• R S Krishnan

Articles written in Proceedings – Section A

• On the plotnikow effect or longitudinal light scattering in liquids

Experiments on the “Plotnikow” effect or the longitudinal light scattering in liquids were carried out with liquids not free from dust and also with liquids free from dust. The effect was observed conspicuously in the case of dusty liquids. The effect completely disappeared in the liquids so far examined when they were freed from dust by repeated distillation in vacuo. It thus seems clear that the Plotnikow effect is not a molecular phenomenon but is due to the presence of dust.

• The scattering of light by particles suspended in a medium of higher refractive index

A short review of the theoretical as well as experimental work done on the scattering of light by particles suspended in a medium of lower refractive index is given. Using the formula derived by Mie, calculations are extended to the case of particles such as air bubbles suspended in a medium of higher refractive index. Results are obtained for three different values of refractive indices and for three different sizes of the particles. When the particles are small in size, the distribution of intensity is symmetrical about the plane perpendicular to the incident beam. As the size of the particle increase, the intensity in the forward direction becomes greater than that in the backward direction. The angle of maximum polarisation is displaced from 90° in the direction of the incident rays and moreover, it is everywhere incomplete. Experimental observations are also made in the case of water particles dispersed in benzene.

• Optical evidence for molecular clustering in fluids

It is pointed out that the presence of molecular clusters in fluids of size comparable with the wave-length of light should be capable of detection by two optical methods, one based on the asymmetry of scattering, and the second based on the anomalous depolarisation of scattered light. In this investigation the second method is developed. Ordinary liquids did not give any positive evidence for the existence of large molecular aggregates at room temperatures. A mixture of carbon disulphide and methyl alcohol when examined gave definite evidence for the formation of molecular clusters in the mixture by the observation of the anomalous depolarisation. This method is so delicate that it has made it possible to detect the pressence of molecular clusters in the mixtures even at temperatures considerably removed from the critical solution temperature.

• On the depolarisation of Tyndall scattering in colloids

In this paper, an attempt has been made to obtain theoretically the relationship between the depolarisationsρu, ρv andρh of the Tyndall scattering in colloids whereρu, ρv andρh are the depolarisations of the transversely scattered light when the incident light is (1) unpolarised, (2) horizontally polarised, and (3) vertically polarised respectively, the plane of observation being the horizontal plane. The total scattering observed in the case of colloids and emulsions is divisible into two parts, (1) the scattering arising from the finite size of the particles assumed to be spherical and isotropic and (2) the scattering arising from the varying orientations of the actual non-spherical anisotropic particles. It is assumed that the law of distribution in direction of the orientation scattering by larger particles is the same as that for very small ellipsoidal particles. The following numerical relationship is then derived betweenρu, ρv andρh, namely,ρu = (1 + 1/ρh)/(1 + 1/ρv) Using the experimental values for the depolarisation obtained with arsenic sulphide sols, castor oil emulsions and sulphur suspensions, it is found that the formula obtained is in satisfactory agreement with observations. In the case of protein solutions, however, certain anomalies are observed. The method of separating the observed scattering into the two parts as stated, and of calculating the size of the particles from the observations is also indicated.

• The reciprocity theorem in colloid optics

A simple method of deriving the following algebraic relation betweenρu,ρv andρh is given,ρu=(1+1/ρh)/(1+1/ρh) applying the general dynamicalPrinciple of Reciprocity formulated by Helmholtz and Rayleigh, whereρu,ρv andρh are the measures of the depolarisation of the Tyndall scattering when the incident light is respectively (1) unpolarised, (2) polarised with its electric vector vertical, and (3) polarised with its electric vector horizontal. The conclusions derived from the reciprocity principle admit of a very simple and direct experimental test which has been carried out and found to be satisfied by all kinds of colloidal solutions, emulsions, suspensions, protein solutions, etc., irrespective of size, shape or structure of the particles contained in them or of their non-uniformity. The principle of the experimental method employed to test the relation consists of splitting the incident unpolarised light by means of a double-image prism into two beams of equal intensity but polarised perpendicularly. The scattered light is also viewed through another doubleimage prism. The four images of the tracks corresponding to the components Vv, Hv, Vh and Hh can be viewed at the same time and their intensities compared. In all the cases studied, Hv is found to be equal to Vh which is equivalent to the relations stated above. The relative intensity of the four track-images furnishes useful indications of the size and shape of the particles in the dispersing medium.

