Brian Van Devener

Collision-induced dissociation of formaldehyde cations: The effects of vibrational mode, collision energy, and impact parameter

We report a study of collision-induced dissociation (CID) of H2CO+, including measurement of the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the CID integral and differential cross sections. CID was studied for collision with both Xe and Ne, and the Ne results provide a very detailed probe of energy transfer collisions leading to CID. The CID appearance threshold is found to depend only on total energy, but for all energies above threshold, vibrational energy is far more effective at driving CID than Ecol, with some mode-specificity. Results are fit with an impact parameter-based mechanism, and considerable insight is obtained into the origins of the Ecol and vibrational effects. A series of ab initio and RRKM calculations were also performed to help interpret the results.

Reaction of formaldehyde cation with methane: Effects of collision energy and H2CO+ and methane vibrations

The effects on the title reaction of collision energy (Ecol), five H2CO+ vibrational modes, and deformation vibrations of methane have been studied, including the measurement of product integral and differential cross sections over a center-of-mass Ecol range from 0.09–3.3 eV. Electronic structure and RRKM calculations are reported, providing an additional mechanistic insight. The total reaction efficiency is well below unity, despite there being two exoergic reaction pathways with no activation barriers. The energetically more favorable channel corresponds to H elimination (HE) from an intermediate complex, however, this channel accounts for only ∼15% of the total reaction cross section at low Ecol and is negligible at high energies. The dominant channel, hydrogen abstraction (HA) by H2CO+ from methane, is dominated by a complex-mediated mechanism at low Ecol, switching over to a direct hydrogen-stripping mechanism at high Ecol. Both HA and HE are inhibited in a strongly mode-specific fashion by H2CO+ vi...

The effects of vibrational mode and collision energy on the reaction of formaldehyde cation with carbonyl sulfide

The effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the title reaction have been studied, including measurements of product ion recoil velocity distributions. A series of ab initio and Rice–Ramsperger–Kassel–Marcus calculations were used to examine properties of various complexes and transition states that might be important. Four product channels are observed. Proton transfer (PT) dominates at low Ecol, and is suppressed by Ecol but mildly enhanced by H2CO+ vibrational excitation. PT occurs by a direct mechanism at high energies, but appears to be mediated by reactantlike complexes at low energies. The other major low energy channel corresponds to H2+ transfer, and the majority of these product ions go on to eliminate CO, producing H2S+. Both H2+ transfer and H2S+ channels are strongly inhibited by Ecol and vibrational excitation, which is interpreted in terms of competition with other channels. Charge transfer occurs in short time scale collisions at all energies, and is...

Pulsed laser induced ohmic back contact in CdTe solar cells

Creating an ohmic back contact has long been a problem for making efficient CdTe solar cells. Current devices utilize some combination of preferential chemical etching, buffer layer, and Cu doping with additional cost, time, and complexity added for each step. In this Letter, these processes are eschewed and replaced with a nanosecond pulsed ultraviolet laser treatment. It is shown that this treatment can eliminate the rollover effect seen in photovoltaic current-voltage (J-V) curves that is indicative of a non-ohmic back contact. Transfer length measurements show that a single UV laser pulse can reduce the specific contact resistivity by a factor of 24 versus untreated samples. X-Ray photoemission spectroscopy shows evidence of increased conductivity and of elemental Te created at the surface by laser pulses. Finally, finite element modeling is used to model the laser-sample interaction, which predicts both the temperature and the amounts of Cd and Te lost during a laser pulse.

Vibrational mode and collision energy effects on reaction of H2CO+ with C2D4

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass E(col) range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition.

Surface stoichiometry of pulsed ultraviolet laser treated polycrystalline CdTe

The effects of nanosecond pulsed ultraviolet laser annealing on the surface stoichiometry of close-space sublimated polycrystalline thin films are investigated using angle-resolved x-ray photoemission spectroscopy (XPS). The raw data suggest the formation of a Cd-rich surface layer, but this is counter to the expectation based on Cd and Te vapor pressures above CdTe that predicts a Te-rich layer and to direct observation of elemental Te at the surface. In order to explain this apparent discrepancy, we analyze our XPS data in the context of prior reports of lateral segregation of Cd and Te at the surface after pulsed laser treatments with a simple model of angular dependent XPS in the presence of surface roughness. This analysis reveals that a uniform Te layer cannot explain our results. Instead, our analysis suggests that Te enrichment occurs near grain boundaries and that a sub-monolayer Cd layer exists elsewhere. These complex yet repeatable results underscore the challenges in measuring surface stoichi...

Automated 3D EDS Acquisition for Spatially Resolved Elemental Characterization of Catalyzed MgH2 Nanostructures

Since the introduction of large area Silicon Drift Detectors (SDD) for EDS, the acquisition time for high resolution EDS maps has decreased by more than an order of magnitude. For example, typical EDS mapping times with good signal to noise for a 256 x 256 pixel map can be now acquired in less than 3 minutes, depending upon the concentration of mapped elements within the sample and incident probe current on the sample. Combined with advances in computer automation of electron microscopes, this has made the acquisition of three-dimensional compositional data via EDS tilt tomography a practical reality.

Te-rich CdTe surface by pulsed UV laser treatment for ohmic back contact formation

Pulsed UV laser treatments have recently been applied to polycrystalline CdTe solar cells to create an ohmic back contact. In this work, we investigate the surface stoichiometry variations produced by pulsed laser excitation using X-ray photoelectron spectroscopy (XPS). These results reveal surfaces that are becoming more Cd-rich, counter to what is expected from calculations and what has been measured by others. To understand this apparent disagreement, we have constructed a geometric model of the sample surface considering surface topography and a segregation of phases at the surface. From this, we infer that the laser treatments do produce a Te enrichment as expected, but that this enrichment is localized near the grain boundaries. Furthermore, the apparent Cd-enrichment occurs because the Te-rich regions contribution to the XPS intensity is lessened by topographic and surface sensitivity effects as well as a sub-monolayer of Cd found elsewhere on the film surface.

Implementing a Software Based User Access System in a Core Analytical Lab: Benefits and Challenges

Abstract: The management of a university analytical lab has many facets. From billing, tool reservations, maintenance and performance metrics, there are many aspects best served by a software based package to assist in day to day operations. As lab instrumentation and usage grow, there is simply too much of the above mentioned information to track without some form of automated system to help improve efficiency. However, not all changes from an informal system are necessarily positive or quantifiable. Creating a sense of ownership and community is a critical part of any Core facility at a university: lab members must feel a sense that the lab is there to serve their needs and help them in every way possible with generating results to guide their research. There is a risk that users may feel alienated with an automated system. Here, a discussion of the move from an informal system, to the implementation of the software package Coral at the Micron™ Microscopy Core at the University of Utah is discussed. Examples of how to meet such challenges with a software based model are presented.