Gaussianbeam. Of
coursethe diodebeamis not Gaussian,but this is a usefulapproximation.In this way,
the spot size as a function of distanceaway from the off-axis parabola can be
determined,and alsowhetherthe beamis actuallybeing focuseddown. This focusing
processis then repeatedfor progressivelyfartherdistancesof the detectorfrom thelaser,as shown in Fig. 2.9. If this processcould be carriedout to infinite distances,the beamwould be nearlyperfectlycollimated.In practice,it is usuallyadequate
to dothis until the detectoris about3 m from the diode. Typically this is accomplishedbymoving the detector50 cm back, then repeatingthe fbcusingprocedure. When thedivergenceof the IR beamis < 0.4" over the first 40 cm after the first turning mirror(seeFig. 2.8), this is found to be adequateto continueon to the next stepof reducing
the beamsize.
It cannotbe over stressed
how importantit is to approachthis task with a calm
mind and caffeine free hands. Write down the final x, y, and z positions of the
micrometerafter eachstep,so that hoursof work can be recoveredif the next stepdoes
not work. Oncethis procedurehasbeencompletedfor a particulardiode in a particular
positionin the dewat,the optimizedsettingscanbe quickly recoveredwhen
realigning
the diode by using the kinematic positioning shown in Fig. 2.8, and the previous
optimizedmicrometersettings,usuallywithout havingto go throughthe entirefocusing
down/steppingback procedure.
Once the beamis collimatedby this procedure,it still has a spot size of = 1 cm,
so it is necessaryto usea 5 to 1 reducerto bring it to a manageablesize. This is

shown

35

in Fig. 2.8. The first elementis a 10 cm focal length, 1.5" diameterplano-convex
CaF2lens,centeredon the beamin a rigid mount. The secondelementis a similarlens,
but with a2 cm focal lengthand 0.5" diameter.The secondlensis mountedon an r,_y.

z translationstageso that its focal point can be alignedwith the first lens. To align thesecondelement,repeatthe focusing/steppingback procedurecarriedout for the off-axis
parabola,but now with this secondlens. Due to the non-Gaussiancharacterof thebeam,it is usuallynot possibleto focusthe beamdown for distancesof > 30 cm fromthe lens. Therefore,a 100 pm diameterspatial filter, also mountedon an x,),,2.

translationstage.is insertedat the focal point of the beamreducerto block any nonGaussianbeam elementsthat do not focus. Once this is done, the beam after thesecondelementcan usuallybe collimatedto a 2 mm spotsize over2 m. The resultant
beamcan be steeredthroughtwo irisesand then overlappedwith a HeNe tracerbeam
via a pellicle; alignmentis relativelytrivial afterthis point.

2.2.4 Opticalfeedback
As opposedto the optical feedbackfrom the cleavedendsof the semiconductorthat is necessary
for lasing,hereopticalfeedbackis due to laserlight that is reflectedback into the activeareaafter the light is emittedfrom the diode. Diode lasersare very
sensitiveto optical feedback,and this has been usedto great advantagein the nearinfrared. For example,one facetof a nearinfrareddiode is anti-reflection(AR) coated,
and a diffractiongratingis set up in a Littrow configurationsuchthat the first orderreflectionis directedbackto the diode.a'5In this way, the lasercavity is definedby theuncoatedfacetof the semiconductor
and the externalgrating. Moreover,the grating
feedbackis frequencyselective,so that the diodelasesin just one frequencymode.
Itis also possibleto tune thesediodescontinuouslyby varying the temperature,current,
andgratingposition.

36

There is no reasonthat such a configurationcould not work for the Pb-salt
diode lasers,and indeed,we have found that a portion of the diode beamcan be fed
back usinga diffractiongratingin a Littrow configuration:changingthe lengthof thisexternalcavity by small amountsenablesone to changethe frequencyof the diode laser(the finesseof the diode lasercavity is quite low, i.e., = 3.J, so it is possibleto tune
the laser by small amountswithin one of the cavity modes). However, no Pb-salt
diodesare commerciallyavailablewith one facetAR coated,so the modestructureof
the semiconductor
cavity dominatesthe feedback,and the gratingfeedbackcannotbe
usedto continuouslyscanthe laser. In addition,the gratingwould ideally be the firstelementthe beamencountersafter a collimatinglens,creatingan externalcavity = I cm
in length. This would haveto be performedin the laserdewar,which would require
somemodification. Leo Hollberg and JosephS. Wells at the NIST laboratoriesare
currentlytrying a gratingfeedbackschemefor Pb-saltlasers,and it will be interestins
to seetheir results.
In the meantime,however,opticalfeedbackis at besta nuisance,and must be
dealtwith to performaccuratefrequencymeasurements
when scanning.The simplest
way to do this is to ensurethat a minimal amountof light is retroreflectedto the laser.
For example,lensescan be Ar coatedor tilted slightly away from the beamdirectionsothat reflectedlight doesnot returnto the laser. In orderto combatopticalfeedback,adiagnosticis neededto tell when it is occurring. Fortunately,the diode lasercan be
rapidly scannedin frequency,providing a real-timediagnostic. To do this, simply
input a sawtoothvoltageramp into the externalmodulationinput of the currentsupply.
Then view the etalonfringesfrom the confocalFabry-Perotcavity on a detector. If the
optical feedbackis bad, the etalonfringeswill look jagged,with stairstepstructurelike
that shownin Fig. 2.10a. This is an indicationthat an externalelementis providinga
secondcavity for the laserlight, suchthat it hasto satisfytwo boundaryconditions. As

!

aJ I

a)

repeatingintensity
"=
a
c)
c

Irequency
frequency

frequency

Fig. 2.10:Indicationsof opticalfeedbackwhen scanningover etalonfringes. a) Detailof.glg.fringe with stairste.p
pattern,indicating-strongieedback. b) Sefiesof fring.,
exhibitingrepeatingintensitypatternand inegula.spaiingsdueto weakfeedback.

the currentis rapidlyscanned,
the diodetriesto hold onto the frequencyof the external
cavityuntil the internalmodestructuredominates;the laserthensuddenlyjumps to thenext externalcavity mode.
ln more mild casesof feedback,the fringepatternwill look like that in Fig.
2.10b. The etalonfringeshaveirregularspacings,oftenrepeating,and varyingheights
due to amplitudevariationsfrom etaloning(discussedbelow). Here the opticalfeedbackis addinga sinusoidalcomponentto the frequencytuning as the current
isbeing scannedlinearly. It is easyto determinewhich opticalelementor elements
areresponsiblefor the feedback.Lightly pushon eachelementalong the beamline until
the stairsteppattern, or sinusoidalpattern on the fringes is seen to move ontheoscilloscope.The last elementthis happensfor along the beamline is the offending
element. Try realigningthis elementto reducethe effect. Another diagnostic
is tosimultaneouslyview the amplitudeof the laserlight on one detectorand theetalonfringes on anotheras the diode is being rapidly scanned. Oftentimestheelement

feedingback to the laseractslike an etalon,and a sinusoidalvarying of the lightlevel
a clear indicationof this. The free spectralrange(fsr) of the externalcavity

iscan be38

determinedfrom this intensity variation, and the offending elementcan be found bymeasuringthe cavity lengthcorrespondingto this fsr from the diode laser.
In the presentstudies,it is found that the feedbackfrom the pinhole aperture(shownin Fig. 2.8) usedto cleanup the spatialprofile of the beamis simply too great,no matter what materialthe pinhole is made from. This is becausethe pinhole isdirectly at the focal point of the first lens. Therefore,it is removedafter aligningthe
laserbeam,and an iris approximatelyI m from the diode is usedto cleanup the spatial
profile. Also, the 6.2 1tmhomostructurediode is found to be far more sensitiveto
optical feedbackthan the 4.3 1tmheterostructure
diode. In order to reducefeedbackfor
the H2O studiesto a manageablelevel.the two elementsof the beamreducershownin
Fig. 2.8 were AR coated. The secondlens in parlicularis a sourceof feedback,andevenafter it is AR coated,a Ge beamsplitteris insertedat the focal point of the reducer
at an angleof 45' with respectto the beam: the intensityof the retroreflected
light isthereforecut by a factorof four, reducingthe feedbackto a manageablelevel.

2.2.5 Scanningthe diode laser
In orderto usethe diode laserfor spectroscopy,
it is necessary
to scanit. This
is not as simple as dialing in some frequencyon a black box and starting a scan.
Instead,it is importantto understandhow the diode scans,what causesmode hops,
why the diodecan lasein multiple modes,and how to "pull" the differentmodesto gain
(nearly)completecoverageof the spectrum.
As mentionedabove, the semiconductorband gap increaseswith rising Z.
However,this is not what scansthe laserrn small steps. The diode can only lase indiscretecavity modeswhen / = n)J2H.where / is the cavity length,n is an integer,2 isthe wavelength,and pris the index of refraction. For lt = 5 and,a cavity length of 250/^fm,this correspondsto a comb of frequenciesseparatedby 4 cm-l. Sincep is both39

mode hop

o
L

()
L

c)
a
(5

garncurve

cavitymodes

f requency

Fig. 2.11: Simple picture for a diode laser mode hop. Increasingthe laser current
heatsthe activeareathroughlocal resistiveheating. The band gap increaseswith Z afactor of = 3 greaterthan the shift in cavity modes,leadingto modehops.

40

temperatureand currentdependent,the cavity modesare shiftedfor a changein I or 1:
for smallchangesin i.r,the changein frequencyis given by Av = -vo(LNil6). As seenin Fig. 2.II,if one of the cavitymodesis coincidentwith the gaincurveabovesome
thresholdvalue,then the diode can laseat this frequency. Small changesin Z due tothe resistiveheatingfrom the changinglasercurrent,or the changein currentdensity,
can move the cavity modesthroughthe garncurve in a continuousfashion,enablins
one to scanthe diode laser.
The lasercannotbe scannedin one cavity mode forever,sincethe gain curve
shiftswith temperatureabouta factor of threefasterthanthe cavity modes.l Therefore,
as the lasercurrent is increased(which also increasesthe temperaturein the active
region)the gain curvemovesthroughthe differentcavity modesas shownin Fig. 2.11.
Continuousscanningis observedfor somefrequencyinterval,thenthe laserhopsto thenextcavity modeat a higherfrequency.This pictureis oversimplitied,however,since
the gain curve not only shiftswith temperature,
but alsoincreasesin magnitudeas thecurrentis increasedand the densityof electron/holepairs grows in the activeregion.
Therefore'the scenarioin Fig. 2.12 is closerto actualdiodebehavior.At currentsjust
abovethreshold,typically the gain curvecan supportlasingin only one cavity mode.
As the currentis increased,
this modeis scanned,but the increasingmagnitudeof thegain curve meanthat additionalcavity modescan supportlasingabovesomecurrent.
The diodebecomesmultimode,first in one othermode,then in severalmodes.
This pictureis consistentwith observeddiode behavior. For example,single
mode lasing is observedfor both lasersusedin the presentexperimentat thresholdconditions. Typically the power is low for threshold,about 1/rW on the signai

detectorfor the heterostructure
diode. As the lasercurrentis increased,
laserpower isobservedto steadilyrise as the gain curve both movesthroughthe cavity modeandincreasesin magnitude. Typically a mode hop is observedafter the currentis scanned

.4 1

- l

multim ode

A A

c

q)
L

J

o
L

q)

a

garncurveth re sholdcavitymodes

frequency
Fig.2.12: Simple.picturefor diodelasermultimodebehavior. At threshold,the gain
curvecan supportlasingin just one cavity mode. As the cuffentis increased,
the laincurve.no.tonly shifts due to resistiveheatingi1
active
area,
but
grows
also
in
the
magnitude.At high enoughcurrents,lasingin-multiple
modesihu, be sup-ported.
"un

-

1 1

+L

over 10 to 20 mA, with the laserjumping anywherefrom 3 to i cm-1to the next mode.
Severaladditionalmodehopsmight be observedat evenhigherlasercurrents,and thelaserbecomesmultimode. For thesehighercurrents,laserpowersof 10 to 20 prW
are
observedon the signaldetector.
To gain maximum coverageof frequencyspace,it is necessaryto ,,pull,,differentmodesof the diode as a function of temperature.From Fig.2.l1,
it canbeseenthat one cavity mode can be scannedcontinuouslyuntil a mode hop
occurs.

At

higher currents,a new cavity mode rs scanned;it beginswith a modewith a modehop. It is possibleto shift at what frequencythesemodehop and ends

hopsoccurfor aparticularcavity mode by varying the temperatureand current;
this is referredto as"pulling" a mode. An exampleof this behavioris shownfor the homostructure
diodein Fig. 2.13. At highertemperaturesand lower currents,the cavity

modesupports(224- 247 mA)

5g+
F(262- 280 mA)(317- 332 mA)

1631.01632.01633.0v (cm-1;
Fig' 2'13: Exampleof pulling a diodemodewith temperarure.
This is for rhe 6.2 1tm
homostructurediode' The cirrent range to scanthe modeis shown in parenthesis.
Each line representsthe samecavity .o"d. that is t.unn.a i-m
a mode hop at the start
until a modehop at the end.

+.-1

lasingat lower frequencies.As can be seen,the converseis true as well; the cavity
modecan be pulled to the blue by decreasingthe temperatureand increasingthe current.
This is due to the changein index of refractionof the activeareawith changingcurrent.
Even though the resistiveheatingin the active areadue to higher currentswould resultin the samerelativepositionof the cavity modeswith respectto the gain curve as thesameeffective temperatureat a lower current,the cavity modes are shifted by anadditionalamountdue to the differencein currentalone.
Normally additionalmodesappearat highercurrents.As long as the majorityof the power is in the primary mode. this is generallynot a problem. Hgwever,secondarymodesare usually3 to 4 cm-l to the red of the primary mode, and strong
spectralfeaturesthat are shiftedby this amountfrom the regionof interestcan showup
as ghost peaks. To test whethera peak is a ghost peak, scanthe sameregion foraslightlydifferentcombinationof temperatureand current;the ghostpeaksshouldshift
relativeto the primary transitions.
To map out the mode structureof a diode, rapidly scan the diode using asawtoothwave, and view both etalon fringes and referencegas absorptionsonanoscilloscope'For a constanttemperature,
the centercurrentof the sawtoothramp canbe varied,and it is possibleto seethe mode structurein real time. Mode hops

show upasdiscontinuities
or inegularitiesin theetalonpeaks,or evenjumps in power. Multiple
modesshow up as additionalpeaksin the etalontrace. By sitting at the startof aand changingthe temperature,it is possibleto watch a particularmodemove
or red. The varioustemperature/current
combinationsthat placethe diodemode

to the blueat the startof

a mode can be recordedfor startingsubsequentscans.The diode is scannedby startingat theseinitial settings,then using a 13 bit D/A controlledby the computer
to senddiscretevoltagestepsto the diode currentsupply,which arethen converted
into currenr
steps.

