Laser-induced reactions of NO2 in the visible region

The 488 nm laser-induced reaction of NO₂ with CO has been investigated and the results computer-modeled. Two reaction mechanisms were considered:1) the direct reaction of vibronically excited NO₂ (NO ₂ ^* ) with CO to form CO₂, and 2), the reaction of an intermediate NO₃ radical formed by the reaction of NO ₂ ^* with NO₂ followed by NO₃+CONO₂+CO₂. The modeling results strongly support the former as the dominant mechanism.Federal Junior Fellow (1980–1984)     

Chemical reactions between CF2HCl and NH3 induced by IR double-beam excitation

Infrared multiphoton dissociation experiments with two wavelengths in different mixtures of chlorodifluoromethane and ammonia have been carried out. It is shown that the presence of ammonia in the sample induces a decrease in the chlorodifluoromethane dissociation yield. It has been observed that the distinct chemical reaction channels are differently activated as the time delay between the two laser pulses is varied. The ratio of the obtained products in the infrared multiphoton dissociation changes with the composition of the initial mixture and are not compatible with the mechanism suggested by Sugita and Arai for this reaction in single IR wavelength excitation if it is assumed to be the only contributing mechanism other than that for direct CF₂HCl dissociation and subsequent C₂F₄ formation from the resulting CF₂ radicals. It appears that, although for simultaneous two-wavelength irradiation the presence of an accumulated solid NH₄Cl deposit does not significantly influence the reaction, this is no longer the case when time delays are introduced between the two beams.     

Observation of OH angular momentum polarization produced by H+O2 at 2.6 eV collision energy

The OH rotational polarization produced by the reaction of fast H atoms from the polarized 193 nm photolysis of HBr with O₂ (2.6 eV collision energy) has been measured by laser-induced fluorescence using polarized analysis light. A strong rotational polarization parallel to the electric vector of the dissociation laser and perpendicular to the flight direction of the H atoms is observed. Implications for the H+O₂ reaction dynamics at high collision energies are discussed.     

CO2-laser-induced breakdown in mono- and diatomic gases

Several monoatomic and homonuclear diatomic gases absorb energy from a focused CO₂-laser photon field. It has been established that the pressure threshold for the energy absorption correlates qualitatively with the known ionization potentials of those gases. The simplified phenomenological theory of the CO₂-laser-induced dielectric breakdown of gases is invoked to explain this phenomenon. In the H₂–D₂ system, the formation of HD is observed under these conditions. The examination of the reaction yields for HD formation demonstrates that the system studied does not reach equilibrium under our experimental conditions. Considerations regarding kinetics of primary processes reveal that ionic species, created originally via an inverse bremsstrahlung mechanism, are converted into atomic transients in fast ionic association processes. The latter species initiate chain reactions with surrounding molecules of substrates leading to the formation of HD. Simple kinetic analysis based on a non-steady-state assumption permitted the derivation of an expression for the yield of HD formation. This equation was fitted to the experimental data assuming that the temperature of the reaction rises with an increase of the amount of D₂ in the mixture. Some other aspects regarding the behavior of this system in a focused CO₂ laser beam are also discussed.U.S. Department of Energy Document No. DE-AS02-76ER03416-37     

Laser-induced fluorescence spectroscopy of FeS in the visible region

The laser-induced fluorescence excitation spectra of jet-cooled FeS molecules have been recorded in the energy range of 18 900-21 600 cm⁻¹, in which four parallel and one perpendicular transitions were identified for the first time. Spectroscopic constants of the observed excited states of FeS were determined by analyzing their rotationally resolved spectra. In addition, the lifetimes of most observed bands were also measured.     

