A molecular look at structures

The advent of modern lasers has led to the resurgence of interest in the Raman Effect and to the discovery of a number of related phenomena.

One of the biggest discovery in the field of science in the past century is that of Raman Effect by Sir C V Raman. It is of little wonder then that this finding, which celebrates its 90th anniversary this year, was awarded the Nobel Prize. Let’s take a look at how this discovery has facilitated various other breakthroughs.

The Raman Effect is a phenomenon observed in the scattering of light which passes through a material medium in a way that the light suffers a change in frequency and a random alteration in phase or change in step. Due to these, Raman scattering is distinctly different from Rayleigh and Tyndall scattering. In Rayleigh and Tyndall scattering, as the scattered light passes through the medium, it has the same frequency as the unscattered light had before entering it and bears a definite phase relation to it. The intensity of normal Raman scattering is roughly one-thousandth of that of Rayleigh scattering in liquids and smaller still in gases. Owing to its low intensity, the Raman Effect was not discovered until 1928. 

Of great value

A few years before the discovery of the Raman Effect, A H Compton had observed frequency changed in X-rays scattered by electrons. Partly prompted by this, Raman and K S Krishnan examined sunlight scattered by a number of liquids. Using complementary filters, they found that there were frequencies in the scattered light that were lower than the frequencies in the filtered sunlight. Then, using the light of a single frequency from a mercury arc they showed that the new frequencies observed in the scattered radiation were characteristic of the scattering medium. The collection of new frequencies in the spectrum of monochromatic radiation scattered by a substance is characteristic of the substance and is called its Raman Spectrum. 

Raman spectroscopy is of great value in determining molecular structure and in chemical analysis, since the manner in which the frequency of the incident light is altered is a tell-tale sign of the nature of the substance through which the light has passed. As molecular rotational and vibrational frequencies can be determined directly and from these frequencies, it is also possible to find the molecular geometry. Raman spectroscopy is a form of vibrational spectroscopy that is used to observe changes in a molecule. Raman spectra also provides information in solid state physics, especially on lattice dynamics and the electronic structure of solids.

The advent of modern lasers has led to the resurgence of interest in the Raman Effect and to the discovery of a number of related phenomena. Owing to the laser beam’s small diameter and high collimation it can be easily used to excite the Raman Effect. Argon and Krypton ion lasers are usually used as they have high continuous wave power but tunable dye lasers are used for excitation of resonance Raman scattering.

The mechanism of the Raman Effect can be considered either by a corpuscular picture of light or from the viewpoint of wave theory. Both pictures merge in the basic quantum theory of radiation. The corpuscular model of light scattering envisages light quanta or photons as particles having linear and angular momenta. On passing through a material medium, these photons collide with atoms or molecules. If the collision is elastic, the photons bounce off the molecules with unchanged energy and momentum and hence, unchanged frequency. Such a process gives rise to Rayleigh scattering as the light frequency remains unchanged. If the collision is inelastic, the light photons may gain energy from, or lose it to, the molecules. A change in the photon energy must produce a change in the light frequency. 

Such inelastic collisions are rare compared to the elastic ones and the Raman scattering is correspondingly much weaker than Rayleigh scattering. If the light frequency is reduced owing to the photon losing its energy to the molecule, we get the Stokes lines and if the light frequency is increased by the photon gaining energy we have the anti-Stokes lines. The temperature of the scattering molecules is an additional factor which affects the intensity of frequencies higher than the exciting frequency (that is, the anti-Stokes lines).

Detecting different types

Development of lasers led to the discovery of a number of kinds of Raman scattering such as the Resonance Raman Effect. When the exiting photon falls within the frequency range of a molecular absorption band in the visible or ultraviolet spectrum, the radiation may be scattered by different processes, resonance fluorescence or resonance Raman Effect. Both these processes give much more intense scattering than the non-resonant Raman Effect.

Thus, the main characteristic of the resonance as compared to the normal Raman Effect is its intensity which may be larger by two or three orders of magnitude. For example, the resonance Raman Spectrum of oxyhaemoglobin may be excited by the 5682-angstrom wavelength of singly ionised krypton. The mechanism of the stimulated Raman Effect depends on the coherent pumping of molecules of the sample into an excited vibrational state by the powerful electric field of a laser beam. 

The coherent radiation so produced is called stimulated Raman scattering. It was first observed by E J Woodbury and W K Ng in 1962. They found the effect in liquid nitrobenzene. The development of tunable lasers has led to a special technique for stimulated Raman scattering called coherent anti-Stokes Raman Spectroscopy (CARS). In this technique, two lasers are used, one of fixed and the other of tunable frequency. The two beams enter the sample at angles differing by about two degrees and simultaneously impinge on the sample molecules.

Whenever the frequency difference between the two lasers coincides with the frequency of a Raman-active vibration of the molecules, emission of coherent radiation is stimulated. Thus, the total Raman Spectrum can be scanned in stimulated emission by varying the frequency of the tunable laser. The advantage of CARS, in addition to the high intensity of scattering, is that its elevated frequency avoids interference from sample fluorescence whose frequencies are always below that of the exciting radiation.

In recent years, there has been the remarkable phenomenon of surface-enhanced Raman scattering (SERS) where a fantastic 14-order of magnitude signal enhancement can occur during Raman scattering from molecules on metallic nanostructures. This turns the normally weak inelastic-scattering effect into a single-molecule spectroscopic probe. Fifty years after the discovery of Raman Effect, Martin Fleischmann and his colleagues reported receiving an unexpectedly strong Raman signal from a monolayer of pyridine adsorbed on electrochemically roughened silver surface electrode.

In recent years, SERS has matured into a powerful spectroscopic method wherein interaction of light, molecules and metal nanostructures boost Raman signals to even 15 or more orders, this being used for resolving the chemical structure of materials even at the single molecule level. It has also contributed to the development of plasmonics, near-field optics which is revolutionising optics and spectroscopy. Thus, remarkably, even 90 years after its discovery, the Raman Effect continues to open up newer avenues to unravel material structures down to single molecule spectroscopy.

(The author is with Indian Institute of Astrophysics, Bengaluru)

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