Energy Threshold for D+H_2→DH+H Reaction

We have been able to measure the threshold energy Eo for the reaction D+H_2→DH+H. The value obtained was (0.33±0.02) eV. Apparently, this is the first direct determination of a threshold energy for a reaction involving the formation and breaking of covalent bond

Determination of Electronic Energy Levels of Molecules by Low-Energy Electron Impact Spectroscopy

This paper describes a new spectroscopic tool in which optically forbidden electronic transitions can usually be detected as clearly as optically allowed ones in a fairly routine manner. It uses the inelastic scattering of low-energy electrons by molecules as the means for determining their electronic energy levels

What is moving in hybrid halide perovskite solar cells?

Organic-inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. In this Account, we discuss the internal motion of methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) and formamidinium lead iodide ([CH(NH$_2$)$_2$]PbI$_3$), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron-hole recombination rates, and current-voltage hysteresis. Multi-scale simulations - drawing from electronic structure, ab initio molecular dynamic and Monte Carlo techniques - have been combined with neutron scattering and ultra-fast vibrational spectroscopy to qualify the nature and timescales of the motions. Recent experimental evidence and theoretical models for simultaneous electron transport and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action. The temporal behaviour found in hybrid perovskites introduces a sensitivity in materials characterisation to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials and device simulations. Herein, we critically discuss the atomistic origin of the dynamic processes.Comment: 29 pages, 3 figure

Dielectric Breakdown of a Mott Insulator

We study the nonequilibrium steady state of a Mott insulator coupled to a thermostat and driven by a constant electric field, starting from weak fields, until the dielectric breakdown, and beyond. We find that the conventional Zener picture does not describe the steady-state physics. In particular, the current at weak field is found to be controlled by the dissipation. Moreover, in connection with the electric-field-driven dimensional crossover, we find that the dielectric breakdown occurs when the field strength is on the order of the Mott gap of the corresponding lower-dimensional system. We also report a resonance and the meltdown of the quasiparticle peak when the field strength is half of this Mott gap.Comment: 5 pages, 5 figures. v2: references adde

Energy dependence of the differential photoelectron cross sections of molecular nitrogen

The angular distribution of photoelectron intensity for molecular nitrogen was studied using He I and Ne I resonance line discharge light sources. Studies of photoelectron angular distributions covering a range of photon energies, and thus a range of photoelectron energies, are possible using the weaker high order lines in each discharge as well as the principal lines. Peaks in three photoelectron bands of N_2 were studied at the photon energies 16.85, 19.78, 21.22, 23.09, and 23.74 eV, where possible. We find that the v′=0 peak of the X^ 2Σ^+_g band has abnormally high intensity and, at the higher photon energies, an abnormally low angular distribution asymmetry parameter, β. Several mechanisms for this anomaly are discussed, including autoionization, the variation of electric dipole transition moments with internuclear distance, and possible shape resonance phenomena. None of these explanations is completely in agreement with all theoretical and experimental evidence

Photoelectron spectroscopy of ethylene, isobutylene, trimethylethylene, and tetramethylethylene at variable angles

Using a HeI line 58.4 nm source lamp, photoelectron angular distributions were measured for the series of four olefins: ethylene, isobutylene (2‐methyl‐propene), trimethylethylene (2‐methyl‐2‐butene), and tetramethylethylene (2,3‐dimethyl‐2‐butene). From these, the asymmetry parameter β as a function of photoelectron energy was obtained for each of these molecules. The following important effects in the behavior of β are observed: (a) In the π orbital ionization regions of the spectrum of each molecule, β increases with increasing electron energy across the vibrational envelopes. (b) With increasing methyl substitution (and at a fixed photoelectron enegy) β for this band decreases. (c) In the region of the spectra of each of the methyl‐substituted ethylenes involving several 2pσ bands, this energy dependence of β behaves as if they constituted a single band, in spite of the widely differing orbital symmetries. (d) Over most of the 2pσ region of each molecule, β decreases with increasing photoelectron energy, except for the high ionization potential end of this region, where β increases instead. We attribute effects (b) and (d) to σ–π orbital mixing

Differences between Low-Energy Electron Impact Spectra at 0º and at Large Scattering Angle

The preceding Comment reports some low-energy electron-impact spectra of helium and ethylene, obtained by a very elegant technique, which are markedly different from the ones we obtained 1 under other experimental conditions. It is quite important to try to understand the reasons for the differences observed. These differences are essentially the following: (a) In our impact spectra of helium obtained with 50-eV electrons we observe pronounced peaks corresponding to the 2 ^3S state and to ionization whereas Simpson and Mielczarek do not. (b) In our spectra of ethylene at this same incident electron energy we observe two pronounced optically forbidden transitions an

Educational Alternatives for Vulnerable Youth: Student Needs, Program Types, and Research Directions

Chapter 1 of this document examines the need for alternative education among vulnerable youth by reviewing the numbers and characteristics of youth who disconnect from mainstream developmental pathways in various ways. The second chapter examines the question of "what is an alternative education school or program" and draws on a variety of elements from the literature to suggest the beginnings of a typology that might be used to define and organize the varieties of educational alternatives that currently exist and might be promoted in the future. Finally, Chapter 3 summarizes the findings of a roundtable on directions for future research on alternative education and describes the types of information and studies that are needed to advance the field of alternative education and foster more support for the development of high quality educational alternatives that all children can choose and benefit from