K G Ramanathan
Articles written in Proceedings – Section A
Volume 19 Issue 3 March 1944 pp 146-148
Volume 20 Issue 1 July 1944 pp 62-69
Volume 24 Issue 1 July 1946 pp 130-136
Using a Residual-ray apparatus, the infra-red absorption coefficients have been measured for 37 cleavage plates of diamond in the 8 μ region. While the absorption coefficients measured with an infra-red monochromator in the region 4–5 μ remain constant within the error of the measurement for all diamonds, those in the 8 μ region show a wide range of variation. The results are correlated with the behaviour of diamond in respect of luminescence and ultraviolet absorption. Some cleavage plates of diamond have also been shown to exhibit patterns of infra-red transparency in the 8 μ region, closely analogous to the patterns of ultraviolet transparency and of luminescence.
Volume 24 Issue 1 July 1946 pp 137-144
The thinnest diamonds of the so-called ultraviolet opaque type have been shown to exhibit transmission upto 2240 Å which is also the limit for the so-called ultraviolet transparent type. In a diamond showing blue luminescence, five new absorption bands have been discovered for the first time in the region below 2300 Å. The variation of absorption coefficient with wavelength for a typical blue-luminescent diamond has been determined for the first time in the region 3030 Å-2570 Å. Finally, the ultraviolet transmission limits for a number of diamonds have been obtained for different photographic exposures and the data correlated with their luminescence and mean infra-red absorption coefficient in the 8 μ region.
Volume 24 Issue 1 July 1946 pp 145-149
The absorption spectrum of diamonds exhibiting a weak blue fluorescence has been investigated in the visible region with large absorption paths secured by the method of multiple transmission. With thicknesses of the order of a few centimetres, the radiation is completely cut-off below 4160 Å. At the temperature of liquid air, a whole series of electronic absorption lines have been observed in the visible spectrum, several of them for the first time. Three absorption bands centred at 4512 Å, 4625 Å and 4776 Å have also been recorded which had been previously noticed only in one specimen having an exceptionally strong absorption.
Volume 24 Issue 1 July 1946 pp 150-161
The absorption spectra of four typical diamonds, three of them luminescent and the fourth non-luminescent, have been investigated in the infra-red in the wavelength range 4·5 μ to 13 μ with a rock-salt prism spectrometer and radiomicrometer. Allowing for reflection, the non-luminescent diamond is found to be completely non-absorbing in the frequency range 770 cm.−1 to 1400 cm.−1. The luminescent diamonds show a band in this region with a number of absorption maxima whose frequencies may be identitied with those of the lattice and superlattice vibrations. The intensity of this band is greatest in the weakly blue-fluorescent diamond, somewhat weaker in the intensely blue-fluorescent diamond and still weaker in the diamond showing both blue and yellow fluorescence. These results are accounted for on the basis of the crystal structure and symmetry of diamond in its various allotropic modifications. The absorption peaks appearing in the region between 1400 to 2700 cm.−1 are accounted for as octaves and combinations of the lattice and superlattice frequencies which are allowed by virtue of the anharmonicity of the vibrations. The appearance of the superlattice frequencies in the 8 μ band is closely associated with the infra-red activity of the fundamental lattice vibration and is in the nature of a resonance effect.
Volume 25 Issue 4 April 1947 pp 314-321
Volume 26 Issue 6 December 1947 pp 469-478
The infra-red absorption spectra of nine cleavage plates of diamond have been investigated in the region of frequencies between 750 and 4000 cm.−1 and their observed spectral behaviours have been elucidated. While the second-order spectrum is similar for all diamonds, the first-order spectrum shows large variations. From the nature of these variations it is concluded that the first-order activity is due neither to chemical impurities nor to mosaicity of structure, but must be attributed to the tetrahedral symmetry of structure possessed by such diamonds.
Volume 26 Issue 6 December 1947 pp 479-480
Photographs of the emission and absorption spectra of blue-luminescent diamonds together with their microphotometric records are reproduced showing the discrete structure of the vibration spectrum.
Volume 26 Issue 6 December 1947 pp 481-492
Exact expressions have been derived for the frequencies of the nine normal modes of vibration of the diamond structure, which take account of the forces of interaction between each atom and its 28 nearest neighbours. The formulæ involve 8 independent constants together with an additional relation between them, and the constants are thus perfectly determinate
Volume 26 Issue 6 December 1947 pp 493-499
Exact expressions for the frequencies of the nine eigenvibrations of rocksalt have been derived in terms of 11 independent constants which take account of the influence of the 26 nearest neighbours of each atom, the constants being connected by two additional relations.
Volume 26 Issue 6 December 1947 pp 500-506
Exact expressions have been derived for the 11 eigenfrequencies of the cæsium chloride structure in terms of 12 constants which take account of the influence of the 26 nearest neighbours of every atom, the constants being connected by two additional relations. Simplified formulae for the frequencies have also been given in terms of 4 independent constants only, which are again connected by an additional relation.
Volume 26 Issue 6 December 1947 pp 507-510
Exact expressions have been derived for the frequencies of the eigenvibrations of the face-centred and body-centred cubic lattices in terms of 6 and 5 force constants respectively, the constants in each case being connected by one additional relation.
Volume 28 Issue 5 November 1948 pp 370-376
A dynamical method has been described for evaluating the acoustic wave-velocities in different directions in a crystal, from the interatomic forceconstants. The method, applied to the case of diamond, yields two transverse velocities for the dodecahedral directions, one of them coming out to be the same as the transverse velocity in a cubic direction. These facts are in full accordance with theoretical expectations.
Volume 28 Issue 5 November 1948 pp 454-458
Exact expressions for the frequencies of the fourteen normal modes of the fluorspar structure are derived in terms of 12 force constants which take account of the influence of the 20 nearest neighbours of each calcium atom and 22 nearest neighbours of each fluorine atom. Simplified formulæ are also given which involve only five constants connected by an additional relation.
Volume 28 Issue 5 November 1948 pp 459-462
Expressions for the frequencies of the nine normal modes of the zinc sulphide structure are derived in terms of 10 force-constants taking account of the interaction of each atom with its 16 nearest neighbours. The formulæ, after simplification contain only six constants which are further connected by two additional relations.
Volume 49 Issue 2 February 1959 pp 55-60