l9
Chapter2

Experimental

2.1 Introduction
This chapterwill describein some detail the designcharacteristicsand constructionof the apparatususedfor vibrationallymediatedphotodissociationof small molecules
and weakly bound molecularcomplexes.The experimenthas been evolving
continuallyover the courseof six years,it is useful,therefore,to collectin one placea
comprehensiveoverview of the critical elementsof the apparatusin the presentstageof developmentand to review the considerationstaken into accountin the design
stage. Each of the major subunitsis discussedseparately,with emphasison thecomponentsthat are unique to this apparatusand thereforenot documentedelsewhere.
We do not include in the current chapterthe experimentaldetailsspecific to each of
the experiments;thesewill be addressedin the experimentalchaptersthat follow.

2.2 Experiment overview
The key componentsof the experimentalapparatusare summarizedin Fig. 2.1. A
slit supersonicexpansion(4 cm by 50 pm, 500 ps pulseduration) is usedto cool theparentHzO moleculesinto the lowest rotationallevels allowed by nuclearspin
statistics,i.e. 06s(para)and lor(ortho)and to producethe HzO containingweakly
bound complexes. As a result of the slower (1/r) drop off in gas density,the slit
expansionprovideshigh concentrationsof HzO per quantum statein the interaction

20

Pump

V a c uumcham ber Photolysis

Probe

Datastoragecom puter

OscilloscoPe

beam isFigure 2.1: Overview of the experimentalapparatus'A cold molecular
jet to the vacuum chamber'preparedby supersonicexpansionthrough a pulsed slit
pump beam
where the molecularbeamis crossedby three laserbeams' First, thearepreparesthe desired initial stateof the reactants;second,the prepared states
via LIF'probedarephotoproducts
pnotoaissociateuby the photolysislaser;third, the21region, as well as providesa long absorptionpath for enhancingthe signalsfrorn
weak overtonevibrationalytransitions.Furthermore,the slit geometryleadsto
efficient collimation of velocitiesalong the slit axis, which leadsto an order ofmagnitudenarrowing of overtoneDoppler widths and thus a correspondinglO-foldenhancementof vibrationally mediatedphotodissociationsignalsfor a sub-Doppler
pump laser.
The jet cooled H2O moleculesor H2O containingcomplexesare then efficiently
pumped into specific rovibrationalstatesin the v6s = 3 OH stretchovertonemanifold
by IR pump pulsesfrom a singlemode,tunableopticalparametricoscillator(OPO).
This OPO is used to producesignaland idler pulsesin the visible (570 nm-650 nrn)
and near-IR (790 nm-950 nm) spectralregion,respectively. By injection seedingthe
resonatorwith a single-modecw dye laser,a Fourier-transform-limitedoutput of thering OPO is achievedwith a spectralwidth of 160 MHz (= 0.005 cm-').
In conjunctionwith output pulse energiesof > 5 mJ/pulse,this OPO light source
provides sufficient spectralbrightnessto appreciablysaturateup to Av = 3 OH stretch
overtonerovibrationaltransitions.In combinationwith efficient jet cooling of parent
speciesinto the few lowest rotationallevels,this permits significantoptical pumping(10-20Vo)into a given rovibrationalquantumstate. For typical stagnationpressuresand distancesdownstreamfrom the slit nozzle,thistranslatesinto excited stateH2O
monomerdensitiesin excessof l0l3 molecules/cm3and 4 x l0ll total moleculesin
the detectionvolume, respectively.For the water containingcomplexesthe estimatednumber densitiesare approximatelya factor of 103lower comparedto the H2Omonomer, due to =l%a complexation efficiency and l0-fold more rotational states

22
thermally populatedin the jet.
The vibrationallyexcited H2O moleculesare then selectivelyphotodissociatedby
the photolysislaserpulse. This pulse is provided either by a KrF excimer laserpulse
at 248 nm, or alternativelyby tunableUV radiationat roughly 222 nm from frequelcy
doubled and sum-frequencymixed output of a pulseddye laser. Typical pulse
energiesof 50 mJ/ pulse and 5 mJ/pulsefor the excimer and dye lasersystents,
respectively,are used in the experiment. At thosewavelengthsthe vibrationallyexcited moleculesare photodissociatedpreferentially,with the photolysiscrosssectionratio o(v6s = 3) / o(voH= 0) > 103. The photolysiscrosssectionsfor H2O areestimatedfrom quantum scatteringcalculationsdescribedin some details in Chapter 3
to be o2a3= 2 x 70-2tcm-2and 6222= 6 x l0-20cm-2,which for typical UV pulse
energiesand a beamsizeof I mm2 translates
into 4x10eand lx10l0 total numberof
photolyzedH2O monomer moleculesin the detectionvolume for the 248 nn and222
nrn photolysis,respectively.
A conventionalfrequencydoubledpulseddye lasersourceprovidestunableradiationin the 280 nm - 310 nm spectralregionto probethe 2I <- 2n electronictransitionin the oH and oD photoproducts.The LIF from a I mm x I mm x 4 cm
detectionvolume is spatiallyfiltered and imaged onto the PMT, using light collection
assetnblyoptics for high scatteredlight rejection( 1: 10e)and high efficiency (abour I 07o)collectionalong the slit dimensionl.To obtainreliableproductstatedistributions
from the measuredLIF spectra,however,considerablecare must be exercisedto
anticipateand correctfor possibleLIF signal saturation.The UV probe laseris
therefore operatedat rather low energiestypically of 2-20 pJ/pulse, a range overz-)which the LIF signalsbegin to show slight curvaturewith increasingpower due topartial saturationof the OH electronictransition,yet still reliableenoughthat OH
state populations can be extractedfrom the spectra'

2.3 The injection seededoptical parametric oscillator(OPO) is theln many respecrsthe injection seededoptical parametricoscillator
infraredheart of the experimentalapparatus.It is the "work house" for producingthe
ofradiationnecessaryfor preparingsingle initial quantumstatesvia optical pumpingforthe overtonetransitionsin the parentmoleculesand molecular cornplexes
dynamicsstudies.This oPo is a one-of-a-kinddevice,designedand built
subsequent
includins
specificallyfor theseexperiments.Therefore,a more detaileddescription,brief overview of the underlyingphysicalprinciplesand descriptionofathe designand

performanceof this device,will be presentedin this section.

2.3.1 Principles of oPeration
nonThe optical parametricprocessis basedon nonlinearoptical propertiesof
at frequencies
centrosymmetriccrystallinematerials. If two electromagneticwaves(D1,&nd 0)2propagatethrough such a material, waves at the sum (ror + rrrz)anddifference (ror - q) frequenciesare produced due to the nonlinear optical
exploited tosusceptibilityof the material.Under certainconditionsthis effect can becreate a net power transfer between the electromagneticfield components

at different

frequencies. Specifically, it can be shown that for the waves olr' 0) and co:(rtlr > roz

. A
L+

> rlrr) propagating
collinearlythroughthenonlinearopticalcrystal,two conditions
powerflow betweenthosewaves:must be fulfilled for the macroscopic( t ) 1= o ) * 0 ) 3(2.1)

nrO)r -nz0)*nt(l')-l(2.2)

where D1,n2 nnd D3are the indicesof refractionat frequencieso)1, to2and trlj,
respectively.
In the framework of quantumfield theory, theseconditionsrepresentrequirementsof energy and momentum conservation,respectively,in a processin which either two
photonsat frequencieso) and ob are annihilatedto produceone photon at frequencyo)l or vice versa,one cDlphoton is annihilatedand photonsat o)2and co3are created.2
The constrainreflectedby equation2.2 is usually referredto as the phasematchingcondition, sinceclassicallyit representsthe requirementof constantphaserelations
betweenthe the three interactingwavesthrough the length of the crystal,which isnecessaryfor a net energytransferbetweenthe different frequencycomponents. For
given set of frequenciescD1,o)2,o.l (alreadyfulfilling the condition o)1= o) + ror ) theindices of refractionmust be adjustedto fulfill the phasematching condition.
Fortunately,this can be accomplishedquite easily utilizing the fact that the nonlinear
crystals are birefringent, and therefore,the index of refraction dependson bothpolarizationand direction of propagationof the wave with respectto the crystal
optical axis. It will be statedwithout further proof that for a range of f:equencies(characteristicfor given nonlinearmaterial),a unique direction of wave propagation

