Damage and molecular changes under a laser beam in SEM‐EDX/MRS interface: a case study on iron‐rich particles

The control of damage to individual environmental particles by a laser beam during Raman spectroscopy carried out in ambient air is generally well understood. The nature and control of damage under vacuum conditions (e.g. in the scanning electron microscopy with energy X‐ray detection combined with micro‐Raman spectroscopy—interfaced SEM‐EDX/MRS) are more complex and less well comprehended. The physical and chemical processes that affect the damage caused to small particles by lasers still remain somewhat unclear, but certainly the atmosphere (vacuum/air) and the beam intensity have very significant influences. Furthermore, it has been determined that some particles (e.g. haematite), although stable under an electron beam, are damaged by the laser beam, hampering their analysis. Additionally, when simultaneous analyses by SEM/EDX and MRS are considered, the correct choice of the collection surface plays a crucial role. As a result, the following collection substrates were tested to determine their influence on the laser beam damage process to the particle: silver and aluminium foils and silicon wafers. A test study was performed using artificial examples of haematite (Fe₂O₃) particles. Exposure of Fe₂O₃ particles in vacuum to 514‐ and 785‐nm laser radiation often leads to their melting, transformation and evaporation. The dependence of the damage caused by the laser beam on the particle structure is reported here. Molecular and crystallographic changes have also been revealed. Formation of magnetite (as an effect of re‐crystallisation) and Raman inactive structures was detected. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of high-energy electron beam irradiation on the surface morphology of CaF2/Si(100) heterostructures

The effect of high-energy electron beam irradiation on the surface morphology of CaF₂ films in the course of molecular-beam epitaxy on Si(100) substrates is studied by atomic-force microscopy. It is shown that not only do morphological defects significantly change their shape in the electron beam spot, but also their average height more than doubles.     

Spectral and energetic characteristics of passing vibrationally excited SF6 molecules through a cooled multichannel plate

We study the spectral and energetic characteristics of SF₆ molecules excited in a pulsed molecular beam by intense IR laser radiation into high-lying vibrational states (with the energy E _v ≥ 0.3–2.0 eV) that pass through a multichannel metal plate that is cooled to T _s ? 80–85 K and is inclined to the beam axis. From comparison of the measured characteristics with the spectral and energetic characteristics of the IR multiphoton absorption of these molecules, we find that they correlate rather well with each other. The results obtained allow us to conclude that, due to different probabilities of passing vibrationally excited and unexcited molecules through the multichannel plate, it is possible to separate molecules in the beam with respect to their isotopic (component) composition.     

Propagation of the off-axis superposition of partially coherent beams through atmospheric turbulence

The propagation properties of the off-axis superposition of partially coherent beams through atmospheric turbulence and their beam quality in terms of the mean-squared beam width w(z) and the power in the bucket (PIB) are studied in detail, where the effects of partial coherence, off-axis beam superposition and atmospheric turbulence are considered. The analytical expressions for the intensity, the beam width and the PIB are derived, and illustrative examples are given numerically. It is shown that the maximum intensity I_max and the PIB decrease and w(z) increases as the refraction index structure constant C_n² increases. Therefore, the turbulence results in a degradation of the beam quality. However, the resulting partially coherent beam with a smaller value of spatial correlation parameter γ and larger values of separate distance x_d and beam number M is less affected by the turbulence than that with a larger value of γ and smaller values of x_d and M. The main results obtained in this paper are explained physically.     

Time-of-flight measurements in a molecular beam generated from a jet of condensing carbon dioxide

The technique is described and results are presented for time-of-flight (TOF) measurements of the number density of molecules using electron-beam fluorescence diagnostics of a cluster-containing molecular beam extracted from a jet of condensing CO₂. The possibility of using these methods to record the velocity distribution function of the molecules (monomers) in a cluster beam is substantiated. Methods for measuring the velocity of the clusters in a CO₂ cluster beam based on their fragmentation upon impact on a solid surface are proposed and implemented. The domains of applicability of these methods and their shortcomings and merits in comparison with the conventional methods are discussed. The proposed methods are used in measurements of the velocity and temperature of the gas and the velocity of the clusters in a jet of condensing CO₂. Zh. Tekh. Fiz. 68, 120–124 (August 1998)     

单口径相干合成系统激光光束的M2因子研究

单口径相干合成系统激光光束的光束质量是一个亟待解决的重要问题.基于二阶矩定义, 文中给出了单口径TEM₀₀, TEM₀₁及TEM₁₀两两相干光束M²因子的解析表达式, 并比较分析了束腰宽度、传输距离、振幅之比,以及源场位置矢量对相干光束M²因子的影响, 得到了诸如源场位置参量d₁<100λ时,各相干光束M²因子恒定,反之, 其随位置参量d₁的增大而增大等一些结论.最后,文章对两TEM₀₀模相干光束M²因子的 部分理论进行了实验验证.     

