Reethamma, Thomas; Nampoori, V P N(Elsevier, December , 1988)
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Abstract:
Photoluminescence, thermoluminescence and phosphorescence studies of cerium and copper doped BaS phosphors are attempted. Cu+ centres in BaS lattice activate red emission while Ce3+ sensitize the blue emission. Results are explained on the basis of superposition theory involving monomolecular kinetics. In Randall and Wilkins model, the decay and TL studies are found to corelate each other.
Nampoori, V P N; Girijavallabhan, C P; Harilal, S S; Riju, C Issac; Bindhu, C V(Journal of Physics D : Applied Physics, January 24, 1997)
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Abstract:
Optical emission studies of C2 molecules in plasma obtained by Nd:YAG
laser ablation of graphite in a helium atmosphere are reported for irradiances in the
range (1–9:2/ x 1010 W cm−2. The characteristics of the spectral emission intensity
from the C2 (Swan band) species have been investigated as functions of the
distance from the target, ambient pressure and laser irradiance. Estimates of
vibrational temperatures of C2 species under various irradiance conditions are
made. Results of measurements performed under different ambient helium gas
pressures are also discussed.
Aneeshkumar, B; Pramod, Gopinath; Vallabhan, C P G; Nampoori, V P N; Radhakrishnan, P(Optical Society of America, July , 2003)
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Abstract:
The nanosecond optical-limiting characteristics (at 532 nm) of some rare-earth metallo-phthalocyanines (Sm(Pc)2, Eu(Pc)2, and LaPc) doped in a copolymer matrix of poly(methyl methacrylate) and methyl-2-cyanoacrylate have been studied for the first time to our knowledge. The optical-limiting response is attributed to reverse saturable absorption due to excited-state absorption. The performance of LaPc in a copolymer host is studied at different linear transmissions. The laser damage thresholds of all the samples are also reported.
Kumar, G A; Vinoy, Thomas; Gin, Jose; Unnikrishnan, N V; Nampoori, V P N(Elsevier, 2002)
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Abstract:
Optical properties of free and substituted porphyrins (PP) doped borate glass matrix are reported for the first time. Absorption spectral measurements of H2TPP, CdTPP, MgTPP and ZnTPP doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed to obtain the optical bandgap (Eg) and other important spectral parameters viz. oscillator strength (f), molar extinction coefficient (ε), electric dipole strength (q2), absorption cross-section (σa) and molecular concentration (N). Intense fluorescence was observed in the region 668–685 nm for CdTPP, ZnTPP and MgTPP doped matrices, whereas no such fluorescence was observed in H2TPP doped matrix. Fluorescence intensity was observed to be almost similar in all the metallated porphyrine matrices. Fluorescence bandwidth (Δλ), decay time (τ), stimulated emission cross-section (σ) and optical gain (G) of the principal fluorescence transitions corresponding to the Q-band excitation were also evaluated and discussed.
Abdul Rasheed, T M; Nampoori, V P N; Sathianandan, K(Chemical Physics, 1986)
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Abstract:
The vibrational overtone spectra 0f the liquid phase 1,2-dichloroethane and 1,2-dibromoethane in the spectral regions of
CH stretching local mode overtones corresponding to delta v CH= = 2 to delta v CH=5 are reported. The observed spectral features are
assigned using the local mode model. LocaI mode frequencies WCH and diagonal local mode anharmonicities XCH are
obtained from an analysis of the spectra. The local-local combinations observed are interpreted on the basis of a coupled CH
oscillator model hamiltonian. Local-normal combinations show complex structures and their possible assignments are given.
Abdul Rasheed, T M; Moosad, K P B; Nampoori, V P N; Sathianandan, K(Springer, September , 1989)
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Abstract:
Vibrational overtone spectra of styrene (liquid) and polystyrene (solid), studied by the laser-induced thermal lens (for ΔV=6) and the conventional near infrared absorption (for ΔV=3–5) techniques, are reported. For polystyrene, the overtone energy-bond length correlation predicts that the aryl CH bonds are ∼0.0005 Å longer than that in benzene, while no such conclusions could be drawn on styrene. Thesp 3 CH overtones in polystyrene are observed on the low energy side of the aryl CH overtones.
Usha, Nair; Bindu M, Krishna; Namboothiri, V N N; Nampoori, V P N(Springer, 2010)
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Abstract:
Machine tool chatter is an unfavorable phenomenon during metal cutting, which results in heavy vibration of cutting tool. With increase in depth of cut, the cutting regime changes from chatter-free cutting to one with chatter. In this paper, we propose the use of permutation entropy (PE), a conceptually simple and computationally fast measurement to detect the onset of chatter from the time series using sound signal recorded with a unidirectional microphone. PE can efficiently distinguish the regular and complex nature of any signal and extract information about the dynamics of the process by indicating sudden change in its value. Under situations where the data sets are huge and there is no time for preprocessing and fine-tuning, PE can effectively detect dynamical changes of the system. This makes PE an ideal choice for online detection of chatter, which is not possible with other conventional nonlinear methods. In the present study, the variation of PE under two cutting conditions is analyzed. Abrupt variation in the value of PE with increase in depth of cut indicates the onset of chatter vibrations. The results are verified using frequency spectra of the signals and the nonlinear measure, normalized coarse-grained information rate (NCIR).
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; 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.
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.
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; Sajan, D George; George, A K(Smart Materials and Structures, July 5, 2007)
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Abstract:
Complete thermal characterization of liquid crystal mixtures in the smectic
phase consisting of various relative volume fractions of cholesterol and
1-hexadecanol have been carried out using the photoacoustic technique.
