Raman Spectroscopy

Analytical Chemistry Lab, Department of Chemistry, Yonsei University

Raman scattering or the Raman effect is the inelastic scattering of a photon . Discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids Raman received the Nobel Prize in 1930

라만 산란이란?

• 정의 : 단색광을 기체 또는 투명한 액체 · 고 체에 쬐면 산란광 속에 파장이 약간 다른 빛이 생기는 현상 • 산란 :빛이 어떤 매질을 통과할 때 빛의 일부는 진행 방향에서 이탈해 다른 방향으 로 진행하는 현상.

Raman Spectroscopy

1923 – vents Inelastic light scattering is predicted by A. Smekel 1928 – ing from quartz 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scatter C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat sol First Raman Spectra: Filtered Hg arc lamp spectrum: C 6 H 6 Scattering http://www.springerlink.com/content/u4d7aexmjm8pa1fv/fulltext.pdf

Raman Spectroscopy

1923 – 1928 – ng from quartz 1928 – Landsberg and Mandelstam see unexpected frequency shifts in scatteri C.V. Raman and K.S. Krishnan see “feeble fluorescence” from neat sol vents 1930 – Inelastic light scattering is predicted by A. Smekel C.V. Raman wins Nobel Prize in Physics 1961 – 1977 – 1997 – Invention of laser makes Raman experiments reasonable Surface-enhanced Raman scattering (SERS) is discovered Single molecule SERS is possible

라만 산란과정의 양자 역학적 이해 광원과 분자의 에너지 상태는 변하지 않음 광원 광원의 에너지는 감소하고 분자의 에너지는 증가한다. 분자의 진동전이가 일어남 광원의 에너지는 증가하고 분자의 에너지는 감소한다. 진동여기 상태에 있던 분자는 바닥상태로 떨어진다.

광원v0-v1 광원v0+v1 Rayleigh scattering v1 Stokes 효과 Anti-Stokes 효과 Raman scattering v1

Raman Spectroscopy

The Raman spectroscopy effect arises when a beam of intense monochromatic light passes through a sample that contains molecules that can undergo a change on molecular polarization as they vibrate.

라만 산란과정은 분자의 진동 전위 변화를 시킨다 . 하지만 직접적으로 v1 을 측정할 수는 없고 산란되는 빛이 rayleigh 산란과 비교해 얼마만큼 에너지를 잃었는가 혹은 얻었는가를 관찰함으로써 스펙트럼은 산란된 빛이 v1 을 측정한다 . Rayleigh scattering 에 대해 얼마만큼 이 shift 되었는가를 Raman shift 로 표시하며 Raman shift 는 분자의 진동 주파수에 해당한다 . 따라서 라만 분광법은 IR 분광법과 같이 분자의 진동 형태 , 회전상태에 대한 정보를 얻기 위해 사용되지만 IR 분광법에서와는 다른 메카니즘과 선택 규칙에 근거하며 측정방법도 다름을 알 수있다 .

Rayleigh Scattering

•Elastic (  does not change) •Random direction of emission •Little energy loss (

E sc

)   8 2   4

d

2 2  )

E

0 Eugene Hecht, Optics , Addison-Wesley, Reading, MA, 1998.

Raman Spectroscopy

1 in 10 7 photons is scattered inelastically virtual state Rotational Raman Vibrational Raman Electronic Raman v” = 1 v” = 0 Infrared (absorption) Raman (scattering)

Presentation of Raman Spectra  ex = 1064 nm = 9399 cm -1 Breathing mode: 9399 – 992 = 8407 cm -1 Stretching mode: 9399 – 3063 = 6336 cm -1

• Spectral peaks in normal Raman spectrometry are commonly 10 -5 to 10 -7 times weaker than the incident radiation • Some Raman lines are closely adjacent to the exciting wavelength • Raman peaks must be observed against a background of stray light originated from Rayleigh scattering.

• Often Raman peaks must be separated from the fluorescence of analytes or sample impurities

Raman Vs IR 분광학 큰 에너지를 주고 빛의 파장이 얼마나 길어졌는지 혹은 짧아 졌는지 측정한다.

v 。 v 。 -v Raman scattering(stokes) v 진동 준위 에너지 차 v에 해당하는 에너지를 가진 빛이 입사한 후 그 파장의 빛의 세기가 얼마나 약해졌는지 측정한다.

