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1、十三、光谱分析十三、光谱分析（Vibrational Spectroscopyfor Molecular Analysis）近代分析实验原理（Introduction of modern analytical methods）12Examine both inorganic and organic materials.metallic materialsstrongly reflect electromagnetic wavesFourier transform infrared spectroscopy (FTIR) Raman microscopymost commonly usedint
2、eraction between electromagnetic radiation and nuclear vibrations in moleculesmuch longer wavelengths, in the order of 107 m31. Theoretical Background1.1 Electromagnetic RadiationEnergy, frequency, wavelength and wavenumber ranges of electromagnetic waves. Frequency range of molecular vibrations is
3、in infrared region close to visible light.cm4several hundreds tothousandsin the order of about102101 eVthe photon energy of X-rays, which is in the order of 10,000 eV.51.2 Origin of Molecular VibrationsDiatomic model of molecular vibration. The centre of gravity does not change during the stretching
4、 vibration.force constantequilibrium distancedisplacementa harmonic vibration6vibrational quantum number7Molecular vibrations200 to 4000 cm1lattice vibrations20300 cm1not as sensitive to temperature changesensitive to temperature changecan be distinguishedthe vibrational spectroscopy covers a wavenu
5、mber range from 200 to 4000 cm1.crystalline solids also generate lattice vibrations in a wavenumber range of about20300 cm1.Coupling between lattice and molecular vibrations can occur if the molecularvibrations lie in such a low wavenumber range.81.3 Principles of Vibrational SpectroscopyTypical IR
6、absorption spectrum of hexanal (醛).1.3.1 Infrared Absorptiondominatesfrom v = 0 to v = 1an individual deep valley represents a single vibration band that corresponds to a certain molecular vibration frequency.91.3.2 Raman ScatteringElastic and inelastic scattering of incident light by molecules. Ray
7、leigh, elastic scattering; Stokes and anti-Stokes, inelastic scattering.100123e电子基态振动能级eeRayleigh 散射eeeRaman 散射Stocks线Anti-Stocks线温度升高概率大！11CCl4的拉曼光谱 Stocks linesanti-Stockes linesRayleigh scattering/cm-1The intensity of anti-Stokes scattering is significantly lower than that of Stokes scattering.12
8、Typical Raman spectrum of polycrystalline graphite.Raman shift131.4 Normal Mode of Molecular VibrationsNormal modes of vibrations in a CO2 molecule.we can decompose any bending mode and express it in terms of v2 and v4, as for decomposing a vector. the two bending modes are indistinguishable in term
9、s of frequency, because the relative movements of nuclei in the two bending modes are physically identical.The normal mode vibration of molecules has the following characteristics:14Normal modes of vibrations in a H2O molecule.Bending in the horizontal plane, will generate rotation of the molecule,
10、which is not considered as a vibration.15Number of Normal Vibration ModesN atomic nuclei in a molecule3N degrees of freedomthree related to translation of a molecule along x, y or z directionthree degrees of freedom related to rotate a molecule3N 6For a linear molecule3N 5Classification of Normal Vi
11、bration Modesrigid bodythe rotation around the bond axisis meaningless16171.5 Infrared and Raman ActivityInfrared Activitya vibration mode must cause alternation of dipole moment in a molecule.the dipole momentCO2 molecule vibrations: dipole moment () as a function of vibration displacement (q).infr
12、ared activeonly some can be detected18Raman Activitya vibration mode must cause polarizability changes in a molecule.determines the deformability of the electron cloud of a molecule by an external electric fieldCO2 molecule vibrations: (b) polarizability () as a function of vibration displacement (q
13、).19Normal modes of H2O vibrations and changes in polarizability ellipsoids. 1, 2 and 3 are all Raman active.Normal modes of CO2 vibrations and changes in polarizability ellipsoids. 1 is Raman active but 3, 2 and 4 are not.polarizability ellipsoid20In summary, we can highlight the features of infrar
14、ed- and Raman-active vibrations:l A vibration can be one of three cases: infrared-active, Raman-active, or both infrared- and Raman-active; lAnd in molecules that have a center of symmetry, infrared- and Raman-active vibrations are mutually exclusive.lFurther, it is helpful for us to know that vibra
15、tions of ionic bonds are strong in infrared spectroscopy (for example O-H), and vibrations of covalent bonds are strong in Raman spectroscopy (for example C=C).212. Fourier Transform Infrared Spectroscopy(FTIR)obtain an infrared spectrum in a whole range of wavenumbers simultaneouslymuch higher sign
16、al-to-noise ratio2.1 Working PrinciplesOptical diagram of a Michelson interferometer in FTIRThe IR radiation from the FTIR source is composed of numerous wavelengths.22Interferograms: (a) sum of (b) and (c); (b) interferogram of light withwavelength 3; and (c) interferogram of light with wavelength
17、.232425Plots of: (a) an interferogram; and (b) a Fourier transform from an interferogram to an IR spectrum.262.2 InstrumentationInfrared Light Sourceheating solid materialsNernst glowerGlobarBeam-Splitterreflect one half portion of infrared light to the moving mirror while transmitting the rest infr
18、ared to a fixed mirror.thin layer of germanium (Ge) between two pieces of potassium bromide (KBr)oxides of rare-earth elementssilicon carbideHighly temperature sensitiveoperated at the temperature where the maximum energy of radiation is near the short wavelength limit of the IR spectrumsubstrate ma
