ABSTRACT
Absorption spectra of gold nanoisland thin film and the composite film of gold having thin coating of Methylene Blue and Rh6G dyes have been studied. Thin gold nanoisland film shows surface plasmon resonance (SPR) peak in the visible wavelength range, which shifts to near infrared with an increase in the thickness of the film. It was found that thin film of gold consists of nanoparticles of different size and shape, particularly nanorods of noncylindrical shapes. A linear relation was found between SPR peak wavelength and the aspect ratio of the nanoparticles in gold thin film. Effective medium refractive index of the gold film is estimated to be ~2.5, which decreases with an increase in film thickness. The coating of dyes on gold films splits the SPR peak with an enhanced absorption. Enhancement in absorption of composite film is maximal when the dye absorption peak coincides with the SPR peak; otherwise enhancement in transmission is observed for all the wavelength range. Absorption amplitude of composite film peaks increase with an increase in the gold film thickness, which tend toward saturation for film thickness of ≥6 nm. A correlation shows that absorption spectra can be described by the Maxwell Garnett theory, when the gold nanoparticles have a nearly spherical shape for very thin film (≤6 nm).
ABSTRACT
A study of aqueous solutions of chromium using single and double pulse laser-induced breakdown spectroscopy (LIBS) is presented. Three atomic emission lines show enhancement in emission under dual pulse LIBS excitation. The temporal evolution of line emission indicates that a shock wave front produced by the first laser pulse plays an important role in determining the decay rate of intensity by excitation transfer in single pulse LIBS and by plasma confinement in double pulse LIBS. The ratio of emission in dual pulse LIBS to single pulse LIBS with time shows a linear increase followed by the onset of saturation. A theoretical calculation of the enhancement is found to be in qualitative agreement with the experimental results, suggesting that material ablation in dual pulse LIBS should be > or = 3.5 times that of single pulse LIBS. There is indication that the increase in ablation and subsequent enhancement in emission may be due to the rarefied gas density inside the region enclosed by the shock wave produced by the first laser pulse. The limit of detection of Cr in aqueous solution has been improved by an order of magnitude with double pulse LIBS.
ABSTRACT
We present a simple theoretical model for the emission from double pulse laser-induced plasmas that was developed to better understand the processes and factors involved in enhancement of plasma emission. In this model, the plasma emission is directly proportional to the square of plasma density, its volume, and the fraction of second laser pulse absorbed through inverse bremsstrahlung absorption by the plasma plume of the first laser pulse. The electron-ion collision frequency determines the profile and location of the peak of emission enhancement with respect to the delay between the two lasers, whereas the amplitude of the enhancement is mainly dependent on the increase in the mass ablation rate after the second laser pulse. The effects of increase in temperature and in plasma volume after the second laser pulse are also discussed in light of this model.
ABSTRACT
Effects of a steady magnetic field on the laser-induced breakdown spectroscopy of certain elements (Mn, Mg, Cr, and Ti) in aqueous solution were studied, in which the plasma plume expanded across an external steady magnetic field (approximately 6 kilogauss). Nearly 1.6 times enhancement in the line emission intensity was observed in the presence of the magnetic field. The temporal evolution of the line emission showed a significant enhancement in plasma emission between 2- and 7- micro(s) gate delays for Mg in the presence of the magnetic field (5-30 micro(s) for Mn). This enhancement in the emission is attributed to an increase in the rate of recombination because of an increase in plasma density due to a magnetic confinement after cooling the plasma. The increase in the optical line emission due to magnetic confinement was absent when the plasma was hot with a dominant background (continuum) emission. The limits of detection of Mg and Mn were reduced by a factor of two in the presence of a steady magnetic field of 5 kilogauss.
ABSTRACT
The optical properties of laser-induced plasma generated firm solid (Al alloy) and liquid (Mn, Cr, Mg, or Ti solutions) samples expanded across an external, steady magnetic field have been studied by atomic-emission spectroscopy. Various line emissions obtained from the constituents of the Al alloy and of the aqueous solution show an enhancement in intensity in the presence of an approximately 5-kG magnetic field. The enhancement of the signal was nearly a factor of 2 for the minor constituents of the solid samples and a factor of 1.5 for the elements in liquid phase. Temporal evolution of the emission from the solid sample showed maximum enhancement in emission intensity at 3-10-micros time delay after plasma formation in the laser energy range 10-50 mJ. However, for the liquid sample the maximum signal was for a gate delay of 3-25 micros the energy range 50-200 mJ. This enhancement in the emission intensity was found to be due to an increase in effective density of the plasma as a result of magnetic confinement when the plasma cooled after expansion. This enhanced emission was due to an increase in the rate of radiative recombination in the plasma.
ABSTRACT
The application of laser-induced breakdown spectroscopy to liquid samples, by use of a Nd:YAG laser in double-pulse excitation mode, is described. It is found that the line emission from a magnesium ion or atom is more than six times greater for double-pulse excitation than for single-pulse excitation. The effect of interpulse separation on the emission intensity of a magnesium ion and a neutral atom showed an optimum enhancement at a delay of 2.5-3 micros. The intensity of neutral atomic line emission dominates the ion emission from the plasma for higher interpulse (>10 micros) separation. A study of the temporal evolution of the line emission from the plasma shows that the background as well as line emission decays faster in double-pulse excitation than in single-pulse excitation. The enhancement in the emission seems to be dominated by an increase in the volume of the emitting gas. The limit of detection for a magnesium solution improved from 230 parts per billion (ppb) in single-pulse mode to 69 ppb in double-pulse mode.
