Raman Spectroscopy –
Raman spectroscopy is a form of vibrational spectroscopy, much like infrared (IR) spectroscopy. However, whereas IR bands arise from a change in the dipole moment of a molecule due to an interaction of light with the molecule, Raman bands arise from a change in the polarizability of the molecule due to the same interaction. This means that these observed bands (corresponding to specific energy transitions) arise from specific molecular vibrations. When the energies of these transitions are plotted as a spectrum, they can be used to identify the molecule as they provide a “molecular fingerprint” of the molecule being observed. Certain vibrations that are allowed in Raman are forbidden in IR, whereas other vibrations may be observed by both techniques although at significantly different intensities, thus these techniques can be thought of as complementary.
Since the discovery of the Raman effect in 1928 by C.V. Raman and K.S. Krishnan, Raman spectroscopy has become an established and practical method of chemical analysis and characterization applicable to many different chemical species.
Samples may be in the form of
- Solids (particles, pellets, powers, films, fibers)
- Liquids (gels, pastes)
The following sections provide a tutorial-style introduction to the basics of Raman Spectroscopy.
A brief look at Raman scattering theory
The Raman Effect and Normal Raman Scattering.
When light is scattered from a molecule, most photons are elastically scattered. The scattered photons have the same energy (frequency), and therefore wavelength, as the incident photons. However, a small fraction of light (approximately 1 in 107 photons) is scattered at optical frequencies different from, and usually lower than, the frequency of the incident photons. The process leading to this inelastic scatter is termed the Raman effect. Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule. Chemists are concerned primarily with the vibrational Raman effect, and thus in this tutorial we use the term Raman effect to mean vibrational Raman effect only.
The difference in energy between the incident photon and the Raman scattered photon is equal to the energy of a vibration of the scattering molecule. A plot of intensity of scattered light versus energy difference is a Raman spectrum.
The Scattering Process
When a beam of light is impinged upon a molecule, photons are absorbed by the material and scattered. The majority of these scattered photons have exactly the same wavelength as the incident photons and are known as Rayleigh scatter. In the scattering process, the incident photon excites an electron into a higher “virtual” energy level (or virtual state) and then the electron decays back to a lower level, emitting a scattered photon. In Rayleigh scattering, the electron decays back to the same level from which it started and thus, Rayleigh scattering is often referred to as a form of elastic scatter. The process of Rayleigh scatterin i visualized in Figure 1.1.
The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole of the molecule. It is a form of electronic (more accurately, vibronic) spectroscopy, although the spectrum contains vibrational frequencies. In classical terms, the interaction can be viewed as a perturbation of the molecule’s electric field. In quantum mechanical terms, the scattering can be described as an excitation to a virtual state lower in energy than a real electronic transition with nearly coincident de-excitation and a change in vibrational energy. The virtual state description of scattering is shown in Figure 1.1a. In the Raman effect, the electron excited in the scattering process decays to a different level than that where it started which is termed inelastic scattering.
The energy difference between the incident and scattered photons is represented by the arrows of different lengths in Figure 1.1a. Numerically, the energy difference between the initial and final vibrational levels, or Raman shift in wave numbers (cm-1), is calculated through equation 1 in which λ incident and λ scattered are the wavelengths (in nm) of the incident and Raman scattered photons, respectively.
The vibrational energy is ultimately dissipated as heat. Because of the low intensity of Raman scattering, the heat dissipation does not cause a measurable temperature rise in a material.
At room temperature, the thermal population of vibrational excited states is low, although not zero. Therefore, the initial state is the ground state and the scattered photon will have lower energy (longer wavelength) than the exciting photon. This Stokes shifted scatter is what is usually observed in Raman spectroscopy. Figure 1.1 (center) depicts Raman Stokes scattering.
A small fraction of the molecules are in vibrationally excited states. Raman scattering from vibrationally excited molecules leaves the molecule in the ground state. The scattered photon appears at higher energy, as shown in Figure 1.1 (right). At room temperature, the anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, and since the Stokes and anti-Stokes spectra contain the same frequency information, most Raman experiments look at Stokes-shifted scatter only.
The energy of a vibrational mode depends on molecular structure and environment. Atomic mass, bond order, molecular substituents, molecular geometry and hydrogen bonding all effect the vibrational force constant which, in turn, dictates the vibrational energy. For example, the stretching frequency of a phosphorus-phosphorus bond ranges from 460 to 610 to 775 cm-1 for the single, double and triple bonded moieties, respectively. Much effort has been devoted to the estimation or measurement of force constants. For small molecules, and even for some extended structures such as peptides, reasonably accurate calculations of vibrational frequencies are possible with commercially available software.
Vibrational Raman spectroscopy is not limited to intramolecular vibrations. Crystal lattice vibrations and other motions of extended solids are Raman-active. Their spectra are important in such fields as polymers and semiconductors. In the gas phase, rotational structure is resolvable on vibrational transitions. The resulting vibration/rotation spectra are widely used to study combustion and gas phase reactions generally. Vibrational Raman spectroscopy in this broad sense is an extraordinarily versatile probe into a wide range of phenomena ranging across disciplines from physical biochemistry to materials science.
