拉曼光谱分析法

1、拉曼光谱分析法拉曼光谱分析法刘和文Remote Raman Analysis on Planetary MissionslTo allow Raman spectroscopy at range of 10s of meters.lThis NASA-funded project is aimed at Mars landers or landers on other planets, but also has terrestrial uses. 激光拉曼光谱基本原理激光拉曼光谱基本原理Rayleigh散射：散射： 弹性碰撞；无能量交换，仅改变方向；Raman散射：散射： 非弹性碰撞；方向改变

2、且有能量交换；Rayleigh散射散射Raman散射散射E0基态， E1振动激发态； E0 + h 0 ， E1 + h 0 激发虚态；获得能量后，跃迁到激发虚态.（1928年印度物理学家Raman C V 发现；1960年快速发展） h E0E1V=1V=0h 0h 0h 0h 0 + E1 + h 0E0 + h 0h( 0 - )激发虚态基本原理基本原理1. Raman1. Raman散射散射Raman散射的两种跃迁能量差： E=h( 0 - )产生产生stokes线；强线；强；基态分子多；基态分子多； E=h( 0 + )产生反产生反stokes线；线；弱；弱；Raman位移：位移：R

3、aman散射光与入射光频率差；ANTI-STOKES 0 - RayleighSTOKES 0 + 0h( 0 + )E0E1V=1V=0E1 + h 0E2 + h 0 h h 0h( 0 - )Rayleigh / Raman TransitionsIR AbsorptionsRayleigh / Raman Transitions and SpectraRayleigh / Raman Transitions and SpectraThe SpectrumA complete Raman spectrum consists of: a Rayleigh scattered peak (h

4、igh intensity, same wavelength as excitation) a series of Stokes-shifted peaks (low intensity, longer wavelength) a series of anti-Stokes shifted peaks (still lower intensity, shorter wavelength) spectrum independent of excitation wavelength (488, 632.8, or 1064 nm)Spectrum of CCl4, using an Ar+ las

5、er at 488 nm.Raman SpectroscopyAnother spectroscopic technique which probes the rovibrational structure of molecules.C.V. Raman discovered in 1928; received Nobel Prize in 1931.Can probe gases, liquids, and solids.Must use a laser source for excitation.Resurgence in recent years due to the developme

6、nt of new detectors with improved sensitivity.Shift back away from FT-Raman to dispersive Raman with multichannel detector systems.Infrared and Raman Spectra of BenzeneIRRaman拉曼光谱与红外光谱分析方法比较拉曼光谱与红外光谱分析方法比较拉曼光谱拉曼光谱红外光谱红外光谱光谱范围光谱范围40-4000Cm-1光谱范围光谱范围400-4000Cm-1水可作为溶剂水可作为溶剂水不能作为溶剂水不能作为溶剂样品可盛于玻璃瓶，毛细管等容

7、器样品可盛于玻璃瓶，毛细管等容器中直接测定中直接测定不能用玻璃容器测定不能用玻璃容器测定固体样品可直接测定固体样品可直接测定需要研磨制成需要研磨制成 KBR 压片压片Some Raman AdvantagesHere are some reasons why someone would prefer to use Raman Spectroscopy. Non-destructive to samples (minimal sample prep) Higher temperature studies possible (dont care about IR radiation) Easily

8、 examine low wavenumber region: 100 cm-1 readily achieved. Better microscopy; using visible light so can focus more tightly. Easy sample prep: water is an excellent solvent for Raman. Can probe sample through transparent containers (glass or plastic bag).Watch for FluorescenceSpectrum of anthracene.

