• Putcha Venkateswarlu

Articles written in Proceedings – Section A

• Emission bands of halogens - Part II. Iodine bands arising in 1g (3Π1g) andΠ2,1g states

The wavelengths of the maxima of all the diffuse bands from 4800 to 2680 A are given. The bands occur in different groups which involve frequency differences of the order of 165 cm.−1, 215 cm.−1 and 360 cm.−1 among the component bands. Five groups of bands with frequency difference of 165 cm.−1 are explained in Paper I. In this paper five groups with frequency differences of 215 cm.−1 and another five groups with frequency differences of 360 cm.−1 among the component bands are discussed. It is shown that they arise in a transition from two initial stable states with frequencies of 215 cm.−1 and 360 cm.−1 to a common set of five different final states which are all unstable. These final states are identified with just those states which are the only theoretically possible ones. The well-known continuum at 3416 A which occurs in emission and in absorption at high temperature is attributed to the transition 1g (3Π1g → Ou (1Σu)) in the iodine molecule. The mechanism of the fluorescent bands corresponding to the emission bands is also discussed.

• Emission bands of halogens - Part III. Iodine bands arising in 1u (3Πu+) and 1g (1Πg) states

The three broad bands at 4747·2, 4662·1 and 4575·2 A are explained as due to transitions from a single stable state 1u (3Σu+) at 44900 cm.−1 to three unstable states Og+ (3Σg), 1g (3Σg) and 1g (1Πg) which dissociate into2P3/2+2P1/2 iodine atoms at 20037 cm.−1 The bands in the region 2687 to 2400 A form three groups which are explained as arising in a level 1g (1Πg) at 58572 cm.−1 and having for their final level the three unstable states 1u (3Δ1u), 2u (3Δ2u) and 1u (1Πu) which dissociate into2P3/2+2P1/2 iodine atoms. The potential energy diagram for iodine molecule is given.

• Emission bands of halogens - Part IV. Diffuse bands of bromine

A large number of new diffuse bands in the emission spectrum of bromine are recorded. The wavelengths of all the diffuse bands of bromine in the region 4200–2000 A are given together with the experimental details. The bands occur in different sets of groups which involve frequency differences of the order of 330, 220 and 480 cm.−1 among the component bands. These are explained, as in the case of iodine, as arising in three initial states 1g (3Π1g), 1g (1Πg) andΠ2,1g with frequencies of about 330, 220 and 480 cm.−1 The final states of all these bands will be repulsive, and they are attributed to just these states which are theoretically possible. Besides these bands, there are nine broad bands whose mutual vibrational frequencies are so irregular that they cannot be considered as belonging to a single group arising in a single transtion. These bands are explained as arising in a state 1u (3Σu+) and having for their final levels nine repulsive states which are identified with just the theoretically possible ones. The approximate position of these four states 1g (3Π1g), 1g (1Πg),Π2,1g and 1u (3Σu+) are at 55534, 61444, 66500 and 47000 cm.−1, whereas for iodine they are at 51583, 58572, 56000 and 44900 cm.−1

• Erratum to: Emission bands of halogens - Part III. Iodine bands arising in 1u (3Πu+) and 1g (1Πg) states

• Emission bands of halogens - Part V. Continuous bands of chlorine

Wavelengths of the maxima of continuous bands of chlorine in the ultra-violet obtained with uncondensed transformer discharge, are given. These are practically the same as were previously recorded by other observers. The chlorine spectrum is shown to be apparently similar to that of iodine and bromine. The bands are conveniently arranged in three sets which are shown to be respectively arising in the three statesσg,πu4,πu4,σu. 1u (3Σu+);σg,πu4,πg3,σu2. 1 (3Π1g);σg,πu4,πg3,σu2. 1g (1Πg). The approximate positions are at about 58000, 67700 and 75000 cm−1 respectively whereas for bromine they are at 47000, 55534 and 61444 cm−1 and for iodine they are at 44900, 51528 and 58572 cm−1. The final states of all these bands will be repulsive, and will have steep potential energy curves. They have been attributed, in analogy with iodine and bromine, to those electronic states which are theoretically possible. The potential energy diagram for chloride molecule has been given.

