ABSTRACT
Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 µs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.
ABSTRACT
Positive and negative third-order optical nonlinearities have been investigated in single-stranded DNA wrapped semiconducting single-walled carbon nanotubes. It is found that the redox reactions of hydrogen peroxide can reverse the sign of the third-order nonlinearity. The observation proves that the lowest unoccupied molecular orbital has a lower density of electronic states than that of the highest occupied molecular orbital. A three-energy-level model is used to explain the effect of the redox reactions. Raman spectroscopy has also been used to investigate the interaction between single-walled carbon nanotubes and single-stranded DNA.
ABSTRACT
The fast boiling dynamics of superheated surface layers of bulk water cavitating under near-spinodal conditions during nanosecond CO2 laser heating pulses was studied using contact broad-band photoacoustic spectroscopy. Characteristic pressure-tension cycles recorded by an acoustic transducer at different incident laser fluences represent (a) weak random oscillations of transient nanometer-sized near-critical bubbles-precursors and (b) well-defined stimulated oscillations of micron-sized supercritical bubbles and their submicrosecond coalescence products. These findings provide an important insight into basic thermodynamic parameters, spatial and temporal scales of bubble nucleation during explosive liquid/vapor transformations in absorbing liquids ablated by short laser pulses in the thermal confinement regime.
ABSTRACT
For the first time to the best of our knowledge, a simultaneous 10.6 mum CO(2) laser pulse has been used to enhance the Laser Induced Breakdown Spectroscopy (LIBS) emission from a 1.064 mum Nd:YAG laser induced plasma on a hard target. The enhancement factor was on the order of 25 to 300 times, depending upon the emission lines observed. For an alumina ceramic substrate the Al emission lines at 308 nm and Fe impurity line at 278 nm showed an increase of 60x and 119x, respectively. The output energy of the Nd:YAG laser was 50 mJ/pulse focused to a 1 mm diameter spot to produce breakdown. The CO(2) laser pulse had a similar energy density of 40 mJ/mm(2). Timing overlap of the two laser pulses within 1 microsecond was important for enhancement to be observed. An observed feature was the differential enhancement between different elemental species and also between different ionization states, which may be helpful in the application of LIBS for multi-element analysis.
ABSTRACT
Microsecond relaxation dynamics in a cavitating surface layer of bulk water superheated by a TEA CO(2) laser was studied using contact broadband photoacoustic spectroscopy. Damped nanosecond and microsecond oscillatory pressure-tension cycles recorded by an acoustic transducer are related to oscillations of steam bubbles of different sizes exhibiting strong dissipative losses and collective (coalescence and percolation) phenomena. These measurements also give important insight into basic parameters, characteristic spatial and temporal scales, and the mechanism of laser ablation of absorbing liquids in the thermal confinement regime.