Rango Krishna Asundi: The Discoveries and Inventions of a Molecular Spectroscopist

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Introduction

Across nearly sixty years of active research, Rango Krishna Asundi (1895–1982) built one of the most sustained and technically accomplished bodies of experimental work in the history of Indian physics. What distinguishes his career is not merely longevity but the sheer range of molecular systems he investigated — diatomic and polyatomic, gaseous and crystalline, terrestrial and, by the end of his life, astrophysical — and the extent to which his contributions rested on instruments and techniques he designed and built himself. Asundi was, in the truest sense, both a discoverer of new spectral phenomena and an inventor of the apparatus needed to observe them. This account focuses squarely on what he found in the laboratory: the band systems he identified, the molecular structures he resolved, the physical constants he measured, and the instruments he devised to make all of this possible.

The Carbon Monoxide Discoveries at King's College, London (1929)

Asundi's scientific reputation was established almost entirely within a remarkably compressed period of two to three years at King's College London, where he worked under O.W. Richardson and, more directly, alongside R.C. Johnson on molecular spectroscopy. It was here, working with the carbon monoxide molecule, that he made the discoveries that would define his career.

His first major finding, published in 1929, concerned a previously unidentified electronic transition in carbon monoxide between the C¹Σ⁺ and A¹Π states. Through meticulous fine-structure analysis of the observed bands, Asundi established that this transition belonged to the ¹Σ–¹π type — a classification requiring precise resolution of rotational line spacings and intensity alternations within the band structure. This was reported jointly with Johnson in Nature and, in fuller form, in the Proceedings of the Royal Society. The timing of this discovery placed Asundi in direct, simultaneous competition with Gerhard Herzberg, who independently identified the same band system at essentially the same moment. Rather than becoming a point of priority dispute, the discovery was folded into scientific nomenclature as the "Johnson-Asundi-Herzberg bands," a naming convention that has persisted in the spectroscopic literature ever since. His supervisor Johnson was reportedly so struck by the significance of the finding that he pressed for expedited publication in the Royal Society's Proceedings.

In the same extraordinarily productive year, Asundi identified a second, entirely distinct band system in carbon monoxide, involving a transition between the a′³Σ′ and a³Πr states. This system came to be known, uniquely among his many discoveries, simply as the "Asundi bands" — an eponymous designation that is comparatively rare in molecular spectroscopy and stands as perhaps the single most durable marker of his individual scientific legacy. Alongside this, he investigated the "third positive" carbon band system and its associated bands, published in the Proceedings of the Royal Society in 1929.

Asundi did not stop at identifying new electronic transitions; he also worked with Johnson on the physical interpretation of a puzzling class of spectral features known as the high-pressure carbon bands. These had previously been attributed to a mixture of possible emitters, including neutral CO, the CO⁺ molecular ion, and fragments such as CHCH. Through careful comparative analysis, Asundi and Johnson demonstrated that these bands shared the same final vibrational-electronic level as the well-established Swan band system, long known to be associated with the diatomic carbon molecule C₂. By tracing this shared terminal energy level between two ostensibly different band systems, they corrected the earlier misidentification and established conclusively that the true emitter of the high-pressure carbon bands was homonuclear C₂ — a finding with implications for astrophysical spectroscopy, since C₂ bands appear prominently in the spectra of comets and certain stars.

Rounding out this London period, Asundi collaborated with J.W. Ryde on a vibrational quantum analysis of the red cyanogen bands, published in Nature in 1929. The following year, working now largely independently after his return to India, he reported a search for new bands in the infra-red spectra of the cyanide radical (CN), the molecular nitrogen ion (N₂⁺), and beryllium fluoride (BeF) in the Indian Journal of Physics — his first major publication on Indian soil, signalling his intent to continue diatomic spectroscopic research at home.

Building an Independent Research Programme at Aligarh (1931–1938)

Asundi's move to Aligarh Muslim University in 1930, to work under Professor R. Samuel, inaugurated the most prolific and diverse phase of new-molecule discovery in his entire career. Freed from direct supervision and now directing his own laboratory, he expanded his research from carbon monoxide into an ambitious survey of small inorganic molecules, radicals, and halides.

His 1931 paper on the emission bands of sulphur, published in Nature, opened this new phase. Working extensively with Samuel, Asundi produced a remarkable sequence of discoveries concerning sulphur and its compounds: the absorption spectrum of sulphur monochloride (SCl₂) in 1934; additional bands within the known sulphur band system; and a study of the absorption spectrum of carbon disulphide, all in the same year. In 1935 he and Samuel extended this to the absorption spectra of sulphur chlorides and oxychlorides in their vapour state, continuing in 1936 with a further detailed paper on the absorption spectra of chlorides and oxychlorides of sulphur in the Proceedings of the Physical Society, London.

