Eric T. Sevy

Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. I. Photofragmentation studies of pyrazine at 248 nm and 266 nm

The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm and 266 nm is determined by IR diode probing of the HCN photoproduct. HCN photoproducts from excited pyrazine are produced via three different dissociation channels, one that is extremely “prompt” and two others that are “late.” The total quantum yield from all reaction channels obtained at low quencher gas pressures, φ=1.3±0.2 for 248 nm and 0.5±0.3 for 266 nm, is in agreement with preliminary studies of this process as well as recent molecular beam studies. To investigate if HCN production is the result of pyrazine multiphoton absorption, this photodissociation process has been further studied by observing the HCN quantum yield as a function of total quencher gas pressure (10 mTorr pyrazine, balance SF6) and as a function of 248 nm laser fluence from 2.8 to 82 mJ/cm2. At the highest SF6 pressures, the HCN quantum yield shows strong positive correlation with laser fluence, indicating that the “prompt” chan...

Translational and rotational excitation of the CO2(0000) vibrationless state in the collisional quenching of highly vibrationally excited 2-methylpyrazine: Kinetics and dynamics of large energy transfers

The relaxation of highly vibrationally excited methylpyrazine (C5N2H6) by collisions with CO2 molecules has been investigated over the temperature range 243–364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot methylpyrazine (E′=40 987 cm−1) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=58–80) of the 0000 ground state of CO2 resulting from collisions with hot methylpyrazine were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 0000 state. In addition, the temperature dependence of the state resolved, absolute rate cons...

Rapid and convenient method for preparing masters for microcontact printing with 1–12 μm features

Mechanical scribing can be employed to create surfaces with recessed features. Through replica molding elastomeric copies of these scribed surfaces are created that function as stamps for microcontact printing. It is shown that this new method for creating masters for microcontact printing can be performed with a computer-controlled milling machine (CNC), making this method particularly straightforward and accessible to a large technical community that does not need to work in a particle free environment. Thus, no clean room, or other specialized equipment is required, as is commonly needed to prepare masters. Time-of-flight secondary ion mass spectrometry confirms surface pattering by this method. Finally, it is shown that feature size in the scribed master can be controlled by varying the force on the tip during scribing.

Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. II. Identifying the dominant energy donor for “supercollisions”

CO 2 bath molecules scattered into J=72 of the 00 0 0 vibrational state at short times after 248 or 266 nm UV excitation of pyrazine are probed using high resolution time resolved IR diode laser spectroscopy as a function of UV laser fluence from ∼3 to 80 mJ/cm2. The implications of pyrazine photodissociation for the interpretation of these collisional energy transfer experiments are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine dissociation may be the source of excitation for collisions that impart a large amount of rotational and translational energy to CO 2 molecules is examined. Transient absorption measurements probing rotationally and translationally excited CO 2 molecules produced following excitation of pyrazine are analyzed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excitation of CO 2 by both translationally hot HCN and vibrationally excited pyrazine. This analysis indicates that vibrationally hot pyrazine, which has sufficient energy to dissociate, is the source of excitation in collisions imparting large amounts of rotational and translational energy to CO 2 .

Porous, High Capacity Coatings for Solid Phase Microextraction by Sputtering

We describe a new process for preparing porous solid phase microextraction (SPME) coatings by the sputtering of silicon onto silica fibers. The microstructure of these coatings is a function of the substrate geometry and mean free path of the silicon atoms, and the coating thickness is controlled by the sputtering time. Sputtered silicon structures on silica fibers were treated with piranha solution (a mixture of concd H2SO4 and 30% H2O2) to increase the concentration of silanol groups on their surfaces, and the nanostructures were silanized with octadecyldimethylmethoxysilane in the gas phase. The attachment of this hydrophobic ligand was confirmed by X-ray photoelectron spectroscopy and contact angle goniometry on model, planar silicon substrates. Sputtered silicon coatings adhered strongly to their surfaces, as they were able to pass the Scotch tape adhesion test. The extraction time and temperature for headspace extraction of mixtures of alkanes and alcohols on the sputtered fibers were optimized (5 min and 40 °C), and the extraction performances of SPME fibers with 1.0 or 2.0 μm of sputtered silicon were compared to those from a commercial 7 μm poly(dimethylsiloxane) (PDMS) fiber. For mixtures of alcohols, aldehydes, amines, and esters, the 2.0 μm sputtered silicon fiber yielded signals that were 3-9, 3-5, 2.5-4.5, and 1.5-2 times higher, respectively, than those of the commercial fiber. For the heavier alkanes (undecane-hexadecane), the 2.0 μm sputtered fiber yielded signals that were approximately 1.0-1.5 times higher than the commercial fiber. The sputtered fibers extracted low molecular weight analytes that were not detectable with the commercial fiber. The selectivity of the sputtered fibers appears to favor analytes that have both a hydrophobic component and hydrogen-bonding capabilities. No detectable carryover between runs was noted for the sputtered fibers. The repeatability (RSD%) for a fiber (n = 3) was less than 10% for all analytes tested, and the between-fiber reproducibility (n = 3) was 0-15%, generally 5-10%, for all analytes tested. The repeatabilities of our sputtered fibers and the commercial 7 μm PDMS fiber are essentially the same. Fibers could be used for at least 300 extractions without loss of performance. More than 50 compounds were identified in a gas chromatography-mass spectrometry headspace analysis of a real world botanical sample with the 2.0 μm fiber.