Determination of the ten second order intensity moments

The general astigmatic beam can be characterized by its ten second order moments in first order approach. Most intensity moments, except the beam twist, can be determined by measuring the intensity in a reasonable number of positions around the waist of the beam. The beam twist is determined by applying a rotated cylindrical lens. The ten intensity moments of two kinds of astigmatic beam were determined: a simple astigmatic TEM_8,0 Hermite–Gaussian beam and a twisted beam generated from the TEM_8,0 mode. The experimental results were compared with the theoretical calculations and demonstrate that the ten second order moments of a beam can be determined in a rather simple way.     

Beam parameters for end diffraction of slab waveguide

Beam parameters including mode-field half-width, divergence half-angle and beam propagation factor are investigated and their relations are derived. Moreover, the beam parameters of end diffraction-limited beam from TE₀ propagating mode are given and influence of change of the near-field distribution on them is researched too.     

Diffraction properties of opaque disks outside and inside a laser cavity

Diffraction of symmetrical Laguerre-Gauss TEM_p0 beams incident on an opaque disk known as a stop is considered. The near- and far-field patterns are studied. Thanks to zero-field occluding, conversion from TEM₁₀ beam to dark hollow beam can be achieved with better efficiency than from a TEM₀₀ beam. It is shown that the fundamental mode of a laser cavity including a diaphragm and a stop can be TEM₀₀- or TEM₁₀-like in shape depending on their size. This result is interpreted from the new divergence hierarchy, which characterises the diffracted TEM_p0 beams emerging from the stop.     

Control of electrical resistance of TiO2 films by short-pulse laser irradiation

Titanium dioxide (TiO₂) films were irradiated with a femtosecond laser beam to alter their electrical resistances. The TiO₂ film was produced by aerosol beam deposition. The wavelength, pulse duration, and repetition rate of the femtosecond laser scanned across the sample surface were 800 nm, 100 fs, and 1 kHz, respectively. By attenuating the laser fluence on the TiO₂ film, a range was found in which the electrical resistance of the TiO₂ film was varied even though the morphology of the film surface was not changed.     

Electron-beam-induced fluorescence of carbon dioxide clusters. II. Molecular beam with clusters

The fluorescence of CO₂ clusters excited by an electron beam in a molecular beam formed from a free jet expansion of carbon dioxide is investigated. Data on the fluorescence of the clusters are obtained by comparing the electron-beam-induced fluorescence with the signal appearing in an electronic-fluorescence detector as a result of reflection of the cluster beam from an obstacle placed after the electron beam, as well as with the intensity of the cluster beam. It is established that, unlike a jet expansion, for CO₂ clusters in a molecular beam the fluorescence yield from a cluster decreases significantly with increasing cluster size and is very small for clusters of large size. It is concluded on the basis of the results from an investigation of the fluorescence of CO₂ clusters that the electron-beam-induced fluorescence technique can be used for measurements in molecular beams with CO₂ clusters. Zh. Tekh. Fiz. 67, 63–71 (July 1997)     

Combustion of micron-sized particles of titanium and zirconium

Particles of titanium and zirconium in the size range of 2–25 μm are ignited while passing through a CO₂ laser beam and their combustion is monitored optically. Prior to ignition, particles pass through a low-power auxiliary laser beam so that the diameter of each ignited particle is measured in situ based on the amplitude of the scattered light pulse. The particles of both Ti and Zr are observed to exhibit micro explosions, similar to those observed for larger size particles of these metals. Particle emission traces are recorded, and a data processing routine is established for discounting emission signals produced by unignited particles and particles partially combusted within the CO₂ laser beam. Burn times and combustion temperatures are measured and compared to earlier measurements for coarser particles of the same metals. For both metals, average combustion temperatures implied by the emission spectra are very close to their respective adiabatic flame temperatures. For both metals, for the particle size range considered, particle combustion temperatures do not depend on the particle size. The particle burn times were found to be only weak functions of the particle size; burn times for Zr are shorter and temperatures are higher compared to the similarly sized Ti particles.     

Optical study of a pulsed molecular beam

By using absorption spectra in a pulsed molecular beam, the rotational temperature and the flow density of the jet are deduced. By using this technique, a comparison between a pulsed and a continuous beam is also reported for NH₃, CF₂Cl₂, and C₂H₃Cl molecular beams. Moreover, the behaviour of the temperature and density inside the pulsed beam is analyzed as a function of time for pure Ammonia. From these measurements, we deduce that a small improvement is obtained for absorption spectroscopy in the jet by using a pulsed molecular beam.     