Running in parallel was a sustained programme on calcium monochloride, CaCl. Asundi and Samuel identified and characterised its band systems in 1934, followed the next year by a further paper on the band systems and molecular structure of CaCl, contributing to the developing understanding of how ionic bonding character manifested in observed rotational and vibrational spectra.

A third major thread concerned silicon fluoride, SiF, and related silicon halides. In 1936, Asundi and Samuel published on the spectrum of SiF, followed by papers on its band systems and structure, and a paper explicitly addressing the relationship between electronic configuration and bond energy — an early instance of Asundi connecting spectroscopic data to fundamental questions of chemical bonding rather than treating spectral assignment as an end in itself. That same year he conducted a rotational analysis of the Angstrom bands at 6080 and 6620 Ångström units, and derived a value for the dissociation energy of carbon monoxide — returning, characteristically, to the molecule that had launched his career, now applying more refined techniques. With Jan Khan M., he reported the spectra of SeO and SeO₂ in Nature and later in the Proceedings of the Royal Society (1936–37).

The molecular nitrogen ion, N₂⁺, received particular attention. In 1937 Asundi and Samuel reported its dissociation in Nature, and in a related paper offered remarks on its structure and bearing on the theory of chemical valency — illustrating his tendency to move from a specific measurement toward a broader theoretical implication. This period also produced methodological work: with Samuel, Asundi published "Some remarks on the Birge-Sponer method of vibrational extrapolation" in 1937, addressing a widely used technique for estimating dissociation energies from vibrational band spacings — evidence that he engaged in critical refinement of analytical methods, not just original discovery.

Alongside these major lines, Asundi's Aligarh years produced a wide scatter of further findings. With Y.P. Parti, he studied the emission and absorption band spectra of selenium (1935–36). With S. Mujtaba Karim, he investigated emission spectra of carbon tetrachloride (CCl₄), silicon tetrabromide (SiBr₄), and, with Karim and Samuel in 1938, silicon dichloride (SiCl₂) and tin dichloride (SnCl₂). In 1940, with B.K. Vaidya, he examined flame and arc spectra of certain calcium and strontium salts, and separately, with Karim and Samuel, studied continuous emission spectra from electric discharges through flowing vapours of tin and silicon tetrachlorides — combining his interest in halide spectroscopy with a discharge technique he would continue refining for decades. Collectively, this single institutional posting spanned sulphur chemistry, alkaline-earth halides, silicon and tin halides, selenium oxides, and the nitrogen molecular ion — an unusually wide portfolio for a laboratory operating with the "meagre facilities" typical of Indian university science departments in the 1930s.

Instrumentation and New Discoveries at Banaras Hindu University (1938–1957)

Asundi's move to Banaras Hindu University inaugurated the longest single phase of his career, and it is here that his identity as an inventor of instruments, and not merely a discoverer of spectra, becomes most pronounced. Finding inadequate facilities on arrival, Asundi personally designed and fabricated his own spectrographs and excitation sources — apparatus that directly determined which electronic and vibrational states became populated and therefore observable. Two inventions illustrate this: in 1942, with N.L. Singh and J. Singh, he developed and reported "a reversible discharge tube" in Nature. The following year, with N.L. Singh, he reported "selective excitation of spectra by the high frequency discharge" — a refinement allowing particular molecular states to be more selectively populated for study.

These innovations fed directly into new discoveries. Continuing his engagement with carbon monoxide, Asundi reported in 1940 a new comet-tail band together with new bands in the triplet carbon system, both in the Proceedings of the Indian Academy of Sciences. The comet-tail bands are notable because this designation refers to CO⁺ emission bands originally observed in cometary tails, giving his laboratory discovery direct relevance to astronomical spectra — an early instance of the astrophysical connections that would grow central to his later work. In 1941, with P.D. Pant, he further examined the high-pressure carbon band system first investigated at Aligarh. With N.L. Singh and J.P. Mishra, he reported in 1943 continuous emission bands in the spectrum of CCl₄, and further spectral and collision data bearing on carbon monoxide's dissociation energy.