!

44

It shouldbe notedthat the heterostructure
diode exhibitsmode hysteresis.Thatis' if one startsthe diode at a higher current and reducesthe current,the mode hop
the next lower cavity mode occursat a currentthat is lower than that for the mode

to

hop

when startingthe diode out in the samelower frequencymode and increasing
thecurrent. Therefore,there exist severaltemperature/current
combinationswhere thediode can lase in one of two possiblemodes,dependingon whether
the mode wasscannedfrom aboveor below this current.This enhances
the coverageobtainablewith
the heterostructure
diodewhencombinedwith the modepulling techniques.Complete
coverageof over 20 cm'l has beenobservedby piecing thesemodes
together.

Thehomostructurediode, however,does not exhibit mode hysteresis,
and therearefrequencygapsthat cannotbe reached.The coverageis sill good,though, with =90Vo
coverageobtained from 1580 to 1640 cm-1. It is possibletoslightly

alter thehomostructure
diode'smode structureby temperature
cycling,i.e., warmingthe diodeto room temperatureand cooling it back down again. This shifts

the modes a f-ew

tenthsof a wavenumberto the blue. Presumably
this is due to smallchangesin the p-njunction due to additionaldiffusion of the impurities

upon temperaturecycling,althoughthis hasnot beencheckedfor otherhomostructure
diodesin thisstudy. The

heterostructure
diode in the presentstudy, however,does not exhibit this behavior,presumablybecausethe p-n junctionis relativelystableto diffusionof impuritiessince
it is formedvia MBE.

2.2.6 Diode laserdewar
The diode laserdewarconsistsof two separatedewars: thefirst is a 1.5liter

capacitydewarwith the lasersmountedon the undersideof
the liquid N2

reservoir,in

vacuum' The secondis a 11.5 liter auxiliary dewar mounted
on top that keepsthe
smallerdewarcompletelyfull. The combinedhold time of thetwo dewarsis > 5 days.

-

45

but for normal day to day use,the liquid N2 level shouldnot be allowed to drop below
the auxiliary dewar. If it does.the upper portion of the lower dewar warms up,
leadingto thermalexpansionof the dewar reservoir. Sincethe diodesare mountedon
the bottom of the lower dewar,this changesthe positionof the diode with respectto thecollimatingoptics,causingmisalignment.
When the dewar is warmedup to room temperature,it shouldbe pumpedon
with a liquid N2 trappedroughingpump. Over a spanof months,condensable
gases
suchas H2O or CO2accumulatein the dewar. As this occurs,blackbodyradiationcan
warm the dewarmoreefficiently,and the ultimatebasetemperatureof 79 K risesby 5
to l0 K. Although no significantpressurerisesare observedin the dewarover a timespan of months, when thesecondensablegasesevaporatethe pressurecan risedramatically.Also, asthe dewaris warmed,it is importantto heatthe outsidewindows
about l0"C warmer than room temperatureso that a mysteriouswhite film doesnot
condenseout on the windows. This can happenif cold vapor from the boiling liquidN2 cascades
down the outsideof the dewarandcoolsthe windowssignificantlybelow
room temperature.Sincethis problemhasbecomelesssevereover time,the white film
is most likely due to somethingused in the manufacturingprocessof the dewar
materialsthat is slowly outgassingand being pumpedaway by incrementsas the dewar
temperatureis cycledand it is pumpedout.

2.3

Detectors
The detectors used to observe the infrared laser light are based onjust like the diode laser. The simplestdetectorsuseonly a pieceof
semiconductors,
intrinsic semiconductor,and are called photoconductive(PC) detectors. As in Fig.
2.2a, when photonswith ftv ) Esupare incident on the detector,they excite electrons

46

from the valencebandinto the conductionband.which increasesthe conductivityof thesemiconductor.This changeof conductivityis measuredto determinethe light level on
the detector. The other detectorsusedin the presentexperimentare photovoltaic(pV)
detectors.Theseare basedon the p-n diodejunction in Fig. 2.3. Photonswith ftv >E*aoincidenton a PV detectorexcitean electroninto the conductionband and a hole inthe valencebandin the depletionregionof the p-n junction. The bias voltagedue to the
p-n junction then whisks the electron/holepair out of the depletionregion,crearinga
photocurrentthat is proportionalto the numberof photonson the detector. Sincethe
quantum yield, i.e., the number of electron-holepairs createdfor each photon, is> 957afor someinfrareddetectors,this providesan extremelysensitiveway to detect
infraredradiation,with backgroundnoiselevelsthat arecloseto shot noiselimited.
Both the signaland referencedetectorsusedfbr the 4.3

1tmCO2 experiments

are InSb PV detectors.Thesedetectorswere purchasedfrom CincinnatiElectronics(model SDD-7854-5l),and are "matched."i.e., the detectorelementsare from thesamewafer,so that the frequencyresponseof the semiconductor
is the same,and theamplificationcircuitsare identical. The detectorelementsare circularand 1.0mm indiameter. Each elementis mountedbehinda 30' filed of view (FOV) cold shield inthermalcontactwith a liquid N2 reservoir. The window on the dewaris CaF2. The
amplifiersare custombuilt by the manufacturer,and havea feedbackresistorof 93 ke.
InSb PV detectorelementsmust be cooled to liquid N2 temperaturesto form thedepletionregion of the p-n junction. When this happens.the resistanceacrossthe
detectorelementis very large,and a staticdischargeacrossthe elementwill damagethe
semiconductoritself. Therefore,these detectorsshould never be connectedor
disconnectedwhen they are cold. When warm, the thermalenergyof the majority
carriersin the impurity region is enoughso that the depletionregion cannotform; the47Johnsonnoise
lrom resistor

totalnoise:
intrinsrcdetector
elementnoise.tcL r-l:f-rct (-'V 1 o=1 ( V 1 2+ Y 2 2 +

\'l/2

cl

shot noisefrom
blackbodyradiation

Fig.2.14: Major noisesourcesfor the InSb detectorandpreamplifier.

conductivityacrossthe junction is thus high. and the elementis not as likely to bedamagedby a staticdischarge.
The InSb detectorsareexceptionallysensitive,but to usethis sensitivityto its
full advantage
it is importantto understandthe noiselimitationsof thesedevices.The
detectorconsistsof the InSb PV elementattachedto a transimpedance
amplifier,shown
schematicallyin Fig. 2.14. The transimpedance
amplifierconvertsthe photocurrent1
from the elementinto a voltagegiven by V = - 1R,whereR is the resistanceusedin thetransimpedance
amplifier. Thereare fluctuationsin this voltage,however,due to noisefrom different parts of the circuit. The four main noise contributionsincludel) thermalJohnsonnoisefrom the resistor,which goesas vnn

no,,,/ffi=(4krR)tlt.

For a 93 kc) resistorat room temperature,
this gives a noiselevel of 39 n\,,,,lNtrt.
2) The amplifier hasboth currentand voltagenoise at the input. The statedspecsfor=7 nY,n,f^/E7andlnoise
the InSbpreampsare Vno1r.
= 1.5x 10-15A,",./JHz. lthe
currentnoiseleadsto a negligiblecontributionacrossthe 93 kO resistor.)3) Intrinsic
noisefrom the detectorelementitself, due to effectssuchas dark current,can add to thenoise. This numberis not providedby the manufacturer,but it is somethingwe canestimatefrom a noisemeasurement.4) Finally, sincethe InSb detectorsare sensttive

48
to light with wavelengthsout to 5.5 prm,blackbodyradiation
incidenton the element
will leadto a photocurrentthat hasshotnoiseon it. Reducing
this shotnoisecan leadto the biggestbenefitsin noisereductron
To calculatethe shot noisefrom backgroundradiation,first

it is necessaryto

determinethe numberof photonsincidenton the detector
element.

The spectralflux

density,i'e.,the power incidenton an areadue to blackbody
radiationat some r. is
given bys(z)=TG+-)(2.t)

The unitsof flux densityarewm3, which meansthatEq. (2.1)
mustbe integratedoversomewavelengthregionto obtainthe powerincident
on an area. Dividing thisbv theenergyof a photonat a given wavelength,we obtain# photons 2rc (sec ' mt

I\/^\I 1r \ ett'
| )'

()

)t

Sincethe bulk of the blackbodyphotonsat room temperature
are at wavelengthslonger
than 3 ptm,and sincethe InSb elementsare sensitive
to photonsout to 5.5 ,um with aquantum efficiency from g0zo to > 95vo overthe 3 to 5.5 pm range, Eq. (2.2) is

integratedover this spectralregionto obtaina photon
flux of

2.5 x l0t6 photons/(seccm2)' Thesephotonsare due to an unrestricted2t steradianview of room temperature
blackbodyradiationabovethe detectorelement.The
numberof incidentphotonscan begreatlyreducedby restrictingthe Fov using
a 77 Kcold shield. The InSb derectors
have a 30" Fov cold shield,which reducesthe room
temperaturephoton flux by afactorof 0'034' with this reductionand a detector
elementsizeof 1.0mm in diameter,
the photonflux on the detectorelementis 6.7 x 1012
photons/sec.Becauseof the highquantumefficiency,it is assumedthat eachof
thesephotonscreatesan electron-holepair' which leads to a photocurrentof l.I
1tA. Finally, the shot noise due to thistcurrentis given by 1,*,,.;,"f ^,lLrz= (2qIo,)'i , whereq is the electronchargeand 16.is

ra

{

a
O

a
0)
d
.d
E

z
o

Ll

F,

o
tt

U

o

F]

3
4
F)

d

lr
6
o
O

A

.

E

l-r
t4

()
L<

a

oo
U)

\n
N

bo
lr

50
the dc photocurrent. Thus 1.r, noise= 5.86 x lQ-i3 A,..f "lHr, which leadsto a
voltagenoiseacrossthe resistorof 54 nY.. /^/Hr .
These main noise contributionsare summed in quadratureto obtain abackgroundnoise level of 67 nY,-,f rlHz for the InSb detectors. This is the rockbottomnoisefloor for theseconditions;the measurednoisewill mostlikely be higher,
especiallysincethe intrinsicnoisedue to the InSb elementis not includedin this sum.
It is possibleto comparethe actualnoiselevelwith this limit. First,a signalsubtraction
schemeis employed,suchthat the output from the signaland referencedetectorsis fed
to the high commonmodeamplifierin Fig.2.l5. By subtractingthe referenceandsignal detectoroutput, amplitudenoise on the diode laserlight from low frequency
technicalcontributions,suchas liquid N2 boiling off in the diode laserdewar,can be
reduced. However,by subtractingthe output of the two InSb detectors,we actually
increasethe noiselevel due to the incoherentnoisecontributionsof eachdetectoras
well. The rock bottom limit forthe subtractionschemewill thus be 95 ny,,,,f^,,lHi,
i.e.,rheabovenumbersummedin quadrature
for the two detectors.
The noisespectraldensityis measuredfor the two InSb detectorswith the full
subtractioncircuit with an FFT, and the resultsare presentedin Fig. 2.16. (Tocomparethesenoiselevelswith the abovecalculations,the scalein Fig.2.l6 hasbeen
dividedby 10 to accountfor the x l0 gain of the subtractioncircuit). As can be seen,
the noisefloor for the detectors
with no laserlight incidenton themis I l8 ny,,,.f^lHz ,with somesmalladditionaltechnicalcontributionsat lower frequencies.This is within
257oof the estimatedrock bottom,which demonstrates
just how quiet thesedetectors
are' If it is assumedthat the discrepancybetweenthesetwo numbersis due solelyto
the intrinsic noise from the detectorelement,then this yields an intrinsic derecror
elementnoiseof 49 nY,^,f^lHz .

51
105

o

N

r

o

Signal- reference.
n o l a s e rl i g h t
S i g n a la l o n e@ l a s e rl i g h t w i t h
Signal- reference@ with laserlight

104

c

=

103

a
c
o

a
'o

lasershot noiselevel foro o s er v e dd e t e c t ornoise floor

v

102

I\

c

predicted
d e t e c t ornoise level

102

103

104

105

f r e q u e n c y( H z )Fig.2.l6: Noisespectraldensityfor InSb pV detectors.