Thermal diffusivity values of these liquid crystal mixtures are evaluated using
the open cell photoacoustic technique whereas the thermal effusivity value is
measured using the conventional photoacoustic technique. From the
measured values of these transient thermophysical parameters, the thermal
conductivity and heat capacity of the sample under investigation are
calculated. Analyses of the results show that all the thermophysical
parameters depend strongly on the volume fraction of the constituents.
Results are interpreted in terms of enhanced hydrogen bonding and the
consequent enhancement in cohesive thermal energy transport with
increasing volume fraction of 1-hexadecanol
Sankara Raman, S; Nampoori, V P N; Vallabhan, C P G; Ambadas, G; Sugunan, S(American Institute of Physics, February 1, 1999)
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Abstract:
The effect of the chemisorbed hydroxyl groups on the thermal diffusivity of gama alumina is determined
by evaluating the thermal diffusivity at various degassing temperatures and by doping it with rare
earth oxide using photoacoustic technique. The thermal diffusivity is found to decrease with the
increase in degassing temperature as well as with the increase in the doping concentration of rare
earth oxide. This decrease has been attributed to the loss of hydroxyl ion from the y-Al2O3.
Annieta, Philip K; Lyjo, Joseph K; Litty, Mathew Irimpan; Radhakrishnan, P; Nampoori, V P N(IOP, 2005)
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Abstract:
The photosensitivity of dye mixture-doped polymethyl methacrylate (PMMA) films are investigated as a function of laser power, concentration of the dyes, modulation frequency and the irradiation wavelength. Energy transfer from a donor molecule to an acceptor molecule affects the emission output of the dye mixture system. Photosensitivity is found to change with changes in donor–acceptor concentrations. PMMA samples doped with the dye mixture are found to be more photosensitive when the dyes are mixed in the same proportion.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1994)
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Abstract:
Laser-induced photoelectric and photoemission optogalvanic effects in a Ne-Nd hollow cathode discharge have been studied using a continuous wave laser source. The potential barrier for photoinduced electron emission from the cathode decreases as the applied voltage is increased. Owing to secondary electron emission in the plasma, the photocurrent is greater than that without discharge. The multiplication of secondary electrons and the quantum efficiency are also investigated.
Lyjo, Joseph K; Dayas, K R; Soniya, Damodar; Bindu, Krishnan; Krishnankutty, K; Nampoori, V P N; Radhakrishnan, P(Elsevier, December 15, 2008)
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Abstract:
The laser-induced luminescence studies of the rare earth titanates (R2Ti2O7) (R = La, Nd and Gd) using 355 nm radiation from an Nd:YAG laser are presented. These samples with submicron or nanometer size are prepared by the self-propagating high temperature synthesis (SHS) method and there is no known fluorescence shown by these rare earths in the visible region. Hence, the luminescence transitions shown by the La2Ti2O7 near 610 nm and Gd2Ti2O7 near 767 nm are quite interesting. Though La3+ ions with no 4f electrons have no electronic energy levels that can induce excitation and luminescence processes in the visible region, the presence of the Ti3+ ions leads to luminescence in this region.
Annieta, Philip K; Lyjo, Joseph K; Litty, Irimpan; Radhakrishnan, P; Nampoori, V P N(Department of Photonics, 2004)
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Abstract:
Laser induced photoacoustic (PA) technique is used in the study of photostability of polymethyl
methacrylate (PMMA) films doped with Rhodamine 6G -Rhodamine B dye system. Energy transfer from a donor
molecule to an acceptor molecule in a dye mixture affects the output of the dye system. Details of investigations on
the role of laser power, modulation frequency and the irradiation wavelength on the photosensitivity of the dye
mixture doped PMMA films are presented.
We have performed thermal diffusion measurements
of nanofluid containing gold and rhodamine 6G dye
in various ratios. At certain concentrations, gold is nearly
four times more efficient than water in dissipating small
temperature fluctuations in a medium, and therefore it will
find applications as heat transfer fluids. We have employed
dual-beam mode-matched thermal lens technique for the
present investigation. It is a sensitive technique in measuring
photothermal parameters because of the use of a lowpower,
stabilized laser source as the probe. We also present
the results of fluorescence measurements of the dye in the
nanogold environment.
V P N Nampoori; Radhakrishnan, P; Girijavallabhan, C P; Sajan, D George(Physical Review, American Physical Society, 2003)
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Abstract:
In this paper, we report the in-plane and cross-plane measurements of the thermal diffusivity of double
epitaxial layers of n-type GaAs doped with various concentrations of Si and a p-type Be-doped GaAs layer
grown on a GaAs substrate by the molecular beam epitaxial method, using the laser-induced nondestructive
photothermal deflection technique. The thermal diffusivity value is evaluated from the slope of the graph of the
phase of the photothermal deflection signal as a function of pump-probe offset. Analysis of the data shows that
the cross-plane thermal diffusivity is less than that of the in-plane thermal diffusivity. It is also seen that the
doping concentration has a great influence on the thermal diffusivity value. Measurement of p-type Be-doped
samples shows that the nature of the dopant also influences the effective thermal diffusivity value. The results
are interpreted in terms of a phonon-assisted heat transfer mechanism and the various scattering process
involved in the propagation of phonons
Nampoori, V P N; Radhakrishnan, P; Girijavallabhan, C P; Kumar, G A; Thomas, J; Unnikrishnan, N V; George, N(Journal of Materials Science ,Kluwer Academic Publishers, 2000)
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Abstract:
Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped
glass matrix are reported for the first time. Absorption spectral measurements of H2Pc,
MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been
made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV
region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other
important optical and physical parameters viz. refractive index (n), molar extinction
coefficient ("), density (½), glass transition temperature (Tg), molecular concentration (N ),
polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported.