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.
Nuclear pore complexes in the nuclear membrane act as the sole gateway of transport of molecules from the cytoplasm to the nucleus and vice versa. Studies on biomolecular transport through nuclear membranes provide vital data on the nuclear pore complexes. In this work, we use fluorescein isothiocyanate-labeled dextran molecules as a model system and study the passive nuclear import of biomolecules through nuclear pore complexes in digitonin-permeabilized HeLa cells. Experiments are carried out under transient conditions in the time lapse imaging scheme using an in-house constructed confocal laser scanning microscope. Transport rates of dextran molecules having molecular weights of 4-70 kDa corresponding to Stokes radius of 1.4-6 nm are determined. Analyzing the permeability of the nuclear membrane for different sizes the effective pore radius of HeLa cell nuclear membrane is determined to be 5.3 nm, much larger than the value reported earlier using proteins as probe molecules. The range of values reported for the nuclear pore radius suggest that they may not be rigid structures and it is quite probable that the effective pore size of nuclear pore complexes is critically dependent on the probe molecules and on the environmental factors.
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.
Abstract.A silver target kept under partial vacuum conditions was irradiated with focused nanosecond pulses at 1.06 µm from a Nd:YAG laser. The electron emission monitored with a Langmuir probe shows a clear twin-peak distribution. The first peak which is very sharp has only a small delay and it indicates prompt electron emission with energy as much as 60 ± 5 eV. Also the prompt electron emission shows a temporal profile with a width that is same as that for the laser pulse whereas the second peak is broader, covers several microseconds, and represents the low-energy electrons (2 ± 0.5 eV) associated with the laser-induced silver plasma as revealed by time-of-flight measurements. It has been found that prompt electrons ejected from the target collisionally excite and ionize ambient gas molecules. Clearly resolved rotational structure is observed in the emission spectra of ambient nitrogen molecules. Combined with time-resolved spectroscopy, the prompt electrons can be used as excitation sources for various collisional excitation-relaxation experiments. The electron density corresponding to the first peak is estimated to be of the order of 10 17 cm −3 and it is found that the density increases as a function of distance away from the target. Dependence of probe current on laser intensity shows plasma shielding at high laser intensities. 52.50.J; 52.40.N; 52.75.R; 34.80.D; 39.90 With the invention of the high-power pulsed lasers, the study of laser-matter interactions has assumed many new dimensions among which the phenomenon of laser ablation has a major role both in fundamental studies and in technological applications [1][2][3][4]. The interaction of laser radiation with solids is a very complex process. It includes light absorption and plasma formation in the vicinity of the target [5], thermalisation of the ablation products due to collisions among themselves and with ambient gas molecules [6-9], evolution and propagation of the plume [10-13], gas phase chemical reactions [14,15], and the eventual deposition of the ablative products on suitable substrates situated at a distance from the target [1,2]. In metals, laser absorption occurs within the skin depth because of the exponential attenuation of electromagnetic radiation in a conducting medium. The light is primarily absorbed by electrons present within the skin depth through inverse bremsstrahlung leading to rapid ionization before significant ablation of the solid occurs. The absorbed energy is transferred to the lattice through electron-phonon (e-ph) interactions which usually happens within a few picoseconds after the absorption [16]. The e-ph relaxation time increases with the laser fluence. This increase in e-ph relaxation time is due to the greater difference between electron and lattice temperatures and hence the greater number of collisions that are required for thermalisation. Most of the theoretical and observational models on laser ablation describe the formation of the plasma and its gas dynamic effects. Consequently, the hot-electron emissio...
Abstract. Analysis of the emission bands of the CN molecules in the plasma generated from a graphite target irradiated with 1-06/~m radiation pulses from a Q-switched Nd:YAG laser has been done. Depending on the position of the sampled volume of the plasma plume, the intensity distribution in the emission spectra is found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as function of distance from the target for different time delays with respect to the incidence of the laser pulse. The translational temperature calculated from time of flight is found to be higher than the observed vibrational temperature for CN molecules and the reason for this is explained.
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