IR spectrum v

예) 이산화탄소(CO2) Symmetric Vibration Raman Vs IR 분광학 Anti-symmetric Vibration Bending Raman ; active IR : inactive Raman ; inactive IR : active Raman ; inactive IR : active 선택률의 차이 적외선 분광학 : 분자의 진동 모드중 이중 극자 모멘트의 변화가 있는 모드만 IR 흡수를 한다.

라만 분광학 : 분자의 진동 모드중 편극도의 변화가 있는 모드만 라만 산란이 일어난다.

Symmetric Vibration : 라만 스펙트럼에서 강하게 일어남 C=C S-S N2 O2 … Anti-symmetric Vibration :IR스펙트럼에서 강하게 일어남 C=O O-H C=N C-H

Raman active Raman inactive Raman inactive

Raman vs IR Spectra

Ingle and Crouch, Spectrochemical Analysis

Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Raman vs Infrared Spectra McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Raman Intensities Radiant power of Raman scattering: 

R ex

) 4

ex E n e

0

i



E i kT

s ( n ex ) – Raman scattering cross-section (cm 2 ) n ex – excitation frequency E n i 0 – incident beam irradiance – number density in state i exponential – Boltzmann factor for state i s ( n ex ) - target area presented by a molecule for scattering

Comparison of Raman with Infrared Spectroscopy 1) Raman spectroscopy can be used to detect and analyze molecules with inactive spectra 2) Raman can be use to study materials in aqueous solution 3) Ability to examine the entire vibrational spectrum with one instrument 4) Spectrum is relatively simple and possible to assign all of Raman bands 5) Sample preparation for Raman is generally simpler 6) Intensity of spontaneous Raman lines increase linearly with concentration

Advantages of IR over Raman • Simpler and cheaper instrumentation.

• Less instrument dependent than Raman spectr a because IR spectra are based on measureme nt of intensity ratio .

• Lower detection limit than (normal) Raman.

• Background fluorescence can overwhelm Ram an.

• More suitable for vibrations of bonds with very low polarizability (e.g. C–F).

Shortcomings of Raman

• Sample must be free from dust particle • Fluorescence background • Photo and heat make sample degradation and/or unwanted side reaction • Relatively high cost and experts needed

Resonance Raman

1) It is important to appreciate that the magnitude of Raman shift are independent of the wavelength of excitation 2) The resonance Raman results from the promotion of an electron into an excited vibrational state, accompanied by immediate relaxation into a vibrational level of the ground state 3) Raman line intensities are greatly enhanced (10 2 ~10 6 time s) by excitation w/ l that is close to labs of a molecule

Resonance Raman Spectra

 ex = 441.6 nm  ex = 514.5 nm http://www.photobiology.com/v1/udaltsov/udaltsov.htm

Spectra from Background Subtraction McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Fluorescence Background in Raman Scattering

McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Basic components of the Raman

• A source, usually a CW gas laser • A sample illuminating system • A sample holder • A double or triple monochromator • A signal processing system-including a PMT and amplification system, an output device

Advantages of laser

• High intensity • Highly monochromatic • Small beam diameter * no dispersion * can apply to microsample • Polarization: 100 % linearity • Excitation frequency can be varied

* Liquid sample

Rotating Raman Cells

Rubinson, K. A., Rubinson, J. F., Contemporary Instrumental Analysis , Prentice Hall, New Jer sey: 2000

Solid Sample

Raman Spectroscopy: PMT vs CCD

McCreery, R. L., Raman Spectroscopy for Chemical Analysis, 3rd ed.

, Wiley, New York: 2000

Dispersive and FT-Raman Spectrometry McCreery, R. L., Spectroscopy for Chemical Analysis, 3rd ed.

Raman , Wiley, New York: 2000

Overall review

1.

2.

3.

Raman spectra give information on molecular vibrations and are obtained from changes in the frequency of light observed in a scattering experiment ( inelastic scattering) The gross selection rule: The physical picture arises from considering changes in polarization (induced dipole moment) that arise if a vibration occurs during the time the electrons are oscillating in response to the applied radiation Camparision of the spectra polarized perpendicular and pararell to the incident light gives information on the symmetry of the vibrational motions

4. Raman spectra can be obtained in water. This is a major advantage over IR 5. Resonance Raman spectra result when the wavelength of the exciting light falls within an electronic absorption band of a chromophore in the molecule. Some vibrations associated with such a chromophore may be enhanced by factors of 1000 or more 6. The experimental parameters of a band in a spectrum are its position ( Dn ) (which is independent of the frequency of the exciting light), its intensity (which is directly propotional to concentration), and its polarization