Browsing by Author "R. Srivastava"

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2S1 excitation of the helium atom
(1983) C.S. Singh; R. Srivastava; D.K. Rai
The variable-charge Coulomb-projected Born approximation proposed by Schaub-Shaver and Stauffer is used to analyze the electron-impact excitation of the 2S1 state of helium. Differential and integrated cross sections for the transition are calculated and compared with the other available theoretical calculations and experimental measurements. © 1983 The American Physical Society.
Brief information about Siddhasāra Nighantu of Ravigupta.
(2001) R. Srivastava; S.D. Dubey; A.L. Srivastava
Siddhasāra nighanti, is authored by 'Bauddhācārya Ravigupta'. On the basis of various evidences, its period is considered as 7th century A.D. Actually Ravigupta has written 'Siddhasāra Samhita', the last part of which is known as "Siddhasāra Nighantu'. The second chapter of Siddhasāra Samhita is 'Dravyasamgrahaniya', where 33 Ganas out of 37 Ganas of Suśruta Samhita of 'Dravyasamgrahaniya", chapter have been mentioned. The Dravyas have been described in Paryāya śaily and with 96 Slokas.
Development of high sensitivity tin oxide based sensors for gas/odour detection at room temperature
(Elsevier Science S.A., 1998) R. Srivastava; R. Dwivedi; S.K. Srivastava
An effort has been made to develop thick film tin oxide gas sensors which could detect various gases/odours at room temperature. To achieve this, the fabricated sensors were annealed in oxygen plasma for various durations. It was then found that, the room temperature sensitivity of such sensors was increased to about ten times as compared to the sensitivity of the non-annealed sensors. Further, plasma annealed sensors are found to be practically independent of temperature and the room temperature sensitivity of these sensors are found to be about 1.5 times the sensitivity of the conventional sensors at its operating temperature of 300°C. Studies on the variation of d.c. resistance, sensitivity, temporal response, current-temperature characteristics and impedance spectroscopy with the annealing time have also been made. These studies reveal that, with the increase in annealing time, there is a permanent gradual reduction in the d.c. resistance of annealed sensors. Further, it is also observed that with the increase in annealing time, the response time improves, barrier height reduces, barrier capacitance increases and the dependence of the sensitivity with temperature reduces while the sensitivity itself improves many-fold.
Double electron excitation in heliumlike ions
(1983) B. Padhy; R. Srivastava; D.K. Rai
A Coulomb-Born-Oppenheimer approach to the calculation of cross sections for electronimpact excitation of the doubly excited 2p2 P3 state in heliumlike ions is presented. It is shown that closed-form expressions can be obtained for both differential and total cross sections. The analysis is applied to calculate the total cross section for the process e-+Li+(1s2 S1)e-+Li+(2p2 P3). © 1983 The American Physical Society.
Electron-impact excitation of H and He
(1978) R. Srivastava; D.K. Rai
The method suggested by Crothers and McCarroll (see Proc. Phys. Soc., vol.86, p.753 (1965)) for the study of electron-impact excitation of neutral atoms has been used to calculate the differential and total excitation cross section in hydrogen and helium. Transitions from the ground state to ns, np and nd (n>or=3) in hydrogen and to m 1P states (m=2 and 3) in helium have been considered. Highly accurate wavefunctions of Hartree-Fock type as well as approximate but simple wavefunctions due to Messmer have been employed respectively for the ground and excited states of helium. The results of the present calculations are compared with the other available theoretical and experimental results.
Excitation of autoionizing levels in the collision of electrons with alkali-metal atoms
(1977) R. Srivastava; D.K. Rai
The method suggested in 1965 by Crothers and McCarroll for the study of electron impact excitation of neutral atoms has been applied to the excitation of autoionizing levels in alkali-metal atoms. The autoionizing levels have been assumed to arise from the excitation of an inner-shell electron to the valence shell and no account has been taken of the electron-electron couplings in the target atoms. The results are compared with the earlier work of Tiwary and Rai (1975) who used the method of Vainshtein et al. (1965).
Excitation of the lowest autoionising states in alkalis
(1982) R. Srivastava; C.S. Singh; D.K. Rai
Total cross sections for electron impact excitation of the lowest autoionising levels in Li, Na and K are calculated in the Glauber approximation. Similar calculations are also performed in the first Born approximation as well as using the method of Crothers and McCarroll (1965). The results obtained from these various approximations are compared with earlier theoretical work.
Implications of tillage and residue management on soil microbial biomass, N-mineralization rate and available-N in a dryland agroecosystem
(2000) C.P. Kushwaha; R. Srivastava; K.P. Singh
[No abstract available]
Ion transport and structural studies on a TEOS-precursor derived silica alcogel doped with acid H3PO4
(2002) R. Srivastava; S. Chandra
Silica alcogels doped with proton conducting acid H3PO4 have been prepared by the sol-gel-process starting from organic precursor tetraethyl orthosilicate (TEOS). Structural (XRD, DTA/TGA, IR) and electrical (transference number, conductivity) studies showed that the solution of dopant remains entrapped in the porous SiO2-gel matrix and can be modelled as "solid-liquid electrolyte" composite. The H3PO4 doped gels have been found to show about five orders higher protonic conductivity than the undoped SiO2-gel. The highest attainable room temperature conductivity ≈1.6 × 10-2 S cm-1 is for the SiO2:H3PO4(40:60) gel composite. The high liquid-like proton transport in this composite has been explained as a result of self dissociation of entrapped H3PO4 or H2PO4-/HPO42- acid-base pair formation.