Next, considerthe noisespectraldensitywith 0.5 pw
light incidenton one detector.As can be seen,theof 2350cm-r diodelaser

diodelaserhasvery little amplitude

noise' with the majority coming from low frequency
technicalcontributionssuch as
liquid N2 boiling off in the diodelaserdewar. This
type of noiseis easilyreducedvia
the subtractionscheme,and ascanbe seenin Fig.
2.16, thenoiselevel of the laserlightis reducedto the shotnoiselimit with respectto
the detectornoisefloor for frequencies
greaterthan a few hundredHz. The shotnoiseis calculatedfor 0.5 pw of 2350 cm-r
light incidenton eitherdetector,and is foundto be 69 ny,^.f^rEi for each derecror.
For lower frequencies,the noisespecsare withina factor of 3 of the shotnoiselimit.
For frequencies> loa Hz, the noisespectraldensitybegins
to decrease.This isdue to the finite bandwidth of the detecrors.
The bandwidth is limited due to thecapacitanceof the Insb p-n junction in combination
with the preamplifier" The

-

52
'1.0

o
c
o
o
q)

o

0.'l
103

i04
105
f r e q u e n c y( H z )106

Fig.2.l7: Frequencyresponseof InSb PV detectors

bandwidthcan be measureddirectly by modulatingthe light of a visible LED with asinewave voltage,and then observinghow the measuredpeak-to-peakamplitudeof themodulationdecreases
with increasingmodulationfrequency. To insurethat thisdecreaseis not due to the LED, first the amplitude as a function of modulation
frequencyis measuredusing a fast FND-100 Si photodiode. It is found that theamplitude modulation is independentof frequencyout to > 200 kHz. The samemeasurement
is thenmadewith the InSb detectors,and the resultsshown inFig.2.l7.
As can be seen,the detector/preamp
combinationhasa -3 dB point of l2O kHz. This is
identicalfor both InSb detectors.
The signaland referencedetectorsfor the 6.2 1tmH2OexperimentsareHgCdTe
PC detectors;at J7 K, the InSb detectorsare sensitiveto light out to 5.5 prm,while the
HgCdTe detectorsare sensitiveout to 12.5 1tm.The HgCdTe detectorelementsare 1.0
x 1.0mm2,and aremountedbehinda 30" FOV cold shieldin a liquid N2 dewarwith a
CaF2 window. These detectorswere purchasedfrom InfraRed Associates(now

\{

J*t

GrasebyInfrared),and came with a custombuilt preamplifier.
This has sincebeenreplacedby the circuit in Fig. 2.18a(the original amplifiers
are labeledand sroredin
B24l and can be replaced). The responseof the detector/amplifier
combinationis
approximately1.9 V per,uW of 4.3
1tmradiation. The intrinsic sensitivityof the
HgCdTe PC detectorsis down by a factor of 5 from the InSb pV
detectors.This isconsistentwith the observedfactor of five sensitivitydifference
between the4.3 1tm
and6'2 ptmexperiments:for the Co2 experimentsthe ultimate
detectionsensitivityis
l'0 x 10-aabsorbance
per pulsetn a20 kHz bandwidth,while it is 5.0 x l0-a
for theH2O experiment.
There are three different detectorsthat can be used

for diagnostics,r.e.,

recordingthe etalonand referencegas traces. oneof theseis a pbSnTepV
which is sensitiveto wavelengthsout to 1l pm anddetector,

can be usedfor eitherfiequency

regionin the presentexperiments.The I .0 x 1.0mm2
elementwas takenfrom an old
detectormanufactured
by Plesseyoptoelectronics
andMicrowave(moclelLXC

1000s)

and remountedbehinda 30" Fov cold shieldin a newliquid N2 dewar with a caF2
window' The amplifiercircuit is homemade,and theschematicis in Fig. 2.lgb. The

backgroundrms noise observedon this detectorisa factorof = 35 greater than

expectedfrom the backgroundblackbody300 K radiation
shot
thanan orderof magnitudegreaterthanfor the InSb

noise. This is more

detectors,whereascomparisonof

the relativesemiconductorsensitivitieswould suggestthat it shouldbe a factor of 5different' This extranoisemight be due to damageof

the detectorelement; its previous

history is unknown. Nevertheless,this sensitivityis more than adequatefor use
as adiagnosticdetector.
The seconddiagnosticdetectoris a PbSePC detector.
semiconductor.
cooling it to 7J K decreases
the bandgap suchextendedto 6'5 1tm.rt can thusbe usedfor boththe H2oSincethis is a pb-salt

that its usefulrangers

and co2 experiments.The

<,|

JA

a) HgCdTedetector
circuitb) PbSnTe detectorcircuilc) PbSe detectorcircuit

Fig.2.18: Hgcdre PC, PbSnTePV, and pbSepc detectorcircuir diasrams.

55

1.0x 1.0mm2elementis from InfraredIndustries(5000series)andis mountedbehinda 30" FOV cold shieldin a liquid N2 dewarwith a CaF2window. The original detector
packagewas hermeticallysealedwith a sapphirewindow. Since sapphiredoesnot
transmitout to 6.2 1tm,this window was removedon the lathe. The amplification
circuit is homemadeand shownin Fig. 2.18c. This detectoris more than adequatetorecordthe referencegastrace; no attemptwas madeto characterizeits noise.
The last detectorusedfor diagnosticsis an InSb PV detector.The elementis
circularwith a diameterof 2.0 mm. It is mountedbehind an AR coatedGe windowcooled to l7 K in a liquid N2 dewar with a sapphirewindow. Details of theconstructionof this detectorand its amplifierarepresented
elsewhere.8
This detectoris
usedin the4.3 um region.

2.4 Frequency measurement
In order to make accuratefrequencymeasurements,
absolutefrequenciesare
determinedfrom ref'erence
gastransitions,and the regulartransmission
fringesfrom a
confocal Fabry-Perotcavity are used to interpolatebetween the referencegas
transitions.The actualalgorithmfor assigningfrequencieswill be coveredin the datacollectionand analysissection;particularsof the referencegas and markercavity arepresentedhere. For the CO2studiesat4.3 ltm,CO2makesan excellentreferencegas.
A singlepassthrougha 6 cm long, 0.1 torr CO2referencecell with sapphirewindows
providesreferencepeakswith anywherefrom 5Voto 90Voabsorptionat intervalsof (0.8 cm-1. The frequenciesof the co2 transitionshavebeenconvenientlytabulatedg
inthe4.3 pm region.
To interpolatebetweenthesetransitions,a 10.3cm confocalFabry-Perotcavlty
is used. A2.54 cm diameter,7.8cm long quartztubewith a2.54 cm diameter.2.54
cm long prezotube mountedon one end is used as a spacerfor the mirrors. The

56

mirrors are 10"3cm radiusof curvature(r.o.c.)CaF2plano-convexlensescoatedon
the convex side to be 957areflectiveat 4.3 Lrm. The mirrors are fortuitouslyreflectiveat 633 nm, so the cavity can be initially alignedwith a HeNe laserby scanningthe
lengthof the piezo tube. Oncethis rs done,the cavity can be alignedfor the infrared
beam by rapidly scanningthe frequencyof the diode and observingthe etalon fringes
on an oscilloscope.The fsr of this cavity is found to be =725 MHz by interpolationfrom referencegasabsorptions.The finesseof the cavity is =17 as measuredwith thegiven that the reflectivitylimited finesseis 30, and that
diodelaser. This is reasonablesomeadditionalbroadeningwill be observeddue to the diode laserfrequencynoise.
Althoughthis cavity is not stabilized,the reproducibilityof repeatedfrequency
measurements
is l2 MHz rms. This level of absoluteaccuracyand relativeprecisionis
obtainedwith a nonstabilizedcavity for three reasons. First, the relatively small
spacingbetweenCO2referencepeaksmeansthat the time to scanbetweenthem is on
the orderof l0 to 20 minutesfor the van der Waalscomplexstudies.On this time scale
the temperatureof the cavity changeslittle. Second,all the CO2studiesarecarriedout
in a purgebox which is slightly overpressured
with N2. The pressurein the cavity isthusbufferedfrom changesin buildingpressureon a l0 to 20 minutetime scale. Lastof all, the majorityof the CO2scanscontainmultiplereferencegastransitions;the fsr
can be recalculatedfor eachscan,thus taking into accountday to day changesdue tooverall differencesin temperatureand pressure.
For the H2O studies,the accuracyand relative precision of the frequency
measurements
is improvedby a factorof two by the useof an actively stabilizedcavity.
Although this is a desirableimprovementin and of itself, it is also necessaryfor the 6.2
pm region given the densityofreferencepeaks. The only frequencystandardat present
for the 1550to 1650cm-l regionis H2O or HOD.e Sincethis is aroundthe origin of
the v2 bend of H2O at 1595 cm-1, relatively few strongtransitionsarepopulatedat 300

51

K, and spacingsbetweenreferencepeakscan be as largeas 3.5 cm-1. Clearly it is notpossibleto includetwo referencepeaksin every scanif the diode can only cover < 1
cm-l betweenmodehops. Therefbre.it rs necessaryto have a constantcavity fsr to use
for frequencymeasurements.
Therearetwo referencegascellsusedin this frequencyregion. The first is 50
cm long with CaF2windows and contains10 torr of H2O vapor. The laserbeam ispassedtwice throughthis cell, giving a total path lengthof I m; this is necessaryto
obtain > l}Vo absorptionfor most of the referencepeaks. The secondcell is 20 cm
long with CaF2windowsandcontains10 torr of a2:l mixtureof H2O:D2O.Becauseof the protonexchangein water,this resultsin a4.4 torr partialpressureof HOD. This
cell is usedfor the HOD absorptions,and can be insertedor removedfrom the beamline to determinewhether an absorptionin the H2O cell is due to HOD in natural
isotopicabundance.
The activelystabilizedconfocalFabry-Perotcavity hasbeendescribedin detailI I on4 a brief summaryof its operation,elsewhere,l0'
plus a more detaileddescriptionof modificationsfor the presentexperiment,is presentedhere. The exceptionalstabilityof this cavity is achieved by locking its length to a transmissionfringe from apolarizationstabilizedHeNe laser.l2 The polarizationstabilizedHeNe provides633
nm light that is stableto within 50 kHz on the time scaleof an hour, and to wirhin 5
MHz overayear.l3
In order to lock the cavity lengthto the HeNe transmissionfringe, the 30 cm
cavity is constructedas shownin Fig. 2.19. An 8 cm long 2.54 cmdiameterstainlesssteeltube is attachedon both endsto 2.54 cmdiameterPyrextubing via a glassto metal
seal. A 17 O heatertapeis wound aroundthe metaltube to provideslow variationof
the cavity length via thermal expansion/contraction.A 2.54 cm long piezo tube
(ChannelIndustries,material:C-5700,silverelectrode,2.54 cmouterdiameter.1.6

58c h o p p e dl R
beam
Ge beamsplitter

15cm f.l.
lens

Piezo

end mtrrors

9 0 / 1 0b e a msplitterpolarization
stabilized
HeNe

Fig' 2'19: opticallayo.ut,forstabilized
confocalFabry-.perot
markercaviry.
Theendmirrorsarecoatedforlc^trj/ely
92vc,eRectiuity; t;A;.t#m and633nm.
mm wall) is mountedon one end to providefast (l kHz)variationof the cavity length.

The end mirrors are 30 cm r.o.c.caF2 plano-convex
lensescoatedon the convexside
to be 92Voreflectiveat both 633 nm and,6.2

1tm.

The cavity is alignedby first mountingone end mirror

to the piezo tube using

cellulosenitrateadhesive(canbe removedwith acetone),
thenattachingthe otherend
mirror to a suitabletranslationstagesuch that it canbe translatedanclrotatedwith

respectto the Pyrex tube/firstmirror for fine adjustments.
The cavity is then aligned
using the HeNe laserby ramping the piezo to viewtransmissionfringes. once thesecondend mirror is in the appropriateposition,it is
attachedto the pyrex tube usrng
the same adhesive. The measuredfinessefor the

HeNe is 14, within 30voof thereflectivitylimitedfinesseof 19.
To actively lock the cavity lengthto a HeNe transmission
fringe, the servoloop
circuit in Figs' 2'20 and'2.21 is used. This circuit

is similar to previousversions,ubut severalsimplifyingmodificationshavebeen
madein the design,and it is included
for completeness'First, a bias currentis sent

to the heatertape to heat the tube toapproximately10'c aboveambienttemperature.
The tube canby changingthe heatercurrent. Next, the piezo

thus be heatedor cooled

is ditheredwith a 4.4 kHz, vnn =

-i
N
(l-r

bo

O
fr
J
t<
(-l

l-r

(-)
I

h
fr

0)
N

ct

0)

ti
-l
at<

bo

O
l-<

r \

C.l

ol
0)
bo
H

;

lr

J4
H
r-

H

c,)
I
L

iJ.
O

N

| .:
l
I

a

t>,

61
0.125 V sine wave. This scansthe cavity by < I MHz, and placesa small amplitude
modulationon the HeNe transmissionfringe. The transmittedHeNe is collectedon an
and this signalis heterodynedand demodulatedwith
FND 100 silicon photodetector.
the 4.4 kHz referenceoscillation to obtain a first derivativeof the HeNe transmission
fringe. Since the first derivative goesto zero aI the peak of the fringe. the servo canlock to this point. Fast,proportionalcorrectionsto the cavity lengthare handledmainlyby the piezo.while slow. integralcorrectionsare handledby the heater.
To simultaneouslyalign the HeNe beamand the infraredlaserbeamthroughthe
cavity, the arrangementin Fig.2.l9 is used. Becausethe infraredlight availablefor
diagnosticsis limited to <257c of the total powerof the diode,it mustbe choppedand
detectedvia lock-in techniquesto obtain adequatereferencegas and etalon fringe
signals.Therefore,the choppedinfraredbeamand unchoppedHeNe arebroughtinto
the cavity via the two differentsidesof the "bow-tie" mode. The HeNe beamis thenfrom the infraredvia a Ge beamsplitterafterthe cavity.
separated
Once the feedbackloop is closed,the servo quickly locks onto a HeNe
fringe; the variousgainsin the loop havebeenpreset,and as long as thetransmissionalignmentof the cavity is fine, thereis nothingto do but switch the servoon afterthetube hasbeengiven a coupleof hoursto warm up to a stabletemperature.The entire
cavity is placedinsidea plexiglassenclosurewith holesfor the beamto enterandexit.
This is to reducethe effectsof air currentson the cavity. With this arrangement,
thecavity has stayedlocked onto the samefringe for over two weeks. This is ascertainedby the intensityof the HeNe light on the detector; if it falls below somesetpoint,thecavity hasmoved off of the fringe, and a warning light comeson in the servo.
The fsr of the cavity is determinedby "splitting the fringe." The fsr is first
determinedto 3 digits by countingthe numberof fringes betweentwo referencepeaks
separatedby = 1 cm-1in the samescan; linear interpolationis usedto determinethe62

fractional fringe spacingsnear the referencepeaks. Since the cavity is stabilized,theabsolutepositionof the fringesdoesnot shift with time, so it is possibleto usethis fsr
to estimatethe relativepositionof a fringe with respectto a referencepeakthat is then2
cm-l away. The actualfringe positionis then measuredand found to be within one
tenth of a fringe spacingfrom the predictedposition. This identifiesthe numberof
integralfringes,and with this information,a refinedfsr can be calculated,which can beusedin turn to estimatea fringe positionevenfartheraway. This processis repeateduntil the fsr is calculated
for a 19 cm-l span. It is foundto be 8.33775x10-3
cm-1,andaccurateto 5 significantdigits. This is a fsr of 250.132MHz, as expectedfor a = 30
cm cavity.
The absolutestabilityof the cavity is measuredover a time spanof 3 daysby
determiningthe fringe frequencywith respectto a referencegasabsorptionin the slit jet
for multiplemeasurements.
The rms deviationin fringefrequencyis 7.5 MHz. This isalmostidenticalto the 8 MHz rms uncertaintyin frequencymeasurementsof ArH2O
transitionsfrom repeatedscans. This is remarkablystableover three days,but is afactorof 5 largerthan the stabilityachievedin the differencefrequencylab.ll Thereareseveralreasonswhy this might be the case. First,the fringesin the presentexperimentare from the infraredlaser,while the differencefrequencyfringesare for the visible
laser. Therearemany atmosphericabsorptions
due to H2O at 6.2prm,and thesemight
affectthe lengthof the cavity in the infraredby changingthe index of refractionnear
anabsorption.To mitigatethis effect, the apparatusis purgedwith dry N2. Secondof all,the infraredand HeNe laserbeamsarenot broughtinto the cavity collinearly. Thisandthe poor spatialmode quality of the infraredbeam might lead to small variations
ininfraredbeampath lengththroughthe cavity as the laseris scanned.This in turnwould
appearas drifts with respectto the HeNe transmissionfringe. A shift of g MHz
could

63

occur for a changein path length of only 5 1l-, which is equivalentto a 0.6 mrad
angularbeamdisplacement.

