Diode-laser-based ultraviolet absorption sensor for nitric oxide

An all-solid-state continuous-wave laser system for ultraviolet absorption measurements of the nitric oxide (NO) molecule has been developed and demonstrated. The single-mode, tunable output of a 10-mW, 395-nm external-cavity diode laser (ECDL) is sum-frequency-mixed with the output of a 115-mW, frequency-doubled, diode-pumped Nd:YAG laser in a beta-barium-borate crystal to produce 40 nW of tunable radiation at 226.8 nm. The wavelength of the 395-nm ECDL is then scanned over NO absorption lines to produce fully resolved absorption spectra. Initial results from mixtures of NO in nitrogen in a room-temperature gas cell are discussed. The estimated NO detection limit of the system for a demonstrated absorption sensitivity of 2×10^-3 is 0.2 ppm per meter of path length for 300 K gas. The estimated accuracy of the measurements is ±10%. Received: 25 February 2002 / Revised version: 31 May 2002 / Published online: 8 August 2002     

Photochemical surface modification of PET by excimer UV lamp irradiation

UV irradiation has interesting potential for the photochemical modification of polymers. In order to study cross-linking effects and/or thin-layer deposition following a treatment in the presence of bi-functional media or in inert atmosphere, irradiation of PET in various atmospheres was performed using a KrCl excimer lamp. Surface properties were investigated by atomic force microscopy, nanoindentation, micro-thermal analysis, and X-ray photo-electron spectroscopy. The studies reveal that surface chemical composition, morphology, adhesion, thermomechanics, and stiffness/modulus are strongly affected by UV irradiation in the presence of bi-functional media. Films treated in octadiene and argon show an increase of surface modulus, much less expansion, and lower soft/melt temperatures, which is an indication of the surface cross-linking effect and a decrease of crystallinity within the near-surface layer. In the case of a diallylphthalate-treated film, depending on the local structure, either a strong decrease of melting temperature or no melting point is found, which is attributed to the irregular cross linking and thickness of the modified layer associated with a decrease of surface modulus. A significant increase of the alkali resistance is found after irradiation, as a result of both wetting and cross-linking effects on the polymer surface.     

Intracavity frequency mixing in pulsed optical parametric oscillators for the efficient generation of continuously tunable ultraviolet radiation

Received: 4 May 1998/Revised version: 8 July 1998     

Generation of subpicosecond vacuum ultraviolet pulses at 126 nm by using harmonics of a subpicosecond Ti:Sapphire laser

Subpicosecond vacuum ultraviolet (VUV) pulses at the wavelength of 126 nm have been generated in rare gases as a result of the 7th harmonic radiation of a subpicosecond Ti:Sapphire laser oscillating at 882 nm. The VUV harmonic intensity was optimized in Xe at the pressure of 1.2 Torr. The behavior of the harmonic emission was qualitatively reproduced by the classical nonlinear optics. The increase of the harmonic intensity was limited by multiphoton ionization of Xe.     

Multi-wavelength excimer ultraviolet sources from a mixture of krypton and iodine in a dielectric barrier discharge

New ultraviolet (UV) sources (excimer lamps) have been demonstrated using mixtures of krypton and iodine in a high-pressure dielectric barrier discharge to provide intense multi-wavelength radiation at λ=183 nm (atomic iodine line, ⁴ P _5/2?² P _3/2), 191 nm (KrI^*, B _1/2?X _1/2), 206 nm (atomic iodine line, ² P _3/2?² P _1/2), and 342 nm (I^* ₂, D _1/2?X _1/2). The characteristics of the optical emission spectra of the atomic species and the excited dimers (excimers) formed for different total gas pressures and in the presence of a buffer gas have been investigated. The highest intensity, at 183 nm, is obtained at pressures up to 1 bar while the overall emission spectra can be controlled by adjusting the total pressure. The results show that these strong multi-wavelength lamps offer an interesting alternative to conventional UV sources. Received: 1 March 2000 / Revised version: 5 April 2000 / Published online: 30 June 2000     

