Padmaja, G; Ravi Kumar, A V; Vidyalal, V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, June , 1991)
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Abstract:
The spectroscopic analysis of the emission from the plasma produced by irradiating a highT c superconducting GdBa2Cu3O7 target with a high power Nd:YAG laser beam shows the existence of the bands from different oxides in addition to the lines from neutrals and ions of the constituent elements. The spectral emissions by oxide species in laser-induced plasma show considerable time delays as compared to those from neutral and ionic species. Recombination processes taking place during the cooling of the hot plasma, rather than the plasma expansion velocities, have been found to be responsible for the observed time delays in this case. The decays of emission intensities from various species are found to be non-exponential.
Thomas, Baby; Sathianandan, K; Nampoori, V P N; Vallabhan, C P G; Ramachandran, T(American Institute of Physics, September , 1991)
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Abstract:
A high power Nz laser of the double-Blumlein type having a modified gas flow system,
electrode configuration, and discharge geometry with minimum inductance is described. By
incorporating a triggere’d-pressurized spark gap switch, arc-free operation was achieved
for a wide E/P range. The device gives a peak power in excess of 700 kW with a FWHM of
3 ns and an efficiency of 0.51%, which is remarkably high for a pulsed nitrogen laser
system. The dependence of output power on parameters such as operating pressure, voltage,
and repetition rate are discussed.
Ravi Kumar, A V; Padmaja, G; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(Springer, October , 1991)
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Abstract:
The acoustic signals generated in solids due to interaction with pulsed laser beam is used to determine the ablation threshold of bulk polymer samples of teflon (polytetrafluoroethylene) and nylon under the irradiation from a Q-switched Nd:YAG laser at 1.06µm wavelength. A suitably designed piezoelectric transducer is employed for the detection of photoacoustic (PA) signals generated in this process. It has been observed that an abrupt increase in the amplitude of the PA signal occurs at the ablation threshold. Also there exist distinct values for the threshold corresponding to different mechanisms operative in producing damages like surface morphology, bond breaking and melting processes at different laser energy densities.
Reji, Philip; Sathy, P; Nampoori, V P N; Jacob, Philip; Vallabhan, C P G(IOP, 1992)
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Abstract:
Two-photon absorption in methanol solutions of Rhodamine 6G is investigated by photoacoustics using the second harmonic of a pulsed Nd:YAG laser. Competition between one-photon and two-photon processes is observed, depending critically on the sample concentration and input light flux.
Santhakumari, N C; Vallabhan, C P G(Elsevier, May , 1992)
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Abstract:
D.C. and a.c. electrical conductivities, dielectric constant and dielectric loss factor in single crystals of ethylenediammonium sulphate, (H3NCH2CH2NH3)(SO4), have been measured axiswise as a function of temperature. Anomalous variations in all the above properties at 480 K indicate the occurrence of a phase transition in the above material at this temperature. The existence of such a phase transition is also confirmed by DSC measurements. Electrical conductivity results are analysed and the activation energies of conduction at different temperature regions have been evaluated from the logσ vs 103T−1 plot. Possible mechanisms for the electrical conduction process are discussed, the available results being in favour of a proton transport model.
Annieta, Philip; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
Non-destructive testing d multilayer dielectric coatings (SiO2/TiO2
structure) has been carried out using the photoacoustic technique. This
technique makes use d a 10 mW He-Ne laser, a photoacoustic cell and a lock-in
amplifier. The chopped He-Ne laser beam is allowed to fall on the sample
placed in a photoacoustic cell. The acoustic signals thus generated are detected
using a microphone and the resulting output is processed by a lock-in amplifier.
The amplitude and phase of the signals were measured as a function of the
chopping frequency. Striking step-like variations are observed in me phase
against frequency plot which dearly reveals the different layers present in the
multilayer structure.
Padmaja, G; Ravi Kumar, A V; Radhakrishnan, P; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
Spatial and temporal analyses of the spectra of the laser induced plasma from a polytetrafluroethylene (PTFE) target obtained with the 1.06 mu m radiation from a Q-switched Nd:YAG laser have been carried out. The spatially resolved spectra of the plasma emission show that molecular bands of C2 (Swan bands) and CN are very intense in the outer regions of the plasma, whereas higher ionized states of carbon are predominant in the core region of the plasma emission. The vibrational temperature and population distribution in the different vibrational levels have been studied as a function of laser energy. From the time resolved studies, it has been observed that there exist fairly large time delays for the onset of emission from all the species in the outer region of the plasma. The molecular bands in each region exhibit much larger time delays in comparison to the ionic lines in the plasma.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A two-photon induced photoemission optogalvanic effect which brings about a change in the discharge voltage when a pulsed dye laser beam is focused on a tungsten electrode has been described. The experiment is performed with N2, NO2 and Ar discharges. The magnitude of the signal voltage is studied as a function of laser energy and discharge current. The effective quantum efficiency in the discharge is found to be larger than that in the vacuum condition.
Sasi Kumar, P R; Nampoori, V P N; Vallabhan, C P G(IOP, 1993)
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Abstract:
A two-photon induced photoemission optogalvanic effect which brings about a change in the discharge voltage when a pulsed dye laser beam is focused on a tungsten electrode has been described. The experiment is performed with N2, NO2 and Ar discharges. The magnitude of the signal voltage is studied as a function of laser energy and discharge current. The effective quantum efficiency in the discharge is found to be larger than that in the vacuum condition.
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.
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; 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.
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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Abstract:
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.
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:
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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Abstract:
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.
Vidyalal, V; Rajasree, K; Vallabhan, C P G(World Scientific Publishing Company, 1996)
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Abstract:
A simple experimental set-up is described to measure the electromagnetic shielding property of high Tc superconducting samples. Measurements were performed using HTSC materials in the form of laser ablated thin films, powders and sintered pellets. Samples used were Gd-123 in pure and doped form as well as a few Bi-based superconducting ceramics. For comparison, similar measurements were carried out on metals like aluminium, copper and μ metal. Very effective shielding was observed for HTSC materials compared to the conventional materials mentioned above. However it also depended on the sample types and poor shielding was observed for powdered HTSC material in comparison to thin films prepared by laser ablation.
Bindhu, C V; Harilal, S S; Geetha K, Varier; Issac, Riju C; Nampoori, V P N; Vallabhan, C P G(IOP, 1996)
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Abstract:
The dual-beam thermal lens technique has been found to be very
effective for the measurement of fluorescence quantum yields of dye solutions. The
concentration-dependence of the quantum yield of rhodamine B in methanol is
studied here using this technique. The observed results are in line with the
conclusion that the reduction in the quantum yield in the quenching region is
essentially due to the non-radiative relaxation of the absorbed energy. The thermal
lens has been found to become abberated above 40 mW of pump laser power.
This low value for the upper limit of pump power is due to the fact that the medium
is a resonantly absorbing one.