9、 A: using Ar+ laser at 514.5 nm. B: using Nd:YAG laser at 1064 nm.Want to use short wavelength because scattering depends on 4th power of frequency.BUTWant to use long wavelength to minimize chance of inducing fluorescence.红外活性和拉曼活性振动红外活性和拉曼活性振动红外活性振动红外活性振动 永久永久偶极矩；极性基团；偶极矩；极性基团； 瞬间偶极矩；非对称分子；瞬间偶极矩；非

10、对称分子；红外活性振动红外活性振动伴有伴有偶极矩变化的振动可以偶极矩变化的振动可以产生红外吸收谱带产生红外吸收谱带. .拉曼活性振动拉曼活性振动 诱导诱导偶极矩偶极矩 = E 非极性基团，对称分子；非极性基团，对称分子；拉曼活性振动拉曼活性振动伴随有极化率变化的振动。伴随有极化率变化的振动。 对称分子对称分子： 对称振动对称振动拉曼活性。拉曼活性。 不对称振动不对称振动红外活性红外活性 EeerSelection Rule for Raman ScatteringvMust be change in polarizabilityNon-Polar groups such as C-S, S-S

11、, C=C, CC (triple bond), N=N and heavy atoms (I, Br, Hg) strong scatterersSymmetric stretching vibrations are much stronger scatterers than asymmetric stretching vibrationsPolarization Effects 对称中心分子对称中心分子CO2，CS2等，选律不相容。等，选律不相容。 无对称中心分子（例如无对称中心分子（例如SO2等），三种振动既是红外活等），三种振动既是红外活性振动，又是拉曼活性振动。性振动，又是拉曼活性振

12、动。选律选律SCSSCSSCS 1 2 3 4拉曼活性拉曼活性红外活性红外活性红外活性红外活性振动自由度：振动自由度：3N- 4 = 4拉曼光谱拉曼光谱源于极化率变化源于极化率变化红外光谱红外光谱源于偶极矩变化源于偶极矩变化Polarization of CCl4 Polarization of CHCl3 Raman位移位移 对不同物质： 不同； 对同一物质： 与入射光频率无关；表征分子振-转能级的特征物理量；定性与结构分析的依据； Raman散射的产生：光电场E中，分子产生诱导偶极距 = E 分子极化率；由拉曼光谱可以获得有机化合物的各种结构信息：由拉曼光谱可以获得有机化合物的各种结构信息

13、：2）红外光谱中，由C N，C=S，S-H伸缩振动产生的谱带一般较弱或强度可变，而在拉曼光谱中则是强谱带。3）环状化合物的对称呼吸振动常常是最强的拉曼谱带。1）同种分子的非极性键S-S，C=C，N=N，CC产生强拉曼谱带， 随单键双键三键谱带强度增加。拉曼光谱与有机结构拉曼光谱与有机结构4）在拉曼光谱中，X=Y=Z，C=N=C，O=C=O-这类键的对称伸缩振动是强谱带，反这类键的对称伸缩振动是弱谱带。红外光谱与此相反。5）C-C伸缩振动在拉曼光谱中是强谱带。6）醇和烷烃的拉曼光谱是相似的：I. C-O键与C-C键的力常数或键的强度没有很大差别。II. 羟基和甲基的质量仅相差2单位。 III.与

14、C-H和N-H谱带比较，O-H拉曼谱带较弱。红外与拉曼谱图对比红外与拉曼谱图对比红外光谱：基团；红外光谱：基团；拉曼光谱：分子骨架测定；拉曼光谱：分子骨架测定；红外与拉曼谱图对比红外与拉曼谱图对比Raman and Infrared Spectra of H-CC-HAsymmetric C-H StretchSymmetric C-H StretchCC StretchVibrational modes of methane (CCl4)Infrared inactive, Raman active vibrationsInfrared active, Raman inactive vibr

15、ations314 cm-1776 cm-1463 cm-1219 cm-1Infrared and Raman Spectrum of CCl4776 cm-1314 cm-1463 cm-1219 cm-1Infrared spectrumRaman spectrum2941，2927cm-1 ASCH22854cm-1 SCH21029cm-1 （C-C）803 cm-1环呼吸环呼吸 1444，1267 cm-1 CH23060cm-1 r-H)1600,1587cm-1 c=c)苯环苯环1000 cm-1环呼吸环呼吸787 cm-1环变形环变形1039, 1022cm-1单取代单取代R

16、aman SpectroscopylRelatively simple and non-destructive structure analysis technique of carbon materialslPowerful tool for the structural characterization of diamond or amorphous carbon materials.5001000150020002500 Intensity (a.u.)Raman Shift (cm-1)5001000150020002500G-modeDiamond: 1335.5cm-1 Inten