t<existsin the crystalfor which the phasematchingconditionis fulfilled.2'3 In
uniaxial crystals,this direction is determinedby an angle called the phasernatchingangle, betweenthe crystal optical axis and propagationdirection of the beams.
Thereforeif a beam at frequencyro1passesthrough the nonlinearoptical material at a
certaindirection with respectto the optical axis, net parametricgain can be createdfor
beamsat unique frequenciesol and co: determinedby the energyconservation
constraintfrom Eq. 2.1 and by the phasematchingcondition in Eq. 2.2. Those three
beamsare usually referredto as pump, signal and idler respectively. As an example,
Fig2.2 shows the dependenceof the signal and idler frequenciesfor a B-Barium
Borate crystal pumped at the 355 nm. The nonlinearmaterialthen acts as a lasergairr
tnedium: a weak beam at the ol or o.j is amplified in the crystal by interactionu,itl-r
the strongpump field at ror. By placingsucha gain rnediurnin an opticaloscillator
resonanteither at ol2or rrrr,a laser-likedevice is producedthat generatescoherent
output at signal (o1) and idler (ah ) frequencies.Such a device is called an opticalparametricoscillator(OPO).
Although the potentialof opticalparametricoscillatorsas broadlytunable,all-solidstatesourcesof coherentradiationhad long beenrecognized,technicallimitations
preventedtheir.widespreaduse as high resolutiondevicesuntil relatively recently.
Two major advanceshave been i) the developmentof pulsed laserswith suitablesingle mode quality to be usedas OPO pump sources,such as now availablewith
narrow band, injection seededNd:YAG lasers,and ii) improved nonlinearoptical
rnaterialssuch as B-Barium Borate (BBg;+-s and Lithium Borate (Lno;s'o *1,n
optical propertiesand high damagethresholdssuitablefor efficient OPO operarion.

26

BBO,typeI phasematchingfor 355nmpump

F

tr
tr
R 800

30

'

31

32

33

phasematchingangle(deg.)

Figure 2.2:Wavelength of the signal and idler waves as a function of the phasematching angle for B-Barium Borate crystalpumped at 355 nm.

34

27
In the absenceof any frequencynarrowing scheme,the linewidth of an opo
outputis limited by the angularphasematchingconditionin the nonlinearmediurn,which can be unacceptablybroad (Av ) I cm-r) for many spectroscopicapplications.
This is especiallytrue for multiple resonancespectroscopies,
where spectral
brightness(i.e.,pulse energyper unit frequencybandwidth)is the limiting quantity,ratherthan simply pulseenergy.Additionally,the unnarrowedOPO line width isstrongly wavelengthdependent,increasingconsiderablyas the degeneracypoint= o)idler=l/2 tDpu,np)(o\ignorfrequencyis approached.A number of narrowing
techniqueshave beenproposedand implementedto reduceOPO line widths closer tothe Fouriertransformlimit. If only modestresolutionis needed,one of the cavity
tnirrorscan be replacedby an opticalgratingin Littrow configuratiop,T,l0-121a,111.thigherresolutionschemesrequiringseveralfrequencyselectiveelementsto tune
t+ In the stateof the art OPO designsof this latterkind, singlemodesynchronously.l3'
operationhas been achievedover a broad tuning range,but at the cost of decreased
efficiency and considerablecomplexity of such devices.
A powerful alternativemethodfor frequencynarrowing is via OPO injectionseedingwith narrow-bandradiationfrom an externallasersource. In an unseeded
OPO, all the cavity modesinitially build up in power at a comparableexponential
rate, typically resultingin multimode oscillation. By way of contrast,if the cavity
alreadyhas circulatingpower due to injection seeding,the power in the seeded
longitudinalmode builds up rapidly during the pump pulse comparedto rhe orher
cavity modes,which then depletesthe parametricgain before the other modes can28reach threshold.Single mode operationof the OPO can thus be achievedin pulsed
operation,with the output frequencylocked onto the injection seedsource.

2.3.2 OPO Cavity Desigrt
The overall layout of the optical parametricoscillatoris schematicallyshown in
Fig.2.3. The setupconsistsof i) a planar four-minor OPO ring cavity, ii) a single
rnode (pulsed)pump laserand iii) a single mode (cw) injection seedlaser. If one
neglectsweak mode pulling effectsto be discussedlater, the signal OPO output
oscillatesat the samefrequencyas the seedlaser,while the idler frequencyis dictated- Qignar).For 355 nm pump radiationand= C\urnp
by energyconservation(0)0r",injection seedingwith a cw R6G dye laser,the idler providesaccessto the 800- 1000
nm region,which overlapsconvenientlywith the Av = 3 overtonebandsin the OH,
CH and NH stretchmanifolds.
Details of the OPO resonatordesignare shown in the Fig. 2.4. The parametricgainof the OPO cavity relieson a pair of l0 mm BBO crystalswith 5 x 6.5 mtn2 entrance
faces.The angle betweenthe crystal optical axesand the entranceface normal is
28.5o,i.e., the crystalsare cut for type I phasematching,with the signal and idler
beamspolarizedalong the 5 mm edge.Both BBO crystalsare cut from the satneboule
to ensurematching optical properties,with input and exit surfacesanti-reflection
coatedat the signal wavelengthto avoid intracavityreflectionlosesin the OPO
resonator.The two crystalsare mountedin precisionrotationalstagesin the walkoff
compensationalrangementdescribedby Bosenbergl5,seelower pancl in Fig.2.4.

29

I N J E C T I OS
NE E D E DoPo

S E E D ED
IN JE C T ION
Nd.YAG

UV(355nm)
P UM PS I N G L EM O D E
CW RINGDYELASEROPOCAVITY
IDLER(830-940nm)

S E E D( 5 7 0 - 6 2 0 n m )SIGNAL(570-620nm)
Figu(oPo). The
re2.3: Schematicof the injection seededopticalparametricoscillator
at 355 nm
laser
Nd:YAG
pulsedOPO cavity is pumped by the third harmonic of aand injection seededby a tunablecw dye laser. The signal output of the OPO
near IR
coincidesin frequencywith the seeddye laser,while the idler accessesthe
resion.

30

C A V I T YL A Y O U TUVSTEERINGMIRRORSIDLERmPZT DRIVENMI RR O R

SEEDER(cw dye)

BBO CRYSTALSUVPUMPS I GN A L

SIGNALS IG N A L

UVPUMPUVPUMP
'\ /'

\ l

@

.PHASEMATCHINGA N GL E

V

O P T I C A LA X E S O F
BBOCRYSTALS
is formedby a fourof theOPOcavity.The opticalresonator
Figure2.4: Schematiccompensating
walk-offinacrystalsflat mirrorring cavitywith two BBO
arrangement.The PZT mountedmirror permitsservoloop lockingof the resonatorto
into theOPOring via a pair of UV
Pumpradiationis introduced
the seedfrequency.
Brewstersteeringmirrors.

Four flat mirrors define the OPO ring cavity, which by choice of mirror
reflectivitiesand servo loop feedbackis singly resonantwith the seedfrequency. The
important advantagesof the ring cavity designare easeof alignment,lack of spatial
hole burning in the gain medium, unidirectionaloutput, and optical isolation of the
OPO pulse from back reflection into the seeddye laser. The free spectralrange (FSR)of the resonatoris 1.35GHz (0.045cm-i), which correspondsto a round trip time of
0.73 ns and thus roughly 10 optical round trips during the 7 ns pump pulse. For servo
loop locking the cavity mode onto the seedlaser,one of the cavity mirrors is mounted
on a piezoelectric(PZT) transducer;this allows tuning of the OPO cavity over two
full free spectralrangeswith negligibledisplacementof the beamover any continuous
single mode scan.The inpuVoutputcoupler is 807oreflectiveat the signal wavelen_sth,
while the other cavity mirrors are highly reflective(> 99Vo). This output coupler
reflectivity dominatesthe round trip loss in the cavity and yields a resonatorfinesseof
8+1. This finesseis alsocloseto the numberof cavity round trips during the pumppulse, which optimizesOPO conversionfrom pump to signal/idler. All four mirrors
have high transmissionin the 800 - 1,000nm idler region to preventdoubly resonant
behavior and possiblemode competitionbetweensignal and idler OPO output.
Furthermore,althoughthe ring cavity mode structureitself can supportoscillation inboth clockwise and counterclockwisedirections,the OPO parametricgain is
exclusively in the direction of the pump pulse/seedlaser. Consequently,the entire
idler output can be efficiently extractedat the first mirror "downstream" of the BBO
crystal pair.
The OPO is pumped by the third harmonicof a Fourier transform limited, pulsed