• Molecular clustering in binary liquid mixtures

The earlier investigations on the intensity and depolarisation of the transversely scattered light in liquid mixtures in the neighbourhood of the critical solution temperature are briefly reviewed. It is pointed out that it is important to make comparative studies of the state of polarisation of the transversely scattered light with the incident light in different states of polarisation in order to get a correct idea of the state of dispersion of the scattering elements in the medium. Measurements of the depolarisationρu,ρv andρh were made, using, respectively, incident unpolarised light and incident light polarised with vibrations vertical and horizontal, for mixtures of eight different polarised with vibrations vertical and horizontal, for mixtures of eight different pairs of liquids over a range of temperature of about 30° above the critical solution temperature. The results obtained fully confirm the author’s preliminary report regarding the existence of clusters in liquid mixtures, not only at the critical solution temperature but also at temperatures considerably removed from it. The validity for such mixtures of the optical reciprocity principle has been verified experimentally. It is also shown that the observed values ofρu,ρv andρh satisfy the relationρu=)1+1/ρh)/(1+1/ρv) The X-ray evidence regarding the state of dispersion of the molecules in liquid mixtures of the type phenol and water is in agreement with the optical evidence presented in this paper. The bearing of the experimental results on the various theories of critical opalescence including those which have been put forward subsequent to the work of Smoluchowski and Einstein is also discussed in the paper.

• Molecular clustering in binary liquid mixtures - Variation with composition and temperature

This paper deals with a comparative study of the intensity and depolarisation of the light scattered transversely by mixtures of phenol and water in different proportions and at different temperatures. The lowest values for the depolarisationsρu,ρv andρh are obtained with the critical composition mixture (in this case 34% phenol mixture) at the critical solution temperature, whereρu,ρv andρh are the depolarisations of the light scattered transversely when the incident light is (1) unpolarised, (2) polarised with electric vector vertical, and (3) polarised with electric vector horizontal respectively. The photoelectric method was employed for the comparison of intensities of scattering by these mixtures at various temperatures above the critical solution temperature. The intensity of scattering is found to increase considerably for the 34% phenol mixture as the critical solution temperature is approached. It is found that the tendency for the formation of clusters and the size of the clusters formed depend on the temperature and composition of the mixture. These phenomena are found to be most pronounced for the critical composition mixture at the critical solution temperature. The bearing of these results on the anomalies of viscosity, magnetic birefringence, etc., noticed in the vicinity of the critical conditions is also pointed out.

• Molecular clustering in liquid fatty acids

The variations in intensity and depolarisation factor of the transversely vScattered light have been studied in detail in the case of the first four fatty acids, namely, formic acid, acetic acid, propionic acid and normal butyric acid, with the incident light in the three different states of polarisation, namely, unpolarised, horizontally polarised and vertically polarised. In the case of formic and acetic acids at the room temperature the depolarisation factor ρh was found to be about 90% when the incident light is polarised with vibrations horizontal. This observation furnishes for the first time definite experimental evidence for the existence of large molecular aggregates in these highly associated liquids, of size not small compared with the wave-length of light.

ρh increases with rise in temperature and at about 90° C. it attains its limiting value of 100% in these two acids. The intensity of scattering also at first diminishes with rise in temperature in these two cases, contrary to the molecular theory of light scattering. It is shown that molecular association is distinctly the cause for the extraordinary behaviour of formic and acetic acids in the initial stages. Propionic and normal butyric acids do not show such anomalies.

• Scattering of light in optical glasses

A comparative study has been made of the intensity and state of polarisation of the light scattered transversely by a series of seventeen glasses of optical quality with the incident light in different states of polarisation (namely, unpolarised, vertically polarised and horizontally polarised). A weak fluorescence was observed in all the glasses. Measurements of the depolarisation factors ρu, ρv and ρh were made with an orange filter in the path of the incident beam to eliminate fluorescence. ρu and ρv are found to be of the same order of magnitude as are usually observed in the case of gases. But on the other hand, ρh is found to be distinctly less than 100% showing thereby the existence of molecular aggregates of size not small compared with the wavelength of light. Since no visible inclusions are observed it is concluded that the scattering in glass is really an internal phenomenon. Measurements of the relative intensity of scattering in these glasses were also made employing the photo-electric method. The influence of composition of the glass on the formation and the size of the molecular aggregates formed is fully discussed. It is found that the tendency for the formation of molecular aggregates increases with an increasing percentage of acidic oxides especially boric oxide, whereas it diminishes when the glass gets more and more basic.

• Molecular clustering in liquid fatty acids

The variations in intensity and depolarization factor of the transversely Scattered light have been studied in detail in the case of the first four fatty acids, namely, formic acid, acetic acid, propionic acid and normal butyric namely, unpolarised, horizontally polarized and vertically polarized. In the case of formic and acetic acids at the room temperature the depolarization factorρh was found to be about 90% when the incident light is polarized with vibrations horizontal. This observation furnishes for the first time definite experimental evidence for the existence of large molecular aggregates in these highly associated liquids, of size not small compared with the wave-length of light.

ρh increases with rise in temperature and at about 90° C. it attains its Limiting value of 100% in these two acids. The intensity of scattering also at first diminishes with rise in temperature in these two cases, contrary to the molecular theory of light scattering. It is shown that molecular association is distinctly the cause for the extraordinary behaviour of formic and acetic acids in the initial stages. Propionic and normal butyric acids do not show such anomalies.

