Louis, Godfrey; Philip, J(The American Physical Society, 1996)
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Abstract:
A detailed ultrasonic study of the elastic properties of lithium ammonium sulfate ~LiNH4SO4! or LAS has
been carried out below room temperature. The elastic constants of LAS at room temperature are reported. The
discrepancy present in earlier elastic constant data associated with the different choice of axes for this orthorhombic
system are clarified. The results of the temperature variation study down to 220 K confirm the
ferroelastic phase transition at 285 K and establish a thermal hysteresis of about 2.5 K between the cooling and
heating cycles. Results of the investigation on the suspected weak phase transition at 256 K suggest that this
transition occurs at 242 K on cooling and at 256 K on heating, thus having a thermal hysteresis of about 14 K.
However, since the observed elastic anomaly for this transition is very small, the nature of this transition still
remains unclear
Unnikrishnan,P; Muraleedharan Nair,S(Department of Chemical Oceanography, School of Marine Sciences, 2000)
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Abstract:
This thesis is a modest attempt in assessing the trace metal levels and their behavior in the aquatic environment of Kuttanad, an aquatic system that is severely affected by man’s intervention on natural processes, by study seriously evaluating the levels of trace metals in dissolved and particulate phases and also in the different chemical fractions of the sediments. Understanding of the distributions, variations and transfer processes of trace metals in different environmental phases in the backwaters of Kuttanad is vital for the assessment of the water pollution problems and study the ecology of the area which contributes 20% of the rice production in the State of Kerala. Kuttand is a low-lying, shallow bay formed as a result of geological uplift. The major economic activity is agriculture involving 40% of the population. About 1.5% of the people are engaged in aquaculture. The trace metal distribution in the Kuttand backwaters is considerably influenced by the tropical features of the location and by human activities including agricultural activities and construction of salinity barrier. In this study an attempt is made to differentiate the metals in the sediment into exchangeable, reducible and resistant fractions in the sediments.
Sreevalsam,Gopinath; Saramma,A V(Faculty of Marine biology, June 19, 2002)
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Abstract:
This study aims to reveal the ability of bacteria isolated from Cochin estuary and the Arabian Sea to produce phosphatases, important characters of the enzymes, its ability to utilize organophosphorus compounds as source of phosphate and also their role in degradation of organophosphorus pesticides. It deals with isolation, identification and screening of bacteria for phosphatase production, and it describes the effect of cultural conditions on growth and phosphatase production. The effect of various factors like pH, NaCl concentration, temperature of incubation, carbon source, period of incubation etc. on growth and phosphatase production by the two selected species were studied to establish suitable environment for phosphatase production by these bacteria. In this study regulation of phosphatase synthesis, characteristics of acid and alkaline phosphatases are discussed.
Ravi Kumar, A V; Padmaja, G; Nampoori, V P N; Vallabhan, C P G(Springer, November , 1989)
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Abstract:
Absorption spectra of formaldehyde molecule in the gas phase have been recorded using photoacoustic (PA) technique with pulsed dye laser at various power levels. The spectral profiles at higher power levels are found to be different from that obtained at lower laser powers. Two photon absorption (TPA) is found to be responsible for the photoacoustic signal at higher laser power while the absorption at lower laser power level is attributed to one photon absorption (OPA) process. Probable assignments for the different transitions are given in this paper.
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; Nibu, A George; George, A K(Journal of Physica: Condensed Matter, Institute of Physics Publishing, November 28, 2000)
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Abstract:
The thermal effusivity values in the isotropic phase of certain comb-shaped
polymers have been evaluated for the first time using an open photoacoustic
cell configuration. The compounds investigated have siloxane and acrylate
backbone and they carry mesogenic groups in their side chain. The results
indicate that the polymer chain length as well as the side chain length have
pronounced influence on the thermal effusivity values in liquid crystalline
polymers.
Sajan D, George; Vallabhan, C P G; Heck, M; Radhakrishnan, P; Nampoori, V P N(Taylor & Francis, 2002)
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Abstract:
An open cell photoacoustic (PA) configuration has been employed to evaluate the thermal diffusivity of intrinsic InP as well as InP doped with tin and iron. Thermal diffusivity data have been evaluated from variation of phase of PA signal as a function of modulation frequency. In doped samples, we observe a reduced value for thermal diffusivity in comparison with intrinsic InP. We also observed that, while the phase of the PA signal varies linearly with the square root of chopping frequency for doped samples, the intrinsic material does not exhibit such behaviour in the experimental frequency range. These results have been interpreted in terms of the heat generation and phonon assisted heat diffusion mechanisms in semiconductors.
