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     

Laser vaporization of carbon in the presence of carbon dioxide

2 . The dependence of the ion production on the laser ablation parameters is investigated and the expansion dynamics of the ablated species is studied through time and space resolved measurements of the ion yield. We discuss our observations on the basis of reactions involving neutral and ionized carbon-based species. Received: 11 March 1997/Accepted: 30 June 1997     

Femtosecond transition state dynamics of cis -stilbene

Femtosecond time-resolved photoion and photoelectron spectroscopy were employed to study the transition state dynamics of cis-stilbene in a molecular beam experiment. Two long-lived photoproducts were found with excitation of 267 nm radiation. Received: 4 August 2000 / Revised version: 11 October 2000 / Published online: 6 December 2000     

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     

Photoassociation Reactions H＋D^＋ and H^＋ ＋D in Ultrashort Pulse Laser Fields

The photoassociation reactions H＋D^＋ and H＋ ＋D induced by the one- and two-colour laser pulses are theoretically studied. The reactions occur through two channels： （i） H（1S）＋D^＋ →HD^＋, and （ii） H^＋ ＋D（1S）→HD^＋. The reaction probability through channel （ii） is higher than through channel （i）. The populations of the products H^＋ ＋D and HD＋ and the ratio of H^＋ ＋D to HD＋ can be controlled by laser pulses. The two-colour laser pulses can more efficiently lower the dissociation probability and therefore enhance the yield ratio of the product HD＋.     

Perspectives of laser processing and chemistry

Laser-induced material processing is reviewed with special emphasis on recent achievements mainly obtained by the Linz group. Among those are investigations using optical fiber tips for nanoscale photophysical etching, laser-induced surface patterning using self-assembled microspheres, the pulsed-laser deposition of thin films of high-temperature superconductors and the modification and cleaning of surfaces. Received: 7 February 2003 / Accepted: 6 March 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at     

Direct measurement of bonded film thickness of A20H lubricant

Currently the bonded lubricant thickness is measured either by Fourier transform infrared spectroscopy (FTIR) or electron spectroscopy for chemical analysis (ESCA) on the remaining lubricant after using fluorocarbon solvent like Vertrel to rinse off the mobile lubricant from magnetic disk surfaces. As the thickness of the lubricant applied on a disk approaches to its molecular dimension (～10 Å), the current measurement methods face tremendous challenges in achieving the desired levels of accuracy and sensitivity. We propose a new method that makes use of a time-of-flight mass spectrometry (TOF-SIMS) to directly measure the bonded film thickness of A20H lubricant by quantifying the ratio of either the hydroxyl end-group or the phenoxy portion of the cyclotriphosphazene end-group with respect to their respective neighboring backbone fragments. The quantified ratios include C₂F₃/CF₂CH₂OH and C₆F₃O/CF₃C₆H₄ measured in the positive polarity and C₂F₃O/OCF₂CH₂OH, and C₇F₅/CF₃C₆H₄O₂H₂ measured in the negative polarity. The transfer function from the quantified ratios to the bonded lubricant thickness (t) is given in the form of t=α×ln?(100×R _T )?β, where α,β are constants for a selected ratio and R _T represents the quantity of the specific ratio. The results correlate very well with the FTIR method currently used in the measurement of the magnetic media during production (R ²>90). The new method can complete the measurement of the bonded lubricant thickness in a one-step process and it has a much higher spatial resolution at sub-micrometers than that of the FTIR or ESCA with order of a few tens of micrometers in resolution. The quantified ratio obtained from this TOF-SIMS technique makes the imaging of the localized bonded lubricant possible, which can be applied in the magnetic media failure analysis.     

Spectroscopy in our age

The development of modern spectroscopy is summarized from Bunsen’s detection of atoms as the beginning of spectral analysis to modern molecular spectroscopies including new high resolution techniques for molecular ions. Recent experiments involving long range charge migration in peptides and proteins are outlined. Received: 2 October 2000 / Published online: 11 October 2000     

Detection of aromatic pollutants in the environment by using UV-laser-induced fluorescence

Fluorescence spectra, decay times, and detection limits of the 16 US-EPA polycyclic aromatic hydrocarbons (PAH) have been measured and the results are discussed in terms of separability of single PAHs in a multicomponent mixture. The results of a mathematical analysis of a 16-component mixture is presented. Spectra from natural water samples contaminated with gasoline and tar are also presented and the potential and limits of the LIF technique are discussed. Received: 19 March 1998/Revised version: 2 June 1998     