Supercollisions, photofragmentation and energy transfer in mixtures of pyrazine and carbon dioxide

The quantum yield for the formation of HCN from the photodissociation of pyrazine excited at 248 nm is determined by IR diode probing of the HCN photoproduct. The quantum yield obtained at low quencher gas pressures, ϕ= 0.81 ± 0.18, is in agreement with the value recently obtained from molecular beam/photofragmentation studies of this process. Analysis of the quenching data within the context of the strong collision model allows an estimate of the first-order rate constant for HCN production from pyrazine excited at 248 nm, kd= 1.6 × 105 s–1. Direct, IR transient absorption measurements of the HCN photoproducts confirm the µs timescale for pyrazine dissociation extracted from the quenching experiments. The implications of this photodissociation process for the interpretation of recent collisional energy-transfer experiments involving pyrazine and CO2 are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine dissociation may be the source of excitation for collisions which impart a large amount of rotational and translational energy to CO2 molecules is examined. Transient absorption measurements of rotationally and translationally excited CO2 molecules produced following excitation of pyrazine are analysed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excitation of CO2 by both translationally hot HCN and vibrationally excited pyrazine. This analysis indicates that vibrationally hot pyrazine, which is above the threshold for dissociation, is the dominant source of excitation in collisions which impart large amounts of rotational and translational energy to CO2.

Alkyl Monolayers on Silica Surfaces Prepared from Neat, Heated 3-Glycidoxypropyldimethylethoxysilane Analyzed by XPS

Silane monolayers on silica, prepared from mono-, di-, and trichlorosilanes, are widely used in industry for surface functionalization and modification. However, unlike di- and trichlorosilanes, monochlorosilanes are particularly easy to work with because they can dimerize, but not polymerize, upon reaction with water. Typically, an organic solvent is used when depositing a silane monolayer. Here we show XPS spectra of monolayers of 3-glycidoxypropyldimethylethoxysilane (CAS# 17963-04-1) on silicon oxide (silicon wafer) prepared using a rapid, solvent-free approach. Reaction conditions are 100 °C for 10 min using the neat (pure) compound, and no inert atmosphere or special treatment of the compound is required.

Alkyl Monolayers on Silica Surfaces Prepared from Neat, Heated (Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-dimethylchlorosilane Analyzed by XPS

Silane monolayers on silica, prepared from mono-, di-, and trichlorosilanes, are widely used in industry for surface functionalization and modification. However, unlike di- and trichlorosilanes, monochlorosilanes are particularly easy to work with because they can dimerize, but not polymerize, upon reaction with water. Typically, an organic solvent is used when depositing a silane monolayer. Here we show XPS spectra of monolayers of (Tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-dimethylchlorosilane on silicon oxide (silicon wafer) prepared using a rapid, solvent-free approach. Reaction conditions are 60 °C for 10 min using the neat (pure) compound, and no inert atmosphere or special treatment of the compound is required.

Alkyl Monolayers on Silica Surfaces Prepared from Neat, Heated ClSi(CH3)2(CH2)6CH=CH2 Analyzed by XPS

Silane monolayers on silica, prepared from mono-, di-, and trichlorosilanes, are widely used in industry for surface functionalization and modification. However, unlike di-and trichlorosilanes, monochlorosilanes are particularly easy to work with because they can dimerize, but not polymerize, upon reaction with water. Typically, an organic solvent is used when depositing a silane monolayer. Here we show XPS spectra of monolayers of ClSi(CH3)2(CH2)6CH=CH2 (octenyldimethylchlorosilane, CAS# 17196-12-2) on silicon oxide (silicon wafer) prepared using a rapid, solvent-free approach. Reaction conditions are 120 °C for 10 min using the neat (pure) compound, and no inert atmosphere or special treatment of the compound is required.