• The ionic character of singly bonded molecules

The overlap factors for bond electrons in various diatomic halides are calculated making use of a simple relation where the overlap factor is an inverse exponential function of the difference between the two atomic radii of the molecule concerned. These overlap factors together with the hybridization values of 15 percent for Cl, and 10 percent for Br and I are used to calculate the ionic characters for various diatomic molecules from the values of the nuclear quadrupole coupling constants obtained by earlier workers. These ionic characters are then plotted against electronegativity differences and the points are found to be fairly well represented by a smooth curve. It is shown that this curve can in general be used for the determination of the ionic characters in the halogen containing single bonds of certain polyatomic molecules. The quadrupole coupling constants are calculated from the ionic characters obtained from the curve and are compared with those observed. The effects of the distortion of the closed shells and of the neighbouring ions oneQq are discussed.

• Emission spectrum of bromine excited in the presence of argon—part I

Bromine was excited by an uncondensed transformer discharge in the presence of argon. The spectrum obtained was found to be different from what one gets without the presence of a foreign gas and consists of (1) a short discrete band system in the region 3150–2970 Å, (2) an extensive discrete band system in the region 2950–2670 Å, (3) a short and weak discrete system in the region 2660–2590 Å and a set of diffuse bands in the region 3340–3190 Å. The wavelengths and wave numbers of the band heads of the system 2950–2670 Å, as obtained from the measurements of the plates taken on the first order 21-ft. grating spectrograph, are given along with the vibrational analysis. This system is shown to be due to a transition from an upper electronic state at 51802 cm.-1 with ω′e = 150·5 cm.-1 and ω′eXe = 1·15 cm.-1 to the well known3IIα (O2+) state at 15918 cm.-1

• Emission spectrum of bromine excited in the presence of argon - Part II. the band system in the Region 3150–2970 Å

The wavelengths and wavenumbers of the band heads of the system 3150–2970 Å as obtained from the plates taken on the first order 21′ grating spectrograph are given along with the vibrational analysis. This system is shown to be due to a transition from an upper electronic state at Te = 48516 cm.-1 with ω′e = 162·0 cm.−1 and ω′eχ′e = 0·29 cm.−1 to the well-known3Πu (Ou+) state at Te = 15918 cm.-1 This lower state is common with that of the system 2950–2670 Å.

• The emission spectrum of iodine bromide excited in the presence of Argon - Part I. The band system in the regions 5425–5360 Å, 4520–4415 Å and 4120–4010 Å

IBr vapour was excited in the presence of argon by an uncondensed transformer discharge. Four band systems were obtained in the regions 5425–5360 Å, 4520–4415 Å, 4120–4010 Å and 3915–3540 Å of which the first three are discussed in this paper. The wavelengths and wavenumbers of the band heads in three systems as measured from the plates obtained with a 3-prism Steinheil glass spectrograph are given along with their visually estimated relative intensities. The three band systems, which are new, are analysed and the following vibrational constants expressed in cm.−1 are obtained:

The lower state of the first two systems has been identified with the (O+) state at 16814 cm.−1 reported earlier by Brown from a study of the absorption bands in the visible region.

• The emission spectrum of IBr excited in the presence of Argon - Part II. The band system in the region 3915–3540 Å

Wavelengths and wavenumbers of the band heads in the region 3915–3540 Å are recorded as obtained from the measurements of the plates taken on a first order 21-feet grating spectrograph. Earlier workers recently reported 40 bands of this system covering the region 3900–3800 Å. All the bands of this system obtained in the present experiments are analysed as involving the3Π (1) state for lower state. The constants for the lower state are such that they represent well the ΔG (v+1/2) values obtained in the present experiments fromv=0 tov=26 as well as those obtained by Brown fromv=9 tov=43. The vibrational constants of the two states involved are:$$\begin{gathered} \begin{array}{*{20}c} {\omega _e ^{\prime \prime } } \\ {137 \cdot 8 cm.^{ - 1} ,} \\ \end{array} \begin{array}{*{20}c} {\omega _e ^{\prime \prime } x_e ^{\prime \prime } } \\ {0 \cdot 571 cm.^{ - 1} } \\ \end{array} \begin{array}{*{20}c} {\omega _e ^{\prime \prime } y_e ^{\prime \prime } } \\ { - 0 \cdot 1156 cm.^{ - 1} } \\ \end{array} \begin{array}{*{20}c} {\omega _e z_e ^{\prime \prime } } \\ {3 \cdot 09 \times 10^{ - 3} cm.^{ - 1} } \\ \end{array} \hfill \\ \begin{array}{*{20}c} {\omega _e ^{\prime \prime } t_e ^{\prime \prime } } \\ { - 2 \cdot 5 \times 10^{ - 5} cm.^{ - 1} ,} \\ \end{array} \begin{array}{*{20}c} {\omega _e ^\prime } \\ {90 \cdot 1 cm.^{ - 1} ,} \\ \end{array} \begin{array}{*{20}c} {\omega _e ^\prime x_e ^\prime } \\ {0 \cdot 15 cm.^{ - 1} } \\ \end{array} \hfill \\ \end{gathered}$$

