G. F. Gribakin

Low-energy positron interactions with atoms and molecules

This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.

Positron-molecule interactions: resonant attachment, annihilation, and bound states

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment, and annihilation. Measurements of annihilation rates resolved as a function of positron energy reveal the presence of vibrational Feshbach resonances VFRs for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules e.g., methyl halides and methanol where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. Molecules that do not bind positrons and hence do not exhibit such resonances are discussed. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom, approximately as the fourth power of the number of atoms in the molecule. While the details are as yet unclear, intramolecular vibrational energy redistribution IVR to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. In connection with IVR, experimental evidence indicates that inelastic positron escape channels are relatively rare. Downshifts of the VFR from the vibrational mode energies, obtained by measuring annihilate rates as a function of incident positron energy, have provided binding energies for 30 species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding-energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule negative-ion cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed. Possible areas for future theoretical and experimental investigation are also discussed.

Multiphoton detachment of electrons from negative ions

A simple analytical solution for the problem of multiphoton detachment from negative ions by a linearly polarized laser field is found. It is valid in the wide range of intensities and frequencies of the field, from the perturbation theory to the tunneling regime, and is applicable to the excess-photon as well as near-threshold detachment. Practically, the formulas are valid when the number of photons is greater than one. They produce the total detachment rates, relative intensities of the excess-photon peaks, and photoelectron angular distributions for the hydrogen and halogen negative ions, in agreement with those obtained in other, more numerically involved calculations in both perturbative and nonperturbative regimes. Our approach explains the extreme sensitivity of the multiphoton detachment probability to the asymptotic behavior of the bound-state wave function. Rapid oscillations in the angular dependence of the n-photon detachment probability are shown to arise due to interference of the two classical trajectories, which lead to the same final state after the electron emerges at diametrically opposite sides of the atom when the field is close to maximal. @S1050-2947~97!03205-8# PACS number~s!: 32.80.Rm, 32.80.Gc, 32.80.Wr In this paper we present an analytical solution to the problem of multiphoton detachment from a negative ion by a linearly polarized laser field. It gives very reliable quantitative results for a wide range of intensities and frequencies of the laser field, from the weak-field regime, where the process is described by the perturbation theory, to the strong fields, where it proceeds as tunneling. The theory is valid when the number of photons n is large, but usually gives good results as soon as n>2. We use it to calculate and examine various characteristics of the problem: the total multiphoton detachment rate, the n-photon detachment cross sections, the spectrum of excess-photon detachment ~EPD! photoelectrons ~the analogue of above-threshold ionization in atoms!, and the peculiar photoelectron angular distributions. There are two important physical properties of the multiphoton detachment process. ~i! The frequency of the laser field is much lower than the electron binding energy v!uE0u, ~1! where E052k 2 /2 is the energy of the bound state ~atomic units are used throughout!. This means that multiphoton detachment is an adiabatic problem. The external field varies slowly in comparison with the period of electron motion in the system. Therefore, the general adiabatic theory @1‐3# is applicable. As long as the laser field is weaker than the atomic field, the detachment probability is exponentially small with respect to the adiabaticity parameter uE0u/v;n. ~ii! The process of multiphoton detachment takes place when the electron is far away from the atomic particle ~see Sec. II! ,a tlarge distances, r;R5S g

Mechanisms of positron annihilation on molecules

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are the following: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate can be related to the positron elastic scattering cross section. This mechanism is characterized by a strong energy dependence of

Many-body calculations of positron scattering and annihilation from noble-gas atoms

{Z}_{\mathrm{eff}}

Positron Annihilation in Molecules by Capture into Vibrational Feshbach Resonances of Infrared-Active Modes

at small positron energies and high

Enhancement of parity and time-invariance violating effects in compound nuclei

{Z}_{\mathrm{eff}}

Quantum chaos in many—body systems: what can we learn from the Ce atom?

values (up to

Many-body theory of gamma spectra from positron-atom annihilation

{10}^{3})

Interaction of an alkaline-earth atom with an electron: scattering, negative ion and photodetachment

for room-temperature positrons, if a low-lying virtual level or a weakly bound state exists for the positron. (ii) Resonant annihilation, which takes place when the positron undergoes resonant capture into a vibrationally excited quasibound state of the positron-molecule complex. This mechanism dominates for larger molecules capable of forming bound states with the positron. For this mechanism