JL

Nd:YAG laserat 355 nm. The oscillatorof the pump laseris injection seededby a
cw single mode YAG laserin order to obtain single mode pulsed operationand asmooth temporal time profile. Maximum available energy from the pump is 200 mJ
in 7 ns pulses,yet to preventexceedingthe damagelimits of the cavity mirrors, only afraction of the full pulse energy (50 - 100 mJ/pulse)is typically utilized. The pumpbeam size at the Nd:YAG laseroutput is 7 mm FWHM and slightly divergent.The
beam is collimated with a Galileantelescope(2:1 reductionratio) to match
approximatelythe size of the BBO crystalsin the OPO resonator.At the output of the
Nd:YAG pump laser,the transversernodequality is betterthan 80% TEMsp. In orderto achievethe far field limit and smooth any potential'hotspots'in the spatialprofile,the pump beam is propagatedfor 4 metersbefore it entersthe OPO resonator.The
pump radiationis coupledinto the crystalsvia two intracavitysteeringlnirrors, coatedfor high reflectivity at 355 nm and high transmissivityat signal and idler wavelengths.
The purnp coupling mirrors are placedat Brewster'sangle with respectto the si-enal
beam to reduceintracavityreflectionlossesas well as to preventthe high intensity
pump radiationfrom exceedingthe damagethresholdfor the broad band cavity
mirrors. Furthermore,thesemirrors serveas polarizationselectiveeletnentsin the
OPO resonator,which prove useful in locking the cw dye laserto the cavity via
polarizationservo loop control, (seesection2.3.3 below).
The OPO cavity is injection seededby an Ar* pumped single mode cw ring dyelaser. Short-termfrequencystability ( < I MHz) of the injection seedis achievedby
locking the dye laser to a fringe of a pressureand temperaturestabilized reference
Fabry-Perotinterferometer.l6Specifically,drift of the seedring dye laserand OPO

33
output is demonstratedto be lessthan * 2MHz over severalhours.The seedlaser
can be single mode scannedunder computercontrol over a range of up to 2 crn-l , rvith
arbitrarily longer frequencyrangescoveredby concatenationof overlappingscans.
Due to the highly nonlinearnatureof the OPO parametricconversionprocess,typicalpower outputs from the cw dye laser (greaterthan 100 mW) representmuch higher
injection seedlevels than necessary;testswith seedpower levels down to a few rnW
indicateno significantdrop off in the OPO pulseoutput.This is qualitatively
consistentwith other OPO systemssuch as reportedby Lehmann,lTBosenberg,l8orMilton,l9 where input power levelseven at the severalmW level proved adequatefbr
achievingsingle mode output. However, this excessof injection seedpower is used
to good advantageby expandingthe ring laserspot size before it entersthe OPO
resonatorto overfill the 355 nm pump beam.

2.3.3 OPO Cavity Locking
For efficient injection seedingof the OPO, the longitudinal mode of the resonatormust be matchedwith the frequencyof the seedradiation. To achievethis, the OPO
resonatoris locked to the seedfrequencyby servo loop control of the cavity lengthwith the internal PZT mountedmirror. The servo looo schemeis basedon
polarizationlocking methodsfirst introducedby Heinschand Couillaud2o (seeFig.
2.5), which rely on the differentialphaseshift betweens-andp-polarizedmodes of thering resonator. Specifically,the linearly polarizedseedlaseris rotatedby 10" awayfrom horizontal (p-polarization)to generatea weak s-polarizedcomponentand
injectedinto the OPO ring. Due to lossy Brewsterelementsand a lower finesse,the

a /

J+

O P O C A V I T Y L O C K I N GS C H E M E
SEEDEREs

Ay = 0P H A S ES H I F T
DETECTION

ERROR
SIGNALP Z TD R I V I N GVOLTAGE

Figure 2.5: Polarizationlocking schemeadaptedfrom ref 22.The optical phaseshift
betweentwo perpendicularlypolarizedlight componentsinduced by the OPO
resonatoris detectedby subsequentpolarizationanalysiswith a),/4 plate,a linear
polarizerand two photodiodes.The resultingerror signal locks the OPO resonator
onto the scanningseeddye laserby servoloopfeedbackto the PZT mounted cavity
mirror.

35s-polarizedcomponentis essentiallyreflectedfrom the input mirror with negligible
phaseshift due to the time spentin the cavity. The reflectedlight of p-polarization,on the other hand, suffersa rapidly changingphasewhich passesthrough zero(rnodulo 2n) as the laseris runedthrough the cavity resonance.Thus, recombinationof the s and phaseshifted p-componentsresultsin an elliptical polarizationfor thelight reflected from the input coupler, where the magnitude and sign of the twopolarizationcomponentscontain resonanceinformation on the cavity. This
information is extractedwith a quarterwave plate plus polarizationbeam splitter (seeFig. 2.5), with the two polarizationsignalsincident onto a pair of balanced
photodiodes.Subtractionof thesetwo photodiodeoutputsresultsin a steeplyvarying
discriminatorsignal suitablefor servo loop locking onto the resonancecavity
condition.Sucha methodhasthe significantadvantageof being "dither-free",i.e.,requiring no modulationof the cavity nor subsequentphasesensitivedetectionof themodulatedsignal.As describedbelow, this permits much simpler servo electronics
and fasterlock recaptureafter the OPO amplified seedpulse.
The error signal from the balancedphotodiodesis processedby analogservo loop
electronics(with both integraland proportionalgain) to generatean amplified or'rtput
voltage on the intracavityPZT mountedmirror. The OPO resonatoris thereby locked
to the zero crossingof the error signal,correspondingto zero optical phaseshift in theresonator.The frequencyresponseof the servoloop is limited by a LC filter to roll off
at 5 kHz, with a unity gain frequencyat 8 kHz. This bandwidthrestrictionis choserl
for two reasons.First, mechanicalresonancesof the piezo driver and mirror assetllbly
occur at approximately10 kHz, which can result in additionalphaseshift

36
instabilities. More importantly,however, the small fraction of the resulting OPO
signal radiationwhich leaksthrough the input coupler exceedsthe seedpower levelby many ordersof magnitude,which saturatesthe balancedphotodiodesand drives
the error signal out of limits during the pump pulse. By limiting the eleoronic
bandwidth to 5 kHz, one preventsthe servo loop from respondingto thesefalse error
signal disturbancesduring the OPO pulse;the cavity mirrors are effectively "frozen"
in place so that the cavity staysin resonancewith the seedfrequency. Consequently,
the servo loop rapidly reacquiresthe lock within 1 msec after the OPO pulse, i.e., on atime scaleshort comparedto the purnp laserrepetitionrate (t,.p= l/10 Hz = 100msec).
An important featureof the currentOPO designis the ability to scanthe output
continuouslyin frequency,which is achievedby scanningthe frequencyof the single
mode seedlaser. This in turn requiresthe OPO cavity to remain actively locked to theseedlaserthroughouta2 cm-t scan. The OPO cavity scanrange,on the other hand, isrestrictedby translationof the PZT mountedmirror; with a 2.54 cm long PZT elernent
and < I kV voltageexcursions,this is currentlylimited to two FSR, i.e.,
approximately2.7 GHz. The PZT voltagemust thereforebe shifted down/up by theequivalentof one FSR wheneverpresethigh/low voltage limits on the PZT arereached. To achievethis, the PZT servo loop i) follows the cavity mode to the end ofits voltagerange and ii) automaticallyreacquiresthe lock on the next cavity mode tocontinue the scan. The time scaleon which the servo loop breaksand reacquiresthe
lock on the next mode is also sufficiently short (< 10 msec) comparedto the OPO
repetitionrate that no output pulsesare influenced.The robustnessof this schemecan

37
be testedby monitoring the error signal (in units of MHz) while scanningthe single
mode dye laserwith and without the OPO cavity lock. As shown in Fig. 2.6, the error
signalin the absenceof the lock executesperiodicexcursionsevery 1.35GHz,
correspondingto the FSR of the OPO cavity. With the cavity lock activated(Fig. 2.6)
on the steepdiscriminatorslope near Av = 0, the error signal is actively driven to zero,with a rms frequencydeviationof < l0 MHz. This is within 6Voof the cavity fringe
width, ensuringthat the injection seedlaseris fully "preresonating"the OPO prior to
the arrival of the 355 nm pump pulse.