The probable electronic configurations and electronic terms for the different observed states of IBr are discussed.

• Errata

• Emission spectrum of chlorine excited in the presence of argon - Part I. The band system in the region 2600–2390 Å

The spectrum of chlorine excited in the presence of argon has been photographed with a 21-ft. grating spectrograph in the first order. Two band systems in the region 2600–2390 Å and 2365–2239 Å are observed which appear to be respectively analogous to the 2950–2670 Å and 2660–2590 Å systems of bromine reported earlier by Venkateswarlu and Verma. The wavelengths and the wavenumbers of all the bands in the system 2600–2390 Å are given. The vibrational scheme along with the corresponding Franck-Condon parabola is also given. The analysis suggests that the lower state of the system is the 3π(Ou+_ state established by Elliott at 17658 cm.−1 and that the upper state is at 67773 cm.−1 The vibrational constants obtained arew0′ = 246·6 cm.−1,w0x0′ = 0·615 cm.−1,w0″ = 255·2 cm.−1,w0x0″ = 5·5 cm.−1,w0y0″ = −0·0155 cm.−1 andw0z0″ = 0·00115 cm.−1

• Halogen nuclear magnetic resonance shifts - Part I. Cl35 resonance in alkali chlorides

The nuclear magnetic resonance shifts (chemical shifts) of Cl35 in aqueous solutions of alkali chlorides were determined using a High Resolution NMR Spectrometer. These small shifts probably represent the effect of the neighbouring atom even in the highly ionic state. A similarity of these results to the quadrupole coupling constants of the halogen atom in these molecules is pointed out.

• Emission spectrum of bismuth monochloride - Part I. The vibrational analysis of the 6170–4220 Å system

Bismuth chloride has been excited in flowing condition with an uncondensed transformer discharge. About 390 bands are observed in the present experiments of which only 140 were recorded by earlier workers. The vibrational constants obtained are the same as those obtained by Morgan from obsorption experiments except for the addition of a cubic term for the upper state. It appears quite likely that the upper state of the system dissociates into Bi (4S3/2) + Cl (2P1/2) while the lower state, which is probably the ground state, dissociates into Bi (4S3/2) + Cl (2P3/2). The rough values of the dissociation energies obtained by extrapolations are D0′=3750 cm.−1 and D0″=24614 cm.−1

• The nuclear magnetic resonance spectra of four spin systems - I. The H1 and F19 resonance spectra of 1-fluoro, 2, 4-dinitrobenzene

High resolution H1 and F19 nuclear magnetic resonance spectra of 1-fluoro, 2, 4-dinitrobenzene were obtained and analyzed to determine all the parameters involved. As the spectrum of one of the protons is well separated from that of the other two, the secular equation has been solved in the first approximation. The analysis gives the following spin-spin coupling constants: J0HH = 8.7 ± 0.3 cps., JmHH = 2.9 ± 0.2 cps., JpHH = 0.6 ± 0.3 cps., J0HH = 10.4 ± 0.2 cps., and the two meta H-F coupling constants were found to be considerably different being 6.5 ± 0.3 cps. and 3.8 ± 0.3 cps. It has been found that theortho andmeta H-F coupling constants have the same sign as theortho andpara H-H coupling constants.

• The nuclear magnetic resonance spectra of four spin systems - III. Proton resonance spectra ofα, β andγ picolines

The ring proton resonance spectra ofα, β andγ picolines have been studied at 40 Mc./sec. The spectra which belong to the classes ABCX, ABXY and A2X2 respectively have been completely analyzed to determine all the involved parameters.