• Scattering of light in optical glasses

A comparative study has been made of the intensity and state of polarisation of the light scattered transversely by a series of seventeen glasses of optical quality with the incident light in different states of polarisation (namely, unpolarised, vertically polarised and horizontally polarised). A weak fluorescence was observed in all the glasses. Measurements of the depolarisation factorsρu,ρv andρh were made with an orange filter in the path of the incident beam to eliminate fluorescence.ρu andρv are found to be of the same order of magnitude as are usually observed in the case of gases. But on the other hand,ρh is found to be distinctly less than 100% showing thereby the existence of molecular aggregates of size not small compared with the wavelength of light. Since no visible inclusions are observed it is concluded that the scattering in glass is really an internal phenomenon. Measurements of the relative intensity of scattering in these glasses were also made employing the photo-electric method. The influence of composition of the glass on the formation and the size of the molecular aggregates formed is fully discussed. It is found that the tendency for the formation of molecular aggregates increases with an increasing percentage of acidic oxides especially boric oxide, whereas it diminishes when the glass gets more and more basic.

• Dispersion of depolarisation of Rayleigh scattering - Part I. fatty acids

The paper describes the results of measurements on the dispersion of polarisation of light scattered by benzene, formic acid, acetic acid, propionic acid and isobutyric acid. The depolarisation factor for benzene is constant over a wide range of wave-length. In the case of formic acid and propionic acid the depolarisation factor is found to increase and in acetic acid it is found to decrease as we proceed towards the ultra-violet. It is pointed out that the formation of large molecular groups in the fatty acids is probably responsible for the observed dispersion of depolarisation.

• Dispersion of depolarisation of light-scattering in colloids - Part I. Gold sols

• Dispersion of depolarisation of lightscattering in colloids - Part II. Silver sols

• Dispersion of depolarisation of light-scattering in colloids - Part III. Platinum, copper, selenium and tellurium sols

• Dispersion of depolarisation of light-scattering in colloids - Part IV. Iodine, graphite, stearic acid, vanadium pentoxide, arsenic trisulphide and ferric hydroxide sols

Measurements of the depolarisation factorsρu,ρv andρh and the extinction coefficients of iodine, graphite, stearic acid, vanadium pentoxide, arsenic trisulphide and ferric hydroxide sols are made for different wavelengths from 2500 A.U. to 7000 A.U. All these sols except the iodine sol do not possess any region of specific absorption. As in the case of gold and silver sols so also in the case of the iodine sol the depolarisation values attain their maximum value in the region of the characteristic absorption,i.e., at 4500 A.U. In stearic acid, vanadium pentoxide and ferric hydroxide sols the depolarisation factors exhibit normal dispersion. The observed dispersion in these cases is a size effect. The behaviour of the graphite sol is slightly different from that of the sols mentioned above, for in this case,ρv andρh both diminish with decrease of wave-length. Unlike all the other sols up till now studied, the depolarisation factors in the arsenic trisulphide sol increasepari passu with wave-length. The depolarisation measurements indicate that the particles in the sols of iodine and arsenic trisulphide are spherical in shape, while, those in vanadium pentoxide, stearic acid, ferric hydroxide and graphite sols are non-spherical in shape.

• Dispersion of depolarisation of light-scattering in colloids - Part V. Colloidal dye-stuffs

The earlier work on the absorption of light by aqueous solutions of organic dye-stuffs has been briefly reviewed. It is pointed out that the study of the optical properties, especially the Tyndall effect, of colloidal dye-stuffs which can be obtained in suspension with the particles graded down from those of microscopic to those of molecular dimensions, is of great importance to understand the connection between the degree of dispersivity and kinetics of the dyeing processes. Measurements of the depolarisation factorsρu,ρv andρh and the extinction coefficients of aqueous solutions of ten typical dye-stuffs are made for different wave-lengths from 2500 to 7000 A.U. As in the case of metallic sols, such as gold and silver, so also in the case of these dye-stuffs the depolarisation values attain their maximum value in the regions of specific absorption. The depolarisation measurements indicate that the sols of aniline blue, congo red, congo rubin and nigrosin contain anisotropic ultra-microscopic micellæ whereas the solutions of indigotin, Hoffman’s blue and iodine green are molecular and not colloidal. The solutions of methyl violet, chryosidine and orange II occupy an intermediate position.

