Spectroscopic observation of the plasma produced by a CO2 laser beam interacting with titanium target under helium and/or argon atmosphere

In many laser applications such as drilling, welding and cutting, the role of the plasma in the transfer of energy between the laser beam and the metal surface appears to be rather important. It depends on several parameters such as laser wavelength, irradiation time and deposited energy but especially on the buffer gas nature. In this work the plasma is initiated by a TEA-CO₂ laser beam perpendicularly focussed onto a Ti target (100 MW/cm²), in a cell containing He, Ar or a He-Ar mixture as buffer gas. The plasma is studied by time and space resolved spectroscopic diagnostics. The results show that helium allows target erosion whereas a highly absorbing breakdown plasma develops in argon shielding the target from the subsequent laser heating. With only 20% Ar in He, a strong quenching of the He plasma by Ar occurs, and the Ar plasma effect is dominant.     

Shock-wave velocity of a femtosecond-laser-produced plasma

Fluorescence measurements have been used to characterize the velocity of atoms in a femtosecond-laser-produced plasma. Nanogram amounts of a copper sample were ablated by the focused radiation (λ=775 nm) of an all-solid-state laser. The laser was operated at a pulse rate of 10 Hz with an energy of 200μJ per pulse. The microplasma expanded into a defined argon atmosphere of pressures between 0.02 and 850 mbar. Atomic fluorescence was excited in the laser plume by a dye-laser pulse with the wavelength set to the line Cu I 282.4 nm. The narrowed beam of the dye-laser was directed into the plasma at different heights above the sample surface. The fluorescence radiation was measured with an échelle-spectrometer, equipped with an intensified-charge-coupled device as the detector. The velocity depends strongly on the pressure of the ambient atmosphere and the distance from the sample surface. The highest velocity found at an argon pressure of 0.02 mbar was 1.0×10⁶ cm s⁻¹.     

Collective behavior of laser-produced plasma from a multicomponent YBa2Cu3O7 target in air

2 Cu₃O₇, using a Q-switched Nd:YAG laser is investigated by time-resolved emission-spectroscopic techniques at various laser irradiances. It is observed that beyond a laser irradiance of 2.6×10¹¹ W cm^-2, the ejected plume collectively drifts away from the target with a sharp increase in velocity to 1.25×10⁶ cm s^-1, which is twice its velocity observed at lower laser irradiances. This sudden drift apparently occurs as a result of the formation of a charged double layer at the external plume boundary. This diffusion is collective, that is, the electrons and ions inside the plume diffuse together simultaneously and hence it is similar to the ambipolar diffusion of charged particles in a discharge plasma. Received: 30 January 1998/Revised version: 12 June 1998     

In uence of ambient conditions on the laser air spark

Received: 17 June 1998/Revised version: 17 September 1998     

Particle velocity, electron temperature, and density profiles of pulsed laser-induced plasmas in air at different ambient pressures

Received: 18 August 1997/Revised version: 9 March 1998     

Detailed Characteristics of Expansion Velocity of Si from Laser Ablated SiC

Optical emission of plasma is used to investigate the characteristics of dynamics distribution in the plume gen- erated by ablation of a SiC sample using Nd：YAG laser. The plume expansion dynamics is characterized by time-of-flight measurement. We find that the profiles of Si （I） （390.55 nm） split into two components and the Si （1I） （634. 71 nm） spectra show two distinct expansion dynamics regions. The time-of-flight measurement of Si（ll） （634. 71 nm） under different laser irradianee conditions, from 0.236 G W/cm^2 to 1.667 G W/cm^2, are presented and discussed.     

Laser-Induced Particle Jet and Its Ignition Application in Premixed Combustible Gases

A hot particle jet is induced as a laser pulse from a free oscillated Nd：YAG laser focused on a coal target. The particle jet successfully initiates combustion in a premixed combustible gas consisting of hydrogen, oxygen, and air. The experiment reveals that the ionization of the particle jet is enhanced during the laser pulse. This characteristic is attributed to the electron cascade process and the ionization of the particles or molecules of the target. The initial free electrons, which are ablated from the coal target, are accelerated by the laser pulse through the inverse Bremsstrahfung process and then collide with the neutrals in the jet, causing the latter to be ionized.     

Mechanisms of absorption in pulsed excimer laser-induced plasma

2 . Received: 20 January 1997/Accepted: 23 June 1997     

Spatial distributions of electron density in microplasmas produced by laser ablation of solids

The spatial distribution of electron density in argon microplasmas produced by laser ablation of solids has been investigated by time-resolved emission spectroscopy. The electron density was derived from Stark broadening and shift of spectral lines. It was found that the radial gradient of the electron density is much smaller than the gradient of the atomic number density of atoms ablated by the laser into the plasma. The almost homogeneous plasma conditions in the centre of the microplasmas are essential for quantitative element analysis of solid samples by laser ablation. On the other hand, because of the homogeneous conditions microplasmas are excellent sources for measurements of reliable Stark broadening and shift parameters of atomic and ionic spectral lines of all elements which can be ablated by lasers from solid samples.     

Generation of magnetic fields by the non-stationary ponderomotive force of electromagnetic waves in plasmas with streaming electrons

It is shown that the non-stationary ponderomotive force of large amplitude electromagnetic waves in plasmas with streaming electrons can spontaneously create magnetic fields. The present result may account for the magnetic fields in laser-produces plasmas, in cosmic plasmas, as well as in galactic and inter-galactic spaces.     

