Ultrafast dynamic ellipsometry, a technique that probes a sample with chirped laser pulses at two angles and with two
orthogonal polarizations, was used to measure the effective refractive index across the ablation region of a Si(111) wafer
exposed to a 100 fs ablation pulse. The resulting refractive index data show a significant increase in the extinction
coefficient, indicative of the melting of silicon.
Ellipsometry is a very useful optical technique to probe the complex index of refraction of a material. We perform
dynamic ellipsometry using ultrafast lasers to probe the complex index dynamics during passage of compressional
shock waves through materials of interest. When used to measure passage of a shock wave through dielectric materials,
dynamic ellipsometry provides a direct measure of the equation of state (except temperature). In addition, the changes in
complex index can be used to measure phase transformations and their kinetics. Using two CCD cameras and two
Wollaston prisms, ellipsometric data at two incidence angles and two polarizations can be obtained simultaneously.
Adding a spectrograph in front of each CCD camera and using chirped probe pulses and frequency domain
interferometry provides a few hundred ps of ellipsometric data from a single shock event.
Shaped femtosecond laser driven shock waves fulfill the requisite time resolution required for following shock induced chemical kinetics, and enable very small scale shock physics experimentation. However, comparison to large scale experiments requires detailed characterization of the generated pressure profile. Spectroscopic studies at pressure and temperature are hindered by fluctuations in shock strength as functions of space and time. Representative time resolved broadband infrared absorption data on shocked polyvinylnitrate thin films are presented, followed by current efforts to alleviate spatial shock strength variations with spatial pulse shaping, and calculations of the interferometric phase shifts expected at long times, during the rarefaction.
For the first time, we have measured the momentum coupling coefficient and plasma expansion velocity (specific impulse) in the femtosecond region, over a laser intensity range from ablation threshold to thirty times threshold. These measurements extend the laser pulsewidth three orders of magnitude relative to previous reports. We studied several pure metals and three organic compounds as targets. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used "tuned absorbers" rather than carbon particles for the laser absorption function. The metals ranged from Li to W in atomic weight. We measured time of flight profiles for ions and found dramatic two-temperature distributions for some conditions. Specific
impulse reached record values for this type of measurement and ablation efficiency was near 100%.
We have used two-dimensional ultrafast microscopic interferometry to observe the hydrodynamic motion and the time dependent changes in the optical properties of two metals (6061-T6 aluminum and ASTM 336 1018 steel) illuminated with 130 fs 800 nm laser pulses. We hve also observed the electron dynamics in optically excited aluminum, gallium, and gold at a metal-glass interface using the same technique. The interferometric technique allows construction of the two-dimensional spatial profile for laser pumped materials with a temporal resolution of < 300 fs and out-of-plane spatial resolution of 0.5 nm using 130 fs probe pulses. Best fits to the diamond turned aluminum data were obtained by assuming physically motivated functional forms for the expected hydrodynamic motion and the time-dependent complex index of refraction. Extraction of changes in the index of refraction provides evidence for melting in the gold targets. These experiments offer a new path for the observation of phase changes and/or for temperature measurements in shocked or laser excited materials, by allowing a determination of the complex index under dynamic conditions and comparing the measured values to those obtained under static conditions.
Nanosecond-interferometry and shadowgraphy is used to observe the dynamic behavior of the etching process during and after the irradiation pulse. Commercially available polymers exhibit quite often poor laser ablation properties for irradiation wavelengths >=248nm. At these wavelengths the absorption is due to the quite photostable aromatic groups. A photolabile triazene polymer was selected to compare the influence of a photolabile group on the laser ablation process. The photochemical active triazene reveals a strong absorption band at 332 nm and is responsible for the observed high etch rates and the low threshold for 308 nm irradiation. The absorption coefficients at 193 nm and at 308 nm are comparable, allowing to study the influence of the different absorption sites by ns-interferometry and shadowgraphy measures. The etching of the triazene polymer starts and ends with the laser beam. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of the ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than for 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer than for the polyimide.
Flat top shocks generated reproducibly by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of Gaussian spatial mode 120 - 400 fs duration incident laser pulses are reproducibly flattened via surface optical breakdown in dielectric substrates at fluences just about the breakdown threshold. These flat top laser profiles have been used to produce shocks flat to 0.7 nm RMS over a 75 - 100 micrometer diameter.
The Cd(DOT)H2 and Cd(DOT)D2 van der Waals complexes have been synthesized by expanding cadmium vapor and He/H2 mixtures into a supersonic free jet. The `half-collision' process, Cd(5s5p 1P1)(DOT)H2 yields Cd(5s5p 3PJ) + H2, was studied by fixing a probe dye laser pulse (delayed 10 ns) onto one of the Cd(5s6s 3S1 $IMP 5s5p 3PJ) transitions while exciting the Cd(DOT)H2(Cd(DOT)D2) complex with a pump dye laser pulse tuned across frequencies near that of the free Cd(5s5p 1P1 $IMP 5s5s 1S0) atomic transition. When the probe laser was tuned to detect Cd(5s5p 3P2), an action spectrum to the red of the atomic transition was obtained for Cd(DOT)H2, consisting of a broad continuum superimposed upon which was an anharmonic series of vibrational transitions with discernible, blue-shaded rotational structure. A similar spectrum was recorded for Cd(DOT)D2, except that only very broadened blue-shaded rotational structure was observed.
A system for the real-time monitoring of emissions from incinerators must be developed which can address the needs of the DOE community and others involved in mixed waste incineration. These needs are an outgrowth of the ever-increasing waste storage problems and the growing concern of the public, as witnessed by the stricter compliance requirements of federal and state agencies, that the products of incineration are hazardous to their health and injurious to the environment. This paper focuses on the technologies being developed here at Los Alamos and other laboratories which address the detection of a broad spectrum of toxic and hazardous chemicals.
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