A New Radiation: C.V. Raman and the Dawn of Quantum Spectroscopy, Part II

In this second installment, we continue examining C.V. Raman’s amazing discovery of the Raman Effect in 1928, a milestone that fundamentally altered the landscape of physics and catalyzed advancements in quantum spectroscopy. Raman’s observation that light scattering could induce wavelength shifts provided empirical support for quantum theory, establishing a foundation for modern spectroscopic methodologies. His pioneering research earned him the 1930 Nobel Prize in Physics and continues to influence disciplines ranging from analytical chemistry to biomedical imaging. This discussion traces Raman’s scientific contributions, emphasizing his transformative role in the revolution of quantum spectroscopy and its contemporary applications. Across this two-part series, we have explored Raman’s quest to characterize what he described as “a new kind of radiation” and its profound impact on scientific progress.

Details of The Raman Effect Papers (1928)

In 1928 Raman introduced a new discovery in light emission from atoms and molecules, laying the groundwork for what would later be known as the Raman Effect. He begins by explaining primary and secondary radiation. Primary radiation occurs when atoms or molecules emit light due to heating or electron bombardment, while secondary radiation arises when substances are illuminated, such as in fluorescence or light scattering (16,17).

Portrait of C.V. Raman in 1930 (Public Domain Photo)

Raman’s team in Calcutta, while investigating light scattering, observed an unexpected phenomenon. When a violet filter was placed in the path of the incident light, the depolarization of scattered light increased. This effect, initially thought to be weak fluorescence, persisted even after careful purification of the substances. Raman’s work in studying this weak fluorescence revealed that certain liquids, especially polar substances like water and alcohols, exhibited this unique behavior (16,17).

These findings set the stage for the discovery of the Raman Effect. The Raman Effect, later demonstrated in 1928, is the inelastic scattering of light where the scattered light has different energy than the incident light, corresponding to shifts in molecular vibrational energy levels. The observations made during these studies of light scattering and fluorescence were key to understanding the molecular interactions involved in the Raman Effect, making this early research crucial in the development of Raman spectroscopy (16,17).

In another paper published in 1928, Raman draws a parallel between X-ray scattering and light scattering, proposing that, just as X-ray scattering by atoms and molecules can be categorized into unmodified (normal) and modified (fluctuating) types, ordinary light scattering should exhibit similar behavior. Raman hypothesized that one type of scattering corresponds to the normal optical properties of atoms or molecules, while the other indicates fluctuations from their normal state. His experiments confirmed this hypothesis, showing that in cases where light is scattered by molecules in dust-free liquids or gases, the scattered light consists of two components: the typical diffuse radiation with the same wavelength as the incident beam and a modified radiation with a shifted (lower) frequency. This modified scattering laid the foundation for the discovery of the Raman Effect, where the frequency shift in scattered light corresponds to energy transitions in the molecules, ultimately leading to the development of Raman spectroscopy (16,17).

Also in 1928, Raman described an analogy to the Compton Effect (18,19) found in X-ray scattering. The presence of new wavelengths in the scattered light, distinct from the incident light, is demonstrated through spectrograms. In his example using toluene, the scattered light shows several new spectral lines, which disappear when specific wavelengths from the incident light are filtered out, confirming that the scattered lines originate from the incident radiation. Raman proposes that the scattering of light by molecules may involve both whole and partial interactions, with the partial interactions leading to a shift to longer wavelengths. The wavelength shift corresponds to the molecular infrared absorption frequencies, and variations in the shift across different molecules support this theory. Raman's observations laid the groundwork for the Raman Effect, where scattered light shows frequency shifts, contributing to the development of Raman spectroscopy (20).

Compton proposes a quantum theory for the scattering of X-rays and γ-rays by light elements, suggesting that an X-ray quantum transfers all its energy and momentum to a single electron, which then recoils while scattering the radiation in a specific direction. Due to this recoil, the scattered quantum has less energy than the incident quantum, resulting in an increase in wavelength given by the relation shown in equation 1 (18–20):

Experimental data confirms that for graphite and molybdenum K radiation, the scattered wavelength is longer than the primary, with observed shifts matching theoretical predictions. Similarly, for γ-ray scattering, the wavelength increases with angle, from 0.022 Å (primary) to 0.068 Å at 135°. The velocity of secondary β-rays excited in light elements aligns with the idea that they are recoil electrons. Furthermore, experimental results on absorption variations and forward scattering distribution for X-rays and γ-rays strongly support the theory. This agreement indicates that scattering is a quantum process, with radiation quanta carrying both energy and momentum. The theory is primarily applicable to light elements, where the constraining forces on electrons are negligible (19).