PET surface modifications by treatment with monochromatic excimer UV lamps

Received: 11 August 1997/Accepted: 14 August 1997     

Pulsed blue-light generation by the frequency doubling of the 4F3/2 to 4I9/2 transition in Nd:YAG and Nd:YAlO3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO₃ lasers emitting at 946 and 930 nm, respectively (⁴F_3/2 to ⁴I_9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively. Received: 23 June 1999 / Revised version: 8 August 1999 / Published online: 3 November 1999     

Photolytic interference affecting two-photon laser-induced fluorescence detection of atomic oxygen in hydrocarbon flames

This study reports on photochemical interferences affecting atomic oxygen detection using two-photon laser-induced fluorescence at 226 nm. In contrast to previous studies in which molecular oxygen was proven to be the relevant photochemical precursor molecule in a hydrogen-fueled flame, the present investigations were carried out in a laminar diffusion flame of methane and air. The most significant interferences were found at the fuel side of the flame in the absence of molecular oxygen, and vibrationally excited carbon dioxide was identified as the most probable precursor molecule for the photochemical production of oxygen atoms. Received: 11 December 2002 / Revised version: 10 March 2003 / Published online: 16 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-925/294-2595, E-mail: tbsette@sandia.gov     

Surface modification of a polymer film by cryogenic laser ablation of organosilicon compounds

An organosilicon compound, hexamethylcyclotrisilazane (HMCTS), was photolyzed in a solid film at 84 K with an ArF excimer laser. A polymeric film was prepared as a result of polymerization and/or crosslinking using a reactive species such as 1-(methylsilyl)methanimine, generated by photolysis. At room temperature in air, the film changed into a polymer having siloxane units. However, fragments generated by laser irradiation of HMCTS in the frozen film made a similar polymer film having a siloxane structure on a PVA film in air. The polymeric film with siloxane units prepared by the cryogenic laser ablation method displayed hydrophobic properties. Received: 23 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Photochemical welding of silica microspheres to silicone rubber by ArF excimer laser

Transparent fused silica (SiO₂) microspheres 2.5 μm in diameter were photochemically welded to transparent, flexible silicone rubber ([SiO(CH₃)₂]_n) substrate by 193 nm ArF excimer laser induced photochemical modification of silicone into silicon oxide. Single layer of silica microspheres was easily formed on an adhesive silicone rubber before laser irradiation after dropping of silica microspheres dispersed in ethanol and subsequent tape peeling. The welding rate, the percentage of welded microspheres tested by ultrasonic cleaning with ethanol, was examined by varying the single pulse fluence and irradiation time of ArF excimer laser. The welding layer underneath microsphere, silicon oxide, was also found to emit white light of strong intensity under UV light illumination.     

ZrO2 films deposited by photo-CVD at low temperatures

Zirconium oxide layers have been successfully deposited by photo-CVD at low temperatures. ZrO₂ growth was observed at temperatures as low as 100 °C. When deposited at 250 °C and above, these films exhibited a polycrystalline structure with a mixture of different crystal phases. Deposition at 300 °C was found to form moisture-free ZrO₂ films with a high refractive index of 2.1, a very low effective density of trapped electrons of ∼8.8×10⁸ cm^-2 and an interface trap density of 6.6×10⁹ cm^-2 eV^-1 being readily obtained. Received: 17 December 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002     

Photocatalytic oxidation of natural organic adsorbates on anatase TiO2 films observed by infrared adsorption spectroscopy with a multiple internal reflection geometry

Photocatalytic oxidation of organic adsorbates on anatase TiO₂ films has been examined in different atmospheres of humid air, dry air and vacuum. The photocatalytic oxidation was observed by IR absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry. The photocatalytic oxidation is the fastest in the air at a humidity of 70% where oxygen and water vapor are consuming to produce OH radicals and O₂⁻ anions on the TiO₂ surface with the UV exposure. In the dry air, a rate of the photocatalytic oxidation is almost 30% of that in the humid air, where only O₂⁻ anions oxidize the organic adsorbates. In vacuum, on the other hand, it is negligible, which suggests that adsorbed H₂O molecules do not play an important role in the photocatalytic reaction. It is suggested that an addition of the water vapor is necessary to achieve the higher catalytic activity.     

