《拉曼光谱分析法》PPT课件.ppt

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1、拉曼光谱分析法,刘和文,Remote Raman Analysis on Planetary Missions,To allow Raman spectroscopy at range of 10s of meters.This 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,Rayleigh散射：弹性碰撞；无能量交换，仅改变方向；Raman散射：非弹性碰撞；方向改变且有能量交换；,Rayleigh散射,Raman

2、散射,E0基态，E1振动激发态；E0+h0，E1+h0 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）,基本原理,1.Raman散射Raman散射的两种跃迁能量差：E=h(0-)产生stokes线；强；基态分子多；E=h(0+)产生反stokes线；弱；Raman位移：Raman散射光与入射光频率差；,Rayleigh/Raman Transitions,IR Absorptions,Rayleigh/Raman Transitions and Spectra,Rayleigh/Raman Transitions and Spect

3、ra,The Spectrum,A complete Raman spectrum consists of:a Rayleigh scattered peak(high 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 excitat

4、ion wavelength(488,632.8,or 1064 nm),Spectrum of CCl4,using an Ar+laser at 488 nm.,Raman Spectroscopy,Another 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 so

5、urce for excitation.Resurgence in recent years due to the development of new detectors with improved sensitivity.Shift back away from FT-Raman to dispersive Raman with multichannel detector systems.,Infrared and Raman Spectra of Benzene,IR,Raman,拉曼光谱与红外光谱分析方法比较,Some Raman Advantages,Here are some re

6、asons 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 examine low wavenumber region:100 cm-1 readily achieved.Better microscopy;using visible light so can focus more tightly.Easy

7、 sample prep:water is an excellent solvent for Raman.Can probe sample through transparent containers(glass or plastic bag).,Watch for Fluorescence,Spectrum of anthracene.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

8、of frequency.BUTWant to use long wavelength to minimize chance of inducing fluorescence.,红外活性和拉曼活性振动,红外活性振动 永久偶极矩；极性基团；瞬间偶极矩；非对称分子；,红外活性振动伴有偶极矩变化的振动可以产生红外吸收谱带.拉曼活性振动 诱导偶极矩=E 非极性基团，对称分子；拉曼活性振动伴随有极化率变化的振动。对称分子：对称振动拉曼活性。不对称振动红外活性,Selection Rule for Raman Scattering,Must be change in polarizabilityNon-P

9、olar groups such as C-S,S-S,C=C,CC(triple bond),N=N and heavy atoms(I,Br,Hg)strong scatterersSymmetric stretching vibrations are much stronger scatterers than asymmetric stretching vibrations,Polarization Effects,对称中心分子CO2，CS2等，选律不相容。无对称中心分子（例如SO2等），三种振动既是红外活性振动，又是拉曼活性振动。,选律,振动自由度：3N-4=4,拉曼光谱源于极化率变化

10、,红外光谱源于偶极矩变化,Polarization of CCl4,Polarization of CHCl3,Raman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：,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。

11、,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,红外光谱：基团；拉曼光谱：分子骨架测定；,红外与拉曼谱图对比,Raman and Infrared Spectra of H-CC-H,Asymmetric C-H Stretch,Symmetric

12、C-H Stretch,CC Stretch,Vibrational modes of methane(CCl4),Infrared inactive,Raman active vibrations,Infrared active,Raman inactive vibrations,314 cm-1,776 cm-1,463 cm-1,219 cm-1,Infrared and Raman Spectrum of CCl4,776 cm-1,314 cm-1,463 cm-1,219 cm-1,Infrared spectrum,Raman spectrum,2941，2927cm-1 ASC

13、H2,2854cm-1 SCH2,1029cm-1（C-C）,803 cm-1环呼吸,1444，1267 cm-1 CH2,3060cm-1r-H),1600,1587cm-1 c=c)苯环,1000 cm-1环呼吸,787 cm-1环变形,1039,1022cm-1单取代,Raman Spectroscopy,Relatively simple and non-destructive structure analysis technique of carbon materialsPowerful tool for the structural characterization of diam

14、ond or amorphous carbon materials.,Remote Raman Analysis on Planetary Missions,To allow Raman spectroscopy at range of 10s of meters.This NASA-funded project is aimed at Mars landers or landers on other planets,but also has terrestrial uses.,NSOM Raman Imaging,Spectrum of potassium titanyl phosphate

15、.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 Mapping,Focus laser to small spot.Tune spectrometer to particular Raman transition peak.Raster scan the

16、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 active i

17、ngredient.Particles are in the 10s of m range.,Chemical Imaging,Now 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 this light on

18、 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 Restoration,This12 century fresco on a church wall in Italy needed to be restored.What paints to use?Raman analysis clearly identif

19、ied 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 Chips,Forensic analysis of paint chips in vehicle accidents.Often multiple layers.Can analyze with IR by strippin

20、g 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.today.Layer 5 was a silic

21、ate based paint.Data arising from a case investigated by LAPD.,Applications-Gem Forgery,In 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 distinguish them.,Applications-B

22、ullet Proof Glass,Identify poly(carbonate)from poly(methylmethacrylate).Both used for shatter-proof glass,Applications-Sunscreen Formulations,Here are the spectra of 5 common sunscreen ingredients.Raman is able to determine from a spectrum on the arm the nature of the sunscreen being used.A:ODPABA(o

23、ctyl 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Raman光谱仪 laser Raman spectroscopy,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：光栅，多单色器；检测器：光电倍增管，光子计数器；

24、,傅立叶变换-拉曼光谱仪,FT-Raman spectroscopy光源：Nd-YAG钇铝石榴石激光器（1.064m）；检测器：高灵敏度的铟镓砷探头；特点：（1）避免了荧光干扰；（2）精度高；（3）消除了瑞利谱线；（4）测量速度快。,Sources,Sources-1,Experiment used to require considerable excitation powerIon lasers,40 W cwHe:Ne,10 W cwYAG,1 J/10 ns pulse(100 MW average pulse)But detectors have improved so much,t

25、he source power requirements have been decreased.Diode laser,25 mWother lasers can be made correspondingly smaller.,Detectors,Scattered light is low intensity,so high gain PMTs have been used in the past.This was used for scanned and FT-Raman instrumentation for many years.Now cooled CCD arrays are used;experiment is now multichannel.,Cooled NIR detector,1024 x 256 pixel array,26 m square pixels.From Jobin Yvon.,