The Compton Effect (18) study examines the scattering of molybdenum Kα rays by graphite at angles of 45°, 90°, and 135°, comparing the scattered spectrum to the primary beam. It was observed that each primary spectral line splits into two components upon scattering: an unmodified line that retains its original wavelength and a modified line with a longer wavelength. The wavelength shift (λ - λ₀) follows the quantum relation (λ - λ₀) = λ(1 - cosθ), where λ = 0.0242 Å. Absorption measurements support this result. Additionally, the modified ray appears less homogeneous than the unmodified ray, with its intensity increasing at larger scattering angles (19).

In another 1928 paper, Raman reports on the discovery that when monochromatic light is scattered through transparent media (gas, liquid, solid), the scattered light ceases to be monochromatic, producing several new spectral lines or bands, often with a continuous spectrum. These new radiations are generally strongly polarized and are distinct from fluorescence, as the effect occurs even when both the exciting radiation and the scattered radiations are far from the medium’s characteristic ultraviolet or infrared frequencies. Using transparent crystalline quartz and mercury’s 4358 Å line as an example, Raman describes how the scattered light generates new spectral lines in the indigo-blue region (21).

Raman concludes that this phenomenon arises from the scattering of light where part of the incident radiation is absorbed by the molecules, causing them to transition to a higher energy level. The remaining part of the light is scattered, and its frequency shift corresponds to this energy change. This process was first theorized by Smekal (22) and is supported by earlier work in quantum mechanics by Kramers (23,24), Heisenberg (25), and Schrödinger (26). Raman’s experiments confirm this phenomenon, which occurs in various substances, including gases, liquids, and crystals. This discovery laid the foundation for the Raman Effect, a key principle in Raman spectroscopy (21).

The phenomenon of anomalous light scattering, originally predicted by Smekal and later examined in greater detail by Kramers and Heisenberg, is explained without reliance on quantum light theories. Instead, the introduction of metastable quantum states that arise during elementary scattering events provides a framework for understanding key optical effects. These transient states help explain the change in the propagation speed of light within a dispersive medium compared to its velocity in a vacuum, offering insight into the fundamental mechanisms of dispersion. Furthermore, this approach allows scattering processes to be analyzed using Einstein’s probability formulations, which were initially developed for stationary quantum states. By extending these principles to non-stationary interactions, the study provides a more comprehensive theoretical foundation for understanding light-matter interactions beyond classical models. (22).

Foundationally, Schrödinger explores the concept of perturbation theory for eigenvalue problems and its application in quantum mechanics. The theory is introduced as a way to extend the eigenvalue problem to situations where exact solutions are not readily available. By considering small perturbations, the eigenvalues and eigenfunctions of a system can be approximated, even for problems that are only closely related to directly solvable ones. The focus is on the continuous dependence of the eigenvalues on the coefficients of the differential equations, rather than on factors like the size of the domain or the boundary conditions (26).

The Schrödinger paper also connects this approach to Lord Rayleigh's earlier work on the vibrations of a string with inhomogeneities in his "Theory of Sound" (1894). The author applies perturbation theory to the study of the Stark effect, which refers to the splitting of spectral lines in the presence of an electric field, specifically focusing on the Balmer lines of hydrogen. The method allows for an approximation of the eigenvalues and eigenfunctions in these cases, making it a valuable tool in quantum mechanics, especially when dealing with small disturbances or external influences on the system (26).

Schrödinger’s focus on perturbation theory and its application to eigenvalue problems is conceptually related to the Raman Effect in that both involve the interaction of light with matter and the resulting changes in the system's properties. The Raman Effect describes the scattering of light by molecules, where the scattered light has a different energy (and therefore wavelength) compared to the incident light, typically due to vibrational or rotational transitions in the molecules (26).

