Laser-induced gas vortices

Recently, several femtosecond-laser techniques have demonstrated molecular excitation to high rotational states with a preferred sense of rotation. We consider collisional relaxation in a dense gas of such unidirectionally rotating molecules, and suggest that due to angular momentum conservation, collisions lead to the generation of macroscopic vortex gas flows. This argument is supported using the Direct Simulation Monte?Carlo method, followed by a computational gas-dynamic analysis.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.     

Laser-induced nuclear excitation

An analysis is presented of the Coulomb excitation of low-lying nuclear levels by the electrons produced by strong-field ionization of atoms. It is shown that the resulting short-lived radioactivity can be as high as on the order of 10³ Ci for certain isotopes excited by using modern laser systems. Relativistic effects are demonstrated that substantially increase radioactivity as compared to that predicted by nonrelativistic theory results.     

Laser-induced doping of GaAs

Thin layers of GaAs are heavily doped locally by laser induced Se or Zn diffusion. H₂Se or diethylzinc gases are used to provide Se or Zn dopant atoms. The surface is locally heated with 3 ns light pulses from a Q-switched frequency doubled Nd-YAG laser. The doping process is described in detail. Doping profiles and sheet carrier concentrations are measured as a function of substrate temperature, laser fluence and processing time. Dopant concentrations of more than 10²¹ cm^–3, with a thickness of the doped layer of less than 20 nm can be achieved.     

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     

Laser-induced fluorescence of estrogens

Spectra of the laser-induced fluorescence of estrogens—estradiol, estriol, and estrone—were measured under excitation by the fourth harmonic (λ=266 nm) of a Nd:YAG laser. Quantum yields of the fluorescence were evaluated by the relative technique to be 0.107 for estradiol, 0.116 for estriol, and 0.0052 for estrone. The water solution of tryptophan is used as the standard.     

Laser-induced fluorescence of GdO

A semiempirical calculation of the energy level diagram of GdO based on the ligand field theory is presented here. The main frame of the resulting scheme is very simple and should be useful in order to link together the numerous observed systems of GdO. Pulsed dye laser-induced fluorescence experiments made possible the evaluation of some of the ligand field theory predictions, particularly that a Y⁷Σ⁻ level is lying 1840 cm⁻¹ above the X⁹Σ⁻ ground state. The I and II systems are shown to arise from the B⁹Σ⁻-X⁹Σ⁻ and B₁⁷Σ⁻-Y⁷Σ⁻ transitions, respectively. Continous wave laser-excitation spectra have been recorded and rotational analysis of the A⁹Π₄ → X⁹Σ⁻ transition carried out, confirming previous results of N. N. Dmitriev, L. A. Kaledin, E. A. Shenyavkaya, and L. V. Gurvich (Acta Phys. Hung. 55, 467–479 (1984)).     

Laser-induced gas-surface interactions

Chemical reactions in homogeneous systems activated by laser radiation have been extensively investigated for more than a decade. The applications of lasers to promote gas-surface interactions have just been realized in recent years. The purpose of this paper is to examine the fundamental processes involved in laser-induced gas-surface chemical interactions. Specifically, the photon-enhanced adsorption, adsorbate-adsorbate and adsorbate-solid reactions, product formation and desorption processes are discussed in detail. The dynamic processes involved in photoexcitation of the electronic and vibrational states, the energy transfer and relaxation in competition with chemical interactions are considered. These include both single and multiple photon adsorption, and fundamental and overtone transitions in the excitation process, and inter- and intra-molecular energy transfer, and coupling with phonons, electron-hole pairs and surface plasmons in the energy relaxation process. Many current experimental and theoretical studies on the subject are reviewed and discussed with the goal of clarifying the relative importance of the surface interaction steps and relating the resulting concepts to the experimentally observed phenomena. Among the many gas-solid systems that have been investigated, there has been more extensive use of CO adsorbed on metals, and SF₆ and XeF₂ interactions with silicon as examples to illustrate the many facets of the electronically and vibrationally activated surface processes. Results on IR laser stimulated desorption of C₅H₅N and C₅D₅N molecules from various solid surfaces are also presented. It is clearly shown that rapid intermolecular energy exchange and molecule to surface energy transfer can have important effects on photodesorption cross sections and isotope selectivities. It is concluded that utilization of lasers in gas-surface studies not only can provide fundamental insight into the mechanism and dynamics involved in heterogeneous interactions, but also offer the possibility for technical innovation for practical applications.     