Excimer-laser etching on silicon

Studies have been made of poly- and single Si etching induced by excimer-laser irradiation of the silicon surfaces in halogenated gases. Etching was investigated for different conduction types, impurity concentrations and crystallographic planes. Chlorine atoms accept electrons generated in photoexcited, undoped p-type Si, thus becoming negative ions which are pulled into the Si. However, the n⁺-type Si is etched spontaneously by Cl^– as a result of the availability of conduction electrons. Fluorine atoms, with the highest electronegativity, take in electrons independent of whether the material is n- or p-type. And thus, the easy F^– ion penetration into Si causes spontaneous etching in both types. New anisotropic etching for n⁺ poly-Si is investigated because of its importance to microfabrication technology. Methyl methacrylate (MMA) gas, which reacts with Cl atoms, produces a deposition film on the n⁺ poly-Si surface. The surface, from which the film is removed by KrF (5 eV) laser irradiation, is etched by Cl atoms, while the film remains on the side wall to protect undercutting. However, with the higher photon energy for the ArF (6.4 eV) laser, the Si-OH bonds are broken and electron traps are formed. These electrontrapping centers are easily annealed out in comparison to the plasma-induced centers. Pattern transfer etching for n⁺ poly-Si has been realized using reflective optics. The problems involved in obtaining finer resolution etching are discussed.     

LIF diagnostics of C2 radical behaviour in a laser CVD environment

A laser-induced fluorescence (LIF) system was developed to diagnose the reaction and transport of radicals in the ArF-laser-assisted CVD environment. The C₂(a ³ II _u) radicals were produced by the multi-photon dissociation of C₂H₂. The transport of the radical was directly measured by LIF. The disappearance rate of the radical in C₂H₂ was also determined. By using the values determined, the in-flux of the C₂ radicals onto a substrate was investigated, based on a diffusion model.     

Reaction of Cu and Cl2 stimulated by synchrotron radiation

Light-induced reactions of poly-Cu with Cl₂ and the formation of CuCl _x films were studied in the spectral range of 105–400 nm by using synchrotron radiation. The efficiencies of the reactions with Cl₂ pressures between 10^–5 and 10^–2 mbar were determined from the height distributions of spatial structures in the CuCl _x films. The heights typically range from 10 to 10⁴ nm. An efficiency of about 10⁷ CuCl _x molecules per dissociated Cl₂ molecule is observed at high Cl₂ exposures. At low Cl₂ pressures CuCl _x formes anisotropically in the irradiated area. The efficiency is determined by light-induced surface processes supporting the build up of CuCl _x in the long wavelength range and competing processes at short wavelengths which reduce the efficiency.     

Isotope-selective IR multiphoton dissociation of CHClF2 in the presence of NO2

2 to the CHClF₂/He mixture irradiated by a Q-switched CO₂ laser leads to oxidation of the dissociation product according to the reaction: CF₂+NO₂→COF₂+NO. The resulting COF₂ with a ¹³C content near 50% is easy to convert to CO₂ or CO for further enrichment by a nonlaser process. We measured the dependence of the fraction of dimerised CF₂ on NO₂ pressure p_NO2 and the amount of NO₂ required to suppress dimerisation on the dissociation yield. Both agree with a kinetic model using known rate constants. For the range of the dissociation parameters (¹³CF₂ yield of 10% per pulse, isotope selectivity of 130) of practical interest, 95% of the CF₂ produced is oxidized at p_NO2≈1/2p_CHClF2. In the absence of NO₂, major (20%–35%) losses of CF₂ at the metal walls of the irradiation system were observed. Addition of NO₂ suppresses them. For comparison, we also used O₂ as a scavenger in CHClF₂ dissociation. NO₂ is by orders of magnitude more efficient. Received: 21 January 1997/Revised version: 23 March 1997     

Determination of diffusion coefficients of C2 radicals by laser induced fluorescence spectroscopy

Diffusion coefficients of C₂ radicals in Ar and H₂ were determined experimentally for the first time. The C₂ radicals were generated by a multi-photon laser photolysis and the spatial behaviour was observed by laser-induced fluorescence spectroscopy. Spatial distributions measured were compared with a theory to determine diffusion coefficients. It is shown that the present technique is suitable to measure diffusion coefficients of short-lived radicals that are difficult to measure by any other means.     

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.     