• Critical opalescence of liquid mixtures

A critical review of the theory of secondary scattering proposed by Rousset in order to explain the finite value of the depolarisation of the opalescence of liquid mixtures is given. Further consequences of the theory of secondary scattering have been worked out and it is found that the experimental results obtained are not in accordance with the conclusions of the theory. From the results obtained it has been concluded that in the neighbourhood of the critical solution temperature of liquid mixtures the molecules have got a tendency to group themselves together in the form of clusters and that the finite value of the depolarisation of the opalescent light arises due to this fact. In the neighbourhood of Tc the secondary scattering has little or no effect on the depolarisation factor, although it may have an appreciable effect for ΔT&lt;0·1° C. where ΔT=T−Tc. The measurements of the depolarisation factorsρu,ρv andρh for three binary mixtures and one ternary mixture have been made over a range of temperature of 30° above Tc. The results obtained fully confirm the author’s earlier report regarding the existence of clusters in liquid mixtures not only at the critical solution temperature but also at temperatures considerably removed from it. The theory proposed by Gans has been discussed in relation to the experimental results obtained by the author and the limitations of the theory have also been pointed out. The bearing of the results on the anomalies of viscosity, electric birefringence, etc., noticed by the earlier investigators in the vicinity of the critical solution temperature has been discussed in detail.

• The reciprocity theorem in colloid optics and its generalisation

• Reciprocity theorem in colloid optics - Case of orientated particles

• Studies on light-scattering in emulsions - Part I. Dilute simple emulsions

• The anomalous depolarisation of light-scattering in optical glasses

• Influence of secondary scattering on depolarisation measurements

• Scattering of polarised light in colloids

• Raman spectra of crystals and their interpretation

An examination of the spectrograms of the Raman effect in rock salt obtained by Rasetti reveals the existence of nine distinct Raman lines both on the Stokes as well as on the antistokes sides. The frequency shifts of these lines have been estimated. From the microphotometer record reproduced in Rasetti’s paper, the relative intensities of the lines as well as the intensity ratios of the Stokes to the antistokes lines have also been evaluated. These nine frequencies have been identified as due to the nine distinct modes of vibration of the sodium chloride lattice to be expected on the basis of the Raman dynamics of crystal lattices. Though these vibrations are forbidden in the Raman effect as fundamentals, they are allowed as octaves and appear as such in the spectrum. It is pointed out that the Born theory of crystal dynamics is not capable of offering any satisfactory explanation for the observed facts.

• The raman spectrum of diamond

The Raman spectra of diamonds of the ultra-violet transparent type have been investigated using the 2536 resonance line of mercury as exciter. Besides the well-known 1332 Raman line, ten others with frequency shifts 1925, 2175·5, 2245, 2267, 2300·5, 2467, 2495, 2518, 2609·5 and 2664·6 cm.−1 have been recorded. These new lines have been identified as the octaves and allowed combinations of some of the eight fundamental frequencies of oscillation of the diamond structure of which the existence is indicated by the Raman theory of crystal dynamics, but which are not themselves permitted to appear in light-scattering by reason of the selection rules.

• Experimental evidence for the existence of the four possible structures of diamond

• Molecular aggregation in optical glasses as revealed by light-scattering

Sir C. V. Raman’s new technique for the study of light-scattering based on the use of a Babinet compensator, has been successfully applied for the detection of the anomalous depolarisation in optical glasses. Thus the existence of the reversal of polarisation when the incident beam is polarised with vibrations horizontal, has been definitely established by the new method which is of an objective character. The present investigation further reveals that in glasses the scattered light is elliptically polarised when the incident beam is linearly polarised in a direction oblique to the scattering plane. These observations support the view that molecular aggregates do exist in glasses. One important point which emerges out of the present investigation is that those glasses which give larger values of Δρu exhibit elliptic polarisation to a correspondingly larger extent. Δρu is the difference between the observed value of ρu and the anisotropic part of ρu which can be considered to be equal to$$\frac{{2\rho _v }}{{(1 + \rho _v )}}$$. It has been pointed out that for making comparative estimates of the sizes of particles in any two scattering media a knowledge of the values of Δρu in the two cases is by far more important than the corresponding values of ρh.

• Raman spectra of the second order in crystals - Part I: Calcite

• Raman spectra of the second order in crystals - Part II. Gypsum

• Raman spectra of the second order in crystals - Part III. Quartz

• Raman spectra of the second order in crystals - Part IV. Barytes

The previous work on the Raman effect in barytes has been briefly reviewed.

The Raman effect in natural crystals of barytes has been studied in detail using the 2536·5 λ mercury resonance radiation as exciter. The recorded spectrum consists of not less than 39 Raman lines nearly half of which have been recorded for the first time. Of these 20 lines belong to the first order Raman spectrum and are distributed as follows:—9 lattice lines and 11 lines due to the internal oscillations of the SO4 ion. The frequency shifts of these lines have been accurately measured. The appearance of 11 Raman lines in the first order spectrum of the SO4 ion has been satisfactorily explained on the basis of the lower order of symmetry of barytes crystal and also due to Fermi resonance splitting.

19 Raman lines of comparatively feeble intensity which constitute the second order spectrum have been assigned as octaves and combinations of some of the 20 principal Raman frequencies.