Laser-induced CVD of rhodium

Stripes of rhodium metal were deposited by focusing an Ar⁺ laser (514.5 nm) onto glass and polyimide substrates in a heated vacuum cell that contained Rh(CO)₂acac vapor. Stripes were characterized by scanning profilometry, electrical resistivity, SEM and Auger measurements. Most stripes were 100–200 m wide and 1–3 m high. Very broad stripes (>500 m) were deposited when the Rh(CO)₂acac vapor pressure was greater than 1 Torr and when the laser power was more than 200 mW. Stripe resistivities were in general around 30 times that of the bulk material. Auger spectra show the presence of carbon in the stripes.     

Laser-induced decomposition of sodium azide

The decomposition of sodium azide, NaN₃, has been studied using a pulsed carbon dioxide laser. Chemiluminescence associated with the decomposition has been measured with and, without, the azide surface in the detector field of view. Near the decomposition threshold, emission has been observed in the wings of the Na 3p → 3s transition at 589 nm that has not been observed in previous azide decomposition experiments. This emission may be a result of surface reactions. Raman and visible absorption spectra have been obtained for the irradiated material; a new, as yet unidentified, band was observed in the Raman spectrum.     

激光诱导血红蛋白分子的多光子荧光

本文首次报道了血红蛋白吸收二个和四个1.06μm的光子后所发射的荧光特性,并分析了其产生的原因。     

Laser-induced surface modification of polystyrene

The modification induced in polystyrene (PS) by the ArF excimer laser radiation has been investigated. Various numbers of the laser pulses of the energies below the material ablation threshold were applied. Changes in the chemical composition of the PS surface layer were studied by the X-ray photoelectron spectroscopy (XPS). Analysis of the morphological changes in the polymer surface layer was performed via the atomic force microscopy (AFM). The contact angles of test liquids (water and diiodomethane) were measured with use of a goniometer while the surface energy (SE) was calculated by the Owens-Wendt method. It was found that the surface energy change was mainly affected by surface roughness caused by the laser radiation and that surface oxidation had not considerably contributed to this change. The increase in the SE was mostly due to its disperse component.     

Laser-induced swelling of transparent glasses

We describe the process of forming bumps on the surface of transparent glasses such as display glasses with moderate thermal expansion ∼3.2 × 10⁻⁶ K⁻¹ and high coefficient of thermal expansion (CTE) glasses, e.g. soda-lime glasses with CTE ∼9 × 10⁻⁶ K⁻¹ using high-power ultra-violet (UV) lasers at a wavelength where glass is transparent. We characterize the effect with optical dynamic measurements. The process relies on increased glass absorption from color-center generation and leads to glass swelling with bumps formation. The bump height may constitute more than 10% of the thickness of the glass sample. The required exposure time is relatively short ∼1 s, and depends on the glass properties, laser power, its repetition rate, and focusing conditions. A brief review of the potential applications for these bumps is provided.     

Laser-induced stimulation of chymotrypsin activity

Experimental evidence supporting Frölich's conjecture of a metastable state in enzymes is provided.     

准分子激光全固态脉冲电源设计与实验研究

针对脉冲能量5~8 mJ的ArF准分子激光器,设计了基于可控硅开关结合三级磁脉冲压缩开关的全固态脉冲电源,采用国产可控硅和磁开关材料,获得了上升时间约150 ns,电压10~14 kV,传递能量0.35~0.68 J的激励脉冲,并实现了对准分子激光器快放电激励。三级磁开关总效率35%,分析表明磁开关损耗较大主要原因为电容能量转移不充分、导线铜损及磁芯材料铁损较大,并提出了相应改进办法。     

Laser-induced breakdown spectroscopic studies of marbles

We present laser-induced breakdown spectroscopic studies of variety of marbles extracted from Quetta region of Boluchistan, Pakistan using a Nd:YAG laser (532 nm) in conjunction with LIBS 2000 detection system. The emission spectra of Onyx (white spot) and Quetta Green Marble samples have been recorded as a function of laser irradiance. The elemental composition and their relative abundance in each sample are fond to be quite different. In Onyx (white spot), calcium dominates while in the Quetta Green marble magnesium is the dominant element. In addition the effect of the laser irradiance on the emission intensity, width and shift of the transition lines have been studied.