Schrödinger’s perturbation theory helps to understand how small changes (such as the introduction of an external electric field in the Stark effect) influence the energy levels of a system. Similarly, in the Raman Effect, the incident light can be considered a perturbation that induces a transition between molecular energy levels, causing the shift in wavelength. Both involve the use of perturbation methods to approximate changes in energy levels due to external influences, making perturbation theory a useful tool in analyzing both the Stark effect (explored in the paper) and the Raman scattering phenomenon (26).

Again in 1928, Raman introduces the historical context of his research on light scattering and its relation to the Raman Effect. He describes how his work, initiated in 1922 at Calcutta, was inspired by Rayleigh’s theory of the blue sky. While initially focused on natural phenomena like the color of the sea and glaciers, Raman's studies revealed much deeper insights into molecular structure, the nature of radiation, and the constitution of solids and liquids (27).

Raman reflects on the early attempts to explain light scattering using classical electromagnetic theory, which focused on fluctuations in optical density due to molecular arrangement. However, by 1922-1923, experimental evidence began to show an unexpected phenomenon: light scattering in purified liquids, such as water and alcohol, was accompanied by radiation at altered wavelengths. This discovery hinted at a new optical effect, which was initially misunderstood as a type of fluorescence. However, Raman recognized that this radiation could not be explained by fluorescence, as it was excited by light far removed from the infra-red and ultra-violet regions typical for molecular fluorescence (27).

This work from the 1920s led Raman to investigate further, with the intention of clarifying the cause of these wavelength shifts, ultimately leading to the identification of what would become known as the Raman Effect, an important principle in molecular spectroscopy.

The Nobel Prize

In 1930, Raman was awarded the Nobel Prize in Physics “for his work on the scattering of light and for the discovery of the effect named after him” (28,29). The Nobel Committee recognized his contributions to advancing molecular physics and exploring the symmetry properties of molecules and nuclear spin in atomic physics (28,29).

Raman’s discovery of the Raman Effect in 1928 was a defining moment in his career. Working at the Indian Association for the Cultivation of Science (IACS) in Kolkata with his collaborator K.S. Krishnan, he demonstrated that when light passes through a medium, a portion of it undergoes a wavelength shift due to interactions with the medium's molecules. This phenomenon provided valuable insights into molecular structures and vibrational energy levels. The discovery’s universality, observed across solids, liquids, and gases, established it as a foundational concept in molecular spectroscopy (1–4).

Light scattering, a phenomenon that had been studied extensively through effects such as the Tyndall effect and Rayleigh scattering, occurs when light is redirected by small particles or molecules in a medium. These earlier studies demonstrated that scattered light is more intense for shorter wavelengths, explaining phenomena such as the blue color of the sky and the reddish hues at sunrise and sunset. However, anomalies in light scattering observed in solids, liquids, and gases could not be fully explained by existing theories. While the Tyndall and Rayleigh effects emphasized the role of dust or atmospheric particles, Cabannes’ 1914 experiments showed that pure and dust-free gases also exhibit scattering. Despite these advancements, the precise mechanisms and their implications for molecular structure remained elusive (28).

In 1928, Raman made a revolutionary discovery while studying the scattering of monochromatic light, such as filtered emissions from a mercury lamp. He observed that the scattered light contained additional spectral lines not present in the original light source. These new lines, symmetrically distributed around the primary wavelength, exhibited frequency shifts that remained constant regardless of the medium used. Raman attributed these shifts to energy exchanges between the incident photons and molecular vibrational or rotational states. This discovery revealed that molecular interactions with light could generate new frequencies, leading to profound implications for molecular spectroscopy (28).

Raman explained the phenomenon using quantum theory, demonstrating that light interacts with molecules by exchanging discrete energy quanta. When a photon transfers energy to a molecule during scattering, the scattered light loses energy, resulting in a Stokes shift. Conversely, when a molecule transfers energy to a photon, the scattered light gains energy, creating an anti-Stokes shift. These shifts correspond to specific molecular vibrational or rotational energy levels in the excited sample, providing a direct link between light scattering and molecular structure. Raman’s findings showed that the frequency differences between incident and scattered light are characteristic of the molecular vibrations in the scattering medium and remain constant across different wavelengths of incident light (28).

