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Automated robotic laser welding is commonly used for the joining of thin-gauge sheet metals for automotive applications due to the precision and speed of the welding process. Without proper process optimization, high-speed laser welding for automotive applications is known to have defects such as porosity, weld concavity, and cyclic humping. Several welding parameters can be adjusted to optimize the laser welding process: decreasing the laser power and the welding speed, changing the intensity distribution of the laser on the surface of the substrate, or by changing the type and flow rate of shielding gas used during welding. However, decreasing the laser power and welding speed leads to a loss in productivity which can be costly in large-scale manufacturing endeavors. This study presents the effect of beam defocusing on the weld pool geometry while holding other parameters constant as the welding mode transitions from full penetration or open keyhole mode (OKM) when the beam is fully focused to partial penetration or closed keyhole mode (CKM) as the beam is defocused. It has been shown that by defocusing the laser beam and controlling the weld pool geometry, the welding process can be stabilized to eliminate defects such as severe concavity without decreasing the laser power or slowing down the welding process. A numerical solution is presented that uses three-dimensional (3D) transient finite element (FE) analysis to calculate the temperature fields and predict the molten weld pool geometry in both the full and partial penetration welding modes using a double conical-Gaussian (DCG) volumetric heat source derived from the classical 3D conical-Gaussian (CG) heat source. The model was calibrated using video imaging of the welding process and known experimental data, and it was validated using an extensive experimental study. The results of this work show that the model can be used to predict the molten weld pool geometry that stabilizes the liquid flow in the melt pool during high-speed laser welding whereby allowing faster speeds to be used in the manufacturing of laser-welded components.

The irradiation of linear chain alkanes in the liquid phase is shown to be an efficient method for the formation of a variety of polyyne, CnH2, species. The chemical products remaining in liquid octane, C8H18, after irradiation with 100 fs 800 nm laser pulses from a Ti:Sapphire laser system have been characterized using surface-enhanced Raman spectroscopy (SERS) and time of flight mass spectrometry (TOFMS). A reaction mechanism is proposed to explain the formation of long chain polyynes from the chemical products created during irradiation. This putative reaction sequence provides an explanation for the generation of polyynes incorporating more carbon atoms than present in the parent alkane.

Solid methane at 77 K was irradiated with laser pulses of wavelength 800 nm and energy of 275 μJ. Laser pulse duration was 100 fs. This irradiation experiment shows formation of higher molecular weight pure carbon clusters C n where n = 1, 2, 3, …, and the formation of alkane molecules of higher molecular mass than the original target molecule. These molecules have the composition (CH 4 ) n where n = 1, 2, 3…. The time of flight spectrum also shows the formation of polyynes and cumulenes as ablation products.

Small alkane and alkene molecules containing a single C C double bond were ionized using intense 800 nm, 100 fs laser pulses. Time of flight mass spectra shows that strong dissociation of the parent molecule occurs in purely single bonded carbon chains while alkenes gain stability from the presence of a single C C bond.

A range of carbon nanoparticles together with polyyne chains are formed when liquid benzene is irradiated with 120 fs pulses of intensity ≈ 1 × 1014 Wcm−2. The morphology and composition of these nanoparticles are reported. Atomic force microscopy (AFM) shows irradiation result in the formation of spherical, ellipsoidal and toroidal nanoparticles with sizes between 1 and 50 nm. Surface enhanced Raman spectroscopy (SERS) indicates that the composition of these particles is primarily amorphous carbon reinforced by polyyne chains with 8–14 carbon atoms. Time of flight mass spectroscopy (TOF-MS) has been used to determine the precursor molecules created in the irradiation of benzene and shows the importance of ring-opening reactions in particle formation. A simple chemical mechanism is proposed to explain the appearance of polyyne chains in irradiated benzene.

The production of polyyne molecules arising from the irradiation of liquid octane with femtosecond (fs) laser pulses has been studied using time of flight mass spectrometry (TOFMS). We attribute the primary formation mechanism for polyynes in solution to the creation of small polyyne and cumulene molecules formed in molecular dissociative ionization, that subsequently increase in length through addition reactions with other products C, C2, C2H, the ions of which are detected in the TOFMS of octane. Polyynes up to C14H2 have been identified and we propose that photo-dissociation limits the length of polyynes produced in solution.

Vibrational bands associated with the out of plane bending modes of CH groups attached to various aromatic ring structures have been identified in surface enhanced Raman spectra of diamond-like amorphous carbon films deposited by femtosecond laser ablation of graphite. The appearance of enhanced modes associated with specific ring structures is assigned to solo-, duo-, trio- and quartet-CH groups and to pendant rings. Analysis of these bands provides insight into the nature of the molecular ring components of fs-DLC films. The resulting composition consists of polycyclic aromatic carbon (PAH)-like ring molecules and polyyne chains stabilized in a sp3-bonded network.