The frequencies corresponding to the maxima observed in the infra-red absorption spectrum of barytes have been compared with those observed in the Raman effect.

A complete bibliography on the Raman effect and infra-red studies in barytes is included.

• The second order Raman spectrum of diamond

• Thermal expansion of diamond

• Temperature variations of the Raman frequencies in diamond

With a high dispersion spectrograph, a diamond of the ultraviolet transparent type and the mercury λ 2536·5 excitation, the principal Raman frequency has been measured and is found to decrease from 1333·2 cm.−1 at 85° T. to 1316·0 cm.−1 at 975° T. Identical results are obtained with a diamond of the ultraviolet opaque type and the λ 4358 excitation. The temperature dependence of the more prominent second order Raman frequencies has also been investigated. The relative change of the principal frequency with relative change of volume decreases from a high value at 85° T. to a constant small value above 500° T. and differs greatly from the so-called Grüneisen constant. The different lattice and superlattice frequencies behave differently in respect of their temperature variation and this has been explained as due to differences in the rate of change with temperature of the different force constants which determine the vibration frequencies of the diamond lattice.

The 1332 line has a finite width which increases with temperature. This may be explained as due to the changes of lattice frequency arising from the thermal fluctuations of density in the volume elements of the crystal, if the effective linear dimensions of a volume element are assumed to be about 8 times as large as the edge of the unit cubic cell.

• The scattering of light in diamond and its Raman spectrum

A detailed study of the scattering of light in diamond and its Raman spectrum has been made using the λ 2536·5 mercury resonance radiation as exciter. The seattered spectrum exhibits two pairs of Doppler-shifted components, one pair due to the longitudinal sound waves, and the other due to the two sets of transverse sound waves which have very nearly the same velocity. The velocities of the longitudinal and transverse sound waves estimated from the observed frequency shifts of the displaced components are in agreement with those calculated from the elastic constants of diamond. The directional dependence of sound velocity in diamond has been quantitatively verified. Contrary to expectation, the longitudinal Doppler components are found to be less intense than the transverse Doppler eomponents.

The second-order spectrum of diamond has been examined with a quartz spectrograph of high dispersion and resolution. It exhibits a whole series of sharply defined Raman lines the frequency shifts of which have been tabulated. The prominent ones which are 15 in number have been satisfactorily explained as octaves of combinations of six of the eight fundamental frequencies of vibration of the diamond lattice to be expected on the basis of the Raman theory, some of which are split due to the removal of degeneracy by anharmonicity and due to resonance effect.

• The Raman spectrum of rocksalt and its interpretation

By a careful microphotometric study of the spectrograms taken under high dispersion, it has been shown that the Raman spectrum of rocksalt consists of an aggregate of discrete lines some of which are more intense than the others. With the aid of the indications given by the theory regarding the vibration frequencies and their relative activities in light-scattering, nine of the more prominent lines in the spectrum have been identified as the octaves and the rest as combinations of the nine fundamental frequencies of oscillation of the rocksalt lattice. The views expressed by Born and Bradburn regarding the nature of the rocksalt spectrum and the explanations offered by them for its appearance have been examined and shown to be untenable.

• Raman spectrum of ammonium chloride and its variation with temperature

Using the λ 2536·5 mercury resonance radiation as exciter, the Raman spectrum of ammonium chloride has been investigated in detail over the range of temperature from 85° T. to 395° T. The recorded spectra at all temperatures exhibit many more frequency shifts than those obtained by the previous workers. The present study reveals the following facts:

The Raman speetrum in the region of frequency shifts 90–200 cm.−1 exhibits seven lattice lines, the frequency shifts and intensities of which show marked variations with temperature. These lines have been identified as the fundamental modes of vibration of the cæsium chloride structure, in which the NH4 ions osciUate as units, as demanded by the new crystal dynamics.

The lattice spectrum persists far above the transition temperature. This fact indicates that Pauling’s hypothesis that there is onset of free rotation of the NH4 ions immediately above the transitiort temperature is not corrcct.

The internal oscillations of the NH4 ions exhibit nine distinct Raman shifts, showing thereby that these osciUafions are not those of a freely vibrating ting tetrahedrally symmetric group.

The spectra recorded at all temperatures reveal the presence of Raman shifts characteristic of HCl and NH3 molecules.

• Raman spectrum of alumina and the luminescence of ruby

Using the γ 2536·5 mercury resonance radiation as exciter, the scattering of light in crystalline alumina has been investigated. The spectrum of the scattered light exhibits two Doppler-shifted components centred round the exciting line and seven Raman shifts. From the measured shift of the Doppler components the velocity of sound in alumina has been estimated. The appearance of seven Raman lines is shown to be in accord with the theoretical work of Bhagavantam and Venkatarayudu. The numerous bands appearing on both sides of the principal fluorescence doublet of ruby have been satisfactorily explained as arising from the combination of the vibration frequencies of the alumina lattice with the electronic transitions of the chromium ions.