The Raman Effect introduced a revolutionary method for investigating molecular structure. By analyzing the frequency shifts in scattered light, researchers gained access to vibrational and rotational energy levels, which were previously challenging to study using infrared (IR) spectroscopy due to limitations of infrared detector sensitivity. The universal nature of the Raman Effect allowed it to be observed in solids, liquids, and gases, enabling detailed studies of molecular behavior under various conditions. It also provided a powerful tool for characterizing chemical bonds, molecular symmetry, and interactions. Raman spectroscopy extended the reach of spectroscopic techniques, offering a non-destructive and precise method for studying the chemical and physical properties of substances, even in aqueous environments since water is a poor Raman scatterer.

The Raman Effect overcame significant challenges in molecular spectroscopy by shifting infrared and ultraviolet frequencies into detectable regions, making it possible to study previously inaccessible parts of the molecular spectrum. This advancement allowed researchers to probe the oscillatory behavior of molecules with unprecedented accuracy. Furthermore, Raman spectroscopy provided a direct means of examining the structural and dynamic properties of matter, including studies of molecular aggregation, electrolytic dissociation, and ultra-red absorption in crystals. It became an essential tool for understanding molecular structure and interactions, with wide-ranging applications across chemistry, physics, and materials science.

Raman’s discovery not only deepened our understanding of molecular and atomic structures but also laid the groundwork for modern spectroscopic techniques. By providing a new method to study the vibrational and rotational states of molecules, the Raman Effect opened up new possibilities for exploring the fundamental nature of matter. The Swedish Royal Academy of Sciences recognized the profound impact of this work by awarding the Nobel Prize to Raman, highlighting the transformative nature of his contributions to light scattering and molecular spectroscopy. Today, the Raman Effect remains a cornerstone of molecular analysis, offering a versatile and powerful approach for investigating the complexities of chemical and physical systems (28)).

Although the Nobel Prize cemented Raman’s legacy, his path to this recognition was not without complexity. Grigory Landsberg and Leonid Mandelstam at Moscow University had independently observed a similar phenomenon, but the Nobel Committee concluded that Raman’s work offered broader interpretation and applicability. Despite some debate, the award reinforced Raman’s status as a pioneering physicist (1–4,28,29).

Other Achievements, Awards, and Recognitions

Throughout his career, Raman received numerous honors, reflecting his significant contributions to physics (1-4,6):

• Curzon Research Award (1912): Acknowledged his early research while in the Indian Finance Service.
• Woodburn Research Medal (1913): Further recognition of his academic achievements during his Finance Service tenure.
• Elected Fellow of the Royal Society of London (FRS) on May 15, 1924 (nominated in 1921).
• Matteucci Medal (1928): Awarded by the Accademia Nazionale delle Scienze in Rome for the international impact of his work.
• Knighthood (1930): Bestowed by Lord Irwin, Viceroy of India, for his contributions to science.
• Hughes Medal (1930): Presented by the Royal Society for his groundbreaking work in light scattering.
• Franklin Medal (1941): Recognized by the Franklin Institute in Philadelphia for his advancements in optics and light scattering.
• Bharat Ratna (1954): India’s highest civilian award, shared with C. Rajagopalachari and Sarvepalli Radhakrishnan.
• Lenin Peace Prize (1957): Honored for his contributions to science and its role in fostering global progress.

Building Institutions and Inspiring Generations

Raman’s contributions extended beyond his research. He was instrumental in building institutions and nurturing talent in India. In 1934, he founded the Indian Academy of Sciences to promote independent research and became its lifelong president. After retiring from the Indian Institute of Science (IISc) in 1948, he established the Raman Research Institute in Bangalore, providing a platform for scientific inquiry (1–4).

His tenure at the IISc and his interactions with the Royal Society reflected his strong commitment to scientific ideals. While some of his relationships with peers and institutions were marked by disagreements, these moments emphasized his dedication to intellectual independence and excellence (1–4)..

A Lasting Legacy

C.V. Raman’s legacy continues to inspire. February 28, the day of his discovery of the Raman Effect, is celebrated annually in India as National Science Day. Roads, institutions, and buildings across the country honor his name, including the C.V. Raman Marg in New Delhi and the Raman Building at IISc. His contributions are also commemorated through stamps issued in his honor in 1971 and 2009 (1–4).

Raman’s life and work exemplify the enduring impact of scientific inquiry and discovery. His achievements not only advanced our understanding of light and matter but also inspired future generations to pursue science with curiosity and determination.