2.5

Optics layout
The completeopticalpath,as shownschematicallyin Fig. 2.1, rs enclosedin a

460 liter dry Nz purgebox. Due to significantatmosphericconcentrationsof CO2 and
HzO, it is not possibleto run the experimentwithout reducingthe atmosphericconcentrationsof thesegasesby at leasta factorof 20. This usuallytakes=l500litersof dry N2, or one quarterof a full lA cylinder. After this point, a small positive
pressureof N2 is maintainedto keepthe opticspath purged.
Unless otherwisenoted, the lens and windows are macleof CaF2 for its
excellent(95Va)transmission
throughthe visibleand out to 7 1tm.BaF2is usedfor thediode laserdewar windows, its propertiesare similar to CaF2. All mirrors are gold
coatedon a flat glasssubstrate,
and arepurchasedfrom EdmundScientific. The beam
splittersrangefrom a pelliclebeamsplitterto overlapthe HeNe tracerbeamwith theinfrared laser,severalcoatedCaF2beam splitters,and Ge beam splitters; theseare
often interchangedfor differentexperiments.
For the sublimationand van der Waals complexstudies,the infraredbeam ismultipassedin the vacuum chamberusing a 40 cm modified Herriott cell.la A
schematicof this is shown in Fig. 2.22 along with the slit pulsed valve on its
translationstage.The opticsconsistof two Au coated40 cm r.o.c.mirrors,5.08cm indiameter. It is relativelysimpleto align the multipassto get anywherefrom l0 to 20
passes.The infraredbeam(overlappedwith the HeNe tracerso that it can be seen)is
coupledinto the multipassvia a 50 cm focal lengthlens. This is placedso that the focal
point is at the centerof the multipass.The beamis broughtin so that it is incidenrupon
the uppercornerof the oppositemirror, and subsequentreflectionsare directedso that64

1 0c mslitjettranslation
srage
JILAmounl

40 cm r.o.c
Au mirror
spot pattern(sideview)
a

out

a

in

Fig.2.22:-.Multipass.layo3!
with slit jet translationstage. The arrowsfor the spot
patternindicatethe orderof the spotsin termsof reflection.s
afterthe input beam.

they traceout the paraboloidspotpatternas shownin Fig. 2.22. The overalllengthof
the multipassis adjustedso that the exiting beamis as tightly focusedas possible.For
fewer passes(e.g., l0), this corresponds
to a shorterdistance(5 to l5 mm decrease
from the 40 cm confocalseparation)betweenthe mirrors,and for more passes(e.g.,20), this corresponds
to a greaterseparation,
approaching
the confocalseparationof40
cm.
Once the laseris alignedthroughthe multipass,the slit jet can be positioned
relativeto it via the translationstage.The slit is slowly loweredand moved side-to-side
until the multipassed
beamis alongthe guideslotsof the slit. The slit is thenraised=5mm fiom whereit beginsto clip the multipassbeam.
Due to the nonorthogonalintersectionof the laserbeamwith the expansionaxrs,
someline broadeningis observedwith the modified Herriott cell. For example,the65

first passof the laserhasthe steepest
angle,i.e.,7o,with respectto a beamorthogonal
to the expansionaxis. For a He expansion,this corresponds
to a velocity componentof 190 m/s along the laserbeam. This in turn correspondsto a Doppler shift of 45
MHz for (COz):. Although this is for the greatestnon-orthogonalangle in themultipass,the linewidthsfor (COz): increasefrom 55 MHz FWHM for a singlepass
througha He expansion,to 75 |dHz FWHM for 20 passesin the modified Herriott cell.
This effect is not as great in an Ar expansion.however. For example,the Doppler
width of ArH2O is l1 MHz at an expansiontemperatureof 6 K. The linewidths
measuredfor 18 passesthroughthe modified Heriott cell are 22MHzFWHM.
As an asidefor the alignmentmeasurements,
the modifiedHerriottcell rotates
the polarizationof the laserlight by 6.8" + 2.0'. This appearsto be independentof thenumberof passes,sincethe samerotationis observedfor 10 or l8 passes.The cell
doesnot significantlychangethe polarizationfrom linearto elliptical the extincrion;
coefficientof 10-3for a secondcrossedlinear polarizer(taking into accountthe 6.g.
rotation)doesnot changeafterpassingthroughthe cell.

2.6 Vacuum system
The vacuumsystemconsistsof an 80 liter aluminumand plexiglasscubeforachamber,evacuatedby a liquid N2 trapped1000liter/s 6" oil diffusion pump.
The
diffusion pump is backedby a23liter/s welch roughingpump (model l39g).

Theroughingpump is vibrationallyisolatedfrom the vacuumchambervia a brassbellows
attachedto a rigid frame,and restson largerubberstoppersfor vibrationisolationfromthe floor' A gate valve is positionedbetweenthe chamberand the liquidN2 rrap.
Pressuresare measuredon an ionization gaugeor thermocouplegauge.
The

base

pressureof the system is 1.0 x 10-6torr. The diffusion pumpis used for the
sublimationand alignmentstudies,while just the liquid N2 trappedroughing
pump

rs

66

usedfor the van der Waals complex studies.The roughingpump aloneis adequateforthesestudies;for example,at a slit jet repetitionrateof 16 Hz for an Ar expansionat Pe= 300 torr, the pressurein the chamber is 75 mTorr. The mach disk for these
conditionsoccurs= 9 cm from the slit. For fasterexpansiongasessuchas He or H2,
backgroundpressuresas high as 2 torr (as measuredon the thermocouplegauge)are
possiblewithout a noticeabledecreasein clustersignal,or increasein expansion
temperature.

2.7 Gas handling
A variety of gaseshaveto be mixed anddeliveredto the experimentat eitherthe
correct pressureor flow rate. This is accomplishedusing the gas handling system
shownin Fig. 2.23. It is constructedof both copperand stainlesssteeltubing,and thevalvesare all stainlesssteel. Connectionsare madewith either Swagelockor VCR
fittings. The systemcanbe evacuatedwith a liqurdN2 trappedmechanicalpump.
Gasmixturesare madein a lA cylindervia a tee insertedon the high pressure
side of the regulator. For the CO2 studies,the mixing tank is first taken throughseveralpurge/pumpcyclesto ensurethatresidualcontaminants
arebelow a part in 105.
Then it is filled with CO2,typically between50 and 100torr; pressuresare measured
using a 100 torr capacitancemanometer(MKS Baratrontype 222CA). Finally, thecarriergasis mixed with the CO2; typically 100 to 200 psi is mixed with the CO2 as
rapidly as possibleto ensureturbulentmixing. Gasmixturesmadeby this processcan
be usedwithin an hour, i.e., no significantvariationsin signal due to incompletely
mixed gas over the time spanof a day are observed. The pressurein the mixing tankcan be readusingthe gaugeson the regulator.For the H2O studies,the mixing tank isreplacedwith an Ar cylinderand the bubbleris usedto introduceH2O vapor into thecarriergas. The bubbleris filled with distilledH2O and is kept at 0"C by immersingit

67

optional
2nd gasto slitjet or
sublimation
doserboxt,I

flowmetersneedleVAIVES't
to 100torr
vacuum Baratron\.2
rare gasor CO2

Fig' 2'23: Schematicof gashandlingsystemforvtovacuum

1 0 , 0 0 0t o r r
Baratronmixingtank,raregas,or

coz

HzQ
bubblerslit jet or sublimationdoserbox.

68
in an ice bath' The vaporpressureof H2o at this temperatureis =concentrationof water vapor
entrainedin
the pressureof the carriergas

5 torr, so the relative

the carrier gas can be controlred
by varying

throughthe bubbler. For thestudiesemproyinga pursed
supersonicexpansion'the gas
mixture is throttreddown tothe appropriatepressureusing a needlevalve' Stagnationpressuresfor the expansion
are measuredwith a
I 0,000torr capacitancemanometer.
To grow the thin films forto the doser box at a precise

the sublimationstudies,itnecessary
to deriver Co2

flow rate. The mixing tankis repracedwith a co2
cylinder'and this gas is passed
througha flowmeter(Tylan,
moderFM3g0), and thenthrottledwith a needlevalve
to regulatethe flow. The
flow rate can be set before
sendinggasto the doserbox
by openingthe gashandringsystemto vacuumright after
the needlevalve; the needle
valve can then be adjusted
to give the properflow rate.
The frowmetersmust be
caribratedfor each gas
mixture used. This is donebymeasuringthe pressurerise
vs' time in a known vorume
fbr a certainflow rate. The
flowmeterreadoutcan thus
be relatedto the actualflow.
2.8

Supersonic expansion
The workhorsefor the majorityof the experimentsperformedin this thesisis
the supersonic
expansion'Pinholeand slit
expansions
arecommonryusedin chemicar
physics studies for theirability to form weakly
bound complexes at rotationaltemperaturesof s 20 K,
and to form molecurar
beams with relativery
narrow
hyperlhermalvelocitydistributions
thatcanbe usedto study
collisiondynamics.Boththe theory and operationof supersonicexpansions
has oeen consideredin
detairl6 1nelsewhere'15'
the presentsection,a brief
overviewof both the principles
behindthe expansion'and the construction
andoperationof the slit
andpinholepulsedvalvesis presented'Lastly' theformationof (co2)n clusters
is considered.

J

69

-t
l x

/rrrt\

v///ll\\\I

P*<PgD

nozzrethroat
supersonic,:1.
l ilt\\\Lbu'|k+ (Skrs/m;r/z/ I llll\\\

Mach
disk

Fig.2.24: Schematicof a supersonic
expansion.

2.8.1 Overviewof theoryand operation
As shownin Fig. 2.24,if a gasat somestagnationtemperature
Z6 and pressure
P0 is expandedthroughan orifice with width d* into a lower pressure(P-) region,thena supersonicexpansioncan be formed. The requirementsfor this to occur are l) that themeanfree path of the gasmoleculesin the nozzleis << d*, and ii) thatPdP* / 2. The
first requirementinsuresthat heat transferbetweenthe walls of the orifice and the
expandinggasis negligible,while the secondrequirementinsuresthat there is someregionover which the expandinggascanbe consideredwithoutthe effectof collisions
from backgroundgas molecules. If theseconditionsare met, then the expansionis
adiabaticand isentropic,and the temperatureof the expandinggasis relatedto the local
densityand pressure,i.e.,
v

T

- t

l

( P \ v|tro ( Pr,

-

( n \
_ t
l
-

I
l

(2.3)

lro J

Here,7= Cp/C,is the ratio of the heatcapacitiesat constantpressureand volume,and
is 5/3 for a monatomicgas. The temperatureof the gas thus drops as it expandsto

70
lower pressures. In addition, the bulk velocity a of the gas increasessuch that it
becomesgreaterthanthe local speedof sound,o = jyRTlm, whereR is the universal
gas constant,and lz is the mass; hence the name supersonicexpansion. As the
expansiondevelops,it approachesits terminal velocitylt uou,n-+,[str,,1mfor amonatomic gas. A rigorous fluid dynamics analysisof the expansioncan be
performedl5'l6 to demonstratethat ultimatetemperatures
as low as a few degreesK arereached.
Although the macroscopicthermodynamicspropertiesof the expansionare
importantto understandthe coolingprocess,it is the microscopicdynamicsthat areof
interestfor processessuchas clusterformationand the creationof rotationalalignment.
In addition to cooling, three-bodycollisionsare neededto form van der Waals
complexes. Hence,the slit expansiongeometryis preferableover the pinhole formaking clusters,sincethe densitydrop is slowerfor a line than a point source. This
leadsto morebinaryandthree-bodycollisions.For the creationof rotationallyaligned
molecules,however,the driving forceis the initial "velocityslip" betweenthe heavier
CO2 moleculesand the lighter carrier gas atoms. The velocity slip decreaseswith
increasingnumberof binarycollisions,andthusfewerbinarycollisionsarerequiredto
createand preservealignedmolecules.Hencethe pinholegeometryis preferableto theslit to createalignedmolecules.Additionalaspectsof the microscopicdynamicswill
comeinto play when consideringhow nonequilibriumpopulationscan be "frozenout,,in the expansion;this will be centralto the discussionof clusterformationdynamicsin
Chapter8, and will be coveredin detail there.
One aspectof the supersonicexpansionthat is relevantto an experimental
sectionis the Mach disk. This is a shock wave where the expandinggas beginsto
comeinto equilibriumwith the backgroundgasin the vacuumchamber.It is refenedtoas the Mach disk becauseit is the point at which the bulk velocity of the expandinggas

1ldropsbelow the local speedof sound. An empirical relatronl7 for the downstream
distancex of the Mach disk for a slit with width d* is givenby

x = 1.23a.(&'lo"'.