2.3.4 Single Mode OPO Spectral Resolutiort
The most importanteffect of cw injection seedingthe OPO resonatoris thedramaticnarrowing of the output frequencyline width, from an unseededline widthof l0 cm-l to singlelongitudinalmodeoperation(Av = 0.005cm-r) with seeding.This
singlemode operationis explicitlydemonstrated
in Fig. 2.7, in which the Iine widthof the pulsed OPO output has beenmeasuredin transmissionusing an independent,high finesse(> 200) Fabry-Perotinterferometer.The measuredline width has a full
width at half maximum (FWHM) of Avpyylly= 160(20)MHz, with a line shapethat isintermediatebetweenLorentzianand Gaussian.The finesselimited line width fromthe Fabry-Perot(2 GHz FSR) is < 10 MHz (as measuredwith the cw seedlaser
source),and thereforethe observedtransmissionprofiles can be entirely ascribedto
OPO line width contributions.It is worth noting that thesemeasuredline widths
reflect severalhundredOPO pulsesand so representa rigorously conservativeupper
limit to single pulse spectralresolution.

38

OPOCAVITYLOCK
t-

N

-J-

=

a
a

100

lJJ

z
F
rF

.L

(5
F

V

o -100o
J

DYE LASERMARKERETALON

-2000

-1000

0

1000

Av lllHz)
Figure 2.6: OPO cavity tracking. a) With the feedbackservo disabled,thepolarizationphaseshift derived error signal variesas the dye laseris scanned.The
rapid downward going signal correspondsto tuning through a cavity resonancewith
the seed laser. b) With the servo enabled,the error signal is driven to zero by PZT
control of the cavity length,thus actively tracking the injection seedfrequency
throughouta single mode scan.The width of the error signal translatesinto a rms
frequency noise on the cavity of < l0 MHz.

' == Hv
q) uy

.

,

o d
- -q=
>\
L
e
A . -

L

ri-

^ - t s

) < F

x.5
/ \
v
a
6

6

-

i

q:.l
L
I F

.FN

.r
F

.

=
L

' ;-: 1 ,=
,v
* c J

=

o

r g -

o

N

y

o

A

N

.E'

=

F

-

IJJ

ll

\;,

r

o

z

,

J

*

2
i"
Q X i

R

-

-

t =r :

.9

s

* f

- ,

o
q.

!

=

.J
at

;;

o
G
o

\
7-

A

aC\

vlr >ax<V- - v . =->> t '.=
s
?
a )

-

'.=
l

?,
X

t s X
l < q
F
X

#i!r
' = t
- i \

5

g

-

YF
v

ci

N

=

l - - f , g

c . l v .
a r ( \ l s

r

F

o

(qtun 'qrE)AISN3INI

= i l
bO a/

o

lf<

_il

rr,

r n =

40

The lower limit of line width for a pulsed light sourceis relatedto the Fouriertransform of the temporal profile, with AvpwHv X trwHu =0.44 predicted for Gaussian
shapes2l.In the time domain, the OPO output pulsesexhibit a Atpy,,sy= 3.5(5) ns,
with a rapidly rising edge followed asymmetricallyby a somewhatslower decay.If
as a purely Gaussiantime profile, then the Fourier
one approximatesthis Atpyyllpa
transformrelation would predict Avrwsv = 130(20)MHz, which is quite close to but
slightly smallerthan the 160(20)MHz line widths observedexperimentally.This isalso consistentwith the asymmetriesin the temporalprofile, which would broadenthe
130(20)MHz predictioninto betteragreementwith experiment.In summary,thesedemonstratethat the OPO output
combined frequencyand time domain measurements
pulsesare essentiallyFourier transformlimited, even averagedover many hundredsof
pulses.

2.3.5 Tunability and injectiort seeding fficiency
Broad band tunability is one of the key advantagesof optical parametricoscillation
methods. With BBO crystalspumpedby a 355 nm laser,the idler radiationcan betuned from the degeneracypoint at 710 nm to the BBO absorptionedge at 2200 nm,
correspondingto a signalfrequencyfrom 710 nm to 420 nm. Since the dielectric
mirrors used in the resonatorprovide reflectivity in a far nalTower spectral window,
severalsetsof cavity mirrors are necessaryto achievethe completetuning range of
the OPO. For the applicationsin this paper,only one set of mirrors is used (seeFig2.8), which limits the currentOPO tuning range somewhatarbitrarily to 550 - 680 nm

41

o
o
o
.F

a,
F

.E
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42
(signal) and 750 - 950 nm (idler). However, this idler rangeconvenientlycovers
the v6s = 3 +- 0 and vcn = 3 <- 0 overtoneregionsin simple speciessuch as HzO and
HCCH, as well as in molecularclusterscontainingthesechromophores.
Injection seedingalso reducesthe build up time in the OPO resonatorcomparedto= 7 nsec)andunseededoperation.Since the pump pulse is relatively short (Tpu,np
correspondsto only 10 round trips of the cavity, this effect dramaticallyincreasesthe
OPO conversionefficiency,especiallywhen operatednearthreshold(25 mJ/pulse,
355nm pump, 3 mm beam diameter).Even at pump powers twice the thresholdvalue,
however,one observesmore than a factor of two enhancementof the OPO output
pulse energy for seededvs. unseededoperation.At 355 nm, the OPO is pumped with< 100 mJ/pulse,limited cunently by the optical darnagethresholdof the cavity
mirrors. At thesemaximum pump energies,OPO output energiesof l6 rnJ/pulsefor
signal and 12 mJ/pulsefor idler beamscan be achieved,correspondingto a total
conversionefficiency of 28Vo.
Of particularrelevanceto high resolutionspectroscopicapplicationsis that
injectionseedingpermitssinglemode scanningof the OPO outputwithout additionalfrequencyselectiveelementsin the cavity. Within the approximatel0 cm-lparametricgain window of the BBO crystal,the OPO frequencyoutput tracks theseedlaserinput, which in turn can be scannedwith single mode resolution.
Furthermore,sincethe typical singlemode tuning range of the seeddye laser(2 cmr)is considerablynarrowerthan this parametricgain window, it is unnecessaryto adjust
the BBO crystalssynchronouslywith the dye laserfor any one scan. Slight cavity
angularadjustmentsare necessaryonly every 4-6 cm-1,with concatenationof

43
arbitrarily long single mode OPO scansmade possiblevia optical transfercavity(TC) methodsbriefly summarizedbelow

2.3.6 Frequency Stability and Calibratiort
To take advantageof single mode OPO performance in high resolution
applications,frequencystability and calibrationare essential.Since the signal andidler frequenciesare controlledby the frequenciesof seedand purnp radiation,thisrequiresthat both seedand pump lasersbe frequencystabilizedusing an absolute
optical transfercavity (TC), seeFig.2.9. As describedin detail elsewhere,l6'22
allopen confocal Fabry-Perottransfercavity (TC) is locked onto the output of a He-Ne
laseroperatingin two adjacentlongitudinalmodes,which is in turn locked to thecenterof the He-Ne gain profile by polarizationstabilizationmethods.The TC
mirrors are speciallycoatedto provide adequatefinesse(roughly 20) fbr both l .064
nm (cw Nd:YAG laser)and visible(dye laser)spectralregions.The 355 nm pumplaserfrequencyoutput is therebyfrequencystabilizedby locking the cw Nd:YAG
laserthat injection seedsthe main Nd:YAG oscillator. Specifically,a small split offof the cw Nd:YAG laseris monitoredin transmissionthrough the TC and locked ontoa cavity resonanceby electronicfeedbackcontrol of the Nd:YAC seedlaser
temperature.Drift in the 355 nm pump frequencycan be reducedto 2 MHz levels,
limited predominantlyby pressuredependentindex of refractiondifferencesbetween
the He-Ne and 1.06nm laserfrequencies.Similarly, the frequencyof the cw seeddyelasercan be locked on anothertransmissionfringe of the sameTC. Although this
could also be done with analogservo loop electronics,this is most readily

AA--

1.064pmcw

Nd:YAG
He-Ne Stabilized

mW
-lMHz

Detec,grge;::il'J'iln"," +-Polarizinq

Polarization
stabilized
H e - N eL a s e r

PulsedNG:YAG(At-8ns)(,)3(pump)
Av- 60 MHz-200 mJro1(seed)
Av - lMHz
P-100mW
Single-modecw
Ring Dye Laser

02 = (l)3- co1(idler)o":'fly"schemefor the injectionseededOPO'
stabilization
Figure2.9:Frequency