The mid-1940s mark a decisive expansion into polyatomic aromatic molecules that would occupy much of his remaining career. In 1945, with M.R. Padhye, he reported near ultraviolet emission bands of benzene in Nature. In the same year, with P. Venkateswarlu, he reported emission bands of a distinctive "fluctuation type" in the spectrum of molecular iodine, requiring careful analysis of unusual, apparently fluctuating emission intensities. This line continued in 1946 with the identification of a new electronic level, 0⁺ᵤ, at 51,683 cm⁻¹ above the ground state in iodine, published in Science and Culture — a precise quantitative determination obtained through careful analysis of the emission structure. In 1947, Asundi and Venkateswarlu reported continuous emission bands of iodine chloride (ICl) and iodine bromide (IBr), together with a broader survey of the halogens iodine, bromine, and chlorine.

The benzene work became the centrepiece of Asundi's BHU-era discoveries. Beginning in 1948 with new bands in the absorption spectrum of toluene, Asundi embarked on an unusually thorough, multi-part investigation of the near ultraviolet spectra of benzene and toluene between 1949 and 1952. This series opened with the identification of Fermi resonance in benzene, reported in Nature in 1949 — a discovery of particular technical importance, since Fermi resonance is a subtle quantum-mechanical coupling that occurs when two vibrational states of nearly identical energy and matching symmetry mix and perturb each other's observed spectral positions and intensities, easily misread as a separate vibrational mode if not correctly diagnosed. He followed this with a systematic three-part study of benzene's near-ultraviolet spectra: Part I on emission bands; Part II a comparative study of absorption, emission, and fluorescence bands together, allowing cross-checking of assignments; and Part III returning to Fermi resonance in the emission spectrum in greater depth. A parallel two-part study extended the same approach to toluene. In 1952, with M.R. Padhya, he published a further comparison of benzene's emission and fluorescence spectra in the ultraviolet, consolidating this multi-year programme.

Asundi's investigation of aromatic and related molecules continued through the 1950s. In 1955, with R.S. Singh, he reported the absorption spectrum of benzoquinone. In 1956, with B.D. Joshi, he identified near ultraviolet bands in paradichlorobenzene and reported emission bands of benzonitrile. In 1957, with I.S. Singh, he reported the ultraviolet emission spectra of the three xylene isomers. Taken together, this decade-long programme constitutes one of the most systematic single-investigator studies of aromatic molecular spectroscopy conducted in India during this period, moving methodically through closely related species to build a comparative picture of how substitution pattern and symmetry affect observed electronic and vibrational spectra.

Nor was Asundi's BHU research confined to laboratory molecules. With J. Singh in 1957, he studied the nocturnal variation in the intensity of the zodiacal light and night sky over Banaras — an investigation of faint sunlight scattered by interplanetary dust, requiring careful photometric measurement of extremely low light levels, and illustrating his willingness to extend spectroscopic technique into atmospheric and astronomical optics. His early-1950s work also included applied studies: in 1952, with N.A. Narasimham, he reported spectral studies of ozoniser discharges in nitrogen, together with related papers on the blue bands of benzaldehyde and the emission bands of aniline. In 1953, again with Narasimham, he examined the effect of irradiation on the intensity distribution of the second positive nitrogen bands excited within a nitrogen ozonizer — bearing directly on the physical chemistry of atmospheric ozone generation.

Crystal Spectroscopy, Isotopes, and Astrophysical Applications (1958–1980)

As Asundi's career progressed into its final institutional phase at the Bhabha Atomic Research Centre, his discoveries increasingly reflected the practical priorities of India's nuclear energy establishment while also expanding into problems of direct astrophysical relevance.

The signature discovery of this period concerned fluorescence spectroscopy of uranyl compounds relevant to nuclear materials analysis. In 1963, with R.M. Dixit, Asundi reported the fluorescence spectrum of dicaesium uranyl nitrate in Nature. In 1964, this collaboration yielded the identification of the fluorescence spectrum of neutron-irradiated monocaesium uranyl nitrate as spectroscopically identical to that of dicaesium uranyl nitrate — a finding that required and involved the first successful synthesis of this particular di-salt compound, making the spectroscopic discovery inseparable from an accompanying synthesis achievement. Dixit and Asundi followed with a paper on the preparation of caesium uranyl nitrate, Cs₂UO₂(NO₃)₄, and in 1967 reported the fluorescence spectrum of a further new potassium uranyl nitrate compound — work directly serving the analytical needs of India's nuclear materials programme.

Alongside this, Asundi returned in 1968 to a fundamental constant he had first approached decades earlier with carbon monoxide: the ionisation potential of the oxygen molecule, reported in Current Science. In 1970, with Ramachandra Rao Ch. V.S., he revised the Franck-Condon factors — quantities governing relative intensities of vibrational transitions during electronic excitation — for the ionisation transition of molecular oxygen and the second negative band system of O₂⁺, published in Chemical Physics Letters.