Infrared multiple photon decomposition of CHBrF2 induced by a free-electron laser

2 . The laser generates an intense infrared macropulse with a duration of 17 μs; the macropulse consists of a train of 380 micropulses, each of which has a duration of a few picoseconds. The fluence of a macropulse was estimated to be about 16 Jcm^-2 at a beam waist. Peak wavelengths were set in the range of 9–10 μm. The macropulse induced the IRMPD of 1 and 5 Torr CHBrF₂; most of molecules in the focal region seemed to decompose at a wavelength of 9.3 μm. The mechanism is the initial decomposition of CHBrF₂ to CF₂ and HBr, followed by the dimerization of CF₂ to form C₂F₄. The decomposition was found to be isotopically selective at 9.7 μm; the final product C₂F₄ had a ¹³C atomic fraction of 6%. Th e addition of CO₂ to CHBrF₂ significantly decreased the yield of C₂F₄. vibrationally excited CHBrF₂ molecules produced by laser pulses were efficiently deactivated by CO₂ molecules. Received: 7 October 1996     

Bias-induced spatially resolved growth and removal of Si-oxide by atomic force microscopy

Three mechanisms for spatially resolved growth and removal of oxide on silicon substrates have been investigated. Thermally grown oxide layers with thicknesses in the range 2–6 nm were the distinctive feature of the system. The layers were characterized and manipulated by methodologies based on atomic force microscopy (AFM) with conducting probes in a vacuum environment of 10^-2–10^-3 Pa. The probe is then effectively a travelling electrode that generates an electrostatic field between the tip and the substrate. Oxide growth was induced for a positive sample bias greater than 5 V, but below the level corresponding to dielectric breakdown. Application of a short pulse of amplitude marginally above that corresponding to dielectric breakdown, on the other hand, had the effect of producing pits of inner diameter of about 10 nm in the pre-existing oxide layer at the point of tip-to-oxide contact. Application of a low positive sample bias (less than that required for measurable oxide growth) in combination with high linear scan speed had the effect of removing a pre-existing oxide layer from the scanned field of view. The most plausible mechanisms are based on transverse ionic diffusion (for oxide growth), controlled dielectric breakdown (for formation of pits) and lateral transport of silicaceous species (for oxide removal). Received: 24 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +617-3875-7656, E-mail: s.myhra@sct.gu.edu.au     

Cavity ring-down spectroscopy of CH and CD radicals in a diamond thin film chemical vapor deposition reactor

A mixture of H₂ and CH₄ is passed over a hot-wire tungsten filament in a diamond thin film chemical vapor deposition reactor. The resulting CH radicals are measured in absorption using cavity ring-down spectroscopy (CRDS). The concentration of the CH radicals increases as the filament is approached. The rotational temperature measurements indicate a large temperature discontinuity between the filament and the CH in the gas phase. The pathways for CH production were investigated by replacing H₂ by D₂ in the feed gas mixture, which resulted in the exclusive production of CD. From this observation it is concluded that rapid H/D isotope exchange dominates in the gas phase. Nonperiodic temporal oscillations in the CH concentration are observed when a rhenium filament is used in place of a tungsten filament. The oscillations are attributed to the nonperiodic changes in the amount of carbon at the filament surface. Received: 21 August 2000 / Accepted: 23 August 2000 / Published online: 23 May 2001     

Study of photoreaction in dichromated gelatin by laser photoacoustic technique

 An experimental method to study the photochemical reaction in solid-state dichromated gelatin during exposure is proposed, in which two laser sources with different wavelengths are used to induce photoreaction and to excite photoacoustic signal, respectively. This method is to obtain the real-time characteristics during the formation of the photoreaction product. The curve is fitted using the Rosencwaig-Gersho (R-G) theory and chemical kinetics. The reaction order and the rate of reaction may be obtained by fitting the parameters. The results show that photochemical reaction in dichromated gelatin conforms to the mechanism suggested by Watanable-Westheimer, and the photosensitivity of the samples, prepared under different conditions, is dependent on the initial concentration of photoactive ions. Received: 18 April 1995/Revised version: 3 May 1996     