45
accomplishedwith a computercontrolled 16 bit D/A transducerwhich scansthe
laserin < I MHz steps. In this fashion,the lasercan be tuned to the top of a TC
transmissionfringe and stabilizedwith digital servo loop control.
However, this method only permits the seeddye laserto be locked to the discrete,uniformly spaced(FSR = 250 MHz) transmissionfringes of the TC. To provide both
continuousand frequencystabilizedtuning capability,a small portion of the dye laser
beam is doubly passedthrough an acousto-opticmodulator (AoM) prior to Iocking
onto the TC. This generatesfrequencyshifted "side band" replicasof the dye laser,with 2,4,... quanraof the 60-140 MHz AOM addedor subtractedfrom the dye laser
frequency.The frequencyof the sidebandcan then be tuned by adjustinga radio
frequency(RF) sourcethat drives the AOM, so long as the tuning range exceedsottehalf of the free spectralrange of the TC. Thus, by "frequency offset locking" one ofrvitltthe side bandsto the TC, the dye lasercan be tuned to an arbitraryfrequency'yet
a long term stability still equivalentto that of the referencecavity fringe'
Although both seedand pump lasersare stabilizedabsolutely,recentstudies
indicatethat the high resolutionOPO output can still be shifted with respectto theseedfrequencydue to nonlinear"mode pulling" effectsin the OPO cavity'17 Indeed,shifts betweenthe seedand signal laserfrequenciesof up to 250 MHz have beenreported,23which if not constantover time, could translateinto OPO frequencydriftsof this magnitude.The magnitudeof the mode pulling effectshave been explicitly
measuredin our OPO cavity designl6 by measurementof the shift betweenthe pulsed
OpO signal and cw seedlasertransmissionfringes in a high finesse(>200) Fabryperot interferometer.Specifically,the OPO output signal is consistentlyblue shifted46
with respectto the seedby approximately100 MHz. The magnitudeof this shiftdependson pump and seedbeam alignmenVpowerin the OPO resonator,but for agiven set of operatingconditionsthis shift is relatively constant(< 10 MHz) on thetime scaleof severalhours.It is worth noting that this mode pulling still represents
only a small fraction of the 160 MHz homogeneousOPO line width. With both pumpand seedstabilizedto <2 MHz, however,thesesmall residualmode pulling effects
currently representthe limiting contributionto our long-termOPO frequencydrifts.
Due to PZT creepand mechanicaldrift, long-termmeasurementsof frequency
stability at this level are notoriouslydifficult to obtain reliably with high finesse
cavities. Thus, independentnrclecularmeasurementsof the absolutefrequency
stability of the signal/idlerhave beenperformed,basedon IR/UV vibrationallymediatedphotodissociationmethodsfirst developedby Crim and coworkers24,2s
andAndresenand coworkers.26Specifically,H2O moleculesare cooled to their Iowest
nuclearspin statesin a slit supersonic
expansion,which by virtue of collisional
quenchingof translationaldegreesof freedomyields sub-Dopplervelocity
distributionswhen probed along the slit dimension. At the l0 K temperatures
measuredin the slit jet, this corresponds
to linewidthsof 170(10)MHz in rhe second
overtoneband region, which is six fold narrowerthan room temperatureDoppler
widths. The OPO idler is used to excite the v6s = 3 secondovertone( 103) <- 100) *)upper state,which is then photolyzedby 248 nm excimer light to form H (2s) + ort(X2n). Since 248 nm excimer photolysisof the vibrationalground stateoccurs via
Franck Condon factors down by at least three orders of magnitude, the overtone
absorptionevent is signalledby the appearanceof OH radicals,which are detectedvia

47
conventionallaserinducedfluorescence(LIF) on the 308 nm A <- X band.27
With this double resonancemethod,the absolutefrequencystability and linewidthof the OPO can be measuredin two ways. First of all, high resolutionscansover
individual rotationallines [(1or)<-(0oo),para H2O and (0oo)<- (lor), ortho H2O] inthe secondovertoneband are performed. Fits to a Gaussianline shapeyield 230( l5)MHz; this is in excellentagreementwith predictionsof 233(22) MHz basedon thequadraturesum of a 170(10)MHz Doppler and 160(20)MHz OPO contributionsto
the line width. Secondly,the long term frequencystability is investigatedby tuning
the idler to the steeplychangingside of a specificsub-Dopplerovertoneline shape.
Frequencyinstability in the idler thereforetranslatesinto excessLIF signal intensity
fluctuations,from which one can establishan upper limit of < l0 MHz nns drift in
the OPO over severalhours of observation.l6

2.4 Photolysislasersystems

2.4.1Excirnerlaser
As oncestatedby my colleague
Bill Chapman,an excimerlaseris basicallya hugesparkplug. While it clearlyis an overstatement,
indeedtheexcimerlaseris fairly
simpledevicecomparedto the sophisticationof thehigh resolutionOPOdescribedin
theprevioussection.Excimerlasersoperatein a gasmixtureat high pressure(P
about3000Torr) containinga halogen(F2or HCI) andan inertgassuchasAr, Xe, orKr in typicallyHe or Ne buffer. By electronbombardment
in a discharge
throughthis
gasmixture,exitedstatemolecule(excimer)suchasArF* areformed,whichare

48
unstablein the ground stateand thereforedissociateimmediatelyupon emitting
light. Thereforealmost no ground statepopulationis formed in this bounddissociativetransition,which resultsin very high gain of this type of laserscheme.
The output frequencyis characteristicfor eachof the gas mix, for experiments
describedin this thesisKrF emissionat 248 nm is used,while for preliminary direct
H2o photodissociationexperiments,ArF mix was usedemitting 193 nm. In thestandardarrangementwith transversedischargeand low-finesseoptical resonator,
those lasersare convenientsourcesof high intensitypulsedultraviolet radiation vvitlt
low spectraland spatialcoherence.28The basicoperationalpararnetersof the Lextra
50 excimerlaserusedin our laboratoryare summarizedin Table 2.1. Under typicalexperimentalconditionswe use 50 mJ/pulseof the excimer energy with the repetition
rate I0 Hz. This laseris fairly closeto a turn-key device in typical day-to-day
operation. The most frequentroutine maintenanceincludesgas mix change,which is
requiredafter typically l0 hours of operationfor the ArF rnix, and 30 days for the KrF
mix, respectively.
The rectangularoutput beam with different divergencein the vertical and horizontal
directionsis difficult to bring to a tight focus with ordinary sphericaloptics. Also thebeam size at the output of the laseris too big to fit convenientlyto standardsize (0 l") optics. Thereforethe horizontaldimensionof the beamis reducedwith a 3:lcompressingtelescopeformed by a pair of quartzcylindrical lenses. The bearnafter
the telescopeis close to squarein shapewith almost equal divergencein vertical andhorizontalplanesand is readily focusedto a well defined spot in the centerof
interactionregion by a single f = 1000mm sphericallens.

49
Table 2.1: Excimer laseroperationalparametersspecifiedby manufacturer29

Laser medium

Ar-F

Kr-F

Wavelength(nm)

193

248

Max. pulse energy (mJ/pulse)

200

300

Max. repetitionrate (Hz)30

30

Pulse-to-pulseenergy stability (7o)

-)

1.8

Beam dimensionsw x h (mm)

-8x23

-8x23

Beam divergencew x h (mrad)

1 x 3

l x 3

2

2

Time jitter (ns)2.4.2 Pulsed dye laser with nonlirrear wave lertgth extensiort
The excimer laserprovidesonly a fixed wavelengthoutput defined by the gas rnix.
In some casesit is importantto have the ability to tune the photolysiswavelengtli.For
that purposewe use a tunablepulseddye laserwith its output shifted to the ultraviolet
region via a nonlinearmixing schemes.One specificexample is schematicallyshorvn
in Fig. 2.10 for generationof the 222 nm photolysiswavelength. The core of thissetupis a pulsed dye laserpumped by the secondharmonicof a YAG laserat 532 nrn.
Two stagesof frequencydoubling and sum frequencygenerationare used to shift thedye laseroutput to the ultravioletregion. Specifically,the dye laseroutput isfrequencydoubledin the first stage,producingradiationin a near UV region around282 nm, which is then mixed with the YAG fundamentalradiationat 1064 nm in a

50

Tunablephotolysisgeneration

Autotracker#1

Autotracker#2

2xCr)

0t 0z

E

E

(\I
@

c{

C\I
TI

A
I
I
{

E
(o
lr)

E
a
@
o
ct)
(!

Pulseddye laser

Yag laser

Figure 2. l0: Generationof tunablephotolysisradiation near222 nm. Output of the
Nd-YAG pumped pulsed dye laseris frequencydoubledin first BBO nonlinearcrystal
and then sum frequencymixed with the Nd-YAG fundamentalbearnat 1064 nm.

