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tems,whereastherateoffluorescenceemissionprovidesthenecessarytime
scalefortheseobservations.
Fig.1.Theschemeshowingthecorrelationbetweendifferentmechanismsofspectral
broadening.Thestronglytemperaturedependenthomogeneousbroadeningalways
exists,eveninensembleofidenticalmoleculesinidenticalenvironments.Inhomo-
geneousbroadening(staticanddynamic)appearsinanensembleofidenticalmole-
culeswhentherearevariationsoftheirinteractionswiththeenvironmentsothatthe
averagepropertiesoftheenvironmentarethesame.Forstaticinhomogeneous
broadeningthecorrelationtimesfordipolarrelaxationsareslowerthantheemis-
sion,τC>>τF,andfordynamicbroadeningτC≤τF.Ifthemoleculesarenotidentical
oriftheyaredistributedbetweendifferentlocalizedenvironments,wewillcallthis
case“theground-stateheterogeneity”
Withthedevelopmentoftime-resolvedtechniquestheresearchergot
auniqueopportunitytofollowthemolecularrelaxationratesbyobservingthe
disappearanceoftheRed-Edgeeffectsasafunctionoftime,inlinewithclas-
sicalresearchworkofNemkovichetal.(Nemkovichetal.1980b).Withthe
applicationoftime-resolvedanisotropyanumberofnewsite-selectiveef-
fectshavebeenobservedintimedomain.Theyincludethesite-selectivityin
fluorophorerotationsinducedbydielectricenvironment(Gakamskyetal.
1983)andintime-resolvedkineticsofexcited-stateenergyhomotransfer
(Nemkovichetal.1981).Sinceboththeseprocessesfailtooccuratthered-
edge,thereappearedthepossibilitytodiscriminatethemfromothermecha-
nismsresponsibleforthekineticsofspectra.Theseeffectsofferadditional
possibilitiesforevaluatingtherelaxationratesandalsooftheratesofexcit-
ed-statereactionsandforthejudgmentonthemutualcouplingbetweenthese
processes.
ThediscoveryoftheRed-Edgeeffectsstimulatedthedevelopmentandappli-
cationofcryogenicmethodsinelectronicspectroscopyofmolecules,where
thezero-phononlineswerespectrallydistinguishedandanalyzed(Wagie
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