Modification of metal-organic interface using F4-TCNQ for enhanced hole injection properties in optoelectronic devices
(2012) O. Rana; R. Srivastava; G. Chauhan; M. Zulfequar; M. Husain; P.C. Srivastava; M.N. Kamalasanan
Incorporation of thin layers of tetrafluro-tetracyanoquinodimethane (F 4-TCNQ) has been found to modify the work functions of a number of substrates. Surface potential measurement using Kelvin probe method (KPM) has been used to monitor the change in work function of the modified substrates. The results support the integer charge transfer model by which the Fermi levels of the substrates are aligned with the negative polaron states of F 4-TCNQ. Further, we found that the work function of the substrates increases with increase in F4-TCNQ thickness from 0 to 7nm and then saturates for further increase in thickness. The variation in work function has been attributed to the low surface coverage of F4-TCNQ islands on the substrates. The ITO and Au substrates with increased work functions were used as the electrode for hole only devices of common hole transport materials. The hole injection property has been found to increase with increase in F 4-TCNQ thickness and for ITO surface modified with 7nm F 4-TCNQ layer, ohmic conduction has been achieved for HTLs with HOMO level up to 5.4eV. When these modified substrates were used as hole injecting contacts in organic light emitting diodes (OLEDs), they gave substantially higher electroluminance, power efficiency and lower operating voltages. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
On the variable-charge Coulomb-projected Born approximation
(Springer-Verlag, 1984) C.S. Singh; S.N. Rai; R. Srivastava; D.K. Rai
Total cross-sections for electron impact excitation of 11S - 23S transition in helium have been calculated using variable-charge Coulomb-projected Born approximation and also using a distorted wave model in which the prior form of the T-matrix is used. The comparison of the two sets of results enables us to make certain observations about the suitability of the variable-charge Coulomb-projected Born approximation. © 1984 Springer-Verlag.
Proton impact excitation of the low lying autoionizing levels in the heavier alkali atoms
(1976) R. Srivastava; D.K. Rai
The total cross-section for proton impact excitation of some low lying autoionizing levels in alkali metal atoms have been calculated using Born approximation. Hartree-Fock atomic orbitals appropriate for the initial and final orbitals have been used. The results are compared with the other available theoretical calculations. © 1976.
PROTON-INDUCED EXCITATION- AUTOIONIZATION
(Indian Academy of Sciences, 1982) R. Srivastava; C.S. Singh; D.K. Rai
[No abstract available]
Restoration of digital mammographic images corrupted with quantum noise using an adaptive total variation (TV) based nonlinear filter
(2012) Subodh Srivastava; Neeraj Sharma; R. Srivastava; S.K. Singh
In this paper, we propose a total variation (TV) based filter adapted to the statistics of quantum noise which follows Poisson distribution, for the enhancement and restoration of the digital mammographic images. The proposed method is developed in a variational framework which reduces to a minimization problem. The proposed model consists of two terms viz. data fidelity term and regularization function and to make a proper balance between these two terms during the filtering process a regularization parameter has been introduced. For digital implementations, the proposed model has been discretized using finite difference schemes. A comparative study of the proposed scheme has also been performed with the other existing techniques in terms of mean square error (MSE), peak signal-to-noise ratio (PSNR), correlation parameter (CP) and mean structure similarity index map (MSSIM). © 2012 IEEE.
Species diversity in dryland and irrigated agroecosystems and marginal grassland ecosystem in dry tropics
(2005) R. Srivastava; K.P. Singh
This study was carried out to compare composition, density and diversity of species between dryland and irrigated agroecosystems and between agroecosystems and the marginal grassland ecosystem in dry tropics. Main management characteristics of these ecosystems are: (1) Dryland cropping, low fertilizer input, rainfed (no irrigation) and seed sown rice; (2) Irrigated cropping, high fertilizer, water (irrigation) inputs, flooded rice by transplanting; (3) Grassland, post-rainy season herbage removal and light grazing. All ecosystems showed comparable number (12-14) of species in winter, but in the rainy season higher number of species (21) were recorded in the grassland ecosystem. The species composition of the grassland was entirely different from both agroecosystems (similarity <1%). Dryland and irrigated agroecosystems showed only 25% and 38% similarity with each other during winter and rainy cropping seasons, respectively. Occurrence of few grasses (e.g., Cynodon dactylon, Dichanthium annulatum and Sporobolus diander) in the agroecosystems depends on the propagules dispersed from the marginal grassland. The change from dryland to irrigated agroecosystem involved elimination of 4-5 species during both cropping seasons and simultaneous recruitment of 2 new species in winter and 6 species in rainy season. In both agroecosystems, forbs (C3 type) dominated over graminoids (C4 type) during winter season but the dominance was reversed during the warm, rainy season. During winter some forbs were abundant in both agroecosystems due to their tolerance to widely varying moisture conditions (e.g., Chenopodium album), others were dominant in either drier soil conditions (e.g., Anagallis arvensis in dryland) or wet conditions (e.g., Melilotus indica and Phalaris minor in irrigated). Higher species diversity occurred in the grassland relative to both agroecosystems. The seasonal trends of species diversity and species evenness were broadly similar in both agroecosystems but irrigated agroecosystem exhibited relatively higher species diversity. The changes in species composition and species diversity in agroecosystems are mainly attributed to differences in water management. The water management in irrigated agroecosystem tends to reduce weed diversity but leads to the dominance of some potentially noxious weeds (e.g., Phalaris minor). © Akadémiai Kiadó, Budapest.
Theoretical calculation of electron-impact resonance excitation of alkali atoms
(Springer India, 1983) C.S. Singh; R. Srivastava; D.K. Rai
Total cross-section for electron impact excitation of the resonance levels in Li, Na and K are calculated using the method due to Crothers and McCarroll. The results are compared with other available theoretical and experimental data. © 1983 Indian Academy of Sciences.