Andrey Yu. Sebrant

Interaction of flows of laser ablation plasmas

Our studies of laser pulsed deposition of thin films, using 20-ns second-harmonic Nd:YAG laser operated at a few GW/cm2, show that introduction of an obstacle like a grid screen into ablation plume with typical density 1018 cm−3 and temperature 3 eV substantially affects dynamics of plasma expansion and results in dramatic changes of plasma parameters. In this work, we report the effects typical for the later stage of plasma-grid interaction—effects related to the interaction of multiple plasma flows. Model experiments, performed employing method of emission spectroscopy, time-of-flight technique, and plasma imaging with CCD camera, show significant variation of plasma composition and transformation of velocity distribution due to plasmas interaction. Two modes of such transformation have been observed: linear growth of density in the resulting flow, and formation of the additional slow peak in the velocity distribution. The effect is shown to be velocity-resonant.

Methods to control plasma parameters in experiments on pulsed laser deposition of carbon films

Studies of laser pulsed deposition of thin films show that introduction of an obstacle like a grid screen into ablation plume substantially affects dynamics of plasma expansion and results in dramatic changes of plasma parameters. Two regimes of plasma-grid interaction have been studied in detail -- regime of the free-molecular flow and the throttling. The grid screen may break plasma plume into several interacting with each other flows. Some features of plasma-grid interaction (like transformation of velocity distribution function and variation of plasma temperature and density) can be attributed to the interaction of plasma flow with the generated shock wave. Different dimensional effects controlling plasma-grid interaction have been considered. We report the effect of velocity distribution transformation on the stage of interaction between multiple plasma flows behind the screen. Two modes of such transformation have been observed: linear growth of density in the resulting flow, and formation of the additional slow peak in the velocity distribution. We associate these two modes of transformation with two different regimes of plasmas interaction -- with regime of lateral loss reduction, and regime of expansion in the background gas.

In-situ monitoring of carbon dimer formation within ablation plume

Carbon dimer formation during ablation plume expansion and subsequent interaction with a substrate is recognized to affect substantially the process of pulsed laser deposition (PLD) of thin films. In this work we report formation of C2 molecules during laser ablation from graphite targets under conditions typical for PLD. Two types of lasers were used for ablation -- 5 microsecond carbon-dioxide laser operated at 1 J and 10 ns Nd-YAG laser operated at 100 mJ in the second harmonic. We present results of time-resolved (50 ns) emission spectroscopy, plasma imaging and time-of-flight techniques. C2 molecules formation has been studied in a variety of experimental conditions, including ablation plume interaction with target surface, plume interaction with the surface of a substrate and two-plasmas interaction. Particularly no molecules formation was observed at maximum energy and there was an optimum laser pulse energy for dimers formation for short pulses of Nd:YAG laser. The plumes in both cases had shell like structure with high ions inside and neutrals located in the outer layers. Molecules formation occurred to be more efficient in the outer layers of the plume in the case of carbon-dioxide laser.

Interaction of laser-induced quasi-stationary plasma flow with substrates

The effects of interaction of quasi-stationary carbon plasma flow with a substrate have been studied using time-integrated and time-resolved emission spectroscopy and ion time-of-flight technique. Plasma flow with typical electron density of order of 1018 cm-3 and temperature of 5 eV was created with 5 ms pulses of TEA carbon-dioxide laser. Plasma- substrate interaction has been shown to result in significant variation of plasma parameters and plasma composition near the surface of the substrate.

Structure, dynamics and optical properties of multicomponent plasma produced near a surface by pulsed CO2 and excimer lasers

Results of the experimental studies of dynamics and optical properties of laser-induced plasmas are presented. Lasers with microsecond pulse width at (lambda) equals 0.308 and 10.6 micrometers were used. Spatially and temporally resolved plasma emission spectroscopy and laser beam absorption measurements were used to reveal plasma plume structure. It has been shown that in the case of low absorption at the plasma front, a two component plasma is formed with gas plasma temperature above 3 eV and electron number density in both gas and target vapor plasma in the range 1018 - 3 (DOT) 1019 cm-3. Comparison of the experimental data with computer model predictions is presented.