t),1\

\P_,/
This relation can be usedto estimatewhat backgroundpressurescan be toleratedfor agiven stagnationpressurebeforethe expansionis disruptedin the laserproberegion.
For the van der Waals complex studies,a pulsedslit expansionis used. In
additionto the higherdegreeof clusteringobtainablein a slit, the expansiongeometry
also leadsto significantlynarrowerlinewidthsthan in a pinhole expansion. The
velocity distributionparallelto the slit is predominatelydue to thermalmotion of thegas, sincethe slit dimensions(4 cm x 120 ptm) approximatean infinite line over a
majority of the slit, i.e., edge effectsare small. The resultinglinewidthscan be
Dopplerlimited, and widths as narrow as22MHz FWHM are observedin the ArH2O
studies. In contrast,linewidthsin unskimmedpinholeexpansionsare predominately
due to projectionsof the beamvelocity on the laserprobe,and widths on the orderof
100 MHz are observedin the pinhole alignmentstudies. Lastly, the expansionis
pulsedto reducethe gasload on the vacuumsystem,and to createa transientabsorption
signalfor more sensitivedetection.
The pulsedslit valve shownin Fig. 2.25 is usedfor the van der Waals cluster
studies; details of its constructionand operationare presentedelsewhere.l6'l8
Briefly, the valve is constructedin the JILA shopsfrom 300 seriesstainlesssteel,while
the lifting rod is madeof 17-4 ph stainlesssteel. The sealassemblyconsistsof a pieceof Viton o-ring attachedwith cyanoacrylateadhesiveto a titaniumholder(thepoppet).
The poppetis fastenedto the lifting rod by first screwingthe lifting rod into the poppet,
thenholdingthis in placeby pressinga nickel slug againstthe lifting rod threadsvia asetscrewon eithersideofthe poppet. A secondsetscrewis thenusedto lock the first

72
F E R R O M ANGE T I C
PLUN GERSOLENOIDADJUSTABLEGAPG A SI N L E T

W O RMG E A R( 3 )SPUR
G E A R( 3 )LIFTINGc o rLROD

SPRINGNOZZLEHOLDE
RSEALASSEMBLY

I NTERC HA NGABLE N O Z Z L E

Fig.2.25: Cut-awayview of pulsedslit valve(from ref. l6).

in place. The valveis openedby accelerating
the magneticplunger(type440 magnetic
stainlesssteel)againstthe lifting rod with a currentpulsethroughthe solenoid(ratedat
1.8V dc.0.6 A resistance,
370 1tHinductance).The currentpulseis suppliedby the
SCR basedcircuit in Fig. 2.26, andhasa peakof 70 A with a FWHM of 440,us. The
pulsedriveris capableof drivingthe valvear repetitionratesof up to 25 Hz.
A sampleCO2monomertransientabsorptionsignalfrom the slit valve is shownin Fig.2.27. The valvetypicallytakes- 50 prsto open,and - 100prsto close. The flat
middle sectionof the pulsecorresponds
to the valvebeingfully opened.To adjustthe
slit valve, either a monomeror clusterabsorptionis used along with the following
procedure. 1) Evacuatethe vacuum chamberand supply - 1 atmosphereof gas

o
:)
.t
Fr
k

l-r

a

(<

L

r')

74

i

l
it

o
c

\

\
\i

unclusteredp r o fi l e(ga___l L
clusteredp r o fi l e
base
gate

signal
g are

base
g ate
time

pulsefrom pulsedslit valve. (P(4)of CO2.conditions:Fig.2.27: Sampleabsorption
0.057oCO2 in He at Ps = 450 ton.)mixtureto the slit jet. Decrease
the springtension(counterclockwise(c.c.w.)on knob
'S' in Fig. 2.25) untrla pressurerise is observedin the chamber.Then increasethejust to the point wherethe leak stops. 2) Begin pulsingthe valve
springcompression
by providing2S5V acrossthe solenoidas shownon the gaugeof the pulsedriver.
Use the appropriatestagnationpressureto observethe absorptionsignalof interest.3)
While observingthe absorptionpulseon an oscilloscope,adjustthe plungertravel('D'in Fig. 2.25,c.w.= more)until the leadingedgeof the pulseis - 700 ,usafterthe
trigger. 4) Adjust the poppettravel('L' in Fig.2.25,c.w. = more)until the trailingedgeof the pulseis - 500 prsafterthe leadingedge. 5) The trailingedgeof the pulse
can thenbe improvedby increasingthe springcompression
and varyingthe poppet
travel. Increasingthe springcompression
alsoreduces"ringing,"l.e.,secondary
bouncesof the sealassembly. 6) For the final pulse,makesurethe valveis fully open
by verifying that increasingthe pulselengthdoesnot increasethe absorption.

t)Once the valve is adjusted.small correctionsmust be made on a day to day
basis,since the pounding of the plunger againstthe lifting rod slowly changestheplungertravel distance. After a few monthsof operation,the trailing edgeof the pulse
will often degradeto the point whereadjustments
can not bring it back to its originalstate. It is then time to replacethe Viton seal. Another problem experiencedwith thevalve was due to bent lifting rods. In this modeof failure, the leadingedgeof the pulsechangedon nearly a pulse to pulse basis,sometimesby as much as 400 ps. This
problemwas solvedby replacingthe lifting rod.
The transientabsorptionpulsecan also be monitoredfor evidenceof extreme
clustering.This is apparentas a dip in absorptionin the centerof the pulse,as shownin Fig. 2.27. This is due to clusteringof the monomer signal, or even secondary
clusteringif a dimer or trimer signal is being monitored. Becausethe leadingand
trailingedgesof the pulsecorrespondto the valvebeingpartiallyopen,theexpansionis
slightly warmerat thesetimes. Hence,the degreeof clusteringis lessthan when thevalve is fully open,and clusteringor secondaryclusteringdepletesthe signalmore in
the centerof the pulsethanat the warmeredges.If a clustersignalis beingoptimized,
the stagnationpressureis decreased
until the pulseresemblesthe unclusteredprofile.
In addition to the pulsedslit jet, a pulsedpinhole expansionis used for thealignmentstudies.The valveis solenoidactivated(GeneralValve, Series9, 28 V DC)
with a 150pm pinhole aperture. Either Teflon or KEL-F poppets(availablefrom
GeneralValve) are used. The Teflon poppetsonly last for a few days of operation
beforethe pulse shapedegrades,however,so the sturdierKEL-F poppetsare usedfor
most of the measurements.The driver for the pulsedvalve is a home-madesupply
borrowedfrom the crossedbeamexperimentin lab 8214. Adjusting the pinhole valvesis quite simple. After replacingthe magneticslug/spring/poppet
assembly,lightly
screwthe pinholecap onto the valve body. Pressurizethe valve with N2, and placea

76

small amountof leak tracerover the pinhole. Tighten the pinhole capjust until the N2stops leaking through the pinhole. The pulse length can then be adjusted bylengtheningthe currentpulseand monitoringa monomerabsorption.The pinholevalve
takes- 200 1tsto openor close,so total pulselengthsaretypically g00 ps.

2.8.2 (COr, clusterformation
At this point it is interestingto considerthe actualformationof (CO2)nclusters
in the slit expansion. The simpiestpicture for this processis basedon three-bodycollisions. If two particlescollide elasticallyin the expansion,the total energyof the
combinedsystemexceedsthe van der Waalsbindingenergy,and the particlesdo not
stick together. A third particlecan carry off someof this energyduring a collision,
however,leavingthe first two particleswithout enoughenergyto separate,i.e.,they
haveformeda complex. Lovejoyl6hasestimatedthe numberof three-bocly
collisionsaparticleexperiences
in a slit supersonicexpansionfor a varietyof "typical" expansion
conditionsfor differentcarriergases:po = 400 torr for Ar, ps = 1500torr for Ne, andPo = 2000 torr for He. In each instance,somewherebetween lo%oto 50oloof theparticlesin the expansionare estimatedto undergoa three body collision for these
conditions. If every one of thesethree-bodycollisionsresultsin a complex, then azeroeth order guessfor the fraction of monomer ending up in clusterswouldbesomewherebetween107oand50Vo.
For expansionsof CO2 in a He carriergas,however,most of the three-bodycollisionswill probablynot end up forming a complexdue to the difficulty in forming
He - CO2 complexes,as will be coveredin Chapter4. This is in contrastto
Ar
expansions,wheresignificantclusteringis observedevenat backingpressures
as low
as 100torr. If it is assumedthatonly three-bodycollisionscontainingat leasttwo

CO2

moleculeswill form complexes,then it is possibleto estimatethe fraction of monomer

77
5
monomer

N

I

A

X

x40

o 3

o

c

c

ooo

dimer

N

I

o

(s
..)

_ O z

o

E20
a

A
l
_ o l

o

0
500
1000
Po(torr)

1500

500
1000
P6(torr)

1500

16
trimer

N

I

>12

oooo
^

e 120
o

b a c k gr o un d
o

X

X

o

3
80
c

3 B

(u
_o
o 4 0

L

3
- 4

aJ)

_o
(g

o

0
0

500
1000
P 6( t o r r )

1500

0

500
1000
P6(torr)

1500

Fig. 2.28: Curvesof growth for R( 1) of monomer( l70l2g l69;, a-typeQ-branchof( C O ) z , , Q o o f ( C O z ) : , a n d b a c k g r o u n da t 2 3 5 3 . 2 6c m - 1 . T h e o r d i n a t ei s t h e
integratedabsorbanceof the transition,except for the background,where just theabsorbanceis reported.(Conditions:2VoCO2in He).

lost to clusteringfor 27oCO2expandedin He for "typical"expansionconditions;only
one in fifty of the three-bodycollisionsa CO2 moleculeexperienceswill involve asecond CO2. If one factors ln the fraction of moleculesexperiencingthree-bodycollisions,less than l7o of the monomer will be lost to (CO2),,clustersat typical
expansionconditionsfor three-bodyclusterformation.
This can be rigorouslytestedvia curvesof growth and the absoluteinfrared
crosssectionsof the absorbingspecies.Shownin Fig. 2.28 arethe curvesof growth78

monomer,(CO2)2,cyclic (COz):, and a broadbackgroundabsorption
for 17O12C160
that has beenattributedl9to (COz),,clusterswheren - 102. Basically,the integrated
absorbanceof one transitionor Q-branchis monitoredas a function of stagnationpressure.If no monomeris lost to clusters,thenthe monomercurveof growth should
increaselinearly with stagnationpressure,sincethe densityof absorbinggas is also
increasinglinearly. As can be seenin Fig. 2"28,though,the monomersignaldeviatesfrom the lineartrend for Po 2 750 torr. If the linear portion of the curve of growth isextrapolatedto higher pressures.it is simple to estimatethe fraction of monomerc l u s t e r e d a t h i g h e r p r e s s u riet iss;> 5 0 7 o a t P o = 1 2 0 0t o r r . C l e a r l y t h i s e x c e e d s t h eestimateof l7o clusteringbasedon three-bodycollisionsalone.
to follow wherethe clusteredCO2isTo reconcilethis difference,it is necessary
going. As mentionedabove,very little is going into HeCO2clustersfor a room
temperaturestagnationgas, so it must be going to (CO2), clusters. The curvesof
wherethe monomerbegins
growthdemonstrate
this quitewell. At increasingpressures
to cluster,first the dimer,the trimer,then the largerclustersgrow in. This sequential
growth with stagnationpressureis partly consistentwith the three-bodycollisionmodel. After all. a dimer must be the basisfor trimer formation unlesseven rarer fburbody collisionsare to be considered.However,both the dimer and trimer curvesof
indicatingthat thesespeciesare eithernot being as
growth level off at higherpressures,
readilyformed,or are themselvesbeing lost to secondaryclustering.In addition,thecurveof growth for the backgroundcontinuesto increasewith backingpressure.This
suggeststhat as larger and larger clustersare formed, the clusterformationprocess
takeson a qualitativelydifferentform than the three-bodycollision model.
from Fig. 2.28 are turned into
To highlight this, the integratedabsorbancesof CO2 in eachcluster,making the assumptionthat the infraredabsoluteconcentrations
oscillatorstrengthof the CO2v3 asymmetricstretchchangeslittle upon complexation.

79
Table,2..l: legrge_of (COz)n clustering as a function of stagnationpressureps.
(Conditions:2VoCO2in He)
Percentage
ofPercentage
of total CO2in:torr) monomerclustered

400
500
600
700
800
900
1000
l100
1200
I 300
1100

< 5.0b

0.8
1 0
1.2
1.5

< 5.0
5 .2
22.5
54.0
...

r.7

0.2
0.4
0.7
0.8
1.t

1.8

l.l

I1.3

1.7

r.2

1 5 I.

l 1
1"7

1.2
t.2

1,6

l.l

1 9r"
22.6
25"0

r.5

I.0

,?5

0.8
1.8
J.J

6.1

8.s

'The backgroundis a broad absorptioncenteredabout 2353.26+
0.25 cm-l with a
F\\'HM of approximately
4 cm-1.
rThe absoluteaccuracyshouldbe taken as approximately+0.5
of the statedvalues.
Seethe text for assumptions
madein the analysis.
r\leasurements
werenot madeat thesestagnationpressuresfor the monomer.

This is done basedon measuredrotationaltemperaturesin the expansionand H6nlLondonfactorsfrom theory. For the broadbackgroundabsorption,the peakheightat
2353.26(25)cm-t timesthe FWHM of = 4 cm-l is takenas the intesratedabsorbance.
The resultsarepresented
in Table2.1.
As can be seen,when clusteringinitially beginsin the expansionat p1;- 500
torr, only a few percentof the monomerends up in clusters,in agreementwith thethree-bodycollision model.

For higher pressures,the concentrationof CO2 in the

dimer and trimer remainsremarkablyconstantat a few percent,but the largeclusters
take off. (The discrepancybetweenthe monomerlost to clustersand the sum of the

80

1.6

1.2

un c l u s t ree dl i n e a rt r e n d

0)
()
c

(u

-o
0.8
o
a
-o

(E

0.4

0.0
0

10 0 0P o( t or r )2000\9.2.29: CO2 monomercurveof growthfor pinholeexpansion.(Conditions:lOVo
CO2in He,d* = 1501rm).

threeclustersizesconsideredhereis just a statementthat severalintermediatecluster
sizesiue beingpopulatedas well thatarenot consideredin the analysis.)
The rapid growth of the large clusters can be viewed as the onset of"snowballing."Above a certainclustersize,the bindingenergyof the clusterand thedensityof clustervibrationalstatesis high enoughthat the excesscollisionalenergy
involved in a two body collisioncan be readily absorbedby the largecluster. Theselarger clustersthen "sweep"throughthe expansion,picking up smallerclustersand
monomersas they go via the much more frequenttwo body collisions. This is one
possibleexplanationfor the dramaticincreasein clusteringat higherpressuresover thatpredictedby three-bodycollisionrates.
As a final note, the relative (COz)ncluster formation in a pinhole and slit
expansionis compared.Shownin Fig. 2.29 is the CO2 monomercurveof growth for

4--

81

l07o Co2 in He expandedthrougha 150 pm pinhole nozzle. Even though theconcentrationof CO2in the stagnationmixture is a factorof five greaterthan for the slit
expansionmonomer curve of growth in Fig. 2.28, significantclusteringof themonomeris only observedat Po 7 1500torr in the pinholeexpansion.This is a clear
exampleof the enhancement
in clusteringdue to the slowerdensitydrop off in the slit
expansionas opposedto the pinhole.