• The Raman effect in topaz and its relation to the luminescence spectrum

The Raman effect in topaz has been investigated using the 2536·5 mercury resonance radiation as exciter. The spectrum exhibits 32 Raman shifts of which no less than 21 have been recorded for the first time. The results obtained axe discussed with special reference to the infra-red absorption tion and luminescence spectra of topaz. The significance of the complete correspondence noticed between the Raman shifts and the frequency shifts of the vibrational bands in luminescence is pointed out.

• Raman spectrum of ammonium bromide and its variation with temperature

• Sir C. V. Raman and crystal physics

• The theory of the vibrations and the Raman spectrum of the diamond lattice

The observed features of the Raman spectrum of diamond have briefly been summarised and compared with the theoretical conclusions arrived at by Helen Smith on the basis of Born’s postulate of the cyclic lattice. In every respect and especially as regards the form, the extent, the number and sharpness of the lines, the spectrum in the low frequency shift region, etc., the theoretical intensity distribution curve is found to differ widely from what has been observed experimentally. It has been shown that the Born theory as worked out by Smith for the case of diamond is unable to explain the characteristic features of the second-order spectrum of diamond.

• Thermal scattering of light in crystals - Part I. Quartz

• Raman spectra of the second order in crystals - Part V. Aragonite

The Raman spectrum of aragonite crystal has been photographed using λ2536·5 excitation. 25 Frequency shifts have been recorded as compared to 17 observed previously. They are distributed as follows: five due to the internal oscillation of the CO3 ion, fourteen due to lattice oscillation and six belonging to the second order spectrum. The latter have been assigned as some of the octaves and combinations of the fundamental frequencies of vibration of the aragonite structure.

• Raman spectrum of anhydrite

• Intensity ratio of the Raman lines in diamond

The relative intensities of the Raman lines in diamond have been studied by the spectrophotometric method, outlined in an earlier communication, using an ultra-violet Spekker photometer in conjunction with a medium quartz spectrograph.

The values obtained for the fundamental oscillation, 1332 cm.−1 are:— IStokes/Ianti·Stokes=426 IFundamenial/IOctave=270 confirming thereby the dependence of the intensity of a Raman line on the fourth power of the emission frequency.

• The scattering of light in fused quartz and its Raman spectrum

• Elliptic polarisation of light scattered by colloidal solutions

By an analysis of the theoretical conclusions of Mie’s theory concerning the elliptic polarisation of the scattered light when the incident light is plane polarised in an arbitrary azimuth, the significance of the phase difference between the vertical and the horizontal components has been pointed out. It has been shown how the experimental investigations so far carried out have failed to bring out clearly the implications of Mie’s theory. Using suitable experimental methods to analyse the scattered radiation in some emulsions and monodisperse sulphur sols containing particles which were spherically symmetric and of uniform size, the constancy of the phase difference has been established by three different methods of measurement. The value of the phase difference calculated from the available Mie scattering functions for one sulphur sol agreed well with the measured value and the characteristics of the transversely scattered elliptically polarised light for different azimuths of polarisation of the plane polarised incident light, were in accordance with the predictions of Mie’s theory.

• Elastic constants of crystals from light scattering measurements

Details of the methods of evaluating the elastic constants of any crystal from the observed frequency shifts of the Brillouin components arising from the thermal scattering of light have been indicated. From the measurements of the frequency shifts of the Brillouin components recorded using ultraviolet excitation in the case of four cubic crystals and four birefringent crystals, the effective elastic constants have been evaluated and have been compared with the values calculated using the elastic constants determined by other methods. The agreement is found to be very satisfactory. Thus light scattering in crystals affords yet another method of determining the elastic constants.

• Scattering of polarised light by colloids containing anisotropic particles

It is only recently that the theoretical significance of the elliptic polarisation of light scattered by colloids containing particles of large size has been pointed out and an analysis made of the theoretical conclusions regarding ellipticity from Mie’s theory for spherical particles. The earlier qualitative measurements on ellipticity of the scattered light have been explained on the basis of these new ideas and the implications of Mie’s theory brought out more clearly. New experiments have been devised and used for measuring the constant phase difference δ that exists between the vertical and horizontal components of the transversely scattered light when the incident light is polarised in any azimuth. Measurements have been made both for systems of spherical particles and anisotropic particles. It is found that even in the case of large anisotropic particles δ is a constant. It follows therefore, that in the most general case of large anisotropic colloidal particles the polarised scattering due to finite size is elliptic in nature and is superposed on the depolarised part due to anisotropy.

