CHEM 515Spectroscopy Raman Spectroscopy
Light Scattering Phenomenon • When radiation passes through a transparent medium, the species present in that medium scatter a fraction of the beam in all directions.
Raman Effect (or Raman Scattering) • In 1928, the Indian physicist C. V. Raman discovered that the visible wavelength of a small fraction of the radiation scattered by certain molecules differs from that of the incident beam. • Furthermore, he noted that the change (shifts) in frequency depend upon the chemical structure of the molecules responsible for the scattering First photographed Raman spectra
First Report of Raman Observation Nature121, 501-502 (31 March 1928) A New Type of Secondary Radiation C. V. RAMAN & K. S. KRISHNAN Abstract If we assume that the X-ray scattering of the ‘unmodified’ type observed by Prof. Compton corresponds to the normal or average state of the atoms and molecules, while the ‘modified’ scattering of altered wave-length corresponds to their fluctuations from that state, it would follow that we should expect also in the case of ordinary light two types of scattering, one determined by the normal optical properties of the atoms or molecules, and another representing the effect of their fluctuations from their normal state. It accordingly becomes necessary to test whether this is actually the case. The experiments we have made have confirmed this anticipation, and shown that in every case in which light is scattered by the molecules in dust-free liquids or gases, the diffuse radiation of the ordinary kind, having the same wave-length as the incident beam, is accompanied by a modified scattered radiation of degraded frequency.
First Report of Raman Observation Nature121, 501-502 (31 March 1928) A New Type of Secondary Radiation C. V. RAMAN & K. S. KRISHNAN Continue The new type of light scattering discovered by us naturally requires very powerful illumination for its observation. In our experiments, a beam of sunlight was converged successively by a telescope objective of 18 cm. aperture and 230 cm. focal length, and by a second lens was placed the scattering material, which is either a liquid (carefully purified by repeated distillation in vacuo) or its dust-free vapour. To detect the presence of a modified scattered radiation, the method of complementary light-filters was used. A blue-violet filter, when coupled with a yellow-green filter and placed in the incident light, completely extinguished the track of the light through the liquid or vapour. The reappearance of the track when the yellow filter is transferred to a place between it and the observer's eye is proof of the existence of a modified scattered radiation. Spectroscopic confirmation is also available.
Professor Sir C.V. Raman 1888-1970 The Nobel Prize in Physics 1930 "for his work on the scattering of light and for the discovery of the effect named after him" http://nobelprize.org/nobel_prizes/physics/laureates/1930/raman-lecture.pdf
Rayleigh Scattering and Raman Scattering The frequency of the scattered light can be: • at the original frequency (νI) “Rayleigh scattering” very strong. • at some shifted frequency (νs = νI±νmolecule) “Raman scatteringorRaman shift” very weak (~ 10-5 of the incident beam)
Stokes and Anti-Stokes Scattering • Raman shift can correspond either to rotational, vibrational or electronic frequencies. Δν = |νI–νs| • Radiation scattering to the lower frequency side (to the red side) of the Rayleigh line is called Stokes scattering. • Radiation scattering to the higher frequency side (to the blue side) of the Rayleigh line is called anti-Stokes scattering.
Origin of Raman Scattering The electric field strength (E) of an EM radiation (normally laser beam with frequency ν0) with an amplitude E0 fluctuates with time (t) according to: When this light irradiates a diatomic molecule, an electric dipole moment (P) will be induced because of the movement of nuclei and electrons as a response for the applied electric field:
Origin of Raman Scattering There is a linear relationship between the induced dipole moment and the applied electric field. α is the proportionality constant and is called the polarizability. The displacement of the nuclei (q) “think about it as normal coordinates” in an electric is written: Polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, which may be caused by the presence of a nearby ion or dipole or by an applied external electric field.
Origin of Raman Scattering For a small nuclear displacement, polarizability is a linear function of nuclear displacement and can be written as: Polarizability at equilibrium position. The rate of change in polarizability w.r.t. in coordinate evaluated from the equilibrium position
Origin of Raman Scattering Combining these equation gives:
Origin of Raman Scattering Rayleigh scattering term Finally one gets: Raman anti-Stokes scattering Raman Stokes scattering
Origin of Raman Scattering Finally one gets: must not be zero to get a Raman active quantum transition.
Raman Scattering • The theory of Raman scattering shows that the phenomenon results from the same type of quantized vibrational changes that are associated with infrared absorption.
Stoke and Anti-Stoke Raman Spectra Virtual excited states Why do intensities of Stokes and anti-Stokes line differ?
