共查询到20条相似文献,搜索用时 31 毫秒
1.
P. Feiden J. Leygnier Ph. Cahuzac C. Bréchignac 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):49-52
Condensation of nano-droplets in a supersaturating vapor decomposes in two steps: the formation of a nucleation center, also
called critical nuclei or nucleation seed, and the growth sequence, by accretion of further atoms on the nucleation center.
These two steps have been investigated separately through the clustering of homogeneous particles Nan and heterogeneous particles NanX in a helium buffer gas (X = (Na2O)2 or (NaOH)2). The growth sequence is analyzed with preformed molecules X injected in a supersaturating sodium vapor and driving production
of NanX clusters.
Cluster distribution mean sizes are controlled by sodium concentration and by the condensation cell effective length. The
signal intensities observed for homogeneous and heterogeneous clusters are proportional to the homogeneous and heterogeneous
nucleation center numbers respectively. We can measure the efficiency for the homogeneous nucleation center production versus
sodium concentration. This process is the onset of the condensation phase transition. 相似文献
2.
We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C60. The observation indicates that the carbon fragments produced just after laser ablation of the C60 molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of CnH5+ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA. 相似文献
3.
Nanosecond pulsed laser ablation of silicon in liquids 总被引:2,自引:0,他引:2
R. Karimzadeh J. Zamir Anvari N. Mansour 《Applied Physics A: Materials Science & Processing》2009,94(4):949-955
Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This
paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied
laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at
532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface
morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology
shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond
pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences
and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid
by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface
modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or
shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective
lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis
is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the
water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used
to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the
incubation effect is also found to be absent. 相似文献
4.
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. 相似文献
5.
The theory of homogeneous nucleation of liquid drops in supersaturated vapours is reviewed. A new kinetic treatment which accounts for the heating of the growing clusters due to the latent heat of condensation is presented, and the irreversible thermodynamics of non-isothermal nucleation discussed. It is found that growing clusters are generally colder than the surrounding vapour during their sub-critical growth period. Time dependent nucleation is discussed and a simple estimate for the time-lag in establishing the steady state is given. The nucleation in cloud chambers expanding with constant speed is discussed in detail, and the number of droplets formed and their final mean size is calculated as a function of the terminal expansion ratio. Re-evaporation of nucleated clusters is discussed. Numerical results are given for a number of typical situations in experiments on water-vapour. These results for the nucleation rate as corrected by Lothe and Pound predict much lower ‘critical’ expansion ratios than indicated by the limited experimental data available, even when the non-isothermal theory is used. To restore agreement between theory and experiments the surface tension for small droplets (?100 molecules) could be increased some 15% above that of the bulk liquid, or the pre-exponential in the nucleation rate could be decreased by some 10?15. This latter expedient roughly corresponds to use of the ‘classical’ instead of the Lothe-Pound nucleation rate. These two alterations would, however, affect the theoretical curves in different manners and careful experiments could possibly distinguish between them. 相似文献
6.
S. Canulescu E. Papadopoulou D. Anglos T. Lippert M. J. Montenegro S. Georgiou M. D?beli A. Wokaun 《Applied Physics A: Materials Science & Processing》2011,105(1):167-176
Thin films of La0.6Ca0.4CoO3 were grown by pulsed laser ablation with nanosecond and femtosecond pulses. The films deposited with femtosecond pulses (248 nm,
500 fs pulse duration) exhibit a higher surface roughness and deficiency in the cobalt content compared to the films deposited
with nanosecond pulses (248 nm, 20 ns pulse duration). The origin of these pronounced differences between the films grown
by ns and fs ablation has been studied in detail by time-resolved optical emission spectroscopy and imaging. The plumes generated
by nanosecond and femtosecond ablation were analyzed in vacuum and in a background pressure of 60 Pa of oxygen. The ns-induced
plume in vacuum exhibits a spherical shape, while for femtosecond ablation the plume is more elongated along the expansion
direction, but with similar velocities for ns and fs laser ablation. In the case of ablation in the background gas similar
velocities of the plume species are observed for fs and ns laser ablation. The different film compositions are therefore not
related to different kinetic energies and different distributions of various species in the plasma plume which has been identified
as the origin of the deficiency of species for other materials. 相似文献
7.
