Positron and Positronium Chemistry

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Authors: Yan Ching Jean, Hong Min Chen, L. James Lee, Jin Tao Yang, Xiao Hong Gu, Wei Song Hung, Kueir Rarn Lee, Juin Yih Lai, Yi Ming Sun, Chien Chieh Hu
Abstract: Positron and Positronium chemistry has been pursued and advanced by many scientists and engineers in both fundamental understanding of Positronium atom and its applications to chemical and polymeric systems during the last decade. This paper presents our recent results from collaborative investigations of positron annihilation in polymeric membranes. Future perspectives of applying Positronium chemistry to membrane science and technology and other related disciplines of nanotechnology, chemical engineering, materials science, energy research, molecules with positrons, biological and medical sciences appear to be promising.
Authors: J.A. Young, C.M. Surko
Abstract: At incident positron energies below the threshold for positronium atom formation, there are many cases in which annihilation rates for molecules are far in excess of that possible on the basis of simple two-body collisions. We now understand that this phenomenon is due to positron attachment to molecules mediated by vibrational Feshbach resonances. The attachment enhances greatly the overlap of the positron with molecular electrons and hence increases the probability of annihilation. Furthermore, measurements of the annihilation spectra as a function of incident positron energy provide a means of measuring positron-molecule binding energies. In this paper we present an overview of our current understanding of this process, highlighting key results and discussing outstanding issues that remain to be explained.
Authors: D.B. Cassidy, S.H.M. Deng, R.G. Greaves, N. Lopez-Valdez, V. Meligne, H.W.K. Tom, A.P. Mills
Abstract: Recent observations of molecular positronium (Ps2) were based on a correlation between changes in positronium (Ps) lifetime spectra associated with the density of an incident positron beam and the population of a positronium (or positron) surface state. While the evidence for molecule formation is compelling it is nevertheless an indirect observation, and has not provided any information about the properties of Ps2 beyond its likely creation. Here we discuss the prospects for a direct observation via laser spectroscopy of a predicted 1S2P excited molecular state. Such a measurement would provide a direct and unambiguous signal of Ps2 formation and would also allow us to determine some properties of the molecule, namely the lifetime of the excited state and the 1S1S-1S2P energy interval.
Authors: S.H.M. Deng, D.B. Cassidy, A.P. Mills
Abstract: A single crystal Al (111) sample cleaned by repeated cycles of ion bombardment and annealing was irradiated by a subnanosecond high density positron beam [1] and the resulting positronium lifetime spectra were measured using single shot positron annihilation lifetime spectroscopy (SSPALS) [2]. We observed the amount of positronium emitted dependence on the incident positron beam density, which indicates the formation of positronium molecules (Ps2) at the Al (111) surface [3]. The Ps2 formation probability appears to be extremely sensitive to surface contamination and further experiments under improved vacuum conditions are planned to clarify this effect.
Authors: David M. Schrader
Abstract: Dipositronium, Ps2, was recently prepared [1]. This is significant because: • It is the first laboratory observation of a molecule that contains more than one positron; • It is the most symmetrical molecule possible; • It is the most non-rigid (floppiest) molecule possible; • The interval between the theoretical establishment of its existence [2] and its laboratory observation [1] is inordinately long – 60 years; and • An extension of the technology developed for the Ps2 observation may soon lead to an observation of the Bose-Einstein condensation of positronium and the development of a gamma ray laser. We briefly discuss the symmetry of Ps2 and how an understanding of it will underlie its characterization in the future. Ps2O and CPs2 might be the next two-positron compounds to be prepared and characterized in the laboratory. A discussion of the contrasting eigenstates of these two molecules is given. An understanding of these states is required in order to identify them.
Authors: Laszlo Lizkay, C. Corbel, P. Perez, P. Desgardin, Marie France Barthe, Toshiyuki Ohdaira, Ryoichi Suzuki, P. Crivelli, Ulisse Gendotti, A. Rubbia, M. Etienne, A. Walcarius
Abstract: Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.
Authors: H.R.J. Walters
Abstract: The theoretical description of positron and positronium scattering by atoms within the context of the coupled pseuodostate approximation is briefly considered.
Authors: Jerzy Kansy, Radosław Zaleski
Abstract: A new method of analysis of PALS spectra of porous materials is proposed. The model considers both the thermalization process of positronium inside the pores and the pore size distribution. The new model is fitted to spectra of mesoporous silica MCM-41 and MSF. The resulting parameters are compared with parameters obtained from fitting the “conventional” models, i.e. a sum of exponential components with discrete or/and distributed lifetimes.

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