Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, September 15, 1996)
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Abstract:
Laser ablation of graphite has been carried out using 1.06mm radiation from a Q-switched Nd:YAG
laser and the time of flight distribution of molecular C2 present in the resultant plasma is
investigated in terms of distance from the target as well as laser fluences employing time resolved
spectroscopic technique. At low laser fluences the intensities of the emission lines from C2 exhibit
only single peak structure while beyond a threshold laser fluence, emission from C2 shows a twin
peak distribution in time. The occurrence of the faster velocity component at higher laser fluences
is explained as due to species generated from recombination processes while the delayed peak is
attributed to dissociation of higher carbon clusters resulting in the generation of C2 molecule.
Analysis of measured data provides a fairly complete picture of the evolution and dynamics of
C2 species in the laser induced plasma from graphite.
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; 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.
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.
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.
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.
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.
Harilal, S S; Issac, Riju C; Bindhu, C V; Nampoori, V P N; Vallabhan, C P G(Jpn. J. Appl. Phys, 1997)
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Abstract:
We report time resolved study of C2 emission from laser produced carbon plasma in presence of ambient helium
gas. The 1.06µm: radiation from a Nd:YAG laser was focused onto a graphite target where it·produced a transient
plasma. We observed double peak structure in the time profile of C2 species. The twin peaks were observed only
after a threshold laser fluence. It is proposed that the faster velocity component in the temporal profiles originates
mainly due to recombination processes. The laser fluence and ambient gas dependence of the double peak intensity
distribution is also reported.
Bindhu, C V; Harilal, S S; Nampoori, V P N; Vallabhan, C P G(World Scientific, 1999)
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Abstract:
Dual beam thermal lens tecbnique is successfully employed for the determination of absolute
Fluorescence quantum yield of rhodamine 6G lnser dye in different solvents. A
532 nm radiation from a Q-switched Nd:YAG laser was used for the excitation purpose.
The fluorescence quantum yield values are found to be strongly influenced by environmental
effects. It has been observed that fluorescence yield is greater for rhodamine 6G
in ethylene glycol system than in water or in methanol. Our results also indicate that
parameters like concentration of the dye solution, aggregate formation and excited state
absorption affect the absolute values of fluorescence yield significantly.
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.