• Raman spectrum of crystalline α-glycine

• Raman spectrum of crystalline tri-glycine sulphate (NH3.CH3.COO)3H2SO4

• Raman spectrum ofl-asparagine monohydrate

The Raman spectrum ofl-asparagine monohydrate in the form of a single crystal has been recorded for the first time.λ 2537 excitation has been used. Fifty-three Raman frequency shifts have been recorded. They are grouped as follows: Eight Raman lines coming under the lattice spectrum, three Raman lines arising from low-frequency vibrations of the hydrogen bonds and the remaining forty-two arising from the internal oscillations of the asparagine molecule. Appropriate assignments have been given for the observed Raman lines.

• Raman spectrum of lanthanum ethyl sulphate nonahydrate

Raman spectrum of a single crystal of lanthanum ethyl sulphate has been recorded for the first time using the λ 2537 radiation Forty-one lines have been identified out of which eight belong to the lattice oscillations, seven to the internal vibrations of the water molecule and the remaining twenty-six to the internal vibrations of the ethyl sulphate group. The Raman spectrum of ethyl sulphate (liquid) has also been recorded using the λ 4358 excitation and is compared with the spectrum of lanthanum ethyl sulphate. Thirty Raman lines could be identified in the spectrum of ethyl sulphate, of which fourteen are recorded for the first time. Probable assignments of the observed frequencies are also given. The sulphate group is found to have O-SO3 structure in lanthanum ethyl sulphate, while it has a co-ordination$$v = 22959 \cdot 9 + 381 \cdot 0(v' + \tfrac{1}{2}) - 3 \cdot 0(v' + \tfrac{1}{2})^2 + 0 \cdot 1(v' + \tfrac{1}{2})^3 - 512 \cdot 0(v'' + \tfrac{1}{2}) + 2 \cdot 1$$ in ethyl sulphate.

• Raman spectrum of triglycine selenate (G3Se), [(NH2CH2COOH)3 H2SeO4]

The Raman spectrum of a single crystal of triglycine selenate G3Se which is ferroelectric below 22° C. has been photographed usingλ 2537 excitation. 42 Raman lines have been recorded of which 6 belong to the lattice spectrum, 3 are due to NH...O oscillations and the remaining 33 are due to internal oscillations of the ions of glycine and SeO4−−. There is a close similarity between the spectrum of triglycine selenate and the spectrum of its isomorph, triglycine sulphate, the frequency shifts due to the SO4−− ion being replaced by the frequency shifts due to the SeO4−− ion. The existence of glycine in the zwitterion form in the structure of G3Se is substantiated by the appearance in the Raman spectrum of lines which are attributable to NH3+ groups and COO groups. The appearance of the additional C-H line at 2982 cm.−1 in the spectrum of triglycine selenate which is absent in the spectrum ofα-glycine indicates the existence of planar monoprotonated glycine also in the structure, as indicated by X-ray studies.

• Raman spectrum of C-deuteratedγ-glycine (NH3+CD2COO)

The Raman spectrum of C-deuteratedγ-glycine (NH3+CD2COO) in the crystal powder form was taken usingλ 2536·5 excitation. 26 Raman lines were recorded. Of these, eight lines are attributed to the external oscillations and eighteen Raman lines to the internal oscillations. Proper assignments are given to the observed frequencies.

• Raman spectrum of diglycine barium chloride monohydrate

The Raman spectrum of diglycine barium chloride monohydrate in the single crystal form has been recorded using λ 2536·5 excitation. 43 Raman lines (9 lattice and 34 internal) have been recorded. Satisfactory assignments have been given for most of the observed Raman lines. It is concluded from a comparison of the Raman spectrum of this compound with those of glycine and of other addition compounds of glycine, that the glycine unit exists in the zwitterion form in the structure of diglycine barium chloride monohydrate.

• Polarisation studies of the Raman spectrum ofα-glycine and the orientation of CH2 groups

The polarisation characteristics of the intense Raman lines ofα-glycine have been investigated using unpolarised incidentγ 2536·5 radiation. Six different orientations of the crystal with reference to the directions of incidence and observation have been used. From the observed depolarisation values, the classification of the Raman lines to the two different symmetry types Ag and Bg has been made. The orientation of the CH2 groups inα-glycine, deduced from polarisation studies, is slightly different from that given by Marsh by X-ray analysis and a quantitative estimate of this deviation has been made.

• Raman spectrum of hydroxylamine hydrochloride (NH3OH.Cl)

The Raman spectrum of hydroxylamine hydrochloride (NH3OH.Cl) in the form of a single crystal has been photographed usingλ 2536·5 excitation. 32 Raman lines with frequency shifts 40, 57, 78, 88, 111, 125, 135, 156, 187, 217, 250, 330, 550, 575, 1004, 1168, 1204, 1470, 1496, 1565, 1590, 1979, 2636, 2710, 2750, 2789, 2926, 2970, 3000, 3050, 3141 and 3220 cm.−1 have been recorded. Of these, the first 8 low-frequency lines belong to the external oscillation, while the four lines at 187, 217, 250 and 330 cm.−1 should be attributed to the vibrations of the hydrogen bond valence vibrations. The remaining Raman lines have been assigned to the vibrations of the NH3OH ion. The O-H and N-H stretching vibrations are very much influenced by the presence of the hydrogen bonds in the crystal.