Raman Selection Rules and Intensities
A simple classical electromagnetic field description of Raman spectroscopy can be used to explain many of the important features of Raman band intensities. The dipole moment, P, induced in a molecule by an external electric field, E, is proportional to the field as shown in equation 2.
The proportionality constant α is the polarizability of the molecule. The polarizability measures the ease with which the electron cloud around a molecule can be distorted. The induced dipole emits or scatters light at the optical frequency of the incident light wave.
Raman scattering occurs because a molecular vibration can change the polarizability.
The change is described by the polarizability derivative, where Q is the normal coordinate of the vibration. The selection rule for a Raman-active vibration, that there be a change in polarizability during the vibration, is given in equation 3.
The Raman selection rule is analogous to the more familiar selection rule for an infrared-active vibration, which states that there must be a net change in permanent dipole moment during the vibration. From group theory it is straightforward to show that if a molecule has a center of symmetry, vibrations which are Raman-active will be silent in the infrared, and vice versa.
Scattering intensity is proportional to the square of the induced dipole moment, that is to the square of the polarizability derivative.
If a vibration does not greatly change the polarizability, then the polarizability derivative will be near zero, and the intensity of the Raman band will be low. The vibrations of a highly polar moiety, such as the O-H bond, are usually weak. An external electric field cannot induce a large change in the dipole moment and stretching or bending the bond does not change this.
Typical strong Raman scatterers are moieties with distributed electron clouds, such as carbon-carbon double bonds. The pi-electron cloud of the double bond is easily distorted in an external electric field. Bending or stretching the bond changes the distribution of electron density substantially, and causes a large change in induced dipole moment.
Chemists generally prefer a quantum-mechanical approach to Raman scattering theory, which relates scattering frequencies and intensities to vibrational and electronic energy states of the molecule. The standard perturbation theory treatment assumes that the frequency of the incident light is low compared to the frequency of the first electronic excited state. The small changes in the ground state wave function are described in terms of the sum of all possible excited vibronic states of the molecule.
Raman scatter is partially polarized, even for molecules in a gas or liquid, where the individual molecules are randomly oriented. The effect is most easily seen with an exciting source which is plane polarized. In isotropic media polarization arises because the induced electric dipole has components which vary spatially with respect to the coordinates of the molecule. Polarized Raman experiments can be a power tool in studying the mechanism of orientation and the final structure of polymeric films and fibers as well as in the characterization of single crystals.
Resonance-Enhanced Raman Scattering
If the wavelength of the exciting laser is within the electronic spectrum of a molecule, then the intensity of some Raman-active vibrations increases by a factor of 102 – 104. This resonance enhancement or resonance Raman (RR) effect may be useful. Resonance enhancement does not begin at a sharply defined wavelength. In fact, enhancement of 5X-10X is commonly observed if the exciting laser is even within a few hundred wavenumbers below the electronic transition of a molecule. This pre-resonance enhancement may also be experimentally useful.
RR, however, is only observed in molecules possessing vibrations that can be resonantly enhanced and as such this approach is limited to certain chemistries. This limits the generalized applicability of RR for analytical applications. An in-depth review of resonance enhancement is beyond the scope of this tutorial and the interested reader is referred to specific publications on the theory and application of RR.
Surface-Enhanced Raman Scattering
The Raman scattering from a compound (or ion) adsorbed on or even within a few Angstroms of a structured metal surface can be 103 – 106x greater than in solution. This surface-enhanced Raman scattering is strongest on silver, but is observable on gold and copper as well for common excitation sources.[3,4] At practical excitation wavelengths, enhancement on other metals is unimportant. Surface-enhanced Raman scattering (SERS) arises from two mechanisms.
Although SERS allows observation of Raman spectra from solution concentrations in the micromolar (1 X 10-6) range, slow adsorption kinetics, competitive adsorption, and the fact that only certain chemistries and states exhibit enhancements, limits the general applicability of SERS outside the R&D laboratory.
The Raman Spectrum
A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm-1). This difference is called the Raman shift. Note that, because it is a difference value, the Raman shift is independent of the frequency of the incident radiation.
Note that each has a characteristic set of peaks that allows it to be distinguished from the other.
Qualitative vs. Quantitative Raman
Historically, due to the complexity and inefficiency of the optical components used, the challenge for Raman instruments has been to simply measure a useable Raman spectrum. A spectrum is used to chemical fingerprint the sample, providing a qualitative assessment of the chemical composition of the sample. Kaiser has revolutionized Raman spectroscopy by pioneering the holographic technology that has allowed high-throughput, compact analyzers to be developed. Kaiser’s technology has enabled Kaiser and its users to move beyond qualitative Raman spectroscopy to quantitative analysis. Quantitative Raman spectroscopy provides for species’ chemical concentrations to be measured, monitored, and controlled. Real-time ‘in situ‘ qualitative Raman spectroscopy has allowed Kaiser’s Raman analyzers to be deployed beyond the laboratory and provide over 20 years of successful 24/7 process control installations.
Advantages of Raman Spectroscopy
Raman spectroscopy is useful for chemical analysis for several reasons: