Enhancement of the secondary ion emission induced by fast clusters

The emission yields of the secondary ions are measured by using a conventional time of flight (TOF) technique under bombardments of Mg and C₂, Ni and Si₂ with different energies, and C_n, Si_n and Ni_n (n = 1-3) with the different charge states and with energy of 1.5 MeV per atom, respectively. For the bombardments of C_n, Si_n and Ni_n, the enhancements of the secondary ion emissions increase with increasing cluster sizes and charge states. For the bombardments of Mg and C₂, Ni and Si₂, although the mass and the nuclear charges of C₂ and Si₂ are the same as or equivalent to Mg or Ni, respectively, the enhancements of the secondary ion emissions induced by the clusters of C₂ and Si₂ in a wide energy range are also clearly indicated. The instantaneous collective interaction of the cluster constituents plays an important role in the secondary ion emissions.     

Fabrication of Mg-doped ZnO thin films by laser ablation of Zn:Mg target

Mg-doped ZnO thin films were fabricated by laser ablation of Zn:Mg targets consisting of Mg metallic strips and Zn disk in oxygen atmosphere with a goal to facilitate convenient control of Mg contents in the films. The characteristics of the deposited films were examined by analyzing their photoluminescence (PL), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) spectra. Mg contents as analyzed by XPS indicate that the target composition is fairly transferred to the deposited films. The wurtzite structure of ZnO was conserved even for the highly doped ZnO films and there was no Mg- or MgO-related XRD peaks. With increase in the Mg content, the bandgap and PL peak energy shifted to blue and the Stokes shift became larger.     

Detection of neutral products formed during excimer laser ablation of polyimide by UV and VUV laser photoionization/mass spectrometry

Some of the neutral species which are produced in the laser ablation of polyimide have been characterized using multiphoton ionization/time of flight mass spectrometry. Three different wavelengths (193 nm, 157 nm, and 118 nm) have been used in an attempt to effect soft ionization of the products formed during or after the initial laser ablation of the polymer. Neutral photo-ablation products detected using this scheme range from atomic to high molecular weight species which, depending on the probe wavelength, include pure carbon clusters as well as a broad distribution of heteroatom containing clusters. However, there is virtually no overlap in the mass spectra recorded at each probe wavelength. When probing with 193 nm, marked changes are observed in the mass spectra as a function of the probe flux used. At moderate fluxes, pure carbon clusters (fullerenes) are observed. The identification of a large distribution of species other than pure carbon clusters is in dramatic contrast to the recent observation [W.R. Creasy, J.T. Brenna: Chem. Phys. 126, 453 (1988)] of the positively charged ionic species produced, which are solely carbon clusters. These results suggest that the neutral and ionic products observed after ablation of the polymer are due to both condensation of the atomic and molecular fragments which form during the ablation laser pulse and nascent polymer fragments. Various implications of this result for the unambiguous determination of the true ablation product distribution are discussed.     

Thermal model for nanosecond laser sputtering at high fluences

The vaporization effect and the following plasma shielding generated by high-power nanosecond pulsed laser ablation are studied in detail based on the heat flux equation. As an example of Si target, we obtain the time evolution of the calculated surface temperature, ablation rate and ablation depth by solving the heat flow equations using a finite difference method. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the variation of ablation depth per pulse with laser fluence is performed. Our numerical results are more agreed with the experiment datum than other simulated results. The result shows that the plasma shielding is very important.     

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.     

Vaporization and Plasma Shielding during High Power Nanosecond Laser Ablation of Silicon and Nickel

A thermal model to describe the high-power nanosecond pulsed laser ablation is presented. It involves the vaporization and the following plasma shielding effect on the whole ablation process. As an example of Si target, we obtainthe time evolution of the calculated surface temperature, ablation rate and ablation depth. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the ablation depth with laser fluence based on different models is shown. Moreover, we simulate the pulsed laser irradiation Ni target. The evolution of the transmitted intensity and the variation of ablation depth per pulse with laser fluence are performed. Under the same experimental conditions, the numerical results calculated with our thermal model are more in agreement with the experimental data.     

Charged particle emission,photon emission,and plasma formation induced by CO2 laser pulsed irradiation of WC-Co hardmetals in vacuum and in air

The thresholds of charged particle emission, photon emission and plasma formation induced by CO₂ laser pulsed irradiation of WC-Co hardmetals were determined. The influence of the surface roughness and chemical composition on the thresholds and on the mechanism of the plasma formation was considered. The first experimental data were obtained which indicate the possible realization of the emissive mechanism of air optical breakdown in the vicinity of the solid surface.     

Analysis of ionic fragments from 308 nm photoablation of polyimide

The ionic products from excimer laser photoablation (=308 nm) of polyimide (Kapton) film have been studied as a function of fluence. Large ion masses up to about 900a.m.u. are easily observed, the mass distribution depending strongly on the fluence. Velocities of the emitted particles lie between 1400 and 10 000 ms^–1, again dependent on the fluence. A mechanism to explain the high velocities is suggested consisting of ionisation of the surface polymer molecules followed by a Coulomb explosion combined with expansion of the high density gas formed by the photoablation.     

Mass spectroscopic studies of the ArF-laser photoablation of polystyrene

We have investigated the photoablation of polystyrene by ArF-laser radiation (193 nm) with respect to the chemical nature and energy distribution of ejected molecular species. A novel analytical technique combining photoionization by vacuum-ultraviolet laser radiation at 118.4 nm with time-of-flight mass spectrometry has been employed. At near threshold fluences above 15 mJ cm^–2 the main neutral photoablation product is the styrene monomer. Using direct time resolved detection the kinetic energy distribution of the product molecules has been determined, it peaks at about 0.7 eV. From the low fragmention-yield at near threshold fluences we can estimate that the mean internal energy of the monomers is less than 2 eV. This and the narrow time-of-flight distribution can be modeled by an adiabatic expansion of the ablated molecules.     

Formation of a knudsen layer in electronically induced desorption

For intense desorption fluxes, particles desorbed by electronic transitions (DIET) from a surface into a vacuum may thermalize in the gas cloud forming above the surface. In immediate vicinity to the surface, however, a non-equilibrium layer (the Knudsen layer) exists which separates the recently desorbed, non-thermal particles from the thermalized gas cloud. We investigate by Monte Carlo computer simulation the time it takes to form a Knudsen layer, and its properties. It is found that a Knudsen layer, and thus also a thermalized gas cloud, is formed after around 200 mean free flight times of the desorbing particles, corresponding to a desorption of 20 monolayers. At the end of the Knudsen layer, the gas density will be higher, and the flow velocity and temperature smaller, than literature values indicate for thermal desorption. These data are of fundamental interest for the modeling of gas-kinetic and gas-dynamic effects in DIET.     

The role of photoelectronic processes in the formation of a fluorescent plume by 248-nm laser irradiation of single crystal NaNO3

 Visible fluorescent “plumes” are readily produced when nominally transparent ionic materials are exposed to pulsed UV laser irradiation. Over a wide range of laser fluences where plumes are observed, however, the photon and electron densities are inadequate to support multiphoton ionization and inverse bremsstrahlung, which are often used to explain plasma production and excitation of atomic spectral lines. We present evidence that the great majority of charged particles (electrons and positive ions) comprising the plume at the onset of formation in defect-laden NaNO₃ are emitted directly from the surface. A model is described wherein the required electron energy to excite and eventually ionize neutral atoms is provided by electrostatic interactions in the expanding plume. The time evolution of the “overlap” between the expanding charge cloud and thermally emitted neutrals accounts for the time evolution of the atomic line emissions after the laser pulse. Received: 15 August 1996/Accepted: 16 August 1996     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Deuterium Clusters Fusion Induced by the Intense Femtosecond Laser Pulse

Neutrons （2.45MeV） from deuterium cluster fusion induced by the intense femtosecond （3Ors） laser pulse are experimentally demonstrated. The average neutron yield 103 per shot is obtained. It is found that the yield slightly increases with the increasing laser spot size. No neutron can be observed when the laser intensity I 〈 4.3 × 10^15 W/cm^2.     

Mechanisms of absorption in pulsed excimer laser-induced plasma

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

Dynamics of Na clusters in picosecond laser pulses

We investigate the electronic and ionic dynamics of Na clusters under the influence of a laser pulse in the range 100 femtoseconds to picoseconds. The dynamics is described by means of the time-dependent local-density approximation coupled to ionic molecular dynamics (TDLDA-MD). Variation of pulse length allows us to explore the time scales of ionic motion in a manner similar to pump and probe experiments. Resonant enhancement of electron emission serves as a measure for the time scale of Coulomb explosion. Received: 3 July 2001 / Published online: 10 October 2001     

The effect of volumetric weighting in the interaction of intense laser fields with clusters

Silver clusters embedded in helium nanodroplets are exposed to intense femtosecond laser pulses (10¹³ - 10¹⁶ W/cm²). The signal of highly charged (q≤11) atomic fragments is maximized by delayed plasmon enhanced ionization using stretched laser pulses. Further details with respect to the dynamics of the charging process can be obtained, when the intensity distribution within the laser focus is taken into account. For the first time, the z-scan method is applied to clusters which offers a route to investigate the explicit dependence of the ion signals with respect to the laser intensity. By taking advantage of the volumetric weighting effect ionization thresholds are determined, yielding values well below 10¹⁴ W/cm² for Ag^q+ ions with q≤11.     

Hard X-ray generation from microdroplets in intense laser fields

Microdroplets of 15-μm diameter are subjected to ultra-short laser pulses of intensities up to 10¹⁵Wcm⁻² to produce hot dense plasma. The hot electrons produced in the microdroplet plasma result in efficient generation of hard X-rays in the range 50–150keV at an irradiance as low as 8×10¹⁴Wcm⁻². The X-ray source efficiency is estimated to be about 2 ×10⁻⁷%. A prepulse that is about 11ns ahead of the main pulse strongly influences the droplet plasma and the resulting X-ray emission. For a similar laser prepulse and intensity, no measurable hard X-ray emission is observed when the laser is focused on a solid target of similar composition and this indicates that liquid droplet targets are best suited for hard X-ray generation in laser–plasma interactions.     

Near-damage threshold femtosecond laser irradiation of dielectric surfaces: desorbed ion kinetics and defect dynamics

We study the modification of fluoride single crystals after irradiation with femtosecond laser pulses for a range of incident intensities from well below to near damage threshold. The behavior of the desorbed positive ion yields, as analyzed by time-of-flight mass spectrometry, is corroborated with temporal characteristics of radiation induced defects in fluorides. The ion yield evolution upon repetitive irradiation (incubation) exhibits the typical reduction of the multi-shot damage threshold with increasing number of pulses. The experimental data point towards an exponential growth of the transient defect density as the origin of this effect. On the other hand, measurements of the time decay of transient defect fluorescence inside the transparent sample show that the defect lifetime may be even longer than tens of milliseconds. To account for the incubation and the increase of the radiation-target coupling efficiency, a model relating the defect lifetime to a pulse-by-pulse accumulation of transient defects is presented, based on a calculation of the free electron density.     

Laser-induced desorption of Na-dimers

We find that Na-dimers are desorbed in a thermal process if rough Na surfaces are irradiated with pulsed laser light of λ=532 nm. In contrast, for light of λ=355 nm, Na₂ can be detached in a non-thermal reaction at low laser fluences. This is concluded from the kinetic energy distributions of the dimers determined by time-of-flight measurements using a second laser at λ=248 nm for photoionization. The transition from non-thermal to thermal desorption at large fluences of the laser light can also be identified. Received: 23 July 1996 / Accepted: 26 August 1996