• Influence of the hydrogen bond on the N-H stretching frequencies in amino-acids

The spectra of glycine, its addition compounds and other amino-acids exhibit Raman lines in the region from 3250 cm.−1 to 2500 cm.−1 It has been shown that these lines cannot be assigned to N-H...O stretching vibrations, where the N atom has the covalency of three, but to N+-H...O stretching vibration where the N atom has the covalency of four. Using the data obtained with triglycine sulphate which has the largest number of N+-H...O bonds and whose H bond lengths are known, the correlation curve giving the relation between the N+-H...O stretching frequencies and the corresponding H bond lengths has been drawn. Using this correlation curve, the N+-H...O stretching frequencies appearing inα-glycine,γ-glycine, diglycine hydrochloride, diglycine hydrobromide,l-asparagine monohydrate anddl-alanine have been satisfactorily accounted for on the basis of the known hydrogen bond lengths in these substances.

• Raman spectrum of guanidinium aluminium sulphate hexahydrate

The Raman spectrum of guanidinium aluminium sulphate hexahydrate also known as ‘GASH’ which is a ferro-electric crystal and has strong hydrogen bonds has been recorded. 38 Raman lines have been identified in the spectra of GASH. The O-H stretching mode is found to be very much influenced by the hydrogen bond and they appear over a widely extended region from 2240–3600 cm.−1 It can therefore be concluded that all the O-H bonds are hydrogen bonded and some of them are quite strong. The Raman lines due to the N-H vibrations appear with the normal frequency shifts indicating thereby that N-H bonds are not hydrogen bonded. These conclusions are fully supported by the results obtained from the X-ray crystal structure analysis of GASH. The principal vibrations of the Al-(OH2)6 groups have also been identified.

• Raman spectra of ferro-electric crystals - Part IV. Lithium hydrazinium sulphate (LiN2H5SO4)

The Raman spectrum of lithium hydrazinium sulphate has been recorded both in the single crystal form and in aqueous solutions. The crystal exhibits thirty-eight Raman lines having the frequency shifts 52, 70, 104, 146, 174, 220, 260, 302, 350, 454, 470, 610, 630, 715, 977, 1094, 1115, 1132, 1177, 1191, 1260, 1444, 1493, 1577, 1630, 1670, 2205, 2484, 2553, 2655, 2734, 2848, 2894, 2939, 3028, 3132, 3290 and 3330 cm.−1 The aqueous solution gave rise to six Raman lines at 452, 980, 1050–1200, 1260, 1425 and 1570 cm.−1 apart from a maximum at 180 cm.−1 in the ‘wing’ accompanying the Rayleigh line. The observed Raman lines have been assigned as arising from the vibrations of the SO4 ion, N2H5+ ion, Li-O4 group, hydrogen bond and the lattice. The influence of the hydrogen bond on the N-H stretching vibrations has been pointed out. The various features of the observed spectrum strongly support the hypothesis that the NH3 group in the crystal is rotating around the N-N axis at room temperature.

• Raman and infrared spectra of ethylene glycol

The Raman spectrum of ethylene glycol has been investigated in the liquid and solid states and also in aqueous solutions. The infrared absorption spectrum of the liquid has been recorded. 27 Raman lines and 17 infrared maxima in the liquid state and 20 Raman lines in the solid state have been reported. Detailed vibrational assignments have been given. Two strong polarised Raman lines of the liquid which have no corresponding infrared absorption bands, and which disappear on solidification, have been attributed to the trans configuration of the glycol molecule. From a comparative study of the various spectra, it is concluded that in the liquid state, glycol exists as a mixture of the two rotational isomers (trans and gauche) whereas in the solid state only the gauche form is present. In very dilute aqueous solutions, the predominant molecular form is the gauche form.

• Errata

• Raman and infrared spectra of ammonium selenate and lithium ammonium selenate

The Raman and infrared absorption spectra of (NH4)2 SeO4, (ND4)2 SeO4 and Li (NH4) SeO4 have been recorded at 300° K; while the spectra of the non-deuterated compounds have been recorded at 110° K also. (NH4)2 Se04 crystal exhibits 33 Raman lines, while LiNH4SaO4 shows 30 Raman lines at 300° K. Proper assignments have been given to the observed vibration frequencies. The SeO4−− ions and the (NH4)+ ions are found to be highly distorted in these selenates. There is lowering of the N-H vibration frequencies due to hydrogen bonding. The present spectroscopic investigations do not reveal the existence of any phase transition in these selenates in the temperature region investigated.

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