Benchmark experiment and theory for near-threshold excitation of helium by electron impact

A new experimental technique has been applied to measure absolute scattering cross sections for electron impact excitation of the n ≤ 2, 3 states of helium at near-threshold energies. The experimental results are compared with predictions from recent state-of-the-art theoretical calculations. The calculations are performed using the R-matrix with pseudostates, B-spline R-matrix, and the convergent close-coupling methods. Generally, very good agreement is found between the experiment and the three theorie

Electron-impact excitation of the (5d106s)2S1/2-(5d106p)2P1/2,3/2 resonance transitions in gold atoms

Results from a joint experimental and theoretical investigation of electron-impact excitation of the (5 d10 6s) S2 1/2 → (5 d10 6p) P2 1/2,3/2 resonance transitions in gold atoms are presented. The calculations were performed using three fully relativi

Exploring Biorthonormal Transformations of Pair-Correlation Functions in Atomic Structure Variational Calculations

Multiconfiguration expansions frequently target valence correlation and correlation between valence electrons and the outermost core electrons. Correlation within the core is often neglected. A large orbital basis is needed to saturate both the valence and core-valence correlation effects. This in turn leads to huge numbers of CSFs, many of which are unimportant. To avoid the problems inherent to the use of a single common orthonormal orbital basis for all correlation effects in the MCHF method, we propose to optimize independent MCHF pair-correlation functions (PCFs), bringing their own orthonormal one-electron basis. Each PCF is generated by allowing single- and double- excitations from a multireference (MR) function. This computational scheme has the advantage of using targeted and optimally localized orbital sets for each PCF. These pair-correlation functions are coupled together and with each component of the MR space through a low dimension generalized eigenvalue problem. Nonorthogonal orbital sets being involved, the interaction and overlap matrices are built using biorthonormal transformation of the coupled basis sets followed by a counter-transformation of the PCF expansions. Applied to the ground state of beryllium, the new method gives total energies that are lower than the ones from traditional CAS-MCHF calculations using large orbital active sets. It is fair to say that we now have the possibility to account for, in a balanced way, correlation deep down in the atomic core in variational calculations

Calculation of the two-photon decay rates of hydrogen-like ions by using B-polynomials

A new approach is laid out to investigate the two photon atomic transitions. It is based on application of the finite basis solutions constructed from the Bernstein Polynomial (B-Polynomial) sets. We show that such an approach provides a very promising route for the relativistic second- (and even higher-order) calculations since it allows for analytical evaluation of the involved matrices elements. In order to illustrate possible applications of the method and to verify its accuracy, detailed calculations are performed for the 2s_{1/2}-1s_{1/2} transition in neutral hydrogen and hydrogen-like ions, and are compared with the theoretical predictions based on the well-established B-spline-basis-set approach

Electron-impact excitation of the (5s(2)5p) P-2(1/2) -> (5s(2)6s) S-2(1/2) transition in indium: Theory and experiment

We present angle-integrated and angle-differential cross sections for electron-impact excitation of the (5s(2)5p) P-2(1/2) -> (5s(2)6s) S-2(1/2) transition in atomic indium. Experimental data for six incident electron energies between 10 and 100 eV are compared with predictions from semirelativistic and fully relativistic B-spline R-matrix calculations, as well as a fully relativistic convergent close-coupling model. Agreement between our measured and calculated data is, with a few exceptions, found to be typically very good. Additionally, the agreement between the present theoretical predictions is generally excellent, with the remaining small deviations being associated with the slightly different, although still very accurate, descriptions of the target structure. Agreement between the present results and an earlier relativistic distorted-wave computation [T. Das, R. Srivastava, and A. D. Stauffer, Phys. Lett. A 375, 568 (2011)] was, however, found to be marginal, particularly at 10 and 20 eV

Recommended Cross Sections for Electron-Indium Scattering

20 pags., 7 figs., 6 tabs.We report, over an extended energy range, recommended angle-integrated cross sections for elastic scattering, discrete inelastic scattering processes, and the total ionization cross section for electron scattering from atomic indium. In addition, from those angle-integrated cross sections, a grand total cross section is subsequently derived. To construct those recommended cross-section databases, results from original B-spline R-matrix, relativistic convergent close-coupling, and relativistic optical-potential computations are also presented here. Electron transport coefficients are subsequently calculated, using our recommended database, for reduced electric fields ranging from 0.01 Td to 10 000 Td using a multiterm solution of Boltzmann's equation. To facilitate those simulations, a recommended elastic momentum transfer cross-section set is also constructed and presented here.The work of K.R.H., O.Z., and K.B. was supported by the United States National Science Foundation under Grant Nos. OAC-1834740 and PHY-1803844 and by the XSEDE supercomputer Allocation No. PHY-090031. The (D)BSR calculations were carried out on Stampede2 at the Texas Advanced Computing Center. The work of D.V.F. and I.B. was supported by the Australian Research Council and resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. F.B. and G.G. acknowledge partial financial support from the Spanish Ministry MICIU (Project Nos. FIS2016- 80440 and PID2019-104727-RB-C21) and CSIC (Project No. LINKA20085). This work was also financially supported, in part, by the Australian Research Council (Project No. DP180101655), the Ministry of Education, Science and Technological Development of the Republic of Serbia, and the Institute of Physics (Belgrade).Peer reviewe

Transport of electrons and propagation of the negative ionisation fronts in indium vapour

22 pags., 23 figs.We study the transport of electrons and propagation of the negative ionisation fronts in indium vapour. Electron swarm transport properties are calculated using a Monte Carlo simulation technique over a wide range of reduced electric fields E/N (where E is the electric field and N is the gas number density) and indium vapour temperatures in hydrodynamic conditions, and under non-hydrodynamic conditions in an idealised steady-state Townsend (SST) setup. As many indium atoms are in the first metastable state at vapour temperatures of a few thousand Kelvin, the initial Monte Carlo code was extended and generalized to consider the spatial relaxation and the transport of electrons in an idealised SST experiment, in the presence of thermal motion of the host-gas atoms and superelastic collisions. We observe a significant sensitivity of the spatial relaxation of the electrons on the indium vapour temperature and the initial conditions used to release electrons from the cathode into the space between the electrodes. The calculated electron transport coefficients are used as input for the classical fluid model, to investigate the inception and propagation of negative ionisation fronts in indium vapour at various E/N and vapour temperatures. We calculate the electron density, electric field, and velocity of ionisation fronts as a function of E/N and indium vapour temperature. The presence of indium atoms in the first metastable state significantly affects the characteristics of the negative ionisation fronts. The transition from an avalanche into a negative ionisation front occurs faster with increasing indium vapour temperature, due to enhanced ionisation and more efficient production of electrons at higher vapour temperatures. For lower values of E/N, the electron density behind the streamer front, where the electric field is screened, does not decay as one might expect for atomic gases, but it could be increased due to the accumulation of low-energy electrons that are capable of initiating ionisation in the streamer interior.The work of SD, JA, DB, MSR, DS, and BPM was sup- ˇported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, and the Institute of Physics (Belgrade). The work of KRH, OZ, and KB was supported by the United States National Science Foundation under Grant Nos. OAC-1834740, PHY-1803844, and PHY-2110023, and by the XSEDE supercomputer Allocation No. PHY-090031. The work of DVF and IB, was supported by the Australian Research Council and resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. FB and GG acknowledge partial financial support from the Spanish Ministry MICIU (Project Nos. FIS2016-80440 and PID2019-104727-RB-C21) and CSIC (Project No. LINKA20085). This work was also financially supported by the Australian Research Council (Project No. DP180101655).Peer reviewe