The correction of coincidence electron-scattering cross sections for radiative effects

The correction of inelastic electron-scattering cross sections for the effects of radiation is considered for experiments in which one of the final-state hadrons is detected in coincidence with the scattered electron. The results are expressed in terms of the missing mass of the unobserved particles.The accuracy of some commonly used approximations is investigated in the case of the ep → epπ⁰ reaction in the region of the Δ (1236) resonance.     

Unitarizing high-energy scattering amplitudes in field theories I: QED

Using a procedure which incorporates the s-channel unitarity and t-channel unitarity at every step, we calculate the scattering amplitudes of QED in the high-energy limit. We find that all scattering amplitudes, elastic and inelastic, are summarized by a single eikonal formula which is explicitly unitary.     

The mechanism of methanol formation and other reactions following formaldehyde adsorption on Ni(110)

The adsorption/desorption and reactive behavior of formaldehyde was studied on clean single-crystal Ni(110) at adsorption temperatures down to 200 °K. For low exposures of the surface to formaldehyde, hydrogen and CO binding states were populated due to decomposition of the molecule upon adsorption. Higher exposures gave rise to a decomposition-limited hydrogen peak exhibiting an activation energy of 20 kcal/gmol and an apparent frequency factor of 10¹⁴ sec^?1. At initial coverages of H₂CO exceeding about 0.5, monolayer methanol was observed to form. The formation of methanol involved a hydrogen atom transfer between two adsorbed H₂CO molecules and did not occur totally via surface hydrogen. Self-oxidation to form CO₂ was also observed. The surface exhibited reaction heterogeneity, and the surface reactivity was observed to depend on the temperature of adsorption of reactants, suggesting strong adsorbate-induced surface “reconstruction.”     

Magnetic circular dichroism of the U center in KCl

The magnetic circular dichroism (MCD) and absorption spectrum of the U center in KCl have been measured at liquid helium temperature. The results are consistent with the usual ¹S(l s²) → ¹P(ls2p) assignment with the temperature independent MCD yielding an excited state magnetic moment only about 40 per cent that calculated for a hydride ion in an octahedral crystal field.     

Application of direct potential fitting to line position data for the X (1)Sigma(+) and A (1)Sigma(+) states of LiH

A collection of 9089 spectroscopic LiH line positions, of widely varying precision, which sample 84.9% and 98.6% of the A and X state well depths, respectively, have been employed in a direct least-squares fit of the effective potential energy and Born-Oppenheimer breakdown functions for the two states. For the four isotopomers (6)LiH, (7)LiH, (6)LiD, and (7)LiD, the data comprise both pure rotational and vibration-rotational transitions within the ground state, as well as rotationally resolved transitions in the A-X system. Despite the unusual shape and associated anomalous properties of the A state potential, no special features or considerations were required in the direct potential fitting approach. The reduced standard deviation of the fit was close to unity, indicating that the quantum mechanical eigenvalues calculated from the fully analytical functions of the Hamiltonians of the two states, which are characterized by a total of only 53 fitted parameters, represent the line positions, on average, to within the estimated uncertainties. A quantum mechanical calculation of the molecular constants G(nu), B(nu), D(nu), H(nu), L(nu), M(nu), N(nu), and O(nu) from the fitted potential for the A state of (7)LiH confirms that the usual polynomial expansion in J(J+1) is an unsatisfactory representation for the rotational terms of the lowest vibrational levels.     

Color center formation in KCl and KBr single crystals with femtosecond laser pulses

Because of their extremely high instantaneous powers, femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long-term behavior of coloration. In this work, we probe the evolution of color centers generated by femtosecond laser radiation in potassium chloride and potassium bromide single crystals on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we follow the changes in coloration due to individual or multiple femtosecond pulses and the evolution of that coloration for long times after femtosecond laser radiation is terminated.     

Coincidence electroproduction of single neutral pions in the resonance region at q2 = 0.5 (GeV/c)2

Data are presented for the reaction ep → epπ⁰ at a nominal four-momentum transfer squared of 0.5 (GeV/c)². The data were obtained using an extracted electron beam from NINA and two magnetic spectrometers for coincidence detection of the electron and proton. Details are given of the experimental method and the results are given for isobar masses in the range 1.19 – 1.73 GeV/c².     

Accuracy of recent potential energy surfaces for the He-N2 interaction. I. Virial and bulk transport coefficients

A new exchange-Coulomb semiempirical model potential energy surface for the He-N2 interaction has been developed. Together with two recent high-level ab initio potential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and N2. The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initio surface of Patel et al. [J. Chem. Phys. 119, 909 (2003)], but with slightly poorer agreement for the earlier ab initio surface of Hu and Thakkar [J. Chem. Phys. 104, 2541 (1996)].