Feshbach projection operator approach to positron annihilation

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We present a theory of vibrationally enhanced positron annihilation on molecules based on the Feshbach projection operator formalism. A key aspect of the present approach is the fact that no direct vibrational excitation is assumed, i.e., the attachment mechanism is electronic in nature, arising from positron-electron correlation-polarization forces, and energy transfer to the nuclei essentially follows from the difference between the potential-energy surfaces of the isolated target and the positron-molecule compound; moreover, no a priori assumption is made on the character of the transient (bound or virtual state). An approximate relation between the annihilation parameter Z(eff) and the vibrationally summed cross section is presented, as well as a hierarchy of approximations that may allow for elaborate model calculations. We also discuss how important aspects of the annihilation process are taken into account in the present theory, such as isotope effects, vibrational energy redistribution and relative strengths among vibrational resonances. For completeness, semiempirical model calculations for acetylene and ethylene are presented. Despite the stringent approximations employed in this simplest version of the theory, fair agreement with experimental data was obtained in the vicinity of 0 -> 1 thresholds.805Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Low-energy electron scattering by CF4, CCl4, SiCl4, SiBr4, and SiI4

in this paper, we show elastic and rotationally inelastic cross-section calculations of low-energy electron scattering by CF4, CCl4, SiCl4, SiBr4, and SiI4. The fixed-nuclei static-exchange scattering amplitudes were obtained with the Schwinger multichannel method with soft norm-conserving pseudopotentials. We show elastic integral and differential cross sections and discuss the role of the basis set on the nature of some structures seen in a previous publication [A. P. P; Natalense et al., Phys. Rev. A 52, R1 (1995)]. We have attributed these structures to linear dependency in the basis set caused by the symmetric combination (x(2)+y(2)+z(2))exp(-ar(2)). The rotational cross sections were calculated with the help of the adiabatic-nuclei-rotation approximation. Our results are in good agreement with available experimental data. The sums of 0 -->0,3,4,6 rotational cross sections in general show good agreement with the elastic (rotationally unresolved) ones. The rotationally summed integral cross section agrees within 0.3% with the elastic integral cross section for CF4 at 7.5 eV, and within 26% for SiI4 at 30 eV. It was found that rotationally inelastic cross sections are considerably large for such molecules, because the heavy peripheral atoms play a significant role as scattering centers. [S1050-2947(99)00611-3].6053684369

Low-energy electron scattering by CH3F, CH2F2, CHF3, and CF4

We present measured and calculated differential cross sections, as well as calculated integral cross sections, for elastic electron collisions with CH3F, CHF2F2, CHF3, and CF4. The calculated cross sections were obtained with the Schwinger multichannel method, and a Born-closure procedure was used to improve the differential cross sections for polar systems. Polarization effects were found to be relevant even for systems with moderately large permanent dipole moments, such as CH3F and CHF3. In general, there is good agreement between theory and experiment.65

Low-energy electron collisions with acetic acid

We present cross sections for elastic collisions of low-energy electrons with acetic acid. We employed the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange plus polarization approximations, for energies ranging from 0.1 to 10 eV. We found a pi(*) shape resonance around 1.7 eV, corresponding to the A(') symmetry of the C(s) group. This resonant state was assigned to the experimental dissociative electron attachment peak at 1.7 eV yielding CH(3)COO(-)+H. We also performed a series of electronic structure calculations using a small basis set for acetic, formic, and trifluoroacetic acids, which exhibit a similar behavior with respect to the dissociative electron attachment. We believe that hydrogen elimination triggered off by electron capture into a pi(*) resonance could be a general property of carboxylic acids.79

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: Wave-packet dynamics on effective potential surfaces of reduced dimensionality

We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde. In these applications the effective potentials should provide a more realistic description of proton-substrate couplings than the sudden or adiabatic approximations commonly employed in studies of proton transfer. The resulting time-dependent photoelectron signals, obtained here assuming a constant value of the photoelectron matrix element for ionization of the wave packet, are seen to track the proton transfer. (c) 2006 American Institute of Physics.1241

Near-threshold vibrational excitation of H-2 by positron impact: A projection-operator approach

We report vibrational excitation (nu(i)=0 ->nu(f)=1) and total cross sections for positron scattering by H-2. The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near-threshold behavior of the vibrational excitation cross section is in good agreement with available experimental data [Sullivan , Phys. Rev. Lett. 86, 1494 (2001)]. Though our fixed-nuclei calculations do not indicate the existence of a e(+)-H-2 virtual state, a proper description of the T matrix threshold behavior is essential (the adiabatic approximation is inadequate). The projection operator approach has long been a powerful tool for studies of nuclear dynamics in electron-molecule collisions, and its application to positron scattering is timely since couplings to nuclear degrees of freedom are known to be very important at low impact energies.76

Cross sections for elastic scattering of low-energy electrons by trimethylarsine (TMAs)

We report elastic differential, integral and momentum transfer cross sections for eTMAs collisions, obtained at the static-exchange level of approximation. The energy range considered was from 5 to 40 eV. Our calculations were performed with the Schwinger multichannel method with pseudopotentials (M H F Bettega, L G Ferreira and M A P Lima 1993 Phys. Rev. A 47 1111). In order to study how molecular conformation affects the scattering process, we carried out calculations considering two possible conformations of the target. The elastic integral cross sections obtained with the two conformations are essentially equal for incident energies beyond 15 eV. For the 5 eV < E < 15 eV energy range, however, conformation effects are verified. We have also found shape resonances in the elastic cross sections for both conformations, around 8 eV for the more stable conformation and 12 eV for the other one.3282031203

Effective configurations in positron-molecule scattering

In this work, we extend the effective configurations (EC) approach (Azevedo D L, da Silva A J R and Lima M A 2000 Phys. Rev. A 61042702) to positron scattering. EC are pseudo-eigenstates of the (N+1)-body collision Hamiltonian. They provide a much more efficient description of polarization effects in scattering calculations by allowing considerable contraction of configuration spaces. As a result, significant computational time is saved.35163531353

Real-time observation of ground state proton transfer: a model study

We propose a femtosecond dump-probe photoelectron spectroscopy scheme for the real-time observation of ground state proton transfer. Feasibility of such a scheme is investigated with quantum wavepacket dynamics on a model potential. Formation of a proton transfer wavepacket by dumping from an eigenstate or from a delocalized mixture of eigenstates is discussed. Characteristic features of proton transfer observed in the time-dependent ion signal and photoelectron kinetic energy spectra are shown, along with a method of extracting the signals from a background unrelated to proton transfer. (c) 2004 Elsevier B.V. All rights reserved.311325526

Cross sections for rotational excitations of CH4, SiH4, GeH4, SnH4 and PbH4 by electron impact

We report differential and integral cross sections for rotational excitation of XH(4) molecules (X: C, Si, Ge, Sn, Pb) from 7.5-30 eV by electron impact. These cross sections were derived from fixed-nuclei scattering amplitudes (Bettega et al. 1995) obtained using the Schwinger Multichannel Method with Pseudopotentials (SMCPP) (Bettega et al. 1993). Our results represent the first rotational excitation cross sections for molecules as large as GeH4, SnH4 and PbH4 using entirely ab initio procedures. The cross sections for CH4 and SiH4 obtained with pseudopotentials are in very good agreement with all-electron calculations and with other theoretical results. A comparison between our calculated cross sections and experimental data for CH4 is in general encouraging, but some discrepancies remain. We found inelastic rotational cross sections and momentum transfer cross sections to be larger for SiH4, GeH4, SnH4 and PbH4 than for CH4. We could explain this feature.391596