Optical Emission Spectroscopy Investigation of a Surface Dielectric Barrier Discharge Plasma Aerodynamic Actuator

The optical emission spectroscopy of a surface dielectric barrier discharge plasma aerodynamic actuator is investigated with different electrode configurations, applied voltages and driving frequencies. The rotational temperature of N2 （C^3 Ⅱu） molecule is calculated according to its rotational emission band near 380.5 nm. The average electron energy of the discharge is evaluated by emission intensity ratio of first negative system to second positive system of N2. The rotational temperature is sensitive to the inner space of an electrode pair. The average electron energy shows insensitivity to the applied voltage, the driving frequency and the electrode configuration.     

Early stage expansion and time-resolved spectral emission of laser-induced plasma from polymer

In the nanosecond laser ablation regime, absorption of laser energy by the plasma during its early stage expansion critically influences the properties of the plasma and thus its interaction with ambient air. These influences can significantly alter spectral emission of the plasma. For organic samples especially, recombination of the plasma with the ambient air leads to interfering emissions with respect to emissions due to native species evaporated from the sample. Distinguishing interfering emissions due to ambient air represents a critical issue for the application of laser-induced breakdown spectroscopy (LIBS) to the analysis of organic materials. In this paper, we report observations of early stage expansion and interaction with ambient air of the plasma induced on a typical organic sample (nylon) using time-resolved shadowgraph. We compare, in the nanosecond ablation regime, plasmas induced by infrared (IR) laser pulses (1064 nm) and ultraviolet (UV) laser pulses (266 nm). Nanosecond ablation is compared with femtosecond ablation where the post-ablation interaction is absent. Subsequent to the early stage expansion, we observe for each studied ablation regime, spectral emission from CN, a typical radical for organic and biological samples. Time-resolved LIBS allows identifying emissions from native molecular species and those due to recombination with ambient air through their different time evolution behaviors.     

Drilling enhancement by nanosecond-nanosecond collinear dual-pulse laser ablation of titanium in vacuum

Laser ablation of titanium in vacuum was performed using single- and dual-pulse regime in order to study crater formation. Crater profiles were analyzed by optical microscopy. It was found that the repetition-rate plays an important role in a process of laser ablation. The drilling is most effective for the highest repetition-rate. For the same total number of laser pulses clear drilling enhancement was achieved by dual-pulse regime of ablation in comparison to single-pulse regime. The strongest ablation rate in dual-pulse regime was achieved for the delay time between the pulses τ = 370 ns. Results are discussed in terms of decreased ablation threshold due to continuous heating of the target during the experiment.     

Surface modification of the nanoparticles by an atmospheric room-temperature plasma fluidized bed

Using hexamethyldisiloxane (HMDSO) monomer, the magnetic nanoparticles (NPs) of nickel oxide (NiO) were modified by using an atmospheric room-temperature plasma fluidized bed (ARPFB). The plasma gas temperature of the ARPFB was not higher than 325 K, which was favorable for organic polymerization. The plasma optical emission spectrum (OES) of the gas mixture consisting of argon (Ar) and HMDSO was recorded by a UV-visible monochromator. The as-treated NPs were characterized by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results show that the assembling NPs were isolated greatly after modified by the organosilicon polymer. Moreover, this treatment process changed the wettability of the NPs from super-hydrophilicity to super-hydrophobicity, and the contact angle (CA) of water on the modified NPs surface exceeded 150°. Therefore, the ARPFB is a prospective technology for the NPs surface modification according to the different requirements.     

Electron temperature measurement in a surface-wave-produced argon plasma at intermediate pressures

The main objective of this work is to obtain the electron temperature in an argon surface-wave-produced plasma column at intermediate gas pressures. After proving that argon upper excited states remain in Excitation Saturation Balance, the value of electron temperature along the plasma column has been obtained using a modified Saha equation and a corrected Boltzmann-plot. Moreover, the electron energy distribution function has been verified to be nearly Maxwellian in a 0.8-2.8 torr intermediate pressure range. Received 24 July 2000 and Received in final form 19 January 2001     

Effects of transverse magnetic field on a laser-produced Zn plasma plume and ZnO films grown by pulsed laser deposition

By adopting a fast photography and time-resolved optical emission spectrometry, we have investigated the effects of transverse magnetic field on the expansion dynamics and enrichment of Zn atoms and Zn⁺ ions in a plume produced by laser ablation of a Zn target in oxygen atmosphere. Plume splitting due to the magnetic field was apparent but the splitting patterns of Zn and Zn⁺ were totally different. The surface morphology and photoluminescence characteristics also changed significantly. In particular, the growth rate increased by as much as 2.4-4.3 times compared to the conventional PLD method.     

Investigation of the interaction of subpicosecond KrF laser pulses with a preformed carbon plasma

The interaction of a subpicosecond KrF laser pulse with a preformed carbon plasma of various scale lengths is investigated. Two different interaction geometries are chosen. In the first one the propagation vector of the short pulse has a component along the density gradient of the preformed plasma (angle of incidence is 45°). In the second geometry the propagation direction of the short pulse is perpendicular to the density gradient of the preplasma (angle of incidence is 90°). The emitted soft X-ray spectrum in the wavelength interval from 10 to 700 is observed while changing several parameters of the experiment. It is found that the emission in the short wavelength part under 200 results from the radiation of ions created by collisional heating near the critical density region. The long wavelength part above 200 , enhanced up to a maximal factor of 20, is mainly produced by radiating particles field—ionized up to the He-like carbon state in the high-intensity laser field. The short wavelength part is missing in the case of 90° angle of incidence because there is no interaction with the critical layer that results in an insufficient collisional heating.     

Effects of a prepulse on laser-induced EUV radiation conversion efficiency

The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation, resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed the influence of the prepulse on the EUV yield. Received: 25 October 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/947-202, E-mail: duesterer@ioq.uni-jena.de