Alexey A. Morozov Anton B. Evtushenko Alexander V. Bulgakov 《Applied Physics A: Materials Science & Processing》2013,110(3):691-696
An analytical model for time-of-flight (TOF) distributions of particles produced by pulsed laser ablation in vacuum has been proposed. The model takes into account the hydrodynamic expansion stage of the ablation plume and is based on a ‘sudden freeze’ model developed previously for steady-state supersonic jets. It is assumed that a continuum-like expansion of the plume takes place until a freezing time moment t free (or, alternatively, until a ‘freezing distance’ x free) whereupon the collisionless expansion begins. The proposed model is applied for analysis of experimental data on graphite ablation with nanosecond laser pulses. For verification, the analytical distributions are compared with calculated results obtained using a hybrid model combining a thermal model of laser-induced material heating with calculations of the plume dynamics by the direct simulation Monte Carlo (DSMC) method. It is shown that the proposed model can accurately estimate the surface temperature for conditions when the common approach fails. 相似文献
8.
S. Canulescu J. Schou S. F. Nielsen 《Applied Physics A: Materials Science & Processing》2011,104(3):775-780
Thin films of fullerenes (C60) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from
a frozen homogeneous dilute solution of C60 in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm2 up to 3.9 J/cm2. MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid
C60. The fragmentation of C60 fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C60 was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can
be produced with laser fluences ranging from 0.15 J/cm2 up to 1.5 J/cm2. 相似文献
9.
F.A. Fernández-Lima C.R. Ponciano E. Pedrero E.F. da Silveira 《Applied Surface Science》2006,252(23):8171-8177
Experimental results of laser sputtering of cesium and rubidium iodide secondary ions are presented. A TOF mass spectrometer, operating in linear mode, continuous extraction for positive or negative ions, was used for the analysis of (CsI)nCs+, (CsI)nI−, (RbI)nRb+ and (RbI)nI− ion emission as a function of the laser irradiance. Experimental data show that the cluster ion emission yields decrease exponentially with n, for all the laser irradiances applied. Theoretical analysis of the clusters structure was performed using density functional theory at the B3LYP/LACV3P level, for the positive and negative cluster series. A quasi-equilibrium evolution of the clusters is proposed to extract a parameter characteristic of the cluster recombination process: the effective temperature. The hypothesis of the atomic species’ recombination (during the expansion of a high density highly ionized cloud) leading to cluster formation is confirmed to some extent in a second set of experiments: the UV laser ablation of a mixed and non-mixed cesium iodide and potassium bromide targets. These experiments show that the emission yields contain contributions from both the recombination process and from the sample stoichiometry, even for high laser irradiances. 相似文献
10.
Nucleation of diamond on silicon wafers using C60 in the hot filament chemical vapour deposition system
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Scanning electron microscopy and Raman shifts were used to study the process of diamond nucleation and growth using C60 in the hot filament chemical vapour deposition (HFCVD) system.The process of nucleation and growth of diamond films on silicon wafer using C60 as intermediate layer in HFCVD system is described.In order to increase the density of diamond nuclei on the wafers,it is not necessary to use negative bias.The UV-light pre-treatment is not beneficial for improving the diamond nucleation.The multi-layers of C60 molecules,but not a monolayer,can increase the density of diamond nuclei in the presence of H atoms. 相似文献
11.
We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (Mw). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer Mw. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer Mw. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced. 相似文献
12.
The growth of epitaxial Nd:Gd3Ga5O12 (GGG) on Y3Al5O12 (YAG) by femtosecond pulsed laser deposition is reported. We have used a Ti:sapphire laser at a wavelength of 800 nm and pulse length of 130 fs, operating at a repetition rate of 1 kHz. The film properties have been studied systematically as a function of the deposition parameters of laser fluence, spot-size, oxygen pressure, target-substrate distance and temperature. Scanning electron microscopy, atomic force microscopy and X-ray diffractometry were used to characterise the surface structure and crystallinity of the films. X-ray diffraction analysis shows that epitaxial growth has occurred. A comparison between the ion velocities produced by nanosecond and femtosecond laser ablation of the GGG target material has been investigated by the Langmuir probe technique. The results indicate a large difference in the plasma characteristics between femtosecond and nanosecond ablation, with ion velocities up to eight times faster observed in the femtosecond case. 相似文献
13.
