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.
Harilal, S S; Issac, Riju C; Bindhu, C V; Pramod, Gopinath; Nampoori, V P N; Vallabhan, C P G(Elsevier, 1997)
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Abstract:
Time and space resolved studies of emission from CN molecules have been carried out in the plasma produced from
graphite target by 1.06 urn pulses from a Q-switched Nd:YAG laser. Depending on the laser pulse energy, time of
observation and position of the sampled volume of the plasma, the features of the emission spectrum are found to
change drastically. The vibrational temperature and population distribution in the different vibrational levels have
been studied as functions of distance, time, laser energy and ambient gas pressure. Evidence for nonlinear effects of
the plasma medium such as self focusing which exhibits threshold-like behaviour are also obtained. Temperature and
electron density of the plasma have been evaluated using the relative line intensities of successive ionization stages of
carbon atom. These electron density measurements are verified by using Stark broadening method.
Riju, C Issac; Pramod, Gopinath; Geetha K, Varier; Nampoori, V P N; Vallabhan, C P G(American Institute of Physics, July 13, 1998)
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Abstract:
Laser-induced plasma generated from a silver target under partial vacuum conditions using the
fundamental output of nanosecond duration from a pulsed Nd:yttrium aluminum garnet laser is
studied using a Langmuir probe. The time of flight measurements show a clear twin peak
distribution in the temporal profile of electron emission. The first peak has almost the same duration
as the laser pulse while the second lasts for several microseconds. The prompt electrons are
energetic enough ('60 eV) to ionize the ambient gas molecules or atoms. The use of prompt
electron pulses as sources for electron impact excitation is demonstrated by taking nitrogen, carbon
dioxide, and argon as ambient gases.
Girijavallabhan, C P; Radhakrishnan, P; Nampoori, V P N; Nibu, A George; George, A K(Applied Physics B Lasers and Optics, July 18, 2001)
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Abstract:
In this paper, we describe the use of an open cell
photoacoustic configuration for the evaluation of the thermal
effusivity of liquid crystals. The feasibility, precision and reliability
of the method are initially established by measuring
the thermal effusivities of water and glycerol, for which the
effusivity values are known accurately. In order to demonstrate
the use of the present method in the thermal characterization
of liquid crystals, we have measured the thermal
effusivity values in various mesophases of 4-cyano-4 -
octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl
(7OCB) liquid crystals using a variable temperature open
photoacoustic cell. A comparison of the measured values for
the two liquid crystals shows that the thermal effusivities of
7OCB in the nematic and isotropic phases are slightly less
than those of 8OCB in the corresponding phases
Nampoori, V P N; Vallabhan, C P G; George, A K; Radhakrishnan, P; Rajasree, K; Vidyalal, V(Elsevier, July , 1998)
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Abstract:
he phenomenon of single beam mirage effect, otherwise known as photothermal deflection (PTD) effect using a He–Ne laser beam has been employed to detect phase transitions in some liquid crystals. It has been observed that anomalous changes in amplitude occur in the PTD signal level near the transition temperature. The experimental details and the results of measurements made in liquid crystals E8, M21 and M24 are given in this paper.
Girijavallabhan, C P; Radhakrishnan, P; V P N Nampoori; Nibu, A George; George, A K(Journal of Physics D : Applied Physics, October 13, 2000)
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Abstract:
We report on a laser induced photoacoustic study of the nematic-to-isotropic
transition in certain commercial nematic liquid crystal mixtures, namely BL001, BL002,
BL032 and BL035. A simple analysis of the experimental data using the
Rosencwaig–Gersho theory shows that the heat capacities of all these compounds exhibit a
sharp peak as the temperature of the sample is varied across the transition region. Also,
substantial differences in the photoacoustic signal amplitudes in nematic and isotropic phases
have been noticed for all the mixtures. The increased light scattering property of the nematic
phase may be the reason for the enhanced photoacoustic signal amplitude in this phase.