5lsecondstageto producethe desiredlight at 222 nm'
Besidesthe advantagesof tunability, the photolysisbeam producedin this way is,unlike the beam from the excimer laser,highly spatiallycoherentand with minirnal
astigmatism,which makesprecisefocusingin the interactionregion easier. However
the down side is the lower energyof the photolysispulsesand also the higher

overalldye
cornplexityof the setup. Specifically,under typical experimentalconditionsthe
laserpump efficiency is roughly 23Voandtherefore160 mJ/pulseof the dyefundamentalradiationis producedwith 700 mJ/pulseof the pump radiationat 532
nm. Doubling efficiency in the first nonlinearcrystal is typically 20Vo,yielding32

rnJ

yields upof the near UV output. With the excessenergyat 1064nm the secondstage
about 6to 18Voefficiency in conversionof 282 nm input to 222 nm output, and thus
lessrnJ/pulseis finally obtainedat the photolysiswavelength,i'e. about factor of l0
than the energyper pulse typically usedfrom the excimer laserat 248 nm'

2.5 Laser induced fluorescenceproduct detection

2.5.1 Excitation lsser requirements
theComparedto the high spectralresolutionand output energy requiredfor
infrared excitationstep,only a fairly modestlasersystemis neededfor

probing the

OH and OD photoproducts.The threebasic requirementsof tunability'

spectralhere' To
resolutionand output energyfor this lasersystemare briefly considered
tunability over the spectral
cover the relevantbandsof OH and OD radicals2T'30,
region 280 nm and 310 nm is needed,which is readily accessiblefrorn thefrequencyofdoubledpulsed dye lasersoperatingwith rhodaminedyes. The spectrallinewidthAv < 1 cm-' is sufficient to resolvemost of the spectrallines in the OH (OD)
excitationspectrumthat are importantfor the OH and OD populationanalysis.
Finally, the energy of the lasershouldbe sufficientto pump approximately 10 Voof
the OH moleculesin the interactionvolume to the excitedelectronicstate,i.e.
opNpr,/S= 0.1 where oo is OH peak absorptioncrosssection,Npr'is number ofphotonsper laserpulse and S is the excitationbeam crosssectionin the interaction
region. Let us considera specificexampleof the P1(1)line in the voH = 0<-0 subband of the
r6

2te2llOH transition.The integratedcrosssectionforthis line is 5.0x10

crn.3lFor Gaussianlaserline width of full width at half maximum Av = I cm-r the

peak crosssectionis 4.Txl0-16cm2 and thereforefor a beamdiameterof 0.l ctn, thc
requiredphoton output to reach the 10Vopumping level is Npn+ 2.5 x l0r2
at l. = 300 nm. Over the courseof thephoton/pulse,or roughly 2 trrJ/pulse
experimentsdescribedin this thesiswe usedthreedifferent lasersystems,i) thealmost legendarySpectraPhysicsPDL2, ii) new and shining Continuum ND-6000,
and finally iii) a home-madeoscillator-onlydye laserwith grazingincidencea grating
configuration. Indeed,even this final, very simple laserwith typical output energiesof 2 mJ and 50 pJ in the dye fundamentaland secondharmonics,respectively,serves
quite satisfactorlyfor purposesof probing OH radical via LIF.

2.5.2 Collectio n assentbly
The collection assemblyis a set of mirrors and lensesdesignedto direct as much of
the fluoresecencelight as possibleto the sensitiveareaof the detectorand at the sametime preventany backgroundscatteredlight from reachingthe detector.Essentially,it

53
is a high numericalapertureimaging optics that projectsthe image of thefluorescenceexcitationregion to the detectoractive area. In the slit jet arrangement
the fluorescenceis excited along the slit, which providespotentially significantpath
length advantagecomparedto a pinhole expansion,provided that the fluorescenceis
efficiently collectedalong the rvholelength of the slit. Two different collection
assemblysetupshave beenusedin the courseof the experimentalwork; theirrespectiveadvantagesand limitations are briefly discussedbelow.
The first designshown in Fig 2.1 I a is essentiallythe cylindrical analogof thesphericaldesignoptimized for pinhole expansionsby Majewski ctt.a\.32 Front anc
rear UV enhancedaluminum coatedcylindrical mirrors (5.2 cm radius of curvature)
image the cylindrical sectionat the crossingof the jet and laserbeamsto a slit ofdimensions0.1 x 6 cm in the rear mirror, which acts as a spatialfilter for rejection of
scatteredphotonsthat do not originatein the interactionregion. Fluorescencethat
passesthrough the slit is collectedby condenserassemblyformed by sphericalandcylindrical lensesand is focusedon the photomultiplierphotocathode.
With the chamberpressurebelow l0-s Torr, the measuredratio of scatteredphotonsdetectedby the PMT to the total number per pulse is lessthan l0-e. The
detectionefficiency is determinedin a Rayleigh scatteringexperimentusing a knorvn
pressureof Ar gas in the chamber,from which the fraction collectedis estimatedto beabout5Voover the 4 cm slit length.This is about a factor of 2x lower than expectedby geometricconsiderations,but as much as an order of rnagnitudelarger than forconventionalpoint sourceimaging.
For optimum performanceof this collection assembly,the laserbeamsneed to beJ1

condensorb)

M1
detectionis slit geometry
for fluorescence
Figure2.11:The collectionassemblies
perpendicularto theplanes
the
two
cuts
in
represent
The
figures
LIF detection.
excitationbeamdirection.

))positionedcorrectly with respectto the collectionmirrors. Indeed,displacementas
small as 1 mm resultsin almost completeloss of the fluorescencesignal. This puts
rather stringentrequirementson alignmentof the beamsto the vacuum chamber.For
the sakeof simplicity and easeof use we have also designedand used the simplified
collection assemblyshown in Fig. Z.llb. Here the back mirror plays the role of theretro-reflector(i. e. increasingthe solid angle of detectionby a factor of 2). The
fluorescenceis then collectedusing the lens condenser.An image of the interaction
volume is formed in the planejust in front of the PMT. Aperture (A) placed in thisplane plays the role of spatialfilter. There is however one important differencecomparedto the previouslydiscussed
collectionassemblies.In this simplifieddesign
the apertureis placedjust in front of the PMT, i. e. outsidethe vacuum charnberand
therefore,its position and shapecan be relativelyeasily changed. Moving theapertureup and down effectively moves the position of interactionvolume, fromwhich the fluorescenceis collected.Similarly, changingthe height and width of thisaperturealtersthe amount of the spatialfiltering. Dependenceof the signal and/or
scatteredlight on the amount of spatialfiltering can thereforebe easily tested,rvhich
is not straightforwardwith the original design. While the geometriccollection
efficiency is somewhatlower for this simplified design,we find the observed
fluorescencesignalsonly marginally lower than in the previouscase.
A 5 cm squarefilter assemblyin front of the PMT housesfilters that reducethe
Iaserscatteredlight level on the PMT. Usually two dielectriccoatednotch filters areused,each having greaterthan 99Voreflectivity at the photolysiswavelength.Thosenotch filters reflect stray photolysislaserlight and thus help eliminateproblerns

56
emittedat
filterswith fluorescencefrom bulk absorption
typicallyencountered
Colorglassfiltersaretypicallyusedin serieswith thereflective
longerwavelength.
filter UG I I with transmission
filters.Most usefulprovedto be SchottGlassbandpass
maximumnear300nm.

