S. C. Langford

Single asperity tribochemical wear of silicon nitride studied by atomic force microscopy

Nanometer scale single asperity tribochemical wear of silicon nitride was examined by measuring the wear of atomic force microscope tips translated against a variety of substrates in aqueous solutions. We show that the chemical nature of the substrate plays an important role: significant wear was observed only when the substrate surface is populated with appropriate metal-hydroxide bonds. Mica and calcite substrates, whose water-exposed cleavage surfaces lack these bonds, produced little if any tip wear. As a function of contact force FN and scan duration t, the length of the tips in this work decreases approximately as (FNt)0.5. We propose that pressure-induced intermediate states involving hydroxyl groups form on both the tip and the substrate; chemical reactions subsequently form transient bridging chemical bonds that are responsible for tip wear.

Color center formation in soda-lime glass with femtosecond laser pulses

We show that exposure of soda-lime glass to ultrafast laser pulses at 800 nm causes coloration (darkening). We have characterized this coloring with time-resolved measurements of the transmission of 633 nm light through the glass during laser exposure. Reverse processes (partial bleaching) operate on time scales of μs to seconds. The competition between coloration after the femtosecond pulse and the subsequent transmission recovery limits the darkening that can be achieved at a given femtosecond pulse energy and repetition rate. The response of soda-lime glass to 400 and 267 nm ultrafast pulses is quite similar, although much lower pulse energies are required for darkening. We argue that darkening is due to absorption processes that produce mobile charge carriers, which then interact to produce trapped hole centers (H3+) that absorb strongly at 633 nm. Trapped electrons (that form E centers) are the likely cause of the accompanying loss of transmission in the near ultraviolet. Finally, we show that diffra...

Structure property relations of photoreactive polymers designed for laser ablation

Abstract The ablation characteristics of various polymers were studied at low fluences, and structure property relations were obtained. The polymers containing the photochemically most active group (triazene) are also the polymers with the lowest threshold of ablation and the highest etch rates, followed by a designed polyester and then polyimide. No pronounced influences of the absorption coefficients, neither α lin nor α eff , on the ablation characteristics are detected. The thermal properties of the designed polymers are only of minor importance. Intensities of fragments obtained by time-of-flight mass spectrometry measurements show pronounced differences between irradiation at the absorption band of the triazene group (308xa0nm) and irradiation at a shorter wavelength (248xa0nm). The larger fragments reveal lower intensities for 248xa0nm irradiation, due to the additional decomposition of these fragments by the higher energy of 248xa0nm photons and the lower etch rates for 248xa0nm irradiation. An excimer lamp emitting at 308xa0nm is applied to decompose the triazene group without major decomposition of the aromatic system. Irradiation with shorter wavelengths, i.e. 222 and 172xa0nm, causes in addition the decomposition of the aromatic system. Two novel applications for the special designed photopolymers are shown. These polymers can be applied in combination with phase masks for the fabrication of microoptical elements using laser ablation. Another quite different application utilizes near-IR irradiation. The plasma created by laser ablation acts as a micro thruster.

Triboelectric charging of a perfluoropolyether lubricant

Perfluoropolyethers (PFPE) are used extensively to lubricate hard disks for high density information storage. Some PFPE lubricants have been shown to be vulnerable to dissociative electron attachment as well as electron bombardment. Triboelectric charges generated during sliding contact may generate quasi-free electrons capable of degrading the lubricant. We describe measurements of triboelectric charging in a model polymer-metal system with and without a PFPE lubricant and show that negative charge accumulates on the substrate polymer surface, both with and without the lubricant. In the presence of the lubricant, some of this charge moves through and becomes bound to the lubricant. Electron and positive ion emission from PTFE-lubricated surfaces during contact sliding again indicate motion of charge through the lubricant. Ion emission in particular suggests possible degradation of the lubricant.

Polymers designed for laser ablation-influence of photochemical properties

Abstract The ablation characteristics of various polymers were studied at low and high fluences. The polymers can be divided into three groups, i.e. polymers containing triazene and ester groups, the same polymers without the triazene group, and polyimide as reference polymer. At high fluences similar ablation parameters, i.e. etch rates and effective absorption coefficients, were obtained for all polymers. The main difference is the absence of carbon deposits for the designed polymers. At low fluences (at 308xa0nm) very pronounced differences are detected. The polymers containing the photochemically most active group (triazene) exhibit the lowest threshold of ablation (as low as 25xa0mJxa0cm −2 ) and the highest etch rates (up to 3xa0μm/pulse), followed by the designed polyesters and then polyimide. The laser-induced decomposition of the designed polymers was studied by nanosecond-interferometry. Only the triazene-polymer reveals etching without any sign of surface swelling, which is observed for all other polymers. The etching of the triazene-polymer starts and ends with the laser pulse, clearly indicating photochemical etching. The triazene-polymer was also studied by time-of-flight mass spectrometry (TOF-MS). The intensities of the ablation fragments show pronounced differences between irradiation at the absorption band of the triazene group (308xa0nm) and irradiation at a shorter wavelength (248xa0nm).