Since microlenses have to date been fabricated primarily by surface manufacturing, they are highly susceptible to surface damage, and their microscale size makes it cumbersome to handle. Thus, cavity lenses are preferred, as they alleviate these difficulties associated with the surface-manufactured microlenses. Here, it is shown that a high repetition femtosecond laser can effectively fabricate cavity microball lenses (CMBLs) inside a polymethyl methacrylate slice. Optimal CMBL fabrication conditions are determined by examining the pertinent parameters, including the laser processing time, the average irradiation power, and the pulse repetition rates. In addition, a heat diffusion modeling is developed to better understand the formation of the spherical cavity and the slightly compressed affected zone surrounding the cavity. A micro-telescope consisting of a microscope objective and a CMBL demonstrates a super-wide field-of-view imaging capability. Finally, detailed optical characterizations of CMBLs are elaborated to account for the refractive index variations of the affected zone. The results presented in the current study demonstrate that a femtosecond laser-fabricated CMBL can be used for robust and super-wide viewing micro imaging applications.

Irradiation by multi-line average-power argon laser was used for the enhancement of the electrochemical deposition of gold and copper from cyanide solution, and copper and nickel from sulfate solutions. The deposition rate was significantly increased with increasing laser power. The diameter of the laser focus on the cathode was less than 30 μm at a power density (intensity) of 0.1 MW/cm 2 . The increase in deposition rate may be explained by thermal activation of the cathode/electrolyte surface at the interface.

Many industrial applications of high-power lasers involve the creation by vaporization of a keyhole in a solid target. This structure is unstable with respect to collapse and can be maintained only by achieving a balance between pressure terms from laser vaporization and those of surface tension and hydrostatics. In addition, liquid flow and plasma effects also modulate the laser beam intensity resulting in a complex feedback system in coupling laser radiation into the target. In this paper, we report data obtained on the time dependence of structures associated with laser drilling of an absorbing liquid. These data, obtained at low incident laser intensities in the absence of plasma effects, show the complex nature of the laser-keyhole interaction even in a two-phase system. Some results of experiments carried out in low gravity are also presented.

We report infrared absorption spectra of explosion nanodiamonds and compare these with spectra of a-C:H films prepared by laser ablation. Similarities between these spectra are discussed. In particular, we find that a feature near 1620 cm−1 in nanodiamond particle spectra can be associated with the presence of C=C groups. This is supported by the observation of bands arising from CH2. We suggest that these groups are created by surface reconstruction on these extremely small particles that tends to de-emphasize tetrahedral bonding in the surface layer.

This work explores the thermal behavior of silicone rubber filled at different concentrations with alumina trihydrate (ATH) and silica. Three particle sizes, 1.5 /spl mu/m, 5 /spl mu/m and 10 /spl mu/m, were employed in the sample preparation. A near-infrared laser was used to heat the samples. The concentration of the filler plays a major role on improving thermal conductivity when compared to particle size. Nevertheless, better thermal conductivity could be distinguished at 5 /spl mu/m for ATH and 1.5 /spl mu/m for silica, at all concentrations.

An experimental reactor system is described for regenerating deactivated hydrotreating catalysts through oxidative coke burn-off using laser radiation. The laser reactor was designed to process batches of catalyst extrudates using a rotating bed principle involving sequential particle irradiation and agitation steps. The catalyst regeneration system contained dual CO 2 laser and agitation units to enhance processing efficiency. The experimental reactor was used to carry out a systematic study involving laser regeneration of spent commercial catalyst samples from an industrial hydrotreating unit. Carbonaceous deposits on the catalyst surface were removed by the photo-thermally induced chemical reaction of coke molecules with an oxidizing gas mixture. The unique properties of laser heating should reduce the deleterious effects of conventional coke burn-off such as sintering and metals agglomeration. A parametric study of the regeneration process was undertaken using an established experimental design procedure. Important processing parameters were found to be laser intensity, residence time, oxygen concentration, and gas flow rate.

Spectra emitted from 0.1% CO-N2 solids excited with high energy electrons at 4 K show evidence for resonant transfer of vibrational energy from highly excited vibrational levels of N2 to CO in the process N2(X1Σg+, ν) + CO(ν = 0) → N2(X1Σg+, ν - 1) + CO(ν = 1) + phonons. Energy transfer from levels with ν ⩽ 9 has been observed.