17、sity(a.u)Raman Shift (cm-1)DLCDiamondRemote Raman Analysis on Planetary MissionslTo allow Raman spectroscopy at range of 10s of meters.lThis NASA-funded project is aimed at Mars landers or landers on other planets, but also has terrestrial uses. NSOM Raman ImagingSpectrum of potassium titanyl phosph

18、ate. From Hans Hallen at NCSU. Squares are 5 x 5 m square of this material doped with Rb. A near-field scanning microscope was used and the Raman signal was used to key the substrate response.Chemical MappingFocus laser to small spot. Tune spectrometer to particular Raman transition peak. Raster sca

19、n the sample under the laser beam, record intensity changes. Resultant map correlates with substance. Acquire an entire spectrum at every point, then choose the feature with which to key the image.Motorized stage from Renishaw for chemical mapping.This is a drug tablet. The yellow corresponds to the

20、 active ingredient. Particles are in the 10s of m range.Chemical ImagingNow defocus the laser (not a small spot but rather “baths” the sample in laser radiation).Pass the emitted radiation through a narrow bandpass filter, adjusted to a particular wavelength, chosen to be a certain Raman band.Focus

21、this light on the CCD camera. Bright regions correspond to locations of substance giving rise to Raman signal.Mixture of cocaine and sugar. Bright spots are cocaine.Applications - Art RestorationThis12 century fresco on a church wall in Italy needed to be restored. What paints to use?Raman analysis

22、clearly identified the paints and pigments that were originally present, permitting a correct choice of cleaning materials and subsequent repainting to restore its original condition.Applications - Paint ChipsForensic analysis of paint chips in vehicle accidents. Often multiple layers. Can analyze w

23、ith IR by stripping successive layers. Image edge with microRaman.Layers 1 and 3 turned out to be rutile phase TiO2 - a white paint. Layer 2 was a Goethite, a red pigment and corrosion inhibitor. Layer 4 was molybdate orange, a common red paint in the 70s in North America and still used in the U.K.

24、today. Layer 5 was a silicate based paint. Data arising from a case investigated by LAPD.Applications - Gem ForgeryIn 1999 a new process was developed called GE POL whereby brown type IIa diamonds could be treated to become indistinguishable from naturally clear diamonds. Raman presented way to dist

25、inguish them.Naturally clear diamondOriginally brown diamondApplications - Bullet Proof GlassIdentify poly(carbonate) from poly(methylmethacrylate).Both used for shatter-proof glassApplications - Sunscreen FormulationsHere are the spectra of 5 common sunscreen ingredients. Raman is able to determine

26、 from a spectrum on the arm the nature of the sunscreen being used.A: ODPABA (octyl N,N-dimethyl-p-aminobenzoic acid)B: OMC (octyl p-methoxycinnamate)C: BZ3 (oxybenzone)D: OCS (octyl salicylate)E: DBM (dibenzoylmethane)G.R. Luppnow et al., J. Raman. Spec. 34, 743 (2003).激光激光Raman光谱仪光谱仪 laser Raman s

27、pectroscopy激光光源激光光源：He-Ne激光器，波长632.8nm； Ar激光器， 波长514.5nm， 488.0nm； 散射强度1/4 单色器单色器： 光栅，多单色器； 检测器检测器： 光电倍增管， 光子计数器；傅立叶变换-拉曼光谱仪FT-Raman spectroscopy光源：光源：Nd-YAG钇铝石榴石激光器（1.064m）；检测器：检测器：高灵敏度的铟镓砷探头；特点：特点：（1）避免了荧光干扰；）避免了荧光干扰；（2）精度高；）精度高；（3）消除了瑞利谱线；）消除了瑞利谱线；（4）测量速度快。）测量速度快。SourcesRaman intensity is weak and the excitation source must be strong to generate sufficient signal.Source must be monochromatic so that spectrum is sufficiently uncomplicated.Intense lamps can work, but when monochromatized, have very little power.Scattering efficiency increases as 4: the bluer the light, th