• Possible splitting of energy levels of atoms or ions in octahedral crystals due to sets of equivalent neighbouring defects

Divalent ions when doped in alkali halide single crystals will go in alkali sites and form associate pairs with neighbouring cation vacancies or other impurity defects. As there will be sets of equivalent positions for such associated pairs, their energy levels will be degenerate. The reducible representations of these associated pairs involving first to sixth nearest neighbours have been reduced to the corresponding irreducible representations of the point group Oh, for cases having rock-salt structure as well as for cases having cesium chloride structure. It is pointed out that if there is a rapid hopping of the vacancies or the defects in the crystal one may expect because of the possible interactions in the system, a splitting of the energy levels of the pairs as indicated by the different irreducible representations. It is stated that this kind of splitting may be expected also in systems in which associated magnetic pairs are formed by two neighbouring spins.

• Electron spin resonance of Mn2+ in NaCl single crystal and lattice defects

The electron spin resonance spectra of Mn2+ in NaCl single crystals are investigated in detail. Seven different spectra (I, II, III1, III2, IV, V and VI) are observed. The spectra I–IV are the same as those observed by earlier workers, while the spectra V and VI are observed here for the first time. The local symmetry at the paramagnetic ion is orthorhombic for the spectra IV and V and tetragonal for the spectrum VI about the crystallographic [001] direction. The properties of the spectrum IV are explained in terms of an associated pair Mn2+: O22− with O22− molecular ion at the nearest neighbour anion site in the [001] direction with its internuclear axis in the (001) plane. The spectrum V is assigned to the associated pair Mn2+: O22− coupled with a nearby cation vacancy in the [001] direction and the spectrum VI to Mn2+ ion associated with OH ion at the anion site in the [001] direction with a probable second neighbour cation vacancy. All the observed spectra are analysed in terms of the parameters of the usual spin-Hamiltonian.

• Forbidden hyperfine transitions in electron spin resonance of Mn2+ in NaCl single crystal

Forbidden hyperfine transitions are observed in the electron spin resonance spectrum of divalent Mn55 ion in NaCl single crystal for a particular associated pair. From the measurements of the M = + 1/2 → −1/2, Δm = ± 1 transitions the parametersQ′ and Q″ of the nuclear electric quadrupole part of the spin-Hamiltonian Ho = Q′ [Iz2 − 1/3 I (I + 1)] + Q″ (Ix2−Iy2) are found to be + 1.70 × 10−4 cm.−1 and +0.16 × 10−4 cm.−1 respectively.

• The probable iodine molecular lasers in the violet and ultraviolet regions

Iodine when excited in the presence of argon shows three band systems in the regions 4420–4000 Å., 3,460–3015 Å. and 2785–2731 Å. all of which involve OuEmphasis&gt;+ (3πu) state at 15642 cm.−1 as the common lower state. It is pointed out in this article that the transitions involving the first two band systems are suitable for laser action as these transitions are very strong and as their common lower state gets efficiently flushed out due to the crossing or touching of a OuEmphasis&gt;− repulsive state while a continued high frequency discharge through a mixture of iodine and argon would keep the upper state populated all the time. Tuning of the interferometer system is expected to make this laser action possible in a wide range of frequencies.

• Paramagnetic resonance of Mn2+ in KNO3 single crystal: Motional effects

The electron paramagnetic resonance of Mn2+ in KNO3 single crystal is investigated over a temperature cycle through transition temperatures. The hyperfine coupling constant, A, half width,Δ H, and the line intensity, I, are found to show sudden changes at the transition temperature, at whichα-KNO3 changes intoβ-KNO3. The lines are much sharper in the high temperatureβ-phase than inα-phase of the crystal. They are explained, qualitatively, in terms of structure change and rotation of NO3 ions. The spectra in the two phases,α andβ, are analysed in terms of usual spin-Hamiltonian. A search of metastableγ-phase is also made and probable indications for the same are found.