Two-photon laser spectra of the Yb vapor have been obtained. Transitions to highly excited 4f 14 6s ns 1 S 0 and 4f 14 6s n d 1 D 2 states are seen in direct two-photon excitation. Hybrid resonances involving 4f 14 6s6p 1 P 0 1 and 4f 14 5d6s 3 D 2 intermediate states lead to transitions to 4f 14 6s ns 1 S 0 , 4f 14 6s n p 3 P 0 2,1 and 4f 14 6s n d 1 D 2 levels.

Coupling coefficients for 10.6μm laser radiation on stainless steel have been measured experimentally under oxidizing and vacuum conditions. A theoretical model has been developed that relates the growth of an oxide layer to changes in coupling coefficients.

The two-photon absorption spectrum to high-lying states of Li vapour has been obtained using ion-cell detection. 2 S states are observed to n = 32 and the 2 D series to n = 49. Simple heating techniques circumvented the problem of absorption by the blue-green band system of Li 2 which overlaps the series limit at 4600 A. The order of the ionization process was examined and found to vary as the square of the laser intensity for two-photon transitions. A conflict with data obtained with a space-charge-limited diode is examined and possible ionization mechanisms are discussed.

The effects caused by collisional interactions of He and Ar on the high lying states of Rb are presented. Using both direct and hybrid two-photon excitation we report the broadening and shift of the Rb 5 2 S→ n 2 S(10⩽ n ⩽21) and 5 2 S → n 2 D (8⩽ n ⩽35) transitions. The perturber gas concentrations ranged from 0.40 relative density (r.d.) to 2.64 r.d. and the rate of broadening and shift has been calculated for the above transitions. When compared with previous results at much lower perturber concentrations our broadening and shift rates are similar for all n 2 S and 19 2 D levels except for the shift rate of the 19 2 D level. The broadening and shift of the hybrid series are found to be relatively unaffected by the participation of a real intermediate state.

Dye laser spectroscopy of Ga vapor in an ion cell has lead to the identification of high lying n p 2 P o and n f 2 F o levels. A hybrid resonance involving the 5s 2 S 1 2 level has been observed together with resonant enhancement of members of the 4 2 P 0 3 4 → np 2 P 0 series due to thislevel.

Calcium atoms codeposited with N2, Ar, Kr, and Xe on a sapphire plate at 10, 17, or 29 K exhibited very strong, sharp absorptions due to the resonance transition of isolated Ca atoms and broad bands in the green region which show Ca concentration dependence characteristic of aggregated species. A red absorption contained sharp vibronic fine structure with 117 cm−1 spacings in solid krypton which decreased to 111 cm−1 with 44Ca deposition. The structured absorption is assigned to the A 1 Σ u + ( 1 S + 1 P) ← X 1 Σ g + ( 1 S + 1 S) transition of Ca2.

The states n 2 P 0 1 2 , n 2 P 0 3 2 , and n 2 F 0 5 2 , 7 2 have been observed in Tl to n = 18, 21 and 26 respectively, using two photon laser spectroscopy and an ionization detector. An analysis of the quantum defects for these states and the spin-orbit splitting of the n 2 P 0 states is given.

Impact broadening and shifts of Ba transitions to parity forbidden Rydberg states have been measured using two-photon laser spectroscopy techniques. Broadening and shift rates for the 6s n s 1 S 0 (10 n n d 1 D 2 (8 n

A quantitative analysis of cutting thin stainless steel sheets with cw CO 2 lasers using an oxygen gas jet assist is reported. Results are interpreted theoretically using a point source model.

Reflection spectra of solid oxygen have been obtained for an angle of incidence of 45° and over the wavelength range 500–2000 A. These spectra are found to depend on deposition temperature, the thermal history of the sample and the amount of ultraviolet radiation absorbed by the sample. Some evidence is presented for the in situ formation and trapping of atomic oxygen in the solid.

Absorption spectra of dilute dispersions of Na atoms in solid C6D6 and (C6H6C6D6) mixtures at 55°K have been obtained. These spectra can be interpreted in terms of vibronic transitions of Na-C6D6 or Na-C6H6 complexes. When Na is trapped in mixed crystals of C6H6 and C6D6 the spectrum appears to be superposition of Na-C6H6 and Na-C6D6 he spectrum appears to be the superposition of Na-C6H6 and Na-C6D6 spectra. This indicates that Na atoms in these solids interact predominantly with one nearby benzene molecule.

The effect of O 2 and N 2 impurities on the spectrum of Hg atoms in solid krypton matrices at 20°K has been investigated. The relative intensity of the components of Hg 1 P 0 1 are shown to depend strongly on the amount of O 2 or N 2 in the matrix. Under certain conditions, the long wavelength triplet component can be completely eliminated. It is shown that the triplet arises from the superposition of spectra of atoms trapped at three distinct trapping sites.