Photochemical etching of GaAs with Cl2 induced by synchrotron radiation

The quality and efficiency of etching at room temperature is determined in the wavelength range from 105 nm to 300 nm by replicating a mask on a GaAs(100) wafer and by wavelength selection of synchrotron radiation with filters and a monochromator. A good anisotropy and selectivity is found for Cl₂ pressures from 10^–2 mbar up to 1.5 mbar, but above 3 mbar the selectivity is lost. Efficiencies for stimulation of the chlorination reaction and for desorption are separated and an optimal efficiency for stimulation of about 100 removed Ga and As atoms per photon is obtained around a wavelength of 122 nm at 1.5 mbar. Growth of reaction products on the surface occurs for short wavelengths and transport processes through layers up to a thickness of 350 nm are relevant. The efficiency and quality of etching can be improved by additional desorption with long wavelengths especially with lasers.     

Chemical reactions following the IRMPD of C2F3Cl

C₂F₃Cl is photolyzed with a TEA-CO₂ laser at 1050.44 cm^–1 with focussed fluences up to 280 J/cm². The stable products in the IRMPD of C₂F₃Cl are determined for up to 10 Torr of C₂F₃Cl being photolyzed both neat and with added O₂. C₂F₄ and trans-C₂F₂Cl₂ are found to occur in the greatest yield though C₃F₅Cl, C₃F₄Cl₂, C₄F₇Cl, and C₂F₃Cl₃ also appear to be primary products. When O₂ is present F₂CO, FClCO, and CF₂ClCOF are the exclusive products. The formation of these products are for the most part consistent with a carbene formation dissociation mechanism for C₂F₃Cl IRMPD. C₂F₃Cl₃ may best be explained by another mechanism competitive with carbene formation. Many products attributed to secondary photolysis mechanisms are observed for long photolysis times.This work was performed at Department of Chemistry and chemical Engineering, Michigan Technological University, Houghton, MI 49931, USA     

Laser-induced chemistry-basic nonlinear processes and applications

Many methods and achievements in chemistry are based on using the interactive of light with atoms and molecules. It is sufficient to mention photochemistry, flashphotolysis, spectrochemistry and others. The advent of laser amplified the connection between chemistry and light. Today laser light has become a very versatile and effective tool, first, to study the dynamics of chemical reactions, secondly, to stimulate chemical reactions and finally, to analyze substance. The unique properties of laser light (high power, monochromaticity, short duration, directivity and temporal coherence) provide quite new instrumental possibilities in all these problems.     

Weathering properties of treated southern yellow pine wood examined by X-ray photoelectron spectroscopy, scanning electron microscopy and physical characterization

In this study the weathering behavior of southern yellow pine (SYP) wood samples pretreated in different solutions has been examined using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and various types of physical characterization regarding material loss and discoloration. The treatment solutions include water as a control, a commercially available water repellent (WR) wood treating additive and polyethylene glycol (PEG) products including PEG PLUS™, PEG 8000 solutions and Compound 20M in varying concentrations. All contained the wood preservative chromated copper arsenate (CCA). One sample was treated with a CCA solution only. The treatments were carried out at 20 °C and 150 psig for 1/2 h after exposure to vacuum (28 mmHg) for 15 min. Simulated weathering was achieved in an Atlas 65-W Weather-Ometer for 2000 h with both light and dark periods and rain. The temperature ranged from 23 °C during the dark cycle to 35 °C during the light cycle. With weathering the XPS O/C ratios increase due to oxidation of the surface. Exposure to UV light results in bond breakage and reaction with oxygen in the presence of air to form organic functional groups such as , , CO and/or O-C-O. These oxidized products can protect the underlying wood from deterioration if they are insoluble in water and remain on the surface as a protective coating. If soluble, rain washes the compounds away and assists in the degradation. Correlated changes are observed in the XPS O/C ratios, the high-resolution XPS C 1s spectra, the SEM micrographs and physical measurements including thickness alteration, weight loss, and discoloration by yellowing or whitening of the weathered wood. The PEG treatments are effective in protecting wood with the 2% PEG PLUS treatment providing the best weathering behavior similar to that of the CCA treatment. The WR and water treatments yield the poorest weathering properties.     