Influence of the irradiation wavelength on the ablation process of designed polymers

Abstract A photolabile triazene polymer was irradiated with a KrF (248xa0nm) and a XeCl (308xa0nm) excimer laser. The resulting ablation fragments were analyzed using time of flight mass spectroscopy (TOF-MS). The peak intensities of the main ablation fragments reveal a pronounced influence of the irradiation wavelength and fluence. After 248xa0nm irradiation the peak intensities of all polymer fragments reach maxima at high fluences whereas after 308xa0nm irradiation only the 168xa0amu peak intensity reaches a maximum. All other analyzed fragments (28, 35 and 76/77xa0amu) do not reveal a maximum. The time of arrival curves of the decomposition products at both irradiation wavelengths are composed of three distinctive components. Two quite sharp peaks are fitted by a Gaussian distribution, whereas a third broad peak is described by a decaying Maxwell–Boltzmann distribution. The first fast peak is attributed to the fragments produced during the laser pulse. The second peak was detected around 100xa0μs after the laser pulse. The peak is very sensitive to the alignment of the setup, but very insensitive to changes in the electrical fields, and was still pronounced after grounding the ionizer. Therefore the peak was assigned to excited neutral nitrogen molecules (metastable N2).

Determination of strain localization in aluminum alloys using laser-induced photoelectron emission

Uniaxial tensile deformation of oxidized aluminum produces low work-function patches of fresh metal which can be probed by measurements of photoelectron emission during exposure to ultraviolet light. We report measurements of photoelectron emission during uniaxial testing of polycrystalline Al(1200), Al–Mn(3003), Al–Mg(5052), and Al–Mg–Si(6061) alloys where the broad face of the gauge section is exposed to pulsed excimer laser radiation (248nm). We show that strain localization alters the distribution of fresh surface metal produced by subsequent deformation. The transition from more homogenous deformation to the principally localized deformation associated with shear bands is associated with a discontinuity in the growth rate of photoelectron intensities versus time. At this transition, the rate of fresh metal production along the illuminated portion of gauge section decreases. In all four materials, the strain at the discontinuity is somewhat below the strain given by the Considere criterion, consistent...

The effect of water vapor and bulk temperature on positive ion emission from wide bandgap single crystals during exposure to 248 nm excimer laser radiation

We investigate the effect of low partial pressures of water vapor and bulk temperature on laser-induced positive ion emission from cleaved, single-crystal sodium nitrate and sodium chloride. Both materials yield more intense Na þ emission in the presence of water vapor ðPH2O � 1 � 10 � 5 PaÞ than samples irradiated under UHV conditions (Ptotal < 1 � 10 � 7 Pa). Emission intensities increase rapidly with increasing temperature in a fashion consistent with activation energies on the order of 0.07–0.08 eV, both with and without water vapor. We attribute Na þ emission to a defect-mediated photoelectronic process. Na þ adsorbed at or near surface anion vacancies (electron traps) is ejected when the underlying or adjacent vacancy is photoionized. Vacancy formation in both materials is expected to have activation energies on the order of 0.08 eV, suggesting that Na þ emission is rate limited by the production of surface anion vacancies. We attribute the effect of water vapor to defect-mediated sorption that increases the ion emission probability. # 2002 Elsevier Science B.V. All rights reserved.

Interaction of wide-band-gap single crystals with 248-nm excimer laser irradiation. IX. Photoinduced atomic desorption from cleaved NaCl(100) surfaces

Neutral atomic sodium and chlorine emissions from cleaved, single-crystal NaCl(100) surfaces due to pulsed, 248-nm excimer laser irradiation have been characterized by time-resolved, quadrupole mass spectroscopy. At laser fluences below the threshold for optical breakdown, the resulting time-of-flight signals are consistent with particles emitted in thermal equilibrium with a laser-heated surface. Activation energy measurements made by varying the substrate temperature are consistent with F-H pair formation under UV excitation. By varying the laser fluence and estimating the effective surface temperature from the time-of-flight signals, additional activation energy measurements were made. The corresponding rate-limiting step is attributed to a thermally assisted, photoelectronic process involving atomic steps. Atomic force microscope images of surfaces irradiated at low fluences show monolayer islands that are created by the aggregation of material desorbed from steps. At somewhat higher fluences, monolay...

Interaction of wide-band-gap single crystals with 248-nm excimer laser irradiation. X. Laser-induced near-surface absorption in single-crystal NaCl

Ultraviolet laser-induced desorption of neutral atoms and molecules from nominally transparent, ionic materials can yield particle velocities consistent with surface temperatures of a few thousand kelvin even in the absence of visible surface damage. The origin of the laser absorption required for this surface heating has been often overlooked. In this work, we report simultaneous neutral emission and laser transmission measurements on single-crystal NaCl exposed to 248-nm excimer laser radiation. As much as 20% of the incident radiation at 248 nm must be absorbed in the near-surface region to account for the observed particle velocities. We show that the laser absorption grows from low values over several pulses and saturates at values sufficient to account for the surface temperatures required to explain the observed particle velocity distributions. The growth of absorption in these early pulses is accompanied by a corresponding increase in the emission intensities. The diffuse reflectance spectra acqui...