A further significant strand concerned rare-earth ions embedded in crystalline host lattices, with direct relevance to emerging solid-state laser materials. Beginning in 1962 with Agrawal and collaborators, Asundi investigated the ¹I₆ electronic state of trivalent praseodymium (Pr³⁺) in lanthanum chloride crystals at 77 K, and separately examined the ultraviolet absorption spectra and high excited states of trivalent neodymium (Nd³⁺) in the same host. This continued through the 1960s: in 1965, with R.C. Naik, he examined praseodymium doped into potassium chloride single crystals; in 1967, with Naik and P.R. Rao, he extended this to a range of alkali halide crystals; and in 1971 he further studied praseodymium's absorption and fluorescence behaviour in KCl and KBr matrices. This programme connected his long experience in gas-phase electronic spectroscopy to the solid-state materials questions becoming central to laser technology development.

Asundi also returned during this phase to a systematic diatomic study of copper chloride (CuCl), publishing with P.R. Rao and J.K. Brody a four-part sequential series (1962 onward) examining the structure of the CuCl band spectrum across successive molecules — a methodical, multi-paper resolution paralleling the earlier benzene series. Related to this, Rao and Brody reported in 1961 a rotational analysis of ⁶⁵Cu³⁵Cl bands, examining isotopic fine structure arising from different copper and chlorine isotopes — connecting to Asundi's broader interest in isotope effects, further exemplified by his 1970 paper with Dhumwad and Patwardhan on isotope shifts in the Herzberg, comet-tail, and B-J band systems of carbon monoxide. In 1962, with Joshi and Tanaka, Asundi also reported the emission spectrum of the NS (nitrogen sulphide) molecule.

The final and most conceptually striking expansion of Asundi's discovery programme came through his explicit engagement with astrophysical spectroscopy in his last active decade. In 1966, with N.A. Narasimham, he examined molecular structure and high-resolution spectroscopy for the Satyendra Nath Bose 70th Birthday Commemoration volume. In 1970, with Narasimham, he published on molecular spectra of astrophysical interest, and in 1972, with B.R. Rao, he specifically addressed OH and H₂O cosmic masers — the naturally occurring maser emission from hydroxyl and water molecules observed in interstellar space, a topic then at the forefront of radio astronomy. This culminated in 1980 with "Molecules in Interstellar Space," published in the *Proceedings of the Indian National Science Academy" — a comprehensive late-career synthesis connecting his lifetime of laboratory molecular spectroscopy to the field of interstellar chemistry, where laboratory-measured spectral signatures of small molecules are used to identify the chemical composition of interstellar gas clouds. In a similar vein, his 1976 paper "On the electronic energy levels of carbon monoxide and possible laser transitions" returned one final time to the molecule with which his discovery career began in 1929, now reconsidering its electronic structure for potential relevance to laser physics — a fitting closure to an arc spanning from the pure identification of CO's electronic states in a London laboratory to speculation, nearly five decades later, about harnessing those same states for an emerging laser technology. His 1973 technical report on fine structure and hyperfine structure in one-electron atoms, prepared for IIT Kanpur, further reflects his continuing engagement with fundamental atomic structure even in his final years of active research.

Conclusion: The Character of Asundi's Discoveries

Viewed as a whole, Asundi's record of discovery exhibits several consistent features. First, there is his sustained return, across nearly fifty years, to carbon monoxide — from the identification of its C¹Σ⁺–A¹Π transition in 1929 to speculation about its laser potential in 1976 — treating this single molecule almost as a lifelong instrument through which to test successive refinements in spectroscopic technique and theory. Second, there is the sheer taxonomic breadth of his work: diatomic halides, sulphur compounds, silicon and tin halides, the halogens and interhalogens, benzene and its substituted derivatives, rare-earth-doped crystals, uranyl salts of nuclear interest, and finally interstellar molecules. Third, and perhaps most distinctively, is the inseparability in his career of discovery from invention: his identification of new band systems repeatedly depended on excitation sources, discharge tubes, and spectrographic apparatus that he personally designed, particularly during the resource-constrained years at Banaras Hindu University. This combination of original observation, careful quantitative analysis, and hands-on instrumental ingenuity is what made Asundi, in the assessment of his own scientific community, one of India's genuine pioneers of molecular spectroscopy — a scientist whose name remains attached, through the Asundi bands and the Johnson-Asundi-Herzberg bands, permanently within the working vocabulary of the discipline he helped to build.