• Effect of temperature and time on paramagnetic resonance of Mn2+ in NaCl single crystal

Exchange of intensities among various spectra of Mn2+ in NaCl single crystal, corresponding to Mn+2 ion associated with various point defects, is described as a function of temperature and time. The variation of line width with temperature is discussed. The fine structure parameter D for a particular spectrum is measured with temperature and is shown to behave like D=D0 (1+bT+cT2). Thermal formation of new complexes and their lifetimes are discussed. A new short-lived complex is found and a model for the same is suggested.

• Paramagnetic resonance of Gd3+ in SmCl3·6H2O single crystals

The Paramagnetic Resonance of Gd3+ in SmCl3·6H2O single crystals, grown from solution, is studied at room temperature. A six line spectrum for H//Z and a seven line spectrum for H//X corresponding to ΔM=± 1 transitions are observed. Their angular variation in ZX plane fromϑ=0° toϑ=90°, is studied and the spin-Hamiltonian analysis is presented. The probable amount of admixture of the next higher electronic state6P7/2 with the ground state8S7/2 is also estimated.

• EPR spectrum of gadolinium in hydrated praseodymium nitrate

The paramagnetic resonance spectrum of Gd3+ in Pr(NO3)3·6H2O single crystals, is studied at room temperature. A seven line spectrum for H//Z as well as for H//X corresponding to ΔM=±1 transitions is observed along with a number of low field transitions (ΔM⩾2). The spin-Hamiltonian analyses is presented.

• Paramagnetic resonance studies of Cu2+ doped in NH4Br

Electron Paramagnetic Resonance studies of Cu2+ doped in NH4Br single crystals have been carried out for the first time. The room temperature data reveal that Cu2+ ions go predominantly to interstitial sites having square planar co-ordination of four Br ions. For this type of complexes the bromine: superhyperfine structure: is observed on the normal hyperfine structure lines of copper. Existence of other weak spectra indicates that a relatively few Cu++ ions go substitutionally to (NH4)+ ions, and are probably associated with a first or a second nearest neighbour cation vacancy, among the two, the latter being more predominant. However, the low temperature studies corresponding to the tetragonal phase of NH4Br indicate that a number of Cu2+ ions at interstitial sites get readjusted to the lattice sites after phase transformation. The spectra in both the phases are analysed by the usual spin Hamiltonian method.

• Electron paramagnetic resonance of Mn2+ in (NH4)2SO4 single crystal

Electron paramagnetic resonance investigation of Mn2+ in (NH4)2SO4 single crystal is discussed both in paraelectric and ferroelectric phases of the crystal. Mn2+ is found to substitute one of the two possible types (α andβ) of NH4+ ions and get associated with the second type of NH4+ vacancy, the vacancy being the second distant neighbour in thebc-plane. As The line joining Mn2+ substituted NH4+ site and NH4+ vacancy lies at an angle of 18° from the crystallographicb-axis in thebc-plane. As the temperature is lowered to − 56° C the crystal becomes ferroelectric and the spectrum in the paraelectric phase splits into two from which it appears that two sets of Mn2+ sites which are magnetically equivalent in the paraelectric phase become inequivalent in the ferroelectric phase. The spin Hamiltonian analysis is presented for the spectrum in the paraelectric phase.

• Electron paramagnetic resonance of Mn2+ in NaF single crystals

The electron paramagnetic resonance of Mn2+ in NaF single crystals is investigated at different temperatures from 573° K to 93° K. Four different spectra designated as I, II, III2 and III4 are observed. Spectrum I consists of a single broad resonance corresponding to precipitated Mn2+ ions. Spectrum II is isotropic and centred nearg = 2.00. This spectrum corresponds to substitutional Mn2+ ions with remote charge compensating sites and therefore with local cubic symmetry. Spectrum III2 withg = 2.014 and spectrum III4 withg = 1.995 are also due to substitutional Mn2+ ions but subjected to tetragonal crystalline fields and are the same as those reported by earlier workers. Superhyperfine structure has been observed in spectra II, III2 and III4. The analysis of that structure in spectra II and III2 has been carried out for the first time and the constants As, and Aσ are given. The spectra are analysed by the usual spin-Hamiltonian method.

