• D D Pant

      Articles written in Proceedings – Section A

    • Luminescence spectra and vibrations in crystal lattices

      D D Pant

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      A general survey of the known luminescence spectra of crystals is given. Those which give discrete spectra have been divided into seven classes. The luminescence spectrum of each of these crystals is examined to see what evidence it gives regarding the nature of the vibrations in the crystal lattices. Wherever possible the information thus obtained is supplemented by the evidence furnished by the complementary absorption spectra. It is shown that there is a mass of striking evidence indicating that vibrational frequencies are sharp and monochromatic in all crystals. Those luminescence spectra, which are not discrete are due to an electronic transition in which one or both the electronic states involved are not sharp and hence no evidence regarding the vibrational frequencies can be gained from them. It is remarkable that over the whole range of crystal structure—from the typical covalent crystal diamond to the typical Van der Waals crystal solid nitrogen— the vibrational frequencies are all sharp and monochromatic. The ionic crystals,e.g., the uranyl salts are also no exception to this rule. In many crystals some of the constituents,e.g., N2 in solid nitrogen, C6H6 in solid benzene, the UO2++ molecule in uranyl salts and so on, behave as if they are in the gaseous state. For example, the vibrational frequency observed in the fluorescence spectrum of solid benzene at very low temperatures is thus the same as that of the ground state of the C6H6 molecule, probably the only difference being that the observed bands are more discrete in the crystal state owing to the absence of the rotational lines.

    • Fluorescence spectrum of uranyl fluoride

      D D Pant N D Sakhwalkar

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      The fluorescence spectrum of solid uranyl fluoride has been studied at liquid-air temperature. The spectrum which consists of 17 sharp and discrete bands is classified into 5 groups and 5 series. The bands have been interpreted as being due to the transitions from (0, 0, 0)′ and (0, 1, 0)′ levels to the various vibrational levels of the ground state. All the three frequencies of the uranyl ion have been taken into account for the interpretation.

    • Some studies in the spectra of single uranyl halides and other uranyl salts

      D D Pant

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      The fluorescence spectra at liquid-air temperature of crystalline uranyl chloride, uranyl fluoride I and II, and of uranyl bromide have been studied for the first time. The spectra of uranyl acetate, nitrate, sulphate and uranyl potassium sulphate have been taken for comparison. Out of the several series present, B, C, D and E are pointed out to be most important. A remarkable shifting of the D band as well as peculiarities in the intensity distribution are noticeable. The similarity between the C and E bands and the extended diffuseness produced by the D bands are also noteworthy.

      A slight rise in temperature from −185°C. causes a great increase in the diffuseness produced by the D bands. With moist crystals the D band appears as a short continuum even at −185° C. but at −110° C. and −70°C. only B, C and E are present, the D bands completely disappear. Uranyl bromide has been found to increase tremendously in intensity on cooling, its spectrum at −185° consisting of very broad bands giving an appearance of a continuous spectrum.

      The absorption spectra of uranyl chloride have been taken with varying thickness of the absorbing layer. With thick layers, some bands which are observed in fluorescence can also be obtained in absorption. In the absorption spectrum of uranyl fluoride II and uranyl bromide the first two A and B bands of the fluorescence spectra appear as A and B band in absorption with a remarkable reversal of intensity.

    • Analysis of the fluorescence bands of the uranyl salts

      D D Pant

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      From some direct experimental evidence, it is concluded that the entire fluorescence spectrum of the uranyl salts arises out of a transition from a single excited state (designated1Πg) to the several vibrational states of two close ground levels (designated1Σg and X) and that the1Πg1Σg transition is much less favoured than the1Πg-X transition. Theν1 andν2 frequencies taking part in the transition agree with those observed in the Raman and infra-red spectra;ν1 is, however, very much more prominent. The vibrational structure of the1Πg-1Σg transition resembles that of a forbidden transition, while the structure of the other is that of an allowed one. The intensity distribution of the A, B and C bands is explained. No specific explanation of the D bands is given. Van Heel’s analysis is shown to be unsatisfactory.