Gas-phase aggregation of fullerenes by laser ablation of fullerenes in reactive and nonreactive matrices

60 molecules, whereas little or no coalescence was found in the nonreactive mixture. In the negative ion TOF channel the maximum peak of twice C₆₀ coalescence by laser desorption was always located at masses greater than 120 carbon mass units, which is different from all previous reports. We suggest that the formation mechanism was due to strong interactions between the C₆₀ molecules and silica particles or surface species such as the H or OH group existing in the matrix. Received: 6 May 1996/Revised version: 22 July 1996     

NO and PO photofragments as trace analyte indicators of nitrocompounds and organophosphonates

Laser photolysis that releases characteristic photofragments, combined with laser-induced fluorescence (LIF) that subsequently monitors them, facilitate detection of trace-vapor analytes in air. The feasibility of the technique is demonstrated for nitrocompounds and organophosphonates detection. Using one-color and two-color lasers, respectively, the representative NO and PO photofragments were tagged. Employment of longer wavelengths for photodissociation, and where possible also for LIF, than for fluorescence collection, obstructs background fluorescence and in the former compounds also prevents interference of ambient ground state NO. Received: 1 February 2000 / Revised version: 27 April 2000 / Published online: 20 September 2000     

The use of rovibrationally excited NO photofragments as trace nitrocompounds indicators

A one-color laser performs photolysis of nitrocompounds and laser-induced fluorescence to monitor the generated NO photofragments and to sensitively detect vapor trace amounts of nitrocompounds. The NO is monitored via excitation and emission through A²Σ⁺(v^′=0)?X²Π(v^′′=2-0) and A²Σ⁺(v^′=0)?X²Π(v^′′=0,1) transitions, respectively. It is found that NO photofragments populate the vibrationless ground state and also the first two vibrationally excited states. The analytical performance of the method is demonstrated on 2,4 dinitrotoluene (DNT) via excitation through A²Σ⁺(v^′=0)?X²Π(v^′′=2). The achieved limits of detection are 3.7 and 2.7 parts per billion (ppb) by weight of gaseous DNT in 100 and 500 Torr of air, for 30 s integration time. The application of this scheme for trace nitrocompound detection has the advantage that no background of ambient ground-state NO interferes and that the fluorescence is collected at shorter wavelengths than the exciting radiation, precluding background fluorescence. Received: 18 August 1999 / Revised version: 22 November 1999 / Published online: 8 March 2000     

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.     

Quantitative laser-induced fluorescence of CH in atmospheric pressure flames

Absolute number densities of the CH radical were determined in a partially premixed methane/air flame (equivalence ratio was 1.36) at atmospheric pressure by exciting a predissociating level via the CH B–X(1,0) transition using a quasi-linear laser-induced fluorescence scheme. The peak number density was (1.0±0.4)×10¹³ cm^-3 or 2.4±1 ppm at 1900 K, with a flame-front width of 250 μm (FWHM). Rotational energy transfer must be considered for correct laser-induced fluorescence signal interpretation. Competition between optical pumping and rotational relaxation in both excited and ground states produces a signal that varies almost linearly with laser pulse energy even for large pumping rates. For these conditions, the population of the initial ground-state rotational level is depleted by optical pumping, and rotational energy transfer collisions rapidly repopulate the level during the laser pulse. Deviations from linear behavior are less than 20%. The effects of spatial resolution and polarization of the fluorescence on the absolute measurements are also discussed. Received: 27 March 2002 / Revised version: 22 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Present address: Lam Research Corporation, Fremont, CA 94538, USA RID="**" ID="**"Present address: Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands RID="***" ID="***"Present address: Mechanical Engineering Dept., Stanford University, Stanford CA 94305, USA RID="****" ID="****"Corresponding author. Fax: +1-650/859-6196, E-mail: smith@mplvax.sri.com     

Laser ablation of polysulfone films: a laser ionization TOF mass spectrometric study

2 . Major peaks were assigned to direct fragments and recombination products ejected from the PS surface. The arrival profiles of these ablation products varied from product to product and were fitted by using a shifted Maxwell–Boltzmann distribution with a center-of-mass flight velocity [(1.4-5.6) ×10⁵ cm/s] and a Knudsen layer temperature (350–3810 K). Two types of ablation products, whose velocities and temperatures showed different dependences on laser fluence, were found to exist. Dynamical aspects in the decomposition of the polymer chain, the ejection of various fragments, and their expansion are discussed on the basis of a photothermal ablation model, where a heated surface layer with a temperature gradient along its depth plays an important role. Received: 11 August 1998/Accepted: 12 August 1998     

