Rajasree, K; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A pulsed Nd-YAG laser beam is used to produce a transient refractive index gradient in air adjoining the plane surface of the sample material. This refractive index gradient is probed by a continuous He-Ne laser beam propagating parallel to the sample surface. The observed deflection signals produced by the probe beam exhibit drastic variations when the pump laser energy density crosses the damage threshold for the sample. The measurements are used to estimate the damage threshold for a few polymer samples. The present values are found to be in good agreement with those determined by other methods.
Rajasree, K; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
The phenomenon of mirage effect suffered by a He-Ne laser beam has been utilized to detect phase transitions in solids. It has been observed that anomalous fluctuations of large amplitude occur in the signal level near the transition temperature. The mean square value of the fluctuation is found to exhibit a well-defined peak at this point. Results of measurements made in the case of crystals of TGS ((NH2CH2COOH)3.H2SO4) and a ceramic sample (BaTiO3) are given to illustrate this technique.
Sasi Kumar, P R; Ambadi, B; Nampoori, V P N; Vallabhan, C P G(Springer, February , 1993)
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Abstract:
Doppler limited high resolution spectrum in the wavelength region 17224 to 17236 cm−1 of the first positive system (B 3Π g −A 3Σ u + ) of the N2 molecule is recorded by optogalvanic spectroscopic technique using a single mode ring dye laser. It is observed that the intensity and line width of the rotational line increase with the discharge current. Dependence of the collision broadening coefficient on the current was also evaluated.
Rajasree, K; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, August 1, 1993)
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Abstract:
Laser‐induced damage and ablation thresholds of bulk superconducting samples of Bi2(SrCa)xCu3Oy(x=2, 2.2, 2.6, 2.8, 3) and Bi1.6 (Pb)xSr2Ca2Cu3 Oy (x=0, 0.1, 0.2, 0.3, 0.4) for irradiation with a 1.06 μm beam from a Nd‐YAG laser have been determined as a function of x by the pulsed photothermal deflection technique. The threshold values of power density for ablation as well as damage are found to increase with increasing values of x in both systems while in the Pb‐doped system the threshold values decrease above a specific value of x, coinciding with the point at which the Tc also begins to fall.
A dynamical system with a damping that is quadratic in velocity is converted into the Hamiltonian format using a nonlinear transformation. Its quantum mechanical behaviour is then analysed by invoking the Gaussian effective potential technique. The method is worked out explicitly for the Duffing oscillator potential.
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.
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.
Sasi Kumar, P R; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1994)
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Abstract:
High resolution optogalvanic spectrum of the (11, 7) band in the first positive system
of nitrogen molecule has been recorded from 17179 to 17376 cm- 1. Assignment of 432 rotational
lines belonging to the 27 branches of this band has been carried out.
Rasheed, T M A; Nampoori, V P N(Springer, March , 1994)
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Abstract:
The CH local mode overtone spectrum of benzyl chloride in the visible and NIR regions studied by laser induced thermal lens and conventional NIR absorption is presented. The analysis shows that the −CH2Cl group is symmetrically oriented with respect to the benzene ring, thus finalizing one of the two possible conformational models predicted by electron diffraction studies. The aryl CH bonds have a slightly larger force constant than that in benzene.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Springer, May , 1994)
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Abstract:
The changes in emission characteristics of a neon hollow cathode discharge by resonant laser excitation of 1s 5→2p 2 and 1s 5→2p 4 transition have been studied by simultaneously monitoring the optogalvanic effect and the laser induced fluorescence. It has been observed that resonant excitation causes substantial variation in the relative intensities of lines in the emission spectrum of neon discharge.
Harilal, S S; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, June , 1994)
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Abstract:
YBa2Cu307 target was laser ablated, and the time-of-flight (TOF) distributions of Y, Y+., and YO in
the resultant plasma were investigated as functions of distance from the target and laser energy
density using emission spectroscopy. Up to a short distance from the target (-1.5 cm), TOF
distributions show twin peaks for Y and YO, while only single-peak distribution is observed for Y+.
