Raman

Billones Lecture Notes

UNIVERSITY OF THE PHILIPPINES MANILA

The Health Sciences Center

Raman Spectroscopy

It is discovered by Indian physicist C.V. Raman in 1928 (Nobel Prize in Physics, 1930)

Raman noted that the wavelength of small fraction of scattered radiation by certain molecules differs from that of the incident beam.

The shifts in wavelength depend upon the chemical structure.

The phenomenon of Raman scattering results from the same type of quantized vibrational changes that are associated with IR absorption.




The difference in wavelength between the incident and scattered radiation corresponds to wavelengths in the mid-IR region.

Thus, the Raman spectrum and IR spectrum for a given species often resemble one another quite closely.




However, distinct differences between the two exist to make these techniques complementary rather than competitive.

Example: Raman spectrum is more useful for highly symmetric molecules.

Important advantages of Raman over IR

Water does not cause interference in Raman.

Glass or quartz cells can be employed.Use of NaCl and atmospherically unstable windows is avoided.

Unlike in IR, aqueous solution of sample can be used.




Disadvantages of RamanSpectrum was hard to obtain before lasers became available in 1960s.

This is now overcome by the use of IR laser source and Fourier transform spectrometers.

Fluorescence of sample or impurities interfere with Raman.

Theory of Raman SpectroscopyRaman spectra are obtained by irradiating a sample with a laser source of visible or IR radiation.During irradiation, the spectrum of scattered radiation is measured at some angle (usually 90 deg)




The intensities of Raman lines are 0.001% of the intensity of the source.

This makes their detection and measurement difficult.

Raman Spectra

Portion of Raman spectrum of CCl4 excited by Ar ion laser having a wavelength of 488.0 nm

Billones Lecture NotesThe Health Sciences Center


The scattered radiation is of three types:Stokes, anti-Stokes, and Rayleigh

The Rayleigh radiation has wavelength exactly the same as that of the excitation source and is significantly more intense than the other two.

In Raman spectra, the abscissa is the wavenumber shift,

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Δν - the difference in wavenumbers (cm-1) between the observed radiation and the source.

In the CCl4 spectrum, three peaks are found on either side of the Rayleigh peak and the pattern of the shifts are identical.



Stokes lines are found at wavenumbers that are 218, 314, and 459 cm-1 smaller than the Rayleigh peak.

The magnitude of Raman shifts are independent of the wavelength of excitation.

- similar spectrum would be obtained using Kr, He:Ne or a Nd:YAG laser

Raman spectral lines at lower energies is analogous to Stokes shifts found in fluorescence.

anti-Stokes lines occur at 218, 314, and 459 cm-1 greater than the wavenumber of the source.

For this reason, negative raman shifts are called Stokes shifts.



Shifts toward higher energies are termed anti-Stokes.

Fluorescence may interfere seriously with Stokes shifts but not with anti-Stokes shifts.

- for this reason, only the Stokes part is generally used.

The abscissa of the Raman spectrum is often labeled simply frequency (cm-1) not wavenumber shift and the negative sign is usually omitted.

anti-Stokes lines are appreciably less intense than the corresponding Stokes lines.

With fluorescing samples, anti-Stokes signals may be more useful despite their lower intensities.




Mechanism of Raman and Rayleigh Scattering

The process is not quantized, the E of the molecule can assume any of the infinite number of values or states called virtual states.

Thicker lines indicate higher probability of occurrence at RT.



The thin (green) arrows show the type of change when the molecule happens to be in the first vibrational level of the electronic ground state.

The Stokes and the anti-Stokes differ from Rayleigh radiation by ±ΔE, the energy of the first vibrational level in the g.s.

The Rayleigh scattering has greater probability; there is no loss of energy (Eexcitation = Escattered radn) i.e. the collision is elastic.

- at RT, the fraction of molecules in this state is small, and the probability of the above process is small.



The relative populations of the two E states are such that Stokes is favored over anti-Stokes.

- the Raman frequency shift and the IR absorption peak frequency are identical.

If the bond were IR active, the E of absorption would also be ΔE.

However, the ratio of anti-Stokes to Stokes intensities will increase with temperature.

- larger fraction of the molecule will be in the first vibrationally excited state at high T.



Assume that a beam of radiation having a frequency νex is incident upon a solution of an analyte.

The electric field of radiation is given by

where Eo is the amplitude of the wave, vex is excitation wavelength

Wave Model of Raman and Rayleigh Scattering

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E = Eo cos 2πν ext( )

When the electric field of the radiation interacts with the electron cloud of an analyte bond, it induces a dipole moment, m, in the bond that is given by

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m =αE =αEo cos 2πν ext( )where α is a proportionality constant called the polarizability of the bond.



In order to be Raman active, the polarizability of a bond must vary as a function of the distance between nuclei according to the equation

where αo is the polarizability of the bond at the equilibrium internuclear distance, req; and r is the internuclear distance at any instant.

Polarizability is a measure of the deformability of a bond in an electric field.

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α =αo + r − req( ) ∂α∂r

The change in internuclear distance varies with the frequency of vibration νv as given by



where rm is the maximum internuclear distance relative to the equilibrium.

Thus polarizability becomes€

r − req = rm cos 2πν vt( )

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α =αo +∂α∂r

rm cos 2πν vt( )

Since induced dipole moment, m is

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m =αEo cos(2πν ext)

Substitution of α gives

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m =αoEo cos(2πν ext) + Eorm∂α∂r

cos(2πν vt)cos(2πν ext)



Recall that

Applying this identity gives

The first term in this equation represents Rayleigh scattering, which occurs at the excitation frequency, νex

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cos x cos y = cos(x + y) + cos(x − y)[ ] / 2

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m =αoEo cos(2πν ext) +Eo

2rm

∂α∂r

cos 2π ν ex −ν v( )t[ ]

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+Eo

2rm

∂α∂r

cos 2π ν ex + ν v( )t[ ]

The second and third terms correspond to the Stokes and the anti-Stokes frequencies.



Note that Raman scattering requires that the polarizability of a bond varies as a function of distance.

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∂α∂r

must be greater than zero if a Raman line is to appear

Raman scattering involves a momentary distortion of electrons distributed around a bond, followed by the reemission of radiation as the bond returns to its g.s.

In its distorted form the molecule is temporaily polarized (i.e. it develops momentarily an induced dipole).

The induced dipole disappears upon relaxation.



For example, the homonuclear molecules such as N2, O2 or H2 has no dipole moment either in the equilibrium or at any instant during stretching vibrations.

μ= qr = 0

N≡N O=O H-H

μ= qr = 0 μ= qr = 0

N N O O H H

The stretching mode is IR inactive in these molecules.

On the other hand, the polarizability of the bond between the two atoms of such a molecule varies periodically in phase with the stretching vibrations.



N≡N O =O H - H

N N O O H H

The stretching mode has a corresponding Raman line in the spectrum.

αincreases

Example:

O=C=O

IR inactiveRaman active

O=C=O

IR activeRaman inactive

symmetric stretch asymmetric stretch




Raman Depolarization Ratios

The 459 cm-1 line in CCl4 spectrum has depolarization ratio of 0.005, indicat ing m i n i m a l depolarization - said to be polarized.

- due to symmetric breathing

Nonsymmetrical vibrations is close to max of 6/7 (or 0.86).- the 218 and 314 lines in CCl4 has p = 0.75




Instrumentation

Three Components:

1. Laser source

2. Sample-illumination system

3. Suitable spectro- photometer

Two sample excitation systems




Sources

Helium/Neon Laser

- most widely used; principal line is 632.8 nm

Argon ion Laser- used when higher sensitivity (3x) is required; principal lines are 488.0 and 514.5 nm

Nd:YAG Laser

- emits NIR at 1.064 μm- can be operated at higher power (up to 50W) without causing photodecomposition of the sample but partially offset by “Raman α ν4” relationship- not energetic enough to bring about electronic transitions (can’t cause fluorescence)




Sample Illumination System

Sample handling for Raman is simpler than for IR.

- glass can be employed for windows, lenses, etc. instead of moisture-sensitive halides

- laser source can be focused on a small sample area (very small samples can be readily examined)

- common sample holder for liquid is an ordinary glass melting-point capillary.

- water is a weak Raman scatterer but a strong absorber of IR radiation.

- aqueous solutions can be studied by Raman but not IR spectroscopy.




Fourier Transform Raman Spectroscopy

A major limitation to Raman spectroscopy is backgroud signal arising from fluorescence of analyte or impurities.

- Raman scattering has low efficiency

- for an incident flux of 108 photons, on average, only one is Raman scattered; - if an impurity with ppm-level absorptivity and quantum yield of 0.1 is present, 10 fluorescent photons could be produced.

It is impossible to obtain a meaningful Raman spectrum with a highly fluorescent impurity or a weakly fluorescent sample.




Sample Raman Spectrum (of Anthracene)

- obtained with conventional Raman- most of the recorded signal arises from fluorescence

- recorded with an FT spectrometer - note the total absence of fluorescence background signal




Optical Diagram of FT Raman Spectrometer

- the interferometer is the same as is used in IR

- the detector is Ge photoconductor

- cut-off filters are employed to detect only the Stokes portion of the spectrum




Applications of Raman Spectroscopy

Inorganic Species

Raman technique is often superior to IR for investigating inorganic systems because aqueous solutions can be employed.

- metal-ligand bonds are generally in the range 100 - 700 cm-1, a region in IR that is difficult to study but Raman peaks with wavenumber shift values in this range are readily observed

Organic Species

Raman spectra are similar to IR (i.e. they have FGR and FPR)

Raman spectra yield more information about certain types of compounds.

- double bond stretch in olefin is weak in IR but intense in Raman.




Quantitative Applications

Raman spectra tend to be less cluttered with peaks than IR spectra are.

- peak overlaps in mixtures are less likely; quantitative measurements are simpler.

Small amounts of water do not interfere.

Quantitative analysis on very small samples is possible.- laser microprobes are employed for this work.

Laser microprobes applications:

- determination of analytes in single bacterial cells.

- determination of components in individual particles of smoke.

- determination of species in microscopic inclusions in minerals.




Resonance Raman Spectroscopy

Resonance Raman scattering refers to a phenomenon in which Raman line intensities are enhanced by excitation with wavelengths that closely approach that of an electronic absorption peak of an analyte.

Raman peaks are enhanced by a factor of 102 to 106

LOD becomes as low as 10-8 M (in contrast to normal Raman’s LOD of 0.1 M)

In resonance Raman, the electron is promoted to an excited electronic state followed by an immediate relaxation to the vibrational level of the electronic ground state.

It differs from fluorescence in that relaxation to the ground state is NOT preceded by prior relaxation to the lowest vibrational level of the excited electronic state.




Energy Diagram

Resonance Raman Fluorescence

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Raman