Box17-3RayleighandRamanScatteringEmission spectra can exhibit confusing features in addition toThe second strongest peak in this example comes at 800 nm,fluorescence and phosphorescence.The chart shows an emissionwhich is exactly twice the excitation wavelength. It is an artifact ofspectrum ofaqueous dichlorofluorescein(colored line)and,forrefer-the monochromator.Grating monochromators designed topasswavelength入alsopass integerfractions/2,/3,and soon,withence,an emission spectrumofpure water(black line).The excitationwavelength is 400 nm. The only difference between the two traces isdecreasing efficiency.When the emission monochromator in Figurefluorescence from dichlorofluorescein with a peak at 522 nm.17-21 is set to pass 800 nm. it also passes some light at 400 nm.Rayleigh scattering at 400nmpasses through the monochromatorRayleighsetto 800nm.We call this second-orderdiffraction fromthemono-scattering(400 nm)chromator. If we used a filter to block 400-nm light between the2nd ordergrating line.sample cell and the emission monochromator in Figure 17-21, therefrom Rayleighoeewouldbenopeakat800nmintheemissionspectrum.二氯荧光素scatteringA weak, but reproducible,peak is observed in both water and(800 nm)dichlorofluorescein solution at 462 nm. The difference in energyRamanFluorescencefromscatteringdichlorofiuoresceinbetween incident light at 400 nm and the peak at 462 nm correspondsfromH,o(522 nm)toavibrationalenergyof H,O.Thepeak at462nm is calledRaman(462 nm)scattering after the Indian physicist C, V.Raman, who discoveredthis phenomenon in 1928 and was awarded the Nobel Prize in 1930.In this type of scattering,which also occurs in a time frame of-10-15s,a smallfraction ofincidentphotons gives up one quantum600400500700800of molecular vibrational energy to H2O.Scatteredradiation emergesEmission wavelength (nm)with less energy than the excitation energy. Vibrational energy is cus-Uppertrace: Emission spectrum of aqueous dichlorofluorescein.Lowertomarily expressed as thewavenumber (cm)of aphoton with thattroce:Spectrum observed from purewater.[CourtesyKris Varazo, Francisenergy.Liquid H,O has a broad range of vibrational energies cen-Marion UnieeR.J.ClatkandA.Oprysaa,FluorescenceandLightScatteringtered near 3404 cm-.Thewavenumber of the exciting radiation is1.Chem.Ed.2004,87,7051/wavelength =1/400 nm =25 000 cm1.In Raman scattering,anWhat are the other peaks? The strongest peak, which is off-incidentphotonwithenergyof25000cm-givesup3404cmscale, is observed at the excitation wavelength of 400 nm.It isand emerges at (25000-3404)=21596 cm1.The wavelength iscalled Rayleigh scattering after the same Lord Rayleigh (J. W.1/(21596 cm-)463nm.Theobservedpeak is at 462nm.Strutt) who discovered argon (page 76). The oscillating electro-What are the lessons from this example? First, compare themagnetic field of the excitation light source causes electrons inspectrum ofpure solvent to the spectrum of a sampleunder study sowater molecules to oscillate at the same frequency as the incidentthat you can disregard peaks due to solvent,Second, fluorescenceradiation. Oscillating electrons emit this same frequency of radia-comes at a fixed location, such as 522 nm for dichlorofluorescein.tion in all directions. The time required for scattering is essentiallyThe wavelength of scattered radiation varies with incident wave-the period of one oscillation of the incoming electromagneticlength.If weusedan excitation wavelengthof 410nminsteadofwave,which is-10-15sfor400-nmlight.Bycomparison,thetime400,thesecond-ordergratinglinewould beseen at820nm and thefor fluorescenceis-10-to 10-s.Rayleigh scattering is alwayswater Raman line would be at an energy that is3404cmless than6present and is usually filtered out so that it is not displayed in thethe exciting light, or 477 nm. Scattered radiation shifts with theemission spectrum.incident wavelength,butfluorescence andphosphorescence do not
6 二氯荧光素
1852年Stokesshift of quinine(奎宁)in CambridgeSolutionof quinineEmission filterExcitation(yellow-glassofwine)filter400nmTransmits>400nm(blue-glassfromchurch window)G.G.StokesFigure1.6.Experinuentai schematicfordetection oftheStokes'shift
1852年Stokes’ shift of quinine (奎宁) in Cambridge 7
硫酸奎宁荧光素WAVELENGTH(nm)6:00250320350400440450520$103T71.010Cuinine SulfateHOOHin1MH5066C-1o.,5-AbsorptionEmissionEm.NCSEXC.300400500600355003150027500235001950015500WAVENUMBER (cm-1)8
硫酸奎宁 8 荧光素
吸收峰与发射峰之间的能级差：21(me- m)n2-1e-l+ cons tan tnA-nF32n2-hc 2e +1 +1a（折射率)，depends onI n: refractive indexthe motion of electrons within the solventmolecules, essentially instantaneous andcan occur during light absorption(介电常数) ε : dielectricconstantdepends on both electronic and molecularmotions,thelatterbeingsolventreorganization around the excited state
吸收峰与发射峰之间的能级差： l n: refractive index (折射率), depends on the motion of electrons within the solvent molecules, essentially instantaneous and can occur during light absorption l ε: dielectric constant (介 电 常 数 ), depends on both electronic and molecular motions, the latter being solvent reorganization around the excited state. 9