2.9 Measuring alignment
As discussedin detail in Chapter3. it is possibleto determinethe second
moment of the M7 distribution,i.e.. the alignment,by measuringthe differencein
absorbancefor light linearly polanzedparalleland perpendicularto the quantization
axis. In the presentsectionwe presentthe detailsfor rotatingthe polarizationof thelaserlight and extractingthe differencein absorbance.
The polarizationof the laserlight is rotatedatl5 kHz betweenorthogonallineu
polarizationsusing a photoelasticmodulator (PEM).

This rapid rotation isadvantageous
for two reasons. First, in the supersonicexpansionexperiments,thepolarizationis rotated= 40 times during the 500 ps pulsefrom the puisedvalve. The
alignmentcan thus be measuredwithin one pulse,minimizing effectsdue to pulseto
pulsefluctuations.Second,the alignmentinformationis encodedatl5 kHz. At thesefrequencies
the laser/detector
noiseis within a factorof two of the shotnoiselimit: bv
using sensitivelock-in techniquesvery weak alignmentsignalscanbe measured.
The PEM (Hinds International,PEM-80 ZS) consistsof an octagonalZnSe
crystalthat is broughtinto a resonantoscillationof 37.5 kHz by two piezotransducers,
as shownin Fig. 2.30. When the crystaloscillates,it experiences
eithera compression
or expansionalong its two principle axes. Due to stressinducedbirefringence,theindex of refractionchangesalong the principle axes. If light is brought into the crystal

82

90o rotation

Z n S ec r y s t a l
Ey

piezo
t r a n s du c er

4- E
Ex

Fig. 2.30: Polarizationrotation by the PEM. The piezo transducersset the ZnSe
crystal into resonantoscillation. When the incident light is brought in at 45" withrespectto the crystalsprinciple axes,the compressionlexpansion
stressinduced
birefringenceleadsto a phaselag betweenthe two polarizations,which is equivalentto
rotatingthepolarization.

linearlypolarized45" with respectto the principleaxes,thenthe electricfield of the light
can be viewed as being decomposedinto a linear combinationof two electricfields
along the principle axes. As shown in Fig. 2.30, the changingindex of refraction
causesthe phaseof theseelectricfields to leador lag one another,i.e.,they are phaseshifted. The amountof phaseshift increaseswith the stress,and at somevalue the shift
is 180'. This correspondsto the polarizationbeing rotated90o. For large enoughoscillationsof the crystal,the 180'phase shift occurstwice during the oscillation
period,so the polarizationis rotatedat 75 kHz.
During the oscillation of the crystal, the stressinduced birefringencegoessmoothly from zero to the value neededto rotatethe polarization90o" Therefore,thepolarizationof the light is also rotatedthrough elliptical and circular polarizations

83

correspondingto differentstressinducedphaseshifts. The PEM must first be set so
to a 90" rotationof the polarization.
that the crystaloscillationamplitudecorresponds
To do so, first the laserlight is broughtinto the PEM linearly polarizedat 45" withby passingthe laser
respectto the principleaxesof the crystal. This is accomplishedfirst througha MgF2 Rochonpolarizer(Karl Lambrecht,MFRV-9). Then a second
MgFz Rochonpolarizeris piacedafter the PEM at right anglesto the first polarizer.
With the PEM off, no light is transmittedthroughthe crossedpolarizerconfiguration.
The PEM is then turnedon, and the oscillationamplitudeincreasedwhile viewing thetransmittedlight on a detector. A15 kHz nearly sinusoidalwave will be observed,
to 90"
which will increasein amplitudeuntil the oscillationmagnitudecorresponding
rotation is reached. The oscillation magnitudecan be increasedfurther, but theamplitudeof the transmittedlight will not increase.Instead,the PEM is now rotating
the light through90", and then on to further ellipticalpolarizations.The transmitted
light will no longerhavea nearsinusoidalpattern.
Oncethe polarizationis beingrotateda full 90', and the PEM is orientedwith
respectto the quantizationaxis of interest,an alignmentmeasurement
can be made.
Consideralignmentin a pulsedpinholeexpansion.The collisionallyalignedmolecules
resultin a greateramountof light linearlypolarizedparallelto the expansionaxis to beabsorbedthan light perpendicularlypoluized. Hencean (exaggerated)absorptionpulse
might look like that in Fig. 2.31. The overallshapeof the absorptionis apparent,but
the differencein absorbance
betweenthe two polarizationsshowsup as an oscillationon the pulse. Two quantitiesareof interestto determinethe alignment. The first is thetotal absorbancefor the two polarizatlons,i.e., A1 * Ar, and the secondis thedifferencebetweenthe two, i.e.,A11-A1. The first quantitycan be determinedby
integratingthe absorptionpulse (discussedfurther in the dataacquisitionsection). The
secondquantitycan be determinedas shownin Fig. 2.31. The raw signalis filtered to84- A.A.
ill{il|llltrvilft
'r
l-

o

f
I

O
c

t
\

i

(6

-----/

lt

absorption
p ur s e\\-Whigh pass"dilnilfiffinlilrr,,.

d e m o d ul a t e

rowpasstrme

Fig. 2.31: Extractingpolarization-dependent
absorbancefrom an absorptionsignal.

isolatethe 75 kHz signal,thenrectifiedby heterodyningit againstal5 kHz reference
signalat the correctphase.The high frequencycomponentsarethenfilteredto obtain- A-1.
an "alignmentpulse"that is proportionalto A11This process is carried out by the phase sensitive polarization signal
demodulator;a circuit diagramis shown in Fig. 2.32. Electronically,the processin
Fig.2.31 is carriedout via standardlock-intechniques.A commercialinstrumentis
not used for this becausethe fast time constantnecessaryto capturethe transientabsorptionpulseis (presently)not availableon standardlock-ins. In order to usethe
demodulator,the correctphasebetweenthe reference 75 kHz signalfrom the pEM andthe measuredsignal must be set. To do this, once again use the crossedpolarizer
configuration. The polarizerscan be set so that the maximum attenuationof lightoccurswhen the polarizationis parallelwith the quantizationaxis. The signalfrom thecrossedpolarizationconfigurationis sentto the demoduiator.Then, using the phasesettingsin Table 2.2,the demodulatedsignaiis first minimizedand the phaserecorded

l!

,t

OJ
3

cq

;

a
o)

-

o
O

.i

E

F

F

fi

o

o

C)

a

o}

!l

3
d

a
H
fl

tr
a
rd

(]
.q

o

lr

I
a't

t

O

(t)
r-'J

-:
-::

5

__l

-,-

-l
;b!

N
H

-J

1

86
Table 2.2: Phaseshift settingsfor polarizationsignaldemodulator(at75 kHz).
Phaseshift:
min -+ 8o

Dial

Phase

0
8

300
q"

118

50"

152

60"

186

90"
100"
I l0'
120"
130"
140"
150"

7tr

221

1600

80"

253

max -> l70o

10"

2tr

/1 <

TJ

82

286
JZI

354
387
42r
456
492
530
s69

at which this occurs. The phaseis then set to a value 90" from this to optimizethe
signal,just like a standardlock-in is optimized. The signalfrom the demodulatorcan
thenbe calibratedto the actualpeakto peakamplitudemodulationby comparingboth
the raw anddemodulated
signalson an oscilloscope.
Although this is everythingthat needsto be done to make an alignment
measurement
as discussedin the data acquisitionsection.there are a f'ew additionalcorrectionsthat needto be madeto the demodulatedsignal. The first is due to the finite
bandwidthof the detector/amplifier
combination.For example,the InSb pV detectors
usedin the CO2 experimentshave a -3 dB point of 120 kHz. This leadsto a l4.6Vo
attenuationof the signalat 75 kHz. The final demodulatedsignalmust be correctedby
this amount.
The secondcorrectionis due to straybirefringencein the opticalpath and strong
absorptionpulses. Straybirefringencecan resultin amplitudemodulationof the laser
light by O.l%o- 0.5Vothat is synchronouswith the PEM modularion. As highly
exaggeratedin Fig. 2.33, this amplitude modulation can lead to an offset ofthedemodulatedsignal. This would not be bad in and of itself,but when thereis astrons

87
Y

l-

c

=
E
(t)

a mp l i t u d e
m o d ul a t i o nfrom straybirefringenceabsorpton

0O

highpass

0 ffi.

demodulate

of f s e t

s p u r i o u ss i g n a l0rowpassFig. 2.33: Spuriousalignmentsignal resultingfrom stray birefringenceplus anabsorption.

absorptionof the light, the peak to peak amplitudeof the modulationdue to srraybirefringencedecreases
by an amountproportionalto the absorption.As shown in Fig.
2.33,this leadsto a dip in the offsetdemodulated
signalthatappearsto be an alignment
signal. To correct for this, the dc offset of the demodulatedsignal and the fractional
attenuationare recordedfor eachpulse. The correspondingcorrectionfor this dip isthen addedto the demodulatedsignal.
As a final note of caution,optical feedbackfrom the PEM or elementsdown
beamfrom the PEM can leadto spuriousalignmentsignals. Becausean absorptionline
shapemakes an excellentfrequencyto amplitudetransducer,this type of spurious
signaloccurswhen scanningover an absorption.The appearanceof this type of signal
variesfrom pulse to pulse,but becausethe sign of the transduceris oppositefor thetwo sidesof the absorptionprofile, the spurioussignalshave a mirror symmetryabout
the peak centerwhen scanningover the absorptionprofile. This can be avoidedby

88
usingthe sametechniquesdiscussedabovefor reducingopticalfeedbackto thediodelaser.

2.10

Sublimation studies
The sublimationexperimentis describedin detailin Chapter9. In the presenr

section we thereforeconcentrateon the constructionof the sublimation
plate,
temperaturemeasurement,
and temperaturecontrol. In addition,detailsconcerningthe
growing of surfaceswill be considered.A drawingof the sublimation
plateassemblyis shownin Fig. 2.34. The platesand thermalconduitfrom rhe liquid

N2 cold fingerto

the sublimationplate are all madeof oxygen-freehigh-conductivity(6IFHC)

copper.

All thermalconnectionsaremadewith indium foil. A thin Teflon spacer
is insertedin
the thermalconduit,as shownin Fig. 2.34,to increasethe thermalresistance
betweenthe cold fingerand the sublimationplate. This enablesus to heatthe

sublimationplatefrom 80 to l2o K while keepingthe catchplareat < 80 K. The 75 pm

wiclthof thisspacer was chosen to maximize cooling rates for the sublimationplate

whilemaintainingsufficientthermalisolationfrom the catchplate.
The temperatureof the sublimationplate is measuredwith two

Si diode

temperature
sensors(omegacY7 Series).The diodesareplacedat the middle

andendof the sublimation plate to check for nonuniform temperatures
acrossthe plate.
Temperature
can be measuredwith the diodesbecausethe voltagedrop acrossthe p-njunction is temperaturedependent.As shownin Fig. 2.3,

thevoltagedrop is due to thedepletionregion in the diode. The lower the temperarure,
the lessthermalenergythemobile electronsand holeshaveto diffuse,and the depletionregion

becomeslarger,

i'e', the voltage drop increases. The diodes used in the present
provided with a voltage to temperatureconversionfactor,

experimentare

and have an accuracyof +0'5 K and precisionof + 1 mK. To operatethe temperature
sensors,l0

lt1t0.o57c

89

F=-----r
5Cm

75 pm Teflon

spacersublimationplate

Si diode
temperature
SENSOTS

plexiglass
doserbox' 12.9f)
heaterfoil

glassfrit
CO2gas inFig' 2.34: Sublimationplate-assemblywith doserbox. The doserbox can be rotated
so that it is flush with the sublimatio_nplate to grow surfaces.It can then be rotatedoutof the opticalpath. An additionalCu^shroud
inot sho*niis thermallyto thebackof thecatchplate.
"tt".t"o90

is driven acrossthe diode,and the voltageis measuredusing a sensitiveinstrumentation
amplifier; the circuit is shownin Fig. 2.35. The currentsuppliedto the diodeis setby
measuringthe voltageacrossa referenceresistorwith a microvoltmeter.The trimpot
for the LM334 currentsupplyis then adjusteduntil the currentfalls within the statedtolerancefor the temperatureconversionsto be valid.
The temperatureof the sublimationplate is controlledvia the proportionalandintegral(PI) servoshown in Fig. 2.36^ This servozeroesthe differencebetweenthe
setpoint and the temperaturesensorvoltage. It can drive up to 0.93 A through the 12.9
C)heaterfoil attachedto the back of the sublimationplate. The temperaturestability ofthe sublimationplatewith the servoloop is measuredto be ! 5 mK over l0 minutes.
With the maximum heatercurrentof 0.93 A, the heatingrateof the sublimationplate
between95 and 100K is 125mK/s, while the maximumcoolinsrateover this ransewith the heateroff is 38 mK/s.
Surfacesare grown on the sublimationplateby swingingthe doserbox in Fig.
2.34 up to the plate,thenpulling it tight againstthe plate. The plexiglassdoserbox has(5mm) glassfrit with an averageporesizeof 10pm. CO2gasis suppliedto
a recessed
the opposite side of the frit, and the surfacesare typically grown at rates of lOa
monolayers(Ml)/sec for 100 sec. During this time, the sublimationplate rises intemperaturefrom 80 to 95 K. The resultingsurfacesappearuniform and "frosty." Asdiscussedin Chapter9, the characteristicsof the surfacedo not dependon the growth
conditions. We note for future reference,however,that identicaltestswere alsocarriedout with surfacesgrown using a frit with an averagepore size of 50,um. At growth

ratesof >104 ML/s, the resultingsurfaceswere shiny and smooth,and highly
fractured,i.e.,a visiblenetworkof crackswere observedacrossthe top of the surface.
Presumablythesecrackspenetratedthroughthe entirethin film thickness.The flux
from thesesurfaceswas considerablylessthan for the surfacesgrown with the fine fritt:t :lol H
tz l H
o €
er
U

;

6

$

.i

3

4

I

A
rd
c)

;l

!.;

|

t'=
I O

t.:

lc)
l p

l o

l3
I ( )
l a

I P
l:J

IE
I O

lO.
I

H

:{

€
C)

=

(h

t\

U

.i
a
q)
l-<

t<

-l

(n
cn
N
hh
fr .