Sajan D, George; Aneeshkumar, B; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, 2004)
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Abstract:
An open-cell configuration of the photoacoustic (PA) technique is employed to determine the thermal and transport properties of intrinsic Si and Si doped with B (p-type) and P (n-type). The experimentally obtained phase of the PA signal under heat transmission configuration is fitted to that of theoretical model by taking thermal and transport properties, namely, thermal diffusivity, diffusion coefficient, and surface recombination velocity, as adjustable parameters. It is seen from the analysis that doping and also the nature of dopant have a strong influence on the thermal and transport properties of semiconductors. The results are interpreted in terms of the carrier-assisted and phonon-assisted heat transfer mechanisms in semiconductors as well as the various scattering processes occurring in the propagation of heat carriers.
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; George, N A; Paul, T; Sebastian, M T(Journal of Materials Science Letters,Kluwer Academic Publishers, 2000)
Ritty J, Nedumpara; Binoy, Paul; Santhi, A; Radhakrishnan, P; Nampoori, V P N(Elsevier, January , 2004)
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Abstract:
In this paper, we present a laser-induced photoacoustic study on the photostability of laser dye Coumarin 540 doped in PMMA matrix and modified by the incorporation of low-molecular weight additives. The dependence of photostability of the dye on various experimental conditions, such as nature of solvents, incident optical power and dye concentration, is investigated in detail. The activation rates for the bleaching process are calculated for different concentrations and they suggest the possibility of two distinct mechanisms responsible for photodegradation. Further, analysis of the data confirms the linear dependence of photodegradation on the intensity of incident radiation. The role of different externally influencing parameters, such as wavelength and modulation frequency of incident radiation, is also discussed.
Sajan D, George; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, Dilna S P(WILEY-VCH Verlag, 2003)
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Abstract:
The photoacoustic technique under heat transmission configuration is used to determine the effect of doping on both the thermal and transport properties of p- and n-type GaAs epitaxial layers grown on GaAs substrate by the molecular beam epitaxial method. Analysis of the data is made on the basis of the theoretical model of Rosencwaig and Gersho. Thermal and transport properties of the epitaxial layers are found by fitting the phase of the experimentally obtained photoacoustic signal with that of the theoretical model. It is observed that both the thermal and transport properties, i.e. thermal diffusivity, diffusion coefficient, surface recombination velocity and nonradiative recombination time, depend on the type of doping in the epitaxial layer. The results clearly show that the photoacoustic technique using heat transmission configuration is an excellent tool to study the thermal and transport properties of epitaxial layers under different doping conditions.
Sathy, P; Reji, Philip; Nampoori, V P N; Vallabhan, C P G(IOP, 1994)
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Abstract:
S1 to S3 excited singlet state absorption and two-photon absorption in Rhodamine 6G at the pump wavelengths of 532 and 1064 nm respectively are investigated. The advantages of employing the pulsed photoacoustic technique for conveniently observing excited singlet state absorption are discussed. It is shown that, since photoacoustics and fluorescence are complementary phenomena, analysis using both techniques will yield a better understanding of optical processes in molecules like Rhodamine 6G.
Riju C, Issac; Harilal, S S; Geetha K, Varier; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Society of Photo-Optical Instrumentation Engineers, February , 1997)
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Abstract:
Pulsed photoacoustic studies in solutions of C70 in toluene are made using the 532-nm radiation from a frequency-doubled Nd:YAG laser. It is found that contrary to expectation, there is no photoacoustic (PA) signal enhancement in the power-limiting range of laser fluences. Instead, the PA signal tends to saturate during optical power-limiting phenomenon. This could be due to the enhanced optical absorption from the photoexcited state and hence the depletion of the ground-state population. PA measurements also ruled out the possibility of multiphoton absorption in the C70 solution. We demonstrate that the nonlinear absorption leading to optical limiting is mainly due to reverse saturable absorption.
Nibu A, George; Jyotsna, R; Thomas Lee, S; Aneeshkumar, B; Jayan, Thomas; Nampoori, V P N; Radhakrishnan, P(Elsevier, 2005)
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Abstract:
Photoacoustic spectrum of samarium phthalocyanine powder is recorded and compared with previously reported UV–vis absorption spectra of the same dissolved in different liquid and solid host media. The Davidov splitting of Q band is observed in the PA spectrum but the two bands are overlapped considerably and the shorter wavelength band is more intense and dominating one in the powder spectrum.