Electron-induced interaction of condensed chlorine with Si(100)

We have studied electron-induced interaction of molecular chlorine with Si(100) surface under ultra-high vacuum conditions. Without electron beam irradiation, chlorine was found to be adsorbed both dissociatively (first step) and molecularly (second step) at 100 K. Illumination of molecular chlorine-covered surface at 100K with electron beam (1–3 keV) led to the formation of silicon tetrachloride on the Si(100) surface. We established that the surface reaction, that is responsible for the SiCl₄ formation, is initiated by Cl₂ dissociation and appearance of the active chlorine radicals. Importance of the such effect is also discussed in a view of their possible applications for selective etching of semiconductor surfaces.     

Mechanism of the quasi-zero axial acoustic radiation force experienced by elastic and viscoelastic spheres in the field of a quasi-Gaussian beam and particle tweezing

The present analysis investigates the (axial) acoustic radiation force induced by a quasi-Gaussian beam centered on an elastic and a viscoelastic (polymer-type) sphere in a nonviscous fluid. The quasi-Gaussian beam is an exact solution of the source free Helmholtz wave equation and is characterized by an arbitrary waist w₀ and a diffraction convergence length known as the Rayleigh range z_R. Examples are found where the radiation force unexpectedly approaches closely to zero at some of the elastic sphere’s resonance frequencies for kw₀ ? 1 (where this range is of particular interest in describing strongly focused or divergent beams), which may produce particle immobilization along the axial direction. Moreover, the (quasi)vanishing behavior of the radiation force is found to be correlated with conditions giving extinction of the backscattering by the quasi-Gaussian beam. Furthermore, the mechanism for the quasi-zero force is studied theoretically by analyzing the contributions of the kinetic, potential and momentum flux energy densities and their density functions. It is found that all the components vanish simultaneously at the selected ka values for the nulls. However, for a viscoelastic sphere, acoustic absorption degrades the quasi-zero radiation force.     

大气湍流中光束束宽扩展和角扩展的比较研究

以厄米-双曲余弦-高斯（H-ChG）光束为例,对H-ChG光束通过大气湍流传输时的束宽扩展和角扩展做了详细研究.用相对束宽和相对角扩展代替束宽和角扩展来研究湍流对光束影响的灵敏程度.研究表明,折射率结构常数C²_n越小,光束束宽扩展和角扩展越小.有较大阶数m,n,较小参数Ω₀和束腰宽度w₀ H-ChG光束的角扩展受湍流影响较小.当传输距离足够远时,这一结论对H-ChG光束的束宽扩展也成立.当传输距离不长时,对H-ChG光束相对束宽随Ω₀和w₀的变化规律做了分析.用数值计算例做了说明,并对结果的正确性做了物理解释.厄米-高斯,双曲余弦高斯和高斯光束在大气湍流中的扩展可作为H-ChG光束的特例来处理. 关键词： 束宽扩展和角扩展 大气湍流 厄米-双曲余弦-高斯光束     

Creation of a sharper focus by using a rectified TEMp0 beam

The superresolution technique is usually used in optical imaging for its ability to make the central diffractive spot smaller than the Airy spot. In this paper, we apply the superresolution technique for transforming a symmetrical TEM_p0 Laguerre-Gauss beam into a Gaussian intensity distribution in the plane of a converging lens. The beam shaping is achieved by an annular binary Diffractive Optical Element having a transmittance, alternatively equal to −1 or + 1, modelled on the p light rings of the incident beam. It is observed that the rectified TEM₃₀ beam at focus has a focal volume 170 times smaller than that of a Gaussian beam.     

Optical fiber cleaved at an angle by CO2 laser ablation: Application to micromachining

Laser ablation can be achieved by delivering short power pulse with durations much smaller than the heat diffusion time. In this investigation, we are collimating and magnifying a beam from a CO₂ laser with a Keplerian telescopic system. Then we study the quality of the cut performed by scanning the beam at a fast speed over an optical fiber just after focusing a well collimated CO₂ beam at λ=10.6 μm. It is found that the best results for cutting optical fibers depend upon both the time required in raising matter temperature to the vaporization point and the scanning speed of the CO₂ laser beam. Some aspects of the laser beam collimation before focusing is reviewed briefly and results for optical fibers being cleaved at low and fast speed under various conditions are also shown and discussed.     

Ramsey-Bordé interferometer and embedded Ramsey interferometer with molecular matter waves of 39K2

A matter wave interferometer based on a molecular beam of K₂ has been designed for observation of both exits: with molecules in the electronically excited state and in the ground state. In addition to the excited state fluorescence the molecular ground state population is detected with a further laser. Two transitions to different electronic states were employed for this purpose and their usefulness is compared. Under the present experimental conditions both interferometer exits show a superposition of different interference patterns due to the influence of transverse and longitudinal overlaps of the interfering matter waves. The interference patterns have been analyzed to be composed of a contribution caused by a two beam splitter Ramsey interference and Ramsey-Bordé pattern with four beam splitters. This overlap of interference signals influences the suitability of the matter wave interferometer for phase measurements of the interferences.