93
underidenticalconditions.Horver,er.
thin films grown at raresof < 10aML/s with thecoarsefrit had fluxesidenticalto thosegrown with the fine frit. We believethat thequickly Srownsurfacesfrom the coarsefrit fracturedand had poor thermalcontactwith
the sublimationplate. Therefore.thefine frit was usedexclusivelyin theexperimenr.

2.ll

Data acquisition
Therearethreedifferenttypesof dataacquisition
routinesusedin the different

experimentsin this thesis.The first is for the acquisitionof high resolutionspectraof
weaklybondcomplexes.It involvessteppingthe laserfrequencyby discreteamounts
andacquiringan absorption
signalas well as diagnostic
informationat eachstep: thiswill be calleda standard
scan.The secondschemeis for themeasurementof alignment
ln supersontc
expansions.
It is qualitatively
similarto the standard
scan.exceptthatan
a l i g n m e n ts i g n a li s a l s o c o l l e c t e da t e a c hf r e q u e n c ys t e p , t h i s w i l l b e c a l l e da nalignmentscan. The final schemeis usedto measurethe cw absorptionsignalfrom
sublimingmolecules,
andinvolvesrapidlvsweepingthelaserfrequencyandrecording
an absorption,alignment.and etalontrace; this will be referredto as a sublimarrons c a n . T h e r e m a i n d eor f t h i s s e c t i o ni s d e v o t e dt o d i s c u s s i ntgh e s es c h e m e isn m o r edetail.

2.11.1 Stotdardsc'an
For the standardscan,the dataacquisitionis controlledby a micro-compurer(Gateway200015 MHz Pentium).which communicateswith a digital oscilloscope(Tektronix2430A) and two analoglock-in detectors(StanfordResearchSystems
S R 5 1 0 )v i a a G P I B c a r d ( N a t i o n a Il n s t r u m e n tG
s P I B - P C I I A( 4 8 8 . 2 ) ) . T h e G p I B(generalpurposeinterfacebus) systemis chosenfor its easeof use and high Ievel
commandsfor controllingthe oscilloscopeand lock-in detectors.The lock-insalso

9-1

havetwo 13bit D/A outputsandfour l3 bit A/D inputsthatarecontrolledvia the GPIB
interface.The diodelaserfrequencvis controlledvia oneof the D/A outputs:the D/Avoltageis fed first to a variableattenuatorfor fine controlof the stepsize.then to theexternalmodulationinput on the diodeiasercurrentsupply. The computercan thensenda commandto the lock-into stepthe D/A voltageby a certainamount,which thenstepsthe diodelaserfrequency.
The procedurefor the standardscanis as follows. First, begin pulsin,rthevalt'e. This is doneb1'sending
a squarewavetriggersignalfrom a functiongenerator(Wavetekmodel l-14)to the slit valve pulsedriver in externaltrigger mode. The
repetitionrate (t),picallybetweenI and 20 Hz dependingon the gasload)of the slit
valveis controlledb1'this functrongenerator.
andthe sametriggersignalis sentto thedigitaloscilloscope.
Next.put theconrputer
into dataacquisition
modeb1,runningthe
programSCAN.BAS. The computerresetsthe laserfrequencyand promptsthe user
for variousscanparameters.
suchas laserstepsize.numbero1'pulses
to average.andgatesfor theabsorption
thepositionof the integration
pulse(discussed
subsequentll').
T h e c o m p u t e rt h e n s e n d sa s i g n a lt o t h e o s c i l l o s c o p teo b e g i n a c q u i r i n g
p u l s e s . T h e o s c i i l o s c o pies t r r g g e r e df r o m t h e f u n c t i o ng e n e r a t oar n dabsorption
acquiresthe numberof absorption
tracesspecifiedby the computer.Thesetracesare
averaged,digitized,and sentto thecomputer.Simultaneouslv,
the computeracquires
the demodulated
referencegasandetalonfringesignalfrom the lock-in detectors.The
absorptionpulseis thenintegrated.
and the absorption,referencegas,and etalonsignal
arestoredin thecomputermemory. The computerthenstepsthe diodelaserfrequency.
and repeatsthis process.The diodelaserstepsizeis chosentobe at leasta factorof
threesmallerthanthe FWHM of the absorptionprofile. A samplescancontaininethe
absorption
signal,etalonfringes,andreference
gastraceis shownin Fig. 2.37. When

95

l-

-'
-

l-

cl

r i

It l

L
I

!

tI

o.l)

^, o>
; ;

J

a

6
. N

-

5 c s
: o

C
al

o

-

i

!
I

> C o
< ( / ) J

a
t

3 N

t
t-

-

!

o

5

E
2

r

0
c
c
G

-

!

-c2
l!z
'E
c,
\
J

t

O

c)

/,

--

t

=

i

U

l-

I
I
I
Rioo trir ? = Ar-I-liO

t

c.^i

'J:

utO
O

A

L

t

=

I
6
Ot

).

c
o
o

:

F

!

O G ,':
C J ^

\ *
(t

( s l u n q r e )a o u e q r o s q v

'

II

.-

t-- .:; .] r'col96
the scanis finished.a final datafile rs transferred
to floppy disk. The formarof the data
file is presented
in AppendixA.
To extractthe absorptioninformationfrom the transientabsorptionpulse.an
integrationschemeis used. The digitizedpulseis brokenup into threeparts: tu,obaselinegatesand a signaigate.as shownin Fi-e.2.27. The absorption
pulseis thenintegratedby subtracting
a baseline.fit betweenthe two basegates.from the si_enal
gate' l'e'
r

\..

r

i \^.,,.\n,.

\j ) b a . s , , \t v1
s r g n a=l 1 . ) s i ( ' ( . r ) - ^ : ', ) + L) ht r r . r , , l ( r , ) i .
4",,;1A.,.,\;")

rt.5r

Here..sig(-r)
is the arravcontainrn-c
the digittzedsrgnalgateinformation.and1V.1'.
is thenumberof pointsin the array. The definitionsfor the basegatesare similar. The
baselineintegrationschemeeffectivelvfilterslow frequencyfluctuationsin the baseline.
The computerprogramallowsone to adjustthe positronand lengthof the rlifferent
gatesfor maximumnoisereiection.Typically.a signalgateof 400 gs is used.w,ith
basegatesof length200prseach.The base,eates
arepositioned
ascloseas possibletcr
theleadingandtrailingedgeof thepulse.
Finally. informationfor the power scalingof the absorptronsignal is alsocollectedduringthe scan.This cantaketu,otorms.dependingon theexperiment.For
theCO2experiments.
thedetectors
areInSbPV detectors,
rvhichrecorda dc signalthat
is proportionalto the power incidenton the detector.This dc level is recordedby
feedingthe dc signalout (prior to AC filtering and subtraction)from the subtractron
circuitin Fig.2.15to oneof the lock-inA/D inputs.The computerthen
recordsthe dc
light level in volts for eachlaserstep. Sincethe detectorshave a dc offset due tobackgroundblackbodyradiationalone,the computeralso promptsthe userfor thisoffsetvalue(recordedin a separate
measurement)
to subtractfrom the recordeddc levelto obtainthe true dc light level. Sincethe absorptionpulseis alsorecordedin volts bt,

the oscilloscope,
it is simpleto scalethe absorptiondatato retrievethe absorbance.
first
takrnginto accountthe factorof 100gainintroducedin the subtraction
circuit.
For the H2O experiments.
the signaldetectoris a HgCdTePC detector.and theoutputis ac coupled. Hence.the abor.'e
procedurecannotbe usedto recordthe power
level. Instead,the demodulated
referencegassignalis usedfor this purpose.Barnng
beamsteeringeffectsduringthe scan,the Iight le'u'el
on the referencegasdetectoris
proportional
to the light levelon the signaldetector.Sincethe reference
gassignalis
first choppedand then demodulated.
the dc level informationis presenr.Therefore.
beforethe scan.the proportionality'
consrantbetu'eenthe light level on the signal
detectorand the demodulated
rel-erencesas detectoron the lock-in is determinedby
choppingthe light incidenton the sienaldetector.This proportionality
factorcanthen
be usedin the dataanalysissectionto scaiethe recordedabsorption
pulsesto retpeve
the absouluteabsorbance.

2.11.2 Aligrunentsc'cur
T h e s a m ea c q u i s i t i o np r o c e d u r ei s u s e df o r t h e a l i g n m e n ts c a na s f o r t h e
s t a n d a r ds c a n ' T h e o n l y d i f f e r e n c ei s t h a t t h e d e m o d u l a t eadl i g n m e n ts i g n a l( a sdiscussed
in Sec.2.9) is input into channeltwo of the digitaloscilloscope.The scope
thendigitizesboththe absorption
andalignmentpulsesandsendsthemto thecomputer.
The baselineintegrationschemeis carriedout on both the absorptionand alignment
pulses,and the resultingsignalsare recordedalong with the etalon fringes
andreference
gastrace. The programtharhandlesthis is calledALSCAN.BAS:the data
formatfor files from this programis coveredin AppendixA.
Powermeasurements
are alsodifferent. Sinceeachalignmentscanis over a
single- 200 MHz FWHM monomerabsorptionprofile,the light level doesnotchangesignificantlyover the courseof the scan. Thus,the light level at line centerisrecorded

98

beforestartingeachscanby choppingthe signallight. This informationis recorded
along with the data file to accuratelyscalethe measuredsignals. The ALSCAN
programalso usesthis informationto correctfor spuriousalignmentsignalsdue to
stravbirefrinsence.
as discussed
in Sec.2.9.

2.I 1.3 Sublintatiotl
sc'un
As opposedto the otherexperiments.
the absorptionsignaldue to the subliming
moleculesis cw. Hencethe laserfrequencyis sweptoverthe absorption
profile(5 msover - 2.5 GHz) by inputtinga sawtoothvoltagewave from the functrongeneratorinro
the externalmodulationinput of the diodelasercurrentsupply. The oscilloscopeisthensimultaneouslv
trisgeredby the functiongenerator.
and acquiresa scanover theabsorptionprofile and etalonfringes. This is digitizedand sentto the computerfor
storage.For the sublimationalisnmentmeasurements,
first the absorptionsignalis
recorded,
thenthedemodulated
alrgnment
signalin a separate
measurement,
sincethree
traces(includin-etheetalonfringes)cannotbe recordedsimultaneouslv.
The programt h a t h a n d l e st h i s i s c a l l e dS U B L I M E . B A S .a n d t h e d a t a f i l e f o r m a ti s c o v e r e di nAppendixl.

p r o c e d u r et.h o u g hq u i t e s i m p l e ,h a s p r o v e nt o b e
The acquisitron

remarkablystablein lrequency.Althoughonecouldtriggerthedataacquisitron
sweep
from an etalonfringe.this hasnot beenfoundto be necessary.Multiple averages
ofsweepstakenover a l0 s periodshow no discerniblebroadeningof the = 75 MHz
FWHM absorptionprofiie.
Powermeasurements
aremadebv rapidlysublimingawaya thin film so thatthetransitionof interestis optically black. The peak height of this saturatedsignal
corresponds
to the light intensitvon the signaldetector,and can be usedto scalethe
recordedsublimationsignalsto retrieveabsorbanceinformation. This procedureensuresthatthe measured
signalsarecorrectlyscaledto the light level in the diodelaser

99
modeof interest.since- 5Vcof the totaliight poweris sometimes
in secondan,
diodemodes.
For each sweep over an absorptionprofile, both the temperatureof thesublimation
plateandthe backgroundpressure
in the chamberas measured
bv the ion
gauge(VarianVacuumIonizationGauge,model843) arerecordedvia the A/D inputs
on oneof the lock-ins.Theseraw signalsarescaledusingsimplealgorithmssupplied(Omegafor the temperatureby the manufacturers
sensors,
Varian for the ionization
gauge)to obtainthe truetemperature
andpressure.For the remperature
programnreddesorption(TPD) experiments.
the sublimarion
plateis eithercontinuouslyheatedby
prol'idinea constantcurrent.or cooiedbv reducingthe current.and the compurer
monitorsthe temperatureof theplateand startsdataacquisitionat specifiedtemperatureor time intervals.The programthathandlesthis is calledTPD.BAS,and the data
fileformatis coveredin AppendixA.