    • Analysis of the absorption bands of the uranyl salts

      D D Pant

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    • Intensity relationships in thea andb bands in the absorption and fluorescence spectra of some uranyl salts at various temperatures

      D D Pant

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      The effect of temperature on the intensity of thea andb series of bands in absorption and fluorescence is studied. It is found that while in absorption the intensity ofb0 band relative to that ofa0 increases with increase in temperature in fluorescence relative intensity of A series of bands increases relative to that of B series of bands. The increase in the relative intensity ofb0 band is due to an increase in the population of the molecules in the X level from which this band arises. The increase in the relative intensity of the A bands is fluorescence points out that there is relative increase in the transition probability of the A band due to perturbations caused by thermal changes. This is in confirmity with the forbidden nature of the transition as well as with observations connected with phosphorescence decay.

    • The absorption and fluorescence spectra of uranyl nitrate solutions at room temperature

      D D Pant D P Khandelwal

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      The fluorescence spectra of uranyl nitrate solutions at room temperature are shown to give four distinct series of bands, designatedα, β, γ andα′, depending upon concentration, pH, etc. Theα, β andγ series are respectively shown to be probably due to the species UO2++, UO2NO3+ and UO3UO2++. Higher hydrolysed species, when present, shift the spectrum towards the red. The species have been identified with the help of absorption spectra and their comparison with the results of earlier workers.α′ bands are probably due to a species of the type UO2(NO3)2. The intensity distribution in the various series is studied and it is found that inα andγ bands the intensity distribution gets wider on dilution. The various characteristics observed as regards the bahaviour of the different series are discussed on the basis of environmental differences of the species in the solid and solution states. A close resemblance between the spectra of hydrated solid UO3 andγ bands is pointed out.

    • Fluorescence spectra of solutions of uranyl nitrate at liquid air temperature - Part I. Rapidly cooled solutions

      D D Pant D P Khandelwal

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      Photometric studies on the fluorescence spectra of rapidly cooled aqueous and alcoholic solutions of uranyl nitrate are reported. Five distinct series of bands, designated asα, β′, γ, δ andα′ appear, depending upon concentration, pH, solvent, etc. From their conditions of enhancement and suppression and close analogy with corresponding room temperature spectra reported earlier, theα andγ-bands are shown to be due to the species UO2++ (hydrated) and UO3UO2++. Higher hydrolysed species give δ-bands. Theα′-bands, which appear in alcoholic solutions also, are attributed to UO2(NO3)2 ·xH2O. The assignment ofβ′-bands is not certain. The intensity distribution in various series is studied and it is found that relative toα series the distribution is largely shifted away from the (0, 0) band inα′ series and towards the (0, 0) band inγ series. Theα′ andγ bands disappear around −60° C. which corresponds with the transformation temperature of ice structure. Various characteristics in the spectra are discussed on the basis of environmental differences in the species.

    • Electronic transitions of uranyl ion

      D N Sanwal D D Pant

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    • Spectral studies on some organic complexes of uranyl ion - Part I. Absorption spectra of pyridine complexes

      D N Sanwal D D Pant

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      The absorption spectra of new crystalline uranyl complexes with pyridine, at 80° Kare described. Pyridine uranyl chloride crystallises in two forms, both forms give sharp line like bands at low temperatures. In Form I the bands are analysed in terms of five electronic transitions F, M, D, U and SU of which the former four were suggested for uranyl ion by Diekeet al. Results on analogous compounds with deuteropyridine are also reported. Chloride complex seems to have pyridine in the first co-ordination sphere while the acetate complex is of the type of double salt.

    • Spectral studies on some organic complexes of uranyl ion - Part II. Emission spectral behaviour of pyridine uranyl complexes

      D N Sanwal D D Pant

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      The fluorescence spectra of uranyl complexes with pyridine are described and compared with the known spectra of double salts and addition compounds. In some cases the spectra are of the double salt type whereas in others of the type of addition compounds. The fluorescence spectra of uranyl salts have been generalised into three categories,viz., (i) single reversal and single banded fluorescence spectrum (to which most of the uranyl salts belong), (ii) double reversal and double banded spectrum (to which uranyl sulphate trihydrate belongs) and (iii) single reversal and double banded fluorescence spectrum (to which pyridine uranyl chloride form I belongs). This latter situation is also found in the case of uranium activated alkali fluoride phosphors. The decay times of these complexes are also reported.

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