Irradiation of pure MgO crystals with intense XeCl laser radiation yields VUV emission associated with the decay of bi-excitons. This emission exhibits a 4-photon dependence on incident laser intensity. Collisional excitation of luminescence from Cr 3+ and Ni 2+ impurities has been detected in doped samples. This emission also shows a multiphoton dependence on incident laser intensity.

Narrow autoionizing resonances have been observed in Yb and Tl using direct 3-photon excitation of ground state atoms. The linewidth is 0.9 cm -1 in Yb and 0.4 cm -1 in Tl. Autoionizing levels in both atoms correspond to doubly excited electronic states. This is the first observation of such states in Tl.

Collision broadening and shift of the sodium 3S-4D and 3S-5S transitions by He, Ar and N2 has been measured using two-photon absorption and ion detection techniques. The perturber gas pressures ranged from 1 × 105N/m2 to 7 × 105 N/m2 (1–7 atmospheres) and it was found that the fwhm and shift varies linearly with pressure throughout this range. A comparison is made between our results and those of Biraben et al. who have used Doppler-free spectroscopy to study the effect of low pertuber pressures on the above transitions.

The possibility that the diffuse interstellar absorption bands may be due to resonance lines of impurity atoms trapped in low temperature hydrocarbon matrices is examined. Reference is made to the spectrum of Na atoms in solid C 6 H 6 at temperatures below 100 K. Comparison of these spectral features to the diffuse bands at 5780.6 A and 5797.1 A is made. Evidence for vibronic structure in the diffuse interstellar band system is discussed.

It is shown that the depletion of Ar atoms in interstellar dust clouds is well correlated with the depletion of other heavy elements such as C, Si, Fe, and S. Since Ar sticks to dust but does not become incorporated in interstellar molecules, the rate of depletion of Ar can be used to infer the rate at which other heavy atoms collide with dust. Here, the rates of depletion of Ar and other elements are compared and the results are discussed. It is found that: (1) the net sticking efficiency of all elements is less than or equal to that of Ar; (2) Ar first accretes on cores of MgFeSi of oxide or silicate composition; and (3) carbon depletion in diffuse interstellar clouds is also due to accretion on MgFeSi-oxide or MgFeSi-silicate grains. The relative sticking probabilities of the different elements are given in a table. 28 references.

Fluorescence by molecular H/sub 2/ within the Lyman and Werner bands, as well as to continuum levels of the X/sup 1/S/sub g/ /sup +/ state, occurs at wavelengths l< or =0.18 mm. This emission is shown to be of intensity comparable to that scattered by dust over the same wavelength range in diffuse clouds. The implied increase in dust albedo in the vacuum ultraviolet (VUV) is likely due to this effect. Thus the small particles responsible for VUV extinction are of conventional chemical composition, i.e., absorbing oxides, silicates, or carbons.

Possible relationships between chemical reactions on interstellar grains and their optical properties are examined. A variety of reactions that will lead to chemical reduction or oxidation are discussed. To illustrate possible processes, the equilibrium in the Fe oxide series (Fe, FeO, Fe/sub 3/O/sub 4/, Fe/sub 2/O/sub 3/) as a function of ambient conditions has been investigated. One immediate conclusion from this study is that pure Fe grains are unlikely to exist in the diffuse ISM (interstellar medium) because of low-temperature oxidation to Fe oxides by O atoms. It is shown that several optical effects customarily ascribed to size changes in dust can be interpreted in terms of chemically induced modifications in the optical properties of dust. As an example, the optical properties of the silicon oxide series SiO/sub x/, where 1< or =x< or =2, is used to investigate a possible relationship between refractive index variations and variations in the lambda/sub max/, the wavelength of maximum linear polarization. Reduction of silicate or oxide grains in H II regions may be related to OH and H/sub 2/O maser sources observed in these regions.

Laboratory spectra of fine partially hydrogenated amorphous Mg/sub 2/SiO/sub 4/ particles have been shown to exhibit an absorption feature at 217.5 nm that matches the interstellar extinction band at this wavelength. This absorption appears to be due to an electronic transition of OH(-) ions in sites of low coordination. Absorption occurs at a similar wavelength in Fe silicates as well as in silicates of mixed Mg, Fe composition, although the best fit to the observed 217.5-nm profile is obtained with silicates containing little iron. 28 references.

The reaction between surface O/sup 2 -/ and OH/sup -/ ions on oxide grains and neutral atoms of S or Cl is predicted to lead to the depletion of these elements in clouds in which these species exist in the gas as neutrals. Strong depletion is predicted for F, while the depletion of Br and I will depend on the fraction of these atoms that are in neutral form in diffuse clouds. The raction of S and Cl with surface ions may lead to observable optical and chemical effects. Two possible effects are (a) the weakening of the 2175 A band in regions of high gas density and (b) a saturation of the selective depletion of other elements.