Jeremy Moxom

Single and double ionization of neon, krypton and xenon by positron impact

Measurements of the single (σi+) and the ratio of double to single (R2) ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV. By multiplying the R2 values of each target by the appropriate σi+, new data for the corresponding double (σi2+) ionization cross sections have also been obtained. The ratio between positron and electron σi+ maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target. Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron-atom scattering.

Dependence of porosity in methyl-silsesquioxane thin films on molecular weight of sacrificial triblock copolymer

Abstract Positron annihilation spectroscopy is used to characterize pore sizes and their interconnectivity for porous low dielectric constant methyl-silsequioxane thin films, which are templated by triblock copolymers. More than 1 μm of ortho-positronium (o-Ps) diffusion length has been observed for certain films, indicating high interconnectivity of pores. S parameters from Doppler broadening of annihilation photons (DBAP) show homogeneous depth profiles of these films. o-Ps 3γ emission and lifetimes show that the pore interconnectivity depends on the characteristics of the triblock copolymers. Pores generated from polymers with higher molecular mass are smaller and more closed, while pores generated by polymers with lower molecular mass and smaller ethylene oxide fraction are more interconnected.

Porosity in porous methyl-silsesquioxane (MSQ) films

An ideal porous methyl-silsesquioxane (MSQ) film for low-k dielectrics would consist of a network of small, closed pores with homogeneous size distribution. In this work, we studied the pore size and interconnectivity between pores for porous MSQ films templated with various types of triblock copolymers (pore generator) using positrons by measuring ortho-positronium (o-Ps) 3γ emission, Doppler broadening of annihilation photons (DBAP), and o-Ps lifetimes. Results show that the depth profiles of the pore structure depend on the molecular mass of pore generators. DBAP measurements show an interfacial layer between the porous films and the Si substrate, which is believed to be the native oxide on the Si wafer surface. Our results suggest the counter intuitive interpretation that porous MSQ films templated by triblock copolymers with larger molecular-mass have smaller and more closed pores, while those with smaller molecular mass and lower ethylene oxide fractions tend to contain larger and more-interconnected pores.

Characterization of Mg doped GaN by positron annihilation spectroscopy

Metalorganic vapor-phase epitaxially grown GaN films have been studied using beam positron annihilation spectroscopy. Positron lifetime and Doppler broadening of annihilation radiation measurements indicate that defects, probably Ga vacancies, are reduced in concentration when films are doped with Mg, as previously observed by other workers. In lightly doped films a homogeneous layer with a minimum defect concentration is present from just below the surface, to a depth of around 0.1 μm. For heavily doped films, there is evidence of a further substantial decrease in the defect concentration following an anneal to 900 °C. The effects of impurities and defects are discussed.

Fragmentation and ionization of CH3F by positron and electron Impact

Abstract New data for the fragmentation of CH3F by low energy e+ using an improved spectrometer are presented. The positively charged fragment ions are detected and mass analyzed using a time-of-flight mass spectrometer and the ion yields are measured as a function of impact energy. For comparison, corresponding data have been obtained for e− scattering, using a similar apparatus.

Microchip Ion Trap Mass Spectrometry

We have been exploring the use of cylindrical ion traps of 1-mm diameter and smaller for mass spectrometry. These miniature ion traps have dimensions that are comparable to the thickness of the substrates used in microfluidic devices and their simple structure is amenable to microfabrication techniques. Operating voltages are lower than for conventional ion traps while RF frequency and working pressure are higher, making miniaturized electronics and vacuum systems possible as well. Mass resolution comparable to or better than for laboratory ion trap mass spectrometers has been demonstrated. The number of ions that can be stored in traps of equal energy depth scales as the radius of the cavity rather than the volume. This property makes two-dimensional arrays of small ion traps attractive for increased dynamic range. Several examples of miniature ion traps, methods of sample ionization, mass analysis, and ion detection will be described.

Present and future positron-molecular scattering experiments

Abstract Progress to date (June, 1999) of the Oak Ridge positron scattering project is reviewed. Results from our positron time-of-flight mass spectrometer include ionization and fragmentation of some halomethanes, positronium formation below the threshold for double ionization of Ne and the heavier noble gases, and the production of HeH + by positron impact on mixtures of He and H 2 . New directions for future experimental work are discussed. These entail the construction and testing of a new spectrometer which measures both the masses and recoil energies of charged products, including secondary electrons. We hope to use an intense positron beam, such as the one at Lawrence Livermore National Laboratory or a similar beam, as the ionizing agent for the new spectrometer, which contains a pulsed supersonic molecular beam. With such an instrument, a range of new phenomena can be studied.

Low-energy positron-argon elastic scattering at 30°

The energy dependence of the cross section for positrons scattering elastically at 30° from argon in the vicinity of the positronium (Ps) formation threshold has been investigated. Together with previous experimental and theoretical results, the present data indicate the influence of channel-coupling effects on elastic scattering from Ps formation and suggest a similar effect from positron impact excitation.

Fragmentation and ionization of CH3Cl by positron impact

Abstract The yields of positive ionic fragments produced by positron impact from CH3Cl have been studied in the energy range from threshold to 50 eV. The interactions take place whilst the positrons are stored in a Penning trap and the ions are detected using a time of flight mass spectrometer. The cross-sections for production of the parent molecular ion and the fragment CH3+ are presented.