2.12 Data analysis
2.12.1 Standurdsc'an
As seenin Fig. 2.37.the standardscanis recordedas a functionof channel
number,i.e., frequencystepsof the diodelaser. It is necessary
to convertthis rnto
tiequencybeforethe peakpositionsand line shapescanbe derermrned.This isdoneintwo ways,dependingon whetherthe unstabilized
markercavity is beingusedin theCO2experiments.
or the stabilizedmarkercavity'is usedfor the H2O experiments.For
the unstabilizedcavity, the analy'sis
programusedis calledSPECTRA.BAS;

first theetalonfringe peaksarelocatedas a function of channelnumber. A simplepeak
findingalgorithmis implementedthat locatesthe maximum of eachetalonfringe
aboveaspecifiedlevel. The maximumandtwo pointslocatedto eithersideof the maximum
arethenfit to a parabolato more accuratelydetermrnethe fringecenter. The

fringecenter100

in channelnumbersis thenstoredfor all fringesin an array. Next, two referencepeaks
are chosenfrom the referencegas trace. Theseare typically two peaksu'ith uelldefinedcentersand well separated
in the scan: in Fig. 2.37,thel' would be the tw'osmaller"hot band"peakson eithersideof the strongmonomerfundanrental
absorption.
The centerof thesepeaksis determinedusingthe samealgorithmfor findingtheetalon
fringecenter.The channelnumberof the reference
peaksis thenstored.as well as thefrequencveof eachpeak.
A f t e r t h i s i s c o m p l e t et,h e f s r o f t h e e t a l o ni s d e t e r m i n e d
b y d i v i d i n gt h efrequencyseparationof the two referencepeaksb1'the numberof etalon fringes
betweenthem. Fractionalspacingson the endsaredeterminedby linearinterpolation
betweentwo fringes.If therearenot tuo sul'ficientlv
separatedreferencepeaksin thesamescan,the fsr from a scanmadeverv neilrin trnteto the scanof interestis usedinstead. Once thesestepsare finished.it is relativelvstraightforward
to assigna
frequencyto eachchannelnumber.The numberof etalonliingesbetweenthechannel
numberof interestand one of the reference
peaksis determinedby countinginte-eralfringes,and usinslinearinterpolation
for fractionalspacin-ts
on theends. This is thennultiplied by' the fsr and addedor subtracted
from the referencepeak frequencyto
determrne
the frequencl'of thechannelnumber.
F o r t h e s t a b i l i z e dc a v i t v H 2 O e x p e r i m e n t st,h e p r o c e d u r ef o r a s s i g n i n gfrequencies
to eachchannelis very similar. The etalonfringecentersarefoundin thesamemanneras above. But nou' one of the actualetalon fringes is used as thereferencepeak. The only trick is to determinethe frequencyof this "referencefnnge'
in the scan. To do this first an absolutereferencefringe is chosen.this fringe isseparatefrom the referencefringeusedin a particularscan,andis the foundationfor all
frequencymeasurements
madewith the stabiiizedcavity. The absolutereferencefringe
is measuredrelativeto an H2170or Hrl80 monomertransitionin the slit iet. For

101

consistency,
thesereferencefringesare alwavschosento be the first fringe to the red.
i.e.,lowerfrequency,from the reference
peak. Sincethe fsr of the stabilizedcavityis
accuratelyknown, linear interpolationcan be usedto determinethe frequencl,for theabsolutereferencefringeusingtheprogramFRINGE.BAS. This measurement
is madeseveraltimesto assignthe absolutereference
fringeas accuratelyaspossible.
Next,the frequencyof the referencefringein a particularscanis assignedusing
the absolutereferencefringe. First.the approximate
frequencypositionof the reference
fringeis determinedfrom a reference
gasabsorption
or an assigned
absorptionin thes i g n a l t r a c e u s i n g t h e s a m e p r o c e d u r ea s a b o v e . T h e d i f f e r e n c eb e t w e e nt h i s
approximatefrequencvfor the referencefringe and the absolutereferencefringe
frequencvis thendividedby'the fsr to determinethe numberof integralfringesbetweent h e t w o . T h e r e s u l t i n gn u m b e ri s t v p i c a l l vw i t h i n 0 . 1 o f a n i n t e g r a n
l u m b e r .T h i s
u n a m b i g u o u s liyd e n t i f i e st h e n u m b e ro f i n t e g r a fl r i n g e sb e t w e e nt h e t w o , a n d t h ereferencefringefrequencyis thenassigned
by'multiplyingthe numberof fringesb1,thefsr andthenaddingor subtracting
this lrom the absolutefringefrequency.
Oncethefrequencyof eachchannelnumbercanbe assigned,
peakpositrons
canbe determined.The channelnumberof the centerof eachpeakis determinedbv fitting
the peak maximum plus two points to either side to a parabola. Thesepeaksarepresmoothedusinga Pascal's
trianglesmoothingalgorithmto minimizethe effectsof
amplitudefluctuationson this analysis. There is also the option in the programs
SPECTRA.BASfor the unstabilized
cavity and CAVITY.BAS for the stabilizedcavitl,
to specifythe peak positionin termsof integralchannelnumberif necessary.The
frequencyof eachpeakis thencalculatedby countingthe numberof etalonfringesto
the referencepeak or fringe, with fractionalspacingsonceagainbeing determinedbv
linear interpolationbetweentwo fringes. It is also possibleto extractline shapeinformationby savinga particularpeakas a text file of absorbancevs. frequency(this101

is a self explanatoryoptionin the anaivsisproerams)that can thenbe usedu,ith thegroupline shapefittingroutinesrn rhe82l2 account.
Lastly, the vertical scaleof the standardscancan be scaledto obtain theabsorbance.
For the CO2measurements.
the totalinfraredpoweris storedin the data
file for eachfrequencystep.andthe anall''sis
programusesthis to scalethe si_enai.
For
the H2O measurements,
the reference
gastracetimesthe appropriate
scalefactor(seeS e c .2 . 1 1 . 1 i)s u s e dt o s c a l et h e s i g n a l .T h e a n a l y s i ps r o g r a mh a sa n o p t i o nr o e x c l s e
the absorptionpeaksfrom the refercncegastraceso that it canbe usedfor this purpose.
O n c e t h e v e r t i c a la x i s i s s c a l e dt o t h e a b s o r b a n c ei t. i s p o s s i b l et o d e t e r n i n et h e
l n t e g r a t ea
d b s o r b a n coef s p e c t r alle a t u r e su s i n gt h e s e l f e x p l a n a t o rov p t i o n si n t h e
a n a l y s i sp r o g r a m s . T h i s i s v e r v u s e f u l i n f o r m a t i o nf o r d e t e r m i n i n ga b s o l u t e
concentrationsof species
in theexpansion.

2.I 2.2 Alisnrnertt st'urt
For the alignment scans.it is not necessarvto measurethe absolutefrequencyof the scannedCO2 monomer trrnsitions.since theseare alreadyknown. Instead.therelevantfrequencl'axis is the shift fron-riine center. The line shapesare recordedsuch
that thev are surroundedby two etalon fringes. The peak center is obtainedfront thereferencegas trace, and the shift from line center is determined via linear interpolation
between the two etalon fringes. This procedure is carried out using the analysis
program ALIGN.BAS;a sample print out from this program is shown in Fig. 2.3g

with the frequencv shift assignedfor the absorption signal (bottom trace) and thedemodulatedalignment signal (top trace). The results are presentedin actual yolts.
which are then scaledwith the known calibration facror for the demodulatedsignal andthe recordedinfrared light level. In this way. the relevantalignmentinformation. r.e..
P = (Al - Al,)l(Au + A r ), can be determined.

103

-

- > a
= . J

= -

. J 9

:

G
!

d > 2

a

N

=*

;i:1 ,^
=

\ !

G
c
o
a
c
o

- '

r N

6
. R
>
t ^ Z

< : a

5

3 5

<

v s

=
N

O

-

6

-c-

O

J

d

.

=

J

- . , N

O
c

c

t--

c
o

tt

)
E

o

':
tN s' N5 o

o

'.1

i

)
/

o ; ^

,
o
= - O

>-

@
o
a

o
N

j

t
(

(
I

t
r-

L

>

C-s
' a )

H

t t
>rL

- a

L

tI

rt-

t
t-

O
N

c : :
. - !

O

tl
t-

r)-

E;,
@

F

/
- (a c)( s l l o ^ r l I r ! )e c u e q r o s q v# P'J- 7.

104
A "tomographicanaiy'sis"is also carriedout for each scan. In additronromeasuringthe aiignmentat line center.the alignmentas a functionof positionin the
supersonic
expansionis of interest.The positionin the beamcan be relatedto the
Dopplershift from line center Hencebv determiningP as a functionof frequencv
shift, a slice can be takenthroughthe beam. The analvsisprogramcarriesour an
tomographic
analysisby dividingthe demodulated
signalby the absorption
signalfor
eachfrequencyshift. This is an approximate
analysismeantto alertthe userto anv
significantvariationsin alignmentacrossthebeamprofile. The tomographicanalysisis
alsoprintedout for quickvisualreference.
as shownin Fig. 2.38.

2.12.3 SublintuticttsL'en
Sirnilarto the alignmentscans.onll' the frequencyshift from line centeris oft n t e r e sfto r t h e s u b l i m a t i o ns c a n s . T h e s u b l i m a t i o nl i n e s h a p e sa r e a l s o r e c o r d e dsurroundedby two etalonfringes.so the sameprocedure
as aboveis usedto assignthefrequencv
shiftfrom line center.The proerantthathandlesthis is calledSUBAN.BAS.
After the frequencyshift is assigned.
two quantitresare of interest: the integrated
absorbanceof the peak and the line shape. However,for very weak sublimation
signals,therearefrequenct'
dependent
modulationsof thebaseline
duringa rapidscan,
as shownin Fig .2.39. Thesearedueto changingrnfraredlight leveloverthe scan.or
residualetaloningdueto opticalfeedbackto the diodelaser.
Clearlythe fluctuationsin the baselinemustbe accounted
for beforeaccurare
integratedabsorbances
and iine shapescan be determined.To do so, the analysis
programfits the baselinesurroundingthe absorptionpeakand then subtractsthis fromthe entiresignal. The baselineis fit to a quadraticpolynomialto accountfor the grossoffset,slope.and curvature.It is also fit to two sinusoidalcomponentsto accountfor
the rapid oscillationdue to etaloning.The periodof thesetwo oscillationsfrom the

10-s

9

-

$a=:-uEF

I

:

a t

-

E !

^,>:-

t

h - ; <

c >
- 9

d
--t g

a

-,,--/

.>

\

'

r

r )

i

O
e

5E
n
F !i :

i9ry*_* , .
I ili t

\.-

,()

-

i
^ I :n ee.*i
!i

,.''

C

d i

.

oc

=
e

E
t-

€
:

l-

rir0

a

-

c

L

()
e

c-

-

E
0)

L

!

-

!

I

e

j

j

-

9

: g ! j

-:
I

a

- E -

i;];
; 5 : j

--

+

.)

-

;t::

C

i
+

't

L

<

3

I-

E

e

! ' : i

[-"r

;

)6)
E

L

) 7 )

;']

-

'c-

t-e

:a

--=
. > I

.-.

= J

e
t

/

-

t

-

n

t
a,

.3

-

=

L

i

. .C;
' = (

<
.-(

1

>\,
r *

u .,
e

---_---_-.-

l

ta:
!..)

'c

{
{

a '
3

a.)

:

-

l

.t

!

C

'-=

o.i
.;
l1

-'x ^

- . t
r !

s

l-)

z -

106
Ieastsquaresfit is foundto be = .1001r4Hzand= 200 MHz. The resultsof thebaseline
subtraction
are shownFig. 2.39. The absorptionsignalis then scaiedto the infrared
light level to andintegratedto obtainthe integratedabsorbance.Line shapesaresayed
to a text file of absorbancevs. frequencvand then fit using the line shapeanalvsis
programsin the B212 account.Finalil'.smallcorrections
to the integrated
absorbancearemadedueto background
300 K COzgasin the chamber.This processis discussed
in Chapter9.

10References for ChaPter 2
1.

Lasers.2nd ed' (Van Nostrand
G. P. AgrawalandN. K. Dutta.Semiconductor
Reinhold.Neu' York, 1993).

2.

W . W . C h o $ , .S . W K o c h a n dN { .S a r g e nItI I . S e m i c o n d u c t o r - L a sPehr} ' s i c r .
Berlin. I 99-l).
(Springer-Veriag.

3.

L ra s e rD e v i c e s .( W i l e v 'N e $ 'G . H . B . T h o m p s o nP, h ! ' s i c so f S e m i c o n d u c t oYork,t980;.

4.C.E.WiemanandL'Hollberg.Rer'.Sci.Instrum.62.1(1991).
5.

and C. wieman.Am. J. Phys.60. 1098
K . B . M a c A d a m .A . S t e i n b a c h( 1992\.

6.

y l e c t r o n i cfso r S c i e n t i s tasn dE n g i n e e r s1. A l l 1 ' n
R . E . S i m p s o nl.n t r o d u c t o rE
1987 )p. . i 5 l .
B
o
s
t
o
n
.
a n dB a c o n I. n c . .

1.

E l e c t r o nQ' E - 1 8 '2 5 9 ( 1 9 8 2 ) 'C . H . H e n r y ,I E E EJ . Q u a n t u m

8.

. n i v e r s i t yo f C o l o r a d oB. o u l d e r '1 9 9 1 'M . D . S c h u d e rP. h .D . t h e s i sUg.

G . G u e l a c h v i lai n d K . N . R a o .H a n d b o o ko f l n f r a r e dS t a n d a r d s(. A c a d e n l i c. l o r i d a .1 9 8 6 ) .
P r e s sI.n c . ,O r l a n d oF

10.

E . R i e d l e .S . H . A s h w o r t h .J . F a r r e l l .J . T ' a n d D J ' N e s b i t t 'R e r ' ' S c i 'l n s t r u m6. 5 . 4 2 ( 1 9 9 - 1 ) .

11.

J . J . T . F a r r e l lP. h .D . t h e s i sU. n i v e r s i tov f C o l o r a d oB. o u l d e r 1. 9 9 5 '12.

, . K u n z n a n na n dF . L e b o w s k . vA.p p l O p t ' 1 1 . 7 1 2 ( 1 9 1 2 ) 'R . B a l h o r nH

13.

T . M . N i e b a u e rJ. . E . F a l l e r .H . M . G o d w i n ,J . L . H a l l a n d R ' L . B a r g e r .
A p p l O p t .2 7. i 2 8 5 ( 1 9 8 8)

11.

D . K a u r ,A . M . d e S o u z aJ. . W a n n a .S . A . H a m m a dL, . M e r c o r e l l ai n d D . S .
P e r r yA
, P P I O P t .2 9 , 1 1 9( 1 9 9 0 ) .

15.

G. Scoles.Atomic and MolecularBeamMethods,(Oxford Universitl'Press'
New York, 1988).

16.

C. M. Lovejoy,Ph. D. thesis,Universityof colorado,Boulder,1990.

11.

G. Dupeyrat.in RarefiedGas.Dynamics,editedby S. S. Fisher,(AIAA. New, o l . 7 4 .P P . 8 1 2 - 1 9 .
Y o r k ,1 9 8 i ) V

1g.

c . M . L o v e j o ,av n dD . J . N e s b i t tR. e v .S c i .I n s t r u m . 5 8 , 8 0 (71 9 8 7 ) .

lg.

J . A . B a r n e sa n dT . E . G o u g h .J . C h e m .P h y s '8 6 , 6 0 1 2( 1 9 8 7 1 '