Cadmium deposition on transparent substrates by laser induced dissociation of Cd(CH3)2 at visible wavelengths

The laser initiated formation of Cd-deposits from dimethylcadmium (DMCd) on quartz substrates has been investigated at various wavelengths between 337 and 676 nm. In this range substrate and DMCd are both transparent. The deposition mechanism is initiated by multiphoton dissociation of DMCd molecules adsorbed at the glass substrate and continues by pyrolysis of DMCd molecules at the laser-heated metallic deposit. Metal structures with a resolution below 1 m have been obtained.     

Investigations of multiphoton selective dissociation of (CH3)2O in the field of a pulsed CO2 laser

Experimental investigations of multiphoton selective dissociation of (CH₃)₂O induced by a pulsed CO₂ laser have been conducted. Separation of H, C, and O isotopes was performed in enriched mixtures and in samples with the natural abundance. The following coefficients of selectivity have been obtained:K _D/K_H=4.0,K ₁₃/K₁₂=1.7, andK ₁₈/K₁₆≧1.6. We studied the dependences of the selectivity coefficient on ether pressure, on the laser energy and frequency as well as the influence of secondary chemical reactions on the dissociation selectivity. Estimations made by using the RRKM theory have indicated that ether molecules that decompose have an average excitation energy above the dissociation threshold of ∼1.5 kcal/mole.     

Thick gold-film deposition by high-repetition visible pulsed-laser chemical vapor deposition

A highly conductive gold film, over 10 m-thick with well-controlled linewidth, has been successfully deposited from dimethyl-gold-acetylacetonate and its fluorinated derivative by pyrolytic CVD (Chemical Vapor Deposition) with a high-repetition, visible, pulsed laser. The thermal damage to the polyimide substrate has been substantially suppressed by reducing the thermal diffusion length within 0.2 m in pulsed-laser-induced transient heating, in contrast to the cw laser-CVD scheme. Reproducible and low contact resistance as low as 0.5 between the written line and the existing gold line has been obtained. Sufficiently tough adhesion to polyimide has been observed for the deposit from dimethyl-gold-acetylacetonate. Reasonable agreement has been obtained between the observed deposition characteristics and analytical results for precursor supply rate and temperature increase during short-pulse irradiation.     

Laser-induced chemistry in silane-hexafluoroacetone mixtures for production of novel Si/C/F/O and C/F/O materials

Chemical reactions induced by CO₂-laser radiation in mixtures of silane and hexafluoroacetone afford various gaseous silicon- and carbon-containing compounds and result in deposition of microstructures of carbon, C/F/O and Si/C/O/F materials. These products are suggested to be formed by a variety of exothermic reactions initiated through SiH₄-photosensitized decomposition of hexafluoroacetone. Silane is shown to be a very potent reagent for the reduction of C-F bonds.     

Scaling-up13C separation by infrared multiphoton dissociation of the CHClF2/Br2 system

¹³C separation at a laboratory scaled-up level by the¹³C-selective InfraRed MultiPhoton Dissociation (IRMPD) of CHClF₂ in the presence of Br₂ has been investigated in a flow reactor. With a complete scaled-up system including a flow reactor, an industrially reliable TEA CO₂ laser with longer pulse duration and a product-separation set-up for¹³C separation, it has been attempted to optimize the parameters suitable for large-scale production of the carbon isotope. The optimization of¹³C separation parameters, such as laser fluence, laser frequency and the partial pressure of CHClF₂ and Br₂ was tested under static conditions. By irradiation with longer pulses, a lower optimum pressure for a high¹³C-production rate was determined. Furthermore, the separation process was scaled in the flow system to examine the¹³C-production rates,¹³C atomic fractions in the CBr₂F₂ products and¹³C depletions in the CHClF₂ reactants at different flow rates and laser repetition frequencies. The data obtained from the flow tests demonstrated a 40 mg/h production rate for CBr₂F₂ at 65%¹³C by using a 40 W (4 J, 10 Hz) laser beam focused with a lens of 120 cm focal length. If the reliable TEA CO₂ laser is operated with 100 W (10 J, 10 Hz) output, the production rate of CBr₂F₂ for¹³C at 60% of 200 mg/h can be attained. The measurements of the spatial profile of the focused laser beam imply a 2 g/h production rate for the 60%¹³C product for an incident power of 200 W (20 J, 10 Hz).