2.5.3 Fluorescence detectiort
The fluorescencegatheredwith the collection optics is detectedby aphotomultiplier (PMT). A PMT detectslight at the photocathodewhich emits
photoelectronsby the photoelectriceffect. Thesephotoelectronsare electrostaticallyacceleratedand focusedon the first dynode of the electronmultiplier chain. On
impact, eachelectronliberatesa number of secondaryelectronswhich are, in turtt,
acceleratedand focusedonto the next dynode. The processis repeatedat eachsubsequentdynode and the electronsfrom the last dynode are collectedat the anode.
The PMT used throughoutthis work is from ElectronTubes Inc. model 98l3QB, withl4-dyodes in a linear focus arrangement,which exhibits large gain, fast transienttitne,and exceptionallinearity. The bialkali photocathodeand quartz input window
provide blue-sensitiveresponsedown to 200 nm. Specificationsfor this PMT
provided by manufacturerare listed in Table 2.2.
There are two basic operationalmodesin which a PMT can operate,i) photon
counting and ii) analogmeasurementmodes. Both of thosemodes will be consideredbelow from the point of view of achievablesignal/noiseratio and dynamic range ofthe detectablesignal. The goal of this discussionis to determinethe optimum

57of the9813QBPMT
Table2.2: Characteristics
Nominalgain( @ 2060V)

7l x 106

voltage(V)Maximum cathode-anode

2500

Dark current(Q-t)

500

Risetirne(ns)2.2

PeakquantumefficiencY(7o)26detectionin presentexperiments,i.e. for fluorescencesignalswith a decay time ofabout I ps following a short excitationpulse,and to identify contributing sourcesof
noiseand discusstheir relativeimportance.For eachcasewe will consideroff-line
noise,i.e. the noise observedon the detectorwhen there is no fluorescencesignal
present (seeFig. 2.12) andthe on-line noise which is observedwhen the fluorescetrce
signal is present.
The photon counting detectionschemeis basedon the ability of a PMT to generate
observablecurrentpulsesthat correspondto single detectedphotons.Those singleelectronicsand theirphoton pulsesare detectedwith a discriminator/counterfrequencyis proportionalto the light intensityimpinging on the PMT photocathode'The off-line noise is in principle limited by the PMT dark counts,which are specified
or 5 x l0-acountsin the I ps detectionwindow. The on-line noise
to be 500 counts's-r
is primarily determinedby the statisticaldeviationsin the number of photons
observedwithin the detectionwindow. For the averagenumber of detectedphotons

Therefore,theby Poissonstatistics.
N., this deviationis JN. , determined

58

Gatedintegrator
or photoncouner

backgroundf luorescence
noise

-V

u. A

RL i l ui\noisePreamplifierJohnsonnoise

off-linenoise.
to photomultiplier
Figure2.12:Contributions

59
ratioissignal/noise

q=5=.N-.
N

J

N

,

Y

() 7\

5

The major advantageof the photon counting detectionis that the off-line noise isvery low, limited by the dark count of the PMT, sinceany analog sourcesof noisesuch as preamplifier noise,are very efficiently suppressedby the discrirninator
electronics. If the signalsare averagedover large numbersof pulses,weak signals(with N. much lessthan l) can be readily detected.However the dynarnicrange ofphotoncountingdetectionis limited by the countersaturation.Specifically,thereis asinglephotonpulses,detenninedby theminimum delay betweentwo detectableelectronics.Typically this minimum
frequencybandwidthof the photon-counting
delay is on the order of 5 ns. Therefore,in a 1trtsdetectionwindow a maximum of
200 photonscould be detected,provided they would be regularly spaced. Morerealisticallyrandom photon distributionmust be taken into account. If K, is theaveragephoton flux (in #/second)than the observedsignal count is
N, = K. .T.e-K", whereT is the detectiongatewidth and t is the tirneresolutionof
the photon counter. For example,for T = I 1rs,t = 5 ns, and the averagephoton 11ux
of 100photonsin the lps window K. = l00i 1,000ns = 0.1 ns-r. ThereforeN. = l0 . e-0r x 5 -6l . Thereforedue to the countersaturationonly 60Voof the detectedphotons

are counted.In other words, already40Vosaturationis observed,which lirnits the
dynamic range obtainablewith the photon counting method.This detectionscherneis
thereforemost useful for signalsof averagephoton flux of lessthan 100 detected
photons in the I ps detectionwindow.

60
In the analogdetectionmode the total anodecurrentI is measuredwhich is
proportionalto the detectedphoton flux dN./dt by the relation- / a N' l".)G^ . q I=l
\dt

)

(2.4)

^"

where G is the gain of the PMT dynodechain and q. is the electroncharge.For
measuringlow duty-cyclepulsedsignals,suchas the fluorescencedecays,usually agated integratoris usedto measurean averagevoltageacrossa load resistorRI-,in a
specifiedtime interval At (gate). This gate is set so that the most intensepart of thesignalis sampled.
Both photon-statisticsand analogsourcesof noise must be consideredin thisdetectionscheme. The most straightforwardway to include both thosecontributionsin the signal/noiseanalysisis to expressthe observedaveragevoltage in terlns of thenumberof detectedphotonsN5. FrornEquation(2.4) it directlylbllows that a single
photon voltage signal Vp is

vn= (c *. q.)*(? 5\

where T is the gate length. Specifically,for the gate length At = I ps, the PMT gain G=7.1 x 107,and the load resistorRr-= 50 C),the singlephotonsignalis Vn - 35.5 pV.
Note that this singlephoton signal is inverselyproportionalto the gate length,and fora constantphoton flux the observedsignal Vs = Ns Vp is, as expected,independentof
the gate width. This singlephoton signal is essentiallya conversionfactor between
the observedanalogvoltage signalV5 and the number of detectedphotonsN5.
The photon statisticsPoissonnoise is 6 N 0o,,,= ./\and thereforein an ideal case6lvoltagenoiseis
thecorresponding

fv.)j

- 'l

6 % , , * = vvpP , /. l + l = ( % . % ) t .

(2'6)

\

intensities,theHowever, due to the spreadin the observedsingle-photon-pulse
realisticallyobtainablePoisson-limitedsignal/noiseratio isIs/V^'\:" = 0 . 8 1" t l N
[v'/

(2.7)

The two major contributionsto the analognoise are i) Johnsonnoisein the loadThe
resistorRl and ii) preamplifiernoise on the input to the gatedintegrator'
noise at room temperatureis Vprl5/JH,

Johnson= (4kTRr)l/2.For a 50 f) load resistorthis

noiseis 2.2nVlJHz. Whenthose
into0.9 nV/JHz.Thepreamplifiertranslates
noise iscontributionsare addedin quadrature,the resultinganalogoff-line

gatewidth. Thrs
detection
Z.anV/JHz, whichyieldsto 2.4pV RMS noisefor a 1trrs
is more than an order of magnitudelessthan the signal resulting

from a single phototr

down to one
detectedin the integrationgateUo = 35 pV' Thereforefor signals
photon per gate,the on-line noise is limited by the Poissonratherthan

the analog

of the photonnoise contributions.While the obtainableon-line S/N ratio is only 80va
range is notcounting limit, the advantageof this detectionschemeis that the dynarnic
limited by photoncountersaturation.
A final sourceof noise that has not been discussedis the contribution

from theby the laser
laser scatteredlight and/or from background fluorescencegenerated
vacuumpulses. The strongphotolysisand probe laserpulsesthat passthrough the

62chamber are scatteredby the vacuum impurities and by the optical elements in the
beam path, such as windows and irises and causescatteredlight signalsobservedby
the pMT. It is possibleto efficiently discriminateagainstthe Rayleigh scatteredlightwith the proper settingof the signal integrationgate and by use of reflective andabsorptivefilters in front of the PMT that spectrallydiscriminateagainstthe
excitationlaserpulses. However,the broadbandfluorescencefrom the vacuum
irnpuritiesand/or generatedin the absorptivefilters can not be cornpletelyeliminatedby either method. Typical contributionsfrom thosemechanismsin the present
experimentsare on the order of 1 photon per gate and thus they in fact representthe
limiting sourceof off-line noise.
Under theseconditions,the analogdetectionschemeprovidesdefinite advantages
over the photon counting detectionand is thereforeusedfor all the experitnents
describedin this thesis.The lack of signalsaturationdue to the limitationsin the
counting electronicsleadsto significantlybetterover all S/N ratio for the stronger
signals,such as from the HzO and HOD monomer photodissociation'For weaksignals,such as from photodissociationof the weakly bound complexes,the overallsensitivityis close to I signalphoton per pulse,limited rnostly by the background
fluorescence,not by the analogdetectionnoise contributions'

2.6 Vacuum system
for the vacuumchamber
thedesignconsiderations
In this sectionwe summarize
andpumpingsystemthatis u.ed for the vibrationalmediatedphotodissociation

63
experimentsin slit supersonicjets. As discussedin previous sections,the use of aslit supersonicexpansion,ratherthan a pinhole,enhancessignificantly the observed
LIF signs,which is an importantfactor in the successof the presentexperiments.The
price to be paid for this advantageis the high gas throughputfor a slit jet comparedto
40
typicalpinholejets. Specifically,for thejet dimensionsusedin our experiments,
mm x 50 pm, the limiting orfice areais 2 mm2,more than 102larger then for a typical
below, even with the low dutypinholejet (with diameter@= 100pm). As discussed
cycle pulsedvalve operation,the gas load is considerable.Moreover, the LIF
detectionputs more stringentrequirementson the backgroundpressurethan, forexample,direct absorptionexperimentsin the slit jet. Specifically,for directabsorptionexperiments,backgroundpressuresof roughly 100 rnTorr can be tolerated
without compromisingthe signal/noiseratio. In contrast,broadbandfluorcscencesignalsfrom impuritiesexcitedby the UV photolysisand probe laserpulsesare
readily observedwith LIF detection,which under our experimentalconditions
becomesthe dominant sourceof noise at chamberpressuresabove l0-a Torr.