Infrared and Raman Transitions • Unlike Raman spectroscopy, in IR, we observe direct absorptions corresponding to ground state vibrational states. • Normal Raman spectroscopy, the exciting line (νI) is chosen to be far below the 1st electronic excited state. It is called “virtual” or “quasi-excited” state.
Resonance Raman Scattering • Resonance Raman (RR)scattering uses an exciting line (νI) that has an energy intercepting the manifold of the electronic excites state. • This provides great enhancement in the Raman band intensities as compared to normal Raman.
Fluorescence Spectra • Fluorescence spectra are observed when an excited molecule goes a radiationless decay to the lowest vibrational level then emits vibration. • Life time of the excited state in RR is very short (~10-14 s) and in fluorescence are much longer (~10-8 to 10-5 s)
Nature of Polarizability Polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, which may be caused by the presence of a nearby ion or dipole or by an applied external electric field.
Raman Activity of Molecular Vibrations • In order to be Raman active, a molecular rotation or vibration must cause some change in a component of the molecular polarizability. The change can either be in the magnitude or the direction of the polarizability ellipsoid. • Polarizability ellipsoid is a three-dimensional body generated by plotting 1/√α from the center of gravity in all directions. • This rule must be contrasted with that for IR activity that requires change in the net dipole moment of the molecule.
Raman and Infrared are Complementary Techniques • Interestingly, although they are based on two distinct phenomena, the Raman scattering spectrum and infrared absorption spectrum for a given species often resemble one another quite closely in terms of observed frequencies. The infrared and Raman spectrum of styrene/buta-diene rubber.
Raman vs. Infrared Spectroscopy • Although IR and Raman techniques are similar in that both can be used as tools to characterize the molecular vibrations, there are many advantages and disadvantages unique to each of them. 1. Their selection rules are remarkably different. Some vibrations are only Raman active, some are only IR active and some are both. For molecules having center of symmetry, the rule of mutual exclusion holds.
Rule of Mutual Exclusion • If a molecule has a center of symmetry, then Raman active vibrations are infrared inactive, and vice versa. If there is no center of symmetry, then some (but not necessarily all) may be both Raman and infrared active.
Raman vs. Infrared Spectroscopy 2. Some vibrations are inherently weak in IR and strong in Raman spectra. • C≡C , C=C , P=S , S–S and C–S stretching vibrations are normally stronger in Raman (in general those bond with more covalent character). • O–H , N–H are stronger in the IR (in general those bond with more ionic character). • Multiple bonds are normally more intense in the Raman spectrum than single bonds. Raman intensity increases as C≡C > C=C > C–C.
Raman vs. Infrared Spectroscopy 3. Water is a very weak Raman scatterer. Thus, Raman spectra of samples in aqueous solution and hygroscopic air-sensitive compounds can be obtained without major interference from water vibrations and its rotation fine structures that are extremely strong in IR absorption spectra. 4. Sample container in Raman technique is made of glass. In IR technique it is impossible to use glass as it absorbs IR radiation.
Raman vs. Infrared Spectroscopy 5. Raman experiment uses a laser beam of a very small diameter (1-2 mm). Thus a very small quantity of the sample is needed to be characterized. 6. The laser source used by Raman needs to be carefully dealt with. It could cause local heating for the sample, burn the sample, or cause it to decompose. 7. Raman instruments need careful calibration as they record the shift in frequencies, unlike the IR technique.
Raman vs. Infrared Spectroscopy 8. The Raman technique is often superior to infrared for spectroscopy investigating inorganic systems because aqueous solutions can be employed. In addition, the vibrational energies of metal-ligand bonds are generally in the range of 100 to 700 cm-1, a region of the infrared that is experimentally difficult to study. These vibrations are frequently Raman active, however, and peaks with values in this range are readily observed. Raman studies are potentially useful sources of information concerning the composition, structure, and stability of coordination compounds.
9. Raman spectra tend to be less cluttered with peaks than infrared spectra. As a consequence, peak overlap in mixtures is less likely, and quantitative measurements are possibly simpler. 10. Raman spectroscopy has not yet been exploited widely for quantitative analysis. This lack of use has been due largely to the rather high cost of Raman spectrometers relative to that of absorption instrumentation.
Raman Depolarization Ratios • Polarization is a property of a beam of radiation and describes the plane in which the radiation vibrates. • Raman spectra are excited by plane-polarized radiation.
Raman Depolarization Ratios • The scattered radiation is found to be polarized to various degrees depending upon the type of vibration responsible for the scattering.
Raman Depolarization Ratios • The depolarization ratio (ρ) is dependent upon the symmetry of the vibrations responsible for scattering.
Raman Polarized Spectra of SO2 Asymmetric stretching vibration