D. Ashkenasi H. Varel A. Rosenfeld F. Noack E. E. B. Campbell 《Applied Physics A: Materials Science & Processing》1996,63(2):103-107
We have investigated the morphology of CaF2 (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps. 相似文献
14.
W. D. Schneider 《Applied Physics A: Materials Science & Processing》1994,59(5):463-467
The fascination of research with nanometersized objects in contact with a macroscopic surface will be illustrated by two examples: mass-selected supported transition-metal clusters and C60 molecules on metallic single-crystal substrates. The preparation, mass-selection and deposition of the small particles will be described in some detail. The main experimental techniques involved in the characterization of their electronic and structural properties are photoelectron spectroscopy and scanning tunneling methods. For the transition-metal clusters the evolution of the valence band with cluster size reveals a trend to metal formation. When the tip of a Scanning Tunneling Microscope (STM) is placed above individual C60 molecules intense light emission is observed. The diameter of this emission spot is approximately 4 Å. This observation indicates the possibility of an optical spectroscopic analysis on the scale of individual molecules. 相似文献
15.
P.E. Dyer M. Pervolaraki T. Lippert 《Applied Physics A: Materials Science & Processing》2005,80(3):529-536
Experiments and thermal modelling of polyimide ablation using the fundamental 1064-nm emission from a 20-kHz nanosecond diode-pumped solid-state Nd:YVO4 micro-laser are described and compared with findings for the 532-nm doubled output. For exposures restricted to short pulse trains, it is found that micron-scale-size ablation features can be defined with this laser, even though polyimide films have weak absorption at 1064 nm and relatively weak absorption at 532 nm. There is evidence at both wavelengths of an incubation effect, driven by thermal modification of the polymer and, with long-term exposure at 1064 nm, Raman micro-spectroscopy reveals a progressive growth of predominantly amorphous carbon in the ablation site. Calculations of the temperature rise produced in the polymer by exposure to a high-repetition-rate pulse train are described that aid an understanding of the thermal aspects of the interaction at the two wavelengths. PACS 42.55.Xi; 44.10.+i; 52.38.Mf 相似文献
16.
L. Torrisi F. Caridi D. Margarone A. Picciotto A. Mangione J.J. Beltrano 《Applied Surface Science》2006,252(18):6383-6389
A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to Cn neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm2, produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates. 相似文献
17.
A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced.
Selected experiments including LaMnO3 and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion
parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size
of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy. 相似文献
18.
纳秒激光与团簇相互作用产生高价离子逐渐成为分子物理界的热点之一, 为了深入研究团簇电离的本质, 本文以分子结构相似、元素组成相同的苯、环己烯和环己烷的分子团簇为对象, 利用飞行时间质谱研究了其与5 ns的532 nm激光相互作用时电离产物的价态和强度分布. 结果表明: 这三种化合物多光子电离效率苯>环己烯>环己烷, 但其高价离子的价态和比值苯是最低的, 环己烷的碳离子最高价态为4价, C3+和C2+的比值为1.1; 环己烯电离产物C3+和C2+ 的比值降低为0.6; 苯团簇的最高价态只有3价, C3+和C2+的比值约为0.4. 引起这种现象的原因可以归结于高的多光子电离效率会导致团簇多位点的电离, 引起团簇在电子加热到发生碰撞电离之前发生解离, 减小了团簇的尺寸, 进而减少了离子发生碰撞电离产生高价离子的反应时间, 最终阻碍了高价态离子的产生. 相似文献
19.
20.
Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The
silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water.
Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized
the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal
colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning
electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which
are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS
spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted
compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence
at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of
larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become
unstable and start to coagulate.
Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002
RID="*"
ID="*"This work is part of the thesis of H. M?ltgen
RID="**"
ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de 相似文献