2.6.1 Puntpirtg speed consideratiorts

The gas load on the vacuum pumps is determinedby the fluid dynamicsof the slit
expansion.Therefore,a brief overview of the most important resultsrelevantto thepresentdiscussionare first presented,with more detaileddiscussionon the subject
The slit jet throughput(o)dependson the limiting areaof thefound elsewhere.33,31
orfice, stagnationpressure,and the natureof the expandinggas and can be expressedas64( 2.8)

o=P A'v,

jetwhere A is the areaof the jet and P and v are the pressureand velocity at the(P6),throat. For an atomic gas,thesevaluesare relatedto the stagnationpressure
temperature(T6),and mass(M) of the expandinggas by the relations:
(

't

\Tr

p=%l--:,l"\.Y+li(2.e)=0.487.P0(s n r. )j(2.10)

v = l - l

[

4 M

)

The averagerhroughputfor the pulsed slit jet expansionis obtainedby rnultiplyinggas
the continuousthroughputby the duty-cycle6 = t / , where t is the width of thepulse.Table 2.3 summarizesthe throughputs(per unit of stagnationpressure)forHe'

Ne, and Ar buffer gasesat room temperature(To = 300 K) in the stagnationregion'=
calculatedfor a jet areaof A = 2xl0-2 cm2 and a duty cycle 6 l0-2.

for He, Ne, andAr buffergases.
Table2.3:Slit jet characteristics
orfice velocitY

averagethroughPut

Max. StagnationPressure(cm s-')(l/s)(Torr)

Ar

2.7g lo4

2.72'rc-3

I 550")

Ne

3 . 9 51 0 4

3 . 8 5l 0

3

1090

He

8.83'104

8.60l0-3

488

buffer gas

")

f".

"

p"-pr"g speedof 4200l/sanda maximumbasepressureof

Torr.

the
The maximum stagnationpressuresin the last column in the Table 2.3 indicate

r-smaximum stagnationpressurebehind the pulsed valve for a given buffer gas,
assumingthe vacuum chamberis evacuatedby a 10" diffusion pump with purnping
speed 4200l/s and maximum inlet pressureof l0--1Torr. However, in practiceit isobservedthat this maximum inlet pressureis somewhatlower becauseof the pulsed
natureof the gas load. For example,for the Ne buffer gas we observethat the
diffusion pump startsto "choke" at a stagnationpressure800 Torr, ratherthan thepredicted I100 Torr, and the averagediffusion pump inlet pressureis 7 l0-a Torr,lower than expectedfor a diffusion pump. Indeed,under theseexperimental
conditionsa single 500 psec gas pulse with a peak throughputof 308 Torrl/s delivers
the pressurein
0.15 Torrl of gas into the vacuurnchamber.This temporarilyincreases
the 100 liter chambervolume to I .5 I 0'3Torr, which is pushingthe lirniting in letpressurefor an oil diffusion pump.

2.6.2 clruntber desigrt
The overall layout of the vacuum chamberand vacuum pumps assernblyis
schematicallyshownin the Fig 2.13. The vacuumchamberis fonned by a welded
cage-likeframe with o-ring sealedaluminum flangesmounted on all
stainless-steel
six faces,forming a vacuum tight compartmentwith inside flange-to-flange
dimensionof 18". Such a designprovideseasyaccessto the chamberinterior byremoving one of the flangesand a high level of flexibility for different applications:It
is easyto remove any of the side and/ortop flangesand replaceit with anotherflangeof sameoverall dimensionsbut with different ports mountedto it. For example,in tlie
presentapplicationthe fluorescencecollection optics and the PMT are mounted

66

I-l

SIGNALGATEVALVEL I O U I DN I T R O G E N
TRAP

10" I 4200lls
D I F F U S I OP
NU M Psystem.
andpumping
Figure2.13:Vacuumchamber

M E C H A N I C A LP U M P

67
directly to one of the side flanges. If desired,this rvholeflange can be removed andreplacedby an ion-detectionassemblywith a time-of-flight mass spectrometerfor
REMPI detectionscheme. Similarly, the slit pulsed valve assemblymounted to the
top flange can be replacedby, for example,a pinhole pulsedvalve setup. Thus thesamevacuum chambercan be quickly modified for specificapplications.
The vacuum chamberis pumped by a 10" (Varian VHSI0) diffusion pump with aliquid-nitrogencooled trap and a gate valve placedbetweenthe pump and the'tower' configuration.2s
The diffusion pump is backedby a twochamberin a standard
stagemechanicalpump(EdwardsM2-80). The basepressurein the chamberis
from the largealuminumflangesand by
typically lxl0-o Torr, limited by degassing
the large o-rings that sealthe flangesto the basiccage.

2.6.3 Gas lrundlirtg and nixirtg ntanifold
The purposeof gas the handling manifold is to preparea gas mixture of wellcontrolledcompositionand deliverit to the pulsedvalve at a desiredstagnationpressureand/or flow rate. Two basic arrangementshave beenused in our
experiments.In the first arrangement,the gas mixture is preparedbeforehandin alarge mixing tank so that the compositionis determinedby partial pressuresof
individual components.In the secondarrangement,the individual componentsaredeliveredto the pulsedvalve through separatelines and the mixture composition
determinedby controlling flow in each line with an needlevalveThe setupof the simplegashandlingmanifoldwith the mixing tank is schematicallyshown in Fig 2.14a.A mixing tank of approximatevolume 25 liters is

68->
To pulsed
valve

'tII

Mixingtank

Ar

To pulsed
valve

Mixingtank

AI
Buffergas(Heor Ne)

H2Oin Buffergas(richmix)
Figure2.14:Gashandlingandmixingmanifolds

69connectedto a gas delivery valve manifold with 5 individual inlet ports and to the
input of a pressureregulatorthat establishesthe stagnationpressurefor the pulsed
valve. Two capacitancemanometersM1 and M2 areused to determinethe gas mix
constituentspartial pressures,with 100 and 5000 Torr full ranges,respectively.A
thermocouplevacuum gaugeTC is usedprimarily for leak testingpurposes' A
ismechanicalpump is used for evacuationof the manifold and the mixing tank andbyisolatedfrom the manifold with a liquid Nz cooled trap to preventcontatnination
pump oil. The main advantageof this setupis its simplicity both in constructionandtooperation. The only practicallimitation is that it does not provide the flexibility
easily changethe gas mix compositionwhile monitoring signals,and therefore
optimizing the gas mix can be tedious.
A typical gas mixrure conrain0.5 - I Voof HzO vapor in He buffer gas for tlle

HzO

monomer studies,while I ToHzO and30VoAr in Ne buffer gas is the Inost successl'ul
jet. The rnix preparatiol iscompositionfor producing the Ar-water complexesin theofsrartedwith pumpingout the mixing tank to below 100 mTorr. A liquid sample
deionizedwater is typically purified by threefreeze-pump-thawcycles and thenwith aallowed to evaporateinto the evacuatedmixing tank, monitoring the pressure
100 Torr capacitancemanometer. No more than l5 Torr of water vapor is usedtoinensurethat liquid water is not formed by condensationinside the gas lines and/or
the pulsedvalve. Next Ar gas is introducedto the tank, if required,and finally the
buffer gas is added. The total pressurein the tank can be varied between 1500 and

Figure2.14b
respectively'
HzOfractionalconcentrations,
3000Torr for lVo and0.5Vo
gasdeliverysystemwith flow control. This systernshowsthemoresophisticated70
allows for independentlychangingof the fractionalconcentrationsof two
constituentsin the gas mix A and B, by controlling their relative flows. Two thermal
flowmeters with full rangeof 400 Ton I s-l and 1.6 Torr I s-r are usedfor the major andminor mix constituents,respectively.They are connectedin serieswith two needle
valvesused to control the gas flow in eachof the delivery branches.The third branchconnectedin parallel to the two flow-control lines is equippedwith a pressure
regulatorand is in somecasesusedto deliveradditionalbuffer gasto maintaina
presettotal stagnationpressure.This setupprovidesmuch more flexibility, but at thesametime requiresmuch more attentionand thereforeis best suitedfor gas rnix
optimization on-the-flY.

1l
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