References

(16) Raman, C. V. A New Radiation. Indian J. Phys. 1928, 2, 387–398. Available at: https://repository.ias.ac.in/70648/1/36-PUb.pdf (accessed 2025-02-07)

(17) Raman, C. V.; Krishnan, K. S. A New Type of Secondary Radiation. Nature 1928, 121, 501–502. Available at: https://repository.ias.ac.in/28460/1/367.pdf (accessed 2025-02-07).

(18) Compton, A. H. A Quantum Theory of the Scattering of X-rays by Light Elements. Phys. Rev. 1923, 21 (5), 483. Available at: https://hd.fizyka.umk.pl/~lab2/manual/27/compton.pdf (accessed 2025-02-07).

(19) Compton, A. H. The Spectrum of Scattered X-rays. Phys. Rev. 1923, 22 (5), 409. Available at: http://users.df.uba.ar/giribet/f4/compton.pdf (accessed 2025-02-07).

(20) Raman, C. V.; Krishnan, K. S. The Optical Analogue of the Compton Effect. Nature 1928, 121, 711. Available at https://repository.ias.ac.in/28463/1/371.pdf (accessed 2025-02-07).

(21) Raman, C. V.; Krishnan, K. S. The Production of New Radiations by Light Scattering—Part I. Proc. R. Soc. Lond. A 1929, 122 (789), 23–35. DOI: 10.1098/rspa.1929.0002. Available at: https://web.archive.org/web/20191027022919id_/https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1929.0002 (accessed 2025-02-07).

(22) Smekal, A. Zur Quantentheorie der Streuung und Dispersion. Z. Phys. 1925, 32 (1), 241–244. DOI: 10.1007/BF01331666

(23) Kramers, H. A. The Law of Dispersion and Bohr’s Theory of Spectra. Nature 1924, 113, 673–674. DOI: 10.1038/113673a0

(24) Kramers, H. A. The Quantum Theory of Dispersion. Nature 1924, 114 (2861), 310–311. DOI: 10.1038/114310b0

(25) Heisenberg, W. Quantum-Theoretical Re-Interpretation of Kinematic and Mechanical Relations. Z. Phys. 1925, 33, 879–893. Available at: http://users.mat.unimi.it/users/galgani/arch/heis25ajp.pdf (accessed 2025-02-07).

(26) Schrödinger, E. Quantisierung als Eigenwertproblem. Ann. Phys. 1926, 385 (13), 437–490. DOI: 10.1002/andp.19263861802. Available Available at: https://uni-tuebingen.de/fileadmin/Uni_Tuebingen/Fakultaeten/MathePhysik/Institute/IAP/Forschung/MOettel/Geburt_QM/schrodinger_AnnPhys_385_437_1926.pdf (accessed 2025-02-07).

(27) Raman, C. V. Part II—The Raman Effect: Investigation of Molecular Structure by Light Scattering. Trans. Faraday Soc. 1929, 25, 781–792. DOI: 10.1039/TF9292500781. Available at: https://pubs.rsc.org/en/content/articlepdf/1929/tf/tf9292500781?casa_token=cji669v3QuoAAAAA:ZHhAV-w00WKHpi14ohBW3cg4jCnwxqU2l5Qk-g8dLLiPinP70lNolVFj7susLval1gONUSFWF3b54A (accessed 2025-02-07).

(28) The Nobel Prize–C. V. Raman Web Page. Available at: https://www.nobelprize.org/prizes/physics/1930/raman/facts/ (accessed 2025-02-07).

(29) 1930 Nobel Prize in Physics Award Ceremony Speech. Available at: https://www.nobelprize.org/prizes/physics/1930/ceremony-speech/#:~:text=The%20Academy%20of%20Sciences%2C%20has,the%20effect%20named%20after%20him (accessed 2025-02-07).

About the Author

Jerome Workman, Jr. serves on the Editorial Advisory Board of Spectroscopy and is the Executive Editor for LCGC and Spectroscopy. He is the co-host of the Analytically Speaking podcast and has published multiple reference text volumes, including the three-volume Academic Press Handbook of Organic Compounds, the five-volume The Concise Handbook of Analytical Spectroscopy, the 2nd edition of Practical Guide and Spectral Atlas for Interpretive Near-Infrared Spectroscopy, the 2nd edition of Chemometrics in Spectroscopy, and the 4th edition of The Handbook of Near-Infrared Analysis. Author contact: JWorkman@MJHlifesciences.com●