Laser ablation of phenylazide in an argon matrix: direct observation and chemical reactivity of ablated fragments

Ablation of pentafluorophenylazide (FPA) in an Ar matrix at 8–10 K was carried out upon irradiation with ns-pulsed UV lasers in a vacuum. The plume of ablated products was monitored by a time-resolved imaging/spectroscopic technique using a gated and intensified CCD camera system. A large amount of pentafluorophenylnitrene (FPN) having a high kinetic energy (≈6 eV) was ejected as fragments from the matrix film during ablation. A quantitative formation of triplet FPN from the photolysis of the FPA was observed by spectroscopic measurements in the IR and UV-visible regions, and was confirmed by a theoretical IR spectrum calculated according to density functional theory. A FPN beam is useful for chemical surface modification of organic materials, such as aromatic polyester and alkylthiol. A surface analysis of these materials by X-ray photoelectron spectroscopy and Fourier transform infrared reflection absorption spectroscopy showed that the FPN was immobilized onto the surface through chemical bonds. This technique for the chemical surface modification of materials is made possible by a pulsed beam of reactive fragments with a high density in the laser ablation process. Received: 14 June 1999 / Accepted: 18 June 1999 / Published online: 21 October 1999     

Removal of oxygen atoms from a SiO2 surface by incoherent vacuum ultraviolet excimer irradiation

We have demonstrated the non-thermal removal of oxygen atoms from an oxidized silicon surface (SiO₂) on a silicon wafer by the use of a low-power (0.3 mW cm^-2) incoherent vacuum ultraviolet (VUV) light source at 126 nm. X-ray photoelectron spectroscopy (XPS) has shown that a maximum Si concentration of 80% appears at the surface after a 20-h irradiation with 9.8 eV photons, as a result of oxygen removal from the SiO₂ matrix. The surface morphology, however, indicates no damage or melting on the surface even after the irradiation. Received: 15 April 2002 / Accepted: 17 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Permanent address: Nano-Tech Photon Inc., Shimotomita 4132-1, Shintomi, Miyazaki, 889-1404, Japan RID="**" ID="**"Corresponding author. Fax: +81-985/583-899, E-mail: kubodera@opt.miyazaki-u.jp     

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     

Atomic force microscopy of photochemistry and the Baeyer–Villiger reaction of 4,4′-dimethylbenzophenone in the solid state

Nanostructures are formed by photodimerization of crystalline 4,4^′-dimethylbenzophenone (1) through intermolecular hydrogen abstraction and Baeyer–Villiger reaction of 1 with m-chloroperoxybenzoic acid (MCPBA) (5) in the solid state. Atomic force microscopy (AFM) reveals that the crystal face (001) of 1 during photodimerization exhibits volcanoes, whereas the same face (001) of 1 yields both craters and volcanoes all over the surface from the contact edge of the crystals during the Baeyer–Villiger reaction. All the experimental results are correlated with the bulk crystal structure. Molecular interpretation of the AFM features of 1 is given. Received: 18 April 2000 / Revised version: 26 July 2000; accepted: 27 July 2000 / Published online: 30 November 2000     

CO2 laser CVD of a-Si:H: in situ gas analysis and model calculations

4 and disilane Si₂H₆ induced by continuous wave CO₂ laser irradiation has been investigated under the conditions of chemical vapor deposition (CVD) of amorphous hydrogenated silicon a-Si:H. At the very position of depositing the thin film the stationary chemical composition of the processing gas is probed in situ by an effusive molecular beam which passes through a differential pumping stage into a quadrupole mass spectrometer (QMS). With SiH₄ as educt and SF₆ as a sensitizer, SiH₄ and Si₂H₆ are found in the processing gas while Si₃H₈ or higher silanes are lacking. Si₂H₆ and SF₆ lead to SiH₄, Si₂H₆, and Si₃H₈, but higher silanes are missing. The experimentally determined composition of the processing gas is semi-quantitatively reproduced by model calculations based on the assumption of stationary local equilibrium conditions and applying thermodynamic and spectroscopic data (molecular statistics). The mass balance of the processing gas entering and leaving the CVD chamber states an atomic ratio Si:H of 1:2 for the gas phase species forming the solid deposit on the reactor walls. This finding together with theoretical considerations indicates the intermediate Si₂H₄ to be the dominating gas phase species forming the a-Si:H thin films. Received: 17 July 1998/Accepted: 20 July 1998