19、terialsplit infrared lightlow humidityabsorb water vaportransparent to infrared lightgood mechanical strength27Infrared Detectorpyroelectric crystal deuterated triglycine sulfate (DTGS)Infrared radiationtemperature change in DTGSChanges dielectric constantCapacitance changevoltage changesemiconducto
20、r detector氘化三甘氨硫mercury cadmium telluride (MCT)碲镉汞 10 times more sensitiveneeds to be cooledsaturates easilyconvert infrared light signals to electric signalssimple and inexpensive but less sensitivecauses the electrons to migrate from the valence band to the conduction bandCommonly to liquid nitrog
21、en temperature (196 C),282.3 Fourier Transform Infrared SpectraSingle beam spectrumincludes both spectra from the sample and background.polystyrene聚苯乙烯29(a) Single beam FTIR spectrum of background: a sample single beam FTIR spectrum of polystyrene (vibrational bands of polystyrene are superimposed o
22、n background spectrum); (b) a plot of raw detector response versus wavenumber without sample; and (c) final FTIR spectrum of polystyrene that only contains the vibration bands from the polystyrene sample (absorption intensity is expressed as the transmittance).聚苯乙烯 backgroundfrom water vapor and car
23、bon dioxidetransmittance spectrum30FTIR spectrum where the absorption intensity is expressed as absorbance. In this spectrum the absorbance scale is normalized to a range 01.For quantitative analysis, an absorbance spectrumshould be used because the peaks of a transmittance spectrum are not linearly
24、 proportional toconcentration.31Examination TechniquesTransmittancehigh signal-to-noise ratiossuitable for samples in any phasesDisadvantage: the thickness limitationmost commonlySolid Sample Preparationthin film or powdermaking KBr pellets and making mullsLiquid and Gas Sample Preparationdo not nee
25、d much preparation32ReflectanceReflectance examination techniques refer to methods for obtaining an infrared spectrum by reflecting IR radiation from a solid or liquid sample.bulk and coating samples without destructive preparationmain advantageA reflectance spectrum can only contain the infrared si
26、gnals from the top 1 to 10m of a solid sample surface.the instrumentation more complicated and expensive.332.4 Fourier Transform Infrared MicrospectroscopyOptical paths of FTIR microscope with IR radiation: (a) transmittance; and (b) reflectance. M, mirror; C, Cassegrain lens.The resolution is prima
27、rily determined by the size of the focused IR beam and precision of motorized stage.The FTIR microscope has an optical system that can easily switch between visible light observation and infrared light spectroscopy.342.5 ApplicationsMicrograph of isolated particulate contamination (AM) and cotton fi
28、ber (C).IR spectrum of the isolated particle (lower) and IR spectrum of polystyrene (upper).聚苯乙烯353. Raman MicroscopyOptical diagram of a Raman microscopecollecting a spectrum at each wavenumber separatelycontinuous-wave laserin the visible light range or close tothe rangeblock the laser light enter
29、ing the detector system36Laser Sourcegas continuous-wave lasers（Ar+, Kr+ And HeNe）a range of wavelengthsfilterPrefiltersSpectral response of an ideal notch prefilter in micro-Raman spectroscopy to remove the elastically scattered light with wavenumber .The scattered light from the microscope must be
30、 passed through special filters before reaching the spectral analyzer in order to remove elastically scattered light.37Diffraction GratingPrinciple of optical grating diffractionDetectorconverts photon signals to electric signalscharged-coupled device (CCD)multi-channelOne diffraction grating can co
31、ver a range of 1000 wavenumbers.change its anglespectral resolution of 1 cm1383.1 Fluorescence ProblemColored samples or impurities in polymer samples may absorb laser radiation and re-emit it as fluorescence. The intensity of fluorescence can be as much as 104 times higher than that of Raman scatte
32、red light.1. Irradiate the sample with high-power laser beams for a prolonged time to bleach out the impurity fluorescence;2. Change the wavelength of laser excitation to a longer wavelength (near infrared). The chance of fluorescence is reduced because the excitation energy of the laser beam is low
33、er; (but lead to the intensity reduction)3. Use a pulsed laser source to discriminate against fluorescence because the lifetime of Raman scattering (10121010 s) is much shorter than that of fluorescence (107109 s). Thus, an electron gate can be used to preferentially measure the Raman signals.393.2
34、Raman ImagingPrinciple of Raman imaging. The spectra of A, B, C represent those of substrate, molecule B and molecule C, respectively.Raman imaging is more difficult than other mapping techniques because the Raman scattered light is inherently weak.scanning direct-imaging40Raman image of rubber part
35、icles in epoxy film at 1665 cm1.413.3 Applicationsan examination technique for ceramic and polymeric materialsThis is its advantage over conventional X-ray diffraction spectrometry in which the sample volume cannot be too small.42Characteristic spectra of carbon in different structures: (a) highly o
36、riented graphite; (b) polycrystalline graphite; (c) amorphous carbon; and (d) diamond-like carbon.carbon atoms are bonded with sp2-type orbitals in graphite while they are bonded in sp3-type orbitals in diamond.43Raman spectra of a laminated polymer film obtained with excitation of a 785-nm laser.Polymer Identification聚酯聚乙烯44Raman spectra of graphite and graphite intercalation compounds (GIC) with FeCl3. A lower stage number indicates a higher degree of intercalation.45Associates with Fe ions46