At greater distances (>1.5 cm) all of them exhibit single-peak distribution. The twin peaks are
assigned to species corresponding to those generated directly/m the vicinity of target surface and to
those generated from collisional/recombination process.
Achamma, Kurian; Bindhu, C V; Harilal, S S; Issac, Riju C; Nampoori, V P N; Vallabhan, C P G(Pramana, November , 1994)
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A simple method based on laser beam deflection to study the variation of diffusion
coefficient with concentration in a solution is presented. When a properly fanned out laser
beam is passed through a rectangular cell filled with solution having concentration gradient,
the emergent beam traces out a curved pattern on a screen. By taking measurements on the
pattern at different concentrations, the variation of diffusion coefficient with concentration can
be determined.
Kriashnan Nair, P R; Nandakumaran, V M; Ambika, G(Pramana, December , 1994)
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Abstract:
We establish numerically the validity of Huberman-Rudnick scaling relation for
Lyapunov exponents during the period doubling route to chaos in one dimensional maps. We
extend our studies to the context of a combination map. where the scaling index is found to
be different.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Elsevier, December 26, 1994)
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We report the experimental observation of subcritical Hopf bifurcation and the existence of non-oscillating “windows” in the dynamics of a Ne-Nd hollow cathode discharge current as the control parameter.
Harilal, S S; Riju, C Issac; Bindhu, C V; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(World Scientific Publishing Company, 1995)
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Pulsed photoacoustic measurements have been carried out in liquid CS2 using 532 nm radiation from a frequency doubled Nd:YAG laser. Variation of signal amplitude with laser fluence clearly indicates the role of multiphoton processes in the generation of photoacoustic effect. It is also shown that four photon induced dissociation and five photon induced ionization are likely processes in CS2 and 532 nm radiation.
Bindhu, C V; Harilal, S S; Riju, C Issac; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(World Scientific Publishing Company, 1995)
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Measurement of thermal lensing signal as a function of laser power made in Rhodamine B solutions in methanol give clear evidence of two photon absorption process within certain concentration ranges when 488 nm Ar+ laser beam is used as the pump source. Only one photon process is found to occur when 514 nm and 476 nm beams are used as the pump.
James, Joseph; Ananthakrishnan, T R; Nampoori, V P N; Rudra Warier, M K; Karanjikar, N P; Kamat, M J(World Scientific Publishing Company, 1995)
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The effect of Ce3+ on the fluorescence emission from CaS:Ce3+ phosphor is studied using X-ray excitation. Apart from the emission in the visible region, the phosphor also shows fluorescence emission in the ultraviolet region. Variation in wavelengths and intensities of these emissions due to change in dopant concentration is also analysed.
Bindhu, C V; Harilal, S S; Issac, Riju C; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1995)
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Thermal lens signals in solutions of rhodamine B laser dye in methanol are
measured using the dual beam pump-probe technique. The nature of variations of signal
strength with concentration is found to be different for 514 and 488 nm Ar + laser excitations.
However, both the pump wavelengths produce an oscillatory type variation of thermal lens
signal amplitude with the concentration of the dye solution. Probable reasons for this peculiar
behaviour (which is absent in the case of fluorescent intensity) are mentioned.
Bindhu, C V; Harilal, S S; Issac, Riju C; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(Pramana, March , 1995)
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Abstract:
Pulsed photoacoustic measurements have been carried out in toluene at 532 nm
wavelength using a Q-switched frequency doubled Nd:YAG laser. The variation of photoacoustic
signal amplitude with incident laser power indicates that at lower laser powers one
photon absorption takes place at this wavelength while a clear two photon absorption occurs in
this liquid at higher laser powers. The studies made here demonstrate that pulsed photoacoustic
technique is simple and effective for the investigation of multiphoton processes in
liquids.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(Elsevier, August 1, 1995)
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The photoemission optogalvanic (POG) effect has been investigated in a neon-neodymium hollow cathode discharge using
cw laser excitation. Both positive and negative effects were observed. It was found that the amplitude of the POG signal was
unstable near the instability region of the discharge.