Page 46 - Mike's published articles
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Absorptionasobservedinaspectrometer(lowlightintensityconditions-ca1015photonscm2s')
resultsfromthetransferofenergyfromthelaserbeamtoexcitedstatesofthepigmentmoleculesorions.
Thesemaybelectronic,vibrationalexcitedstates.Eventualy,alofthesexcitedstateswileitheremit
theradiationatlongerwavelengths(fluorescenceorphosphorescence)ortheywilconvertthenergyto
heatandwarmthesample.Fluorescenceandphosphorescencearesignificantonlyforelectronicaly
excitedstatesforwhichthesemayleadtosubstantiallosofenergybeingnon-directional.Weshalshow
thatthesefectsarenegligible.Inaspectrometer,theheatingefectistrivial.Thisisnotsoforlaser
experiments,howeverg,Inlaserworkthelightintensityisveryhigh(ca1Ophotonscm21).ifwe
considerthelifetime'0oftherelevantvibronicexcitedstateas10"s,andamoleculeas102Ainarea
(10'2cm2),itisclearthatatlowlightintensitiesagivenmoleculeinteractswithlO'x1012xlO=
108photonsduringthexcitedstate'sdephasinglifetimeorposibly1O'x10'2x10=106photonsin
thelectronicexcitedstate'stotallifetimeof,typicaly,1ns.Inpracticethismeansthatamoleculecan
onlyinteractwithonephotonatatimeundertheseconditions.However,atthehighlaserintensitiesa
moleculecaninteractwith1028xlO'2x10"=10photonsinthevibroniclifetimeor1Ophotonsinthe
state'stotallifetime.Thisprocesleadstomulti-ormultiple-photonabsorptiondependingonwhether
thexcitedstatehasretainedphaseornotbeforethesecondphotoninteractionocurs.Withreferenceto
thepresentstudy,aQ-switchedrubylaser,operatingat694nmandamaximumenergydensityof
12Jcm2witha25nspulsewidth,corespondsto(hc/X)'xpulsenergyIpulseduration= xlOx3xlO/694xlO.9)4x12/(25xlO9)=1.67 s.Ifthislaserpulseisaplied
(6.626 27photons toa5nmdiameterspot,thelaserfluxbecomes,1.67 /
27 (3.142x0.252)1028photonscm2s'.Clearly thissystem,asusedinclinicalpractice,iswelwithinthemulti-ormultiplephotonregime"'3.
Forconvenience,weshalhenceforthrefertothisasthemultiphotonregimeandignorethedistinction betwentheseintheinterestsofconcisexpresion.
Therelevanceofthelinearorlowlightintensityspectratosuchaprocesisthatanabsorptionis requiredtoinitiatethismechanism.Nowalspeciesabsorbthroughoutthespectrumandsoposessome residualabsorptionwhichwiltrigermultiphotonabsorption'4foranylaserfrequencyifthelaserfluxis highenough.Multiphotonprocesesarenhancedifthereisarealysignificantabsorptionatthelaser wavelengthasobservedinaspectrophotometer.Nevertheles,thewavelengthdependenceofmultiphoton procesesisfarlesthanthatofsinglephoton(lowflux)procesesasseninaspectrophotometer.Thusit ishighlyunlikelythatspectrophotometricdatawilbeamajordeterminantinthefectivenesofagiven laserwavelengthinthelaser-tisueorlaser-pigmentinteraction.
Whenmultiphotonabsorptionocurs,typicalpigmentionisationenergiesof10eV(120nm)can easilybereached(e.g.6photonsofrubylaserlight).Theresultantelectronsliberatedfromthepigment areaceleratedbythelectricalfield(1.4x1OVcm')oftheintenselightstronglyabsorbingthere- mainderofthe25nsofthelaserpulse(inverseBremstrahlung)9"5sothattheyaremultipliedconsider- ablybymultiplecolisionswithadjacentmoleculesandproduceaplasmaevenwithinpicoseconds.The evolutionofthisplasmaintimeandspaceleadstolightemision,spatialexpansionandheatingofthe targetareasurfaceintensofnanoseconds.Thexpansionoftheplasmaleadstoarecoilmomentumof thetargetresultinginapresurewavetravelingwithsupersonicvelocity-i.e.ashockwave.Presures of1Oatmospheres(1Op.s.i.)arestimatedtobegenerated'6.Clearlytheseleadtosignificantmechanical destructionoftargetpigments.
ItisextremelydificulttobtaintheabsorptionspectrumofasolidatUV/visiblewavelengths becausethextinctioncoeficientissohigh.Forexample,a1Omolarsolutioncanexhibitopticaldensi-
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