• Electron paramagnetic resonance studies of VO2+ in single crystals of NaCl, KCl and RbCl

Electron paramagnetic resonance (EPR) of VO2+ radicals incorporated in face-centred single crystals of NaCl, KC1 and RbCl has been studied in the temperature range of 77° K to 330° K. At liquid nitrogen temperature, anisotropic spectra have been recorded in each case while spectra recorded at room and higher temperatures are isotropic suggesting the existence of a fast readjustment of VO2+ molecular ions in the crystals at higher temperature while this motion gets hindered at liquid nitrogen temperatures. Spin-Hamiltonian constants are calculated from the recorded isotropic and anisotropic spectra. The line widths in each case are found to obey a parabolic law originally proposed by Kivelson. The constants which give a close fit have been evaluated. The random orientation and readjustment of V-0 bond in these alkali chloride crystals is explained on the assumption that the medium exhibits a “liquid-like” nature for VO2+ molecular ion as far as the electron paramagnetic resonance absorption is concerned.

• Electron paramagnetic resonance of CU2+ ion in CsCl single crystals

Electron paramagnetic resonance spectrum of the Cu2+ ion in CsCl is studied at different temperatures. The local symmetry at the Cu2+ site is found to be tetragonal. A model of substitutional Cu2+ ion associated with a first neighbour cation vacancy is proposed to explain the observed spectrum.

• Optical absorption of Co2+ doped NH4Cl: Phase transformation studies

The absorption spectrum of Co2+ doped NH4Cl has been studied from the room temperature to the liquid nitrogen temperature. A sudden change in the spectrum is observed between 243° K and 233° K which is attributed to the phase transition in the crystal. From the observed spectrum it is suggested that Co2+ goes in interstitially as well as substitutionally. Both the types of centers exist at room temperature, but with decrease in temperature substitutional ions migrate to interstitial sites, the process being stimulated at the phase transformation point so that the 77° K spectrum seems to be mostly due to the interstitial centers. The 77° K spectrum is analyzed in the approximation of octahedral symmetry for interstitial ions and the band positions are fitted fairly well with B = 870 cm.−1 Dq = 850 cm.−1 and C = 4·4 B. A blue shift of about 100 cm.−1 is observed for4T1 (P) band at the phase transition which is attributed to the increase in Dq value with the anomalous lattice contraction at the phase transition. The decrease in the lattice parameter calculated from this blue shift is around 0·4% which is in good agreement with the results of X-ray measurements. Two possible models for the interstitial complex are examined and the one with fourfold chlorine coordination associated with two neutral water molecules at the first neighbour (NH4)+ site lying along &lt; 100&gt; direction is suggested to be more probable.

• Electron paramagnetic resonance of mn2+ in single crystals of Cesium Sulphate

Electron paramagnetic resonance (EPR) of divalent manganese ion has been studied at 9.5 KMc/sec. These studies reveal that Mn2+ ion substitutes at a β-Cs+ site and gets associated with a vacancy at a neighbouring β-Cs+ site in the aft-plane. Thez-axis of this ββ complex makes an angle of 25° with the crystallographicb-axis. The spectra observed have been described by the spin-Hamiltonian for Mn2+ in rhombic crystalline field. The temperature dependence of the parameters D and E has been studied in the range 293°–77° K. The spectra for the observed complex along its three-principal axes have been analysed using second order perturbation equations. The spin-Hamiltonian parameters obtained from the spectra taken at room temperature are:$$\begin{gathered} g_z = 2 \cdot 000 \pm 0 \cdot 003, g_x = 2 \cdot 015 \pm 0 \cdot 003, g_y = 2 \cdot 000 \pm 0 \cdot 003; \hfill \\ A_z = - 93 \pm 1, A_x = - 91 \pm 1, and A_y = - 91 \pm 1 G; \hfill \\ D = - 941 \pm 3 and E = - 14 \pm 4 G; \hfill \\ b^\circ _4 = \left( {0 \cdot 0} \right), b_4 ^2 = \left( {13} \right), and b_4 ^4 = - 77 \pm 5 G \hfill \\ \end{gathered}$$

• Raman spectrum of Cs2So4 using Ar+ laser excitation

Raman spectrum of a Cs2SO2 single crystal has been recorded and measured using an Ar+ laser as a source of excitation. The polarization behaviour of the Raman lines under different crystal orientations has been studied. Symmetry co-ordinates of the Raman active lattice modes as worked out from group theoretical concepts are presented.

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