In-situ monitoring of UF5 nanoparticle growth in a plug-flow reactor by a low-pressure differential mobility analyzer

The dependence of UF₅ nanoparticle size distribution on growth time was investigated for various initial concentrations of UF₅ monomers. UF₅ nanoparticles prepared by photodissociation of UF₆ in a mixed-flow reactor were allowed to pass through a plug-flow reactor (PFR), and their size distribution was measured by an in-situ size-monitoring system, namely a low-pressure differential mobility analyzer. By changing the length of the PFR, the growth time in the PFR was controlled from 0 to 30 s. An analysis using Lee’s coagulation model revealed that the growth mechanism of the UF₅ nanoparticles involved a Brownian coagulation process and that the sticking probability of the nanoparticles was approximately unity. Received: 27 August 2001 / Accepted: 6 September 2001 / Published online: 20 December 2001     

Structural and electrical characteristics of zirconium oxide layers derived by photo-assisted sol–gel processing

This paper reports the photo-assisted formation of ZrO₂ layers derived by sol–gel processing at low temperatures using intense radiation from ultraviolet (UV) excimer lamps. Excellent layer properties can be readily obtained for these sol–gel layers after 5-min exposure to the UV irradiation at around 300 °C. Analyses of the as-deposited sol–gel layers by UV/VIS spectrophotometry show that the organic species contained in the layers have been removed to a large extent after 5-min irradiation. This is further confirmed by X-ray photoelectron spectroscopy analyses of the same irradiated layers, which indicate the formation of ZrO₂ with little carbon contamination contributed by the organic species and less oxidation of Si at the interface. Electrical measurements of these layers are also reported. Received: 31 July 2001 / Accepted: 6 September 2001 / Published online: 17 October 2001     

Photoemission studies of oxygen adsorbed on a LiAl(1 1 0) alloy surface: Role of Li segregation

We investigated that the effect of the number of segregated Li atoms on the rate of oxidation on a LiAl alloy surface. Oxygen molecules adsorbed on the LiAl alloy react with the surface atoms to form stable oxides. The segregated Li atoms at reconstructed surfaces (c(2×2) and (2×1)) enhance the oxidation rate and form stable LiAlO_x and Li₂O. The degree of enhancement of oxidation by segregated Li atoms varies as a function of O₂ exposure and annealing temperature, where the latter is directly related to the mode of surface reconstruction by Li segregation.     

Compositional studies of near-room-temperature thermal CVD poly(chloro-p-xylylene)/SiO2 nanocomposites

Chemical-vapor-deposited (CVD) nanostructured thin films have been recently developed to overcome the limitations of thin films from one material class. In particular polymer/SiO₂ nanocomposite thin films have been developed to reduce power consumption, cross-talk, and RC delay in the next generation of ultralarge-scale integrated devices. Since polymers mainly possess electronic polarization they inherently have a low dielectric constant. However, they often suffer from poor dielectric anisotropy, low elastic and shear moduli, and have poor resistance to metallic diffusion. As a proof of concept, poly(chloro-p-xylylene)/SiO₂ thermal CVD nanocomposites have been developed to overcome such material deficiencies. Additionally, the CVD process allows for high manufacturing throughput and compositional control in situ, both potentially advantageous for IC fabrication. The study here focuses on the polymeric phase of the nanocomposite, which as a homopolymer can possess ≈60% crystallinity and a positive optical birefringence of 0.034, both post-deposition-annealed just before the polymer’s melting point. With increasing volume percent of SiO₂, the percent crystallinity is reduced, the thin film becomes more isotropic and the index of refraction can be varied depending on the volume percent SiO₂. Received: 15 December 1999 / Accepted: 7 January 2000 / Published online: 5 April 2000