Resonant Positron Annihilation on Molecules

2008 ◽  
Vol 607 ◽  
pp. 9-16 ◽  
Author(s):  
J.A. Young ◽  
C.M. Surko
Keyword(s):  
Positron Annihilation ◽  
Binding Energies ◽  
Current Understanding ◽  
Positronium Atom ◽  
Feshbach Resonances ◽  
Positron Energy ◽  
Incident Positron

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.

Slow Positron Annihilation in Ion-Implanted Silicon

2008 ◽  
Vol 280-281 ◽  
pp. 21-28 ◽  
Author(s):  
Girjesh Singh ◽  
S.B. Shrivastava ◽  
M.H. Rathore
Keyword(s):  
Positron Annihilation ◽  
Shape Parameter ◽  
Positron Energy ◽  
Slow Positron ◽  
Vacancy Clusters ◽  
S Parameter ◽  
Trapping Model ◽  
Line Shape Parameter ◽  
Incident Positron ◽  
Ion Implanted

The mechanism of slow positron annihilation in ion-implanted Si has been discussed in terms of the Diffusion-Trapping model (DTM). The trapping of positron has been considered in native vacancies (monovacancies) and ion induced vacancies i.e. vacancy clusters. The model has been used to calculate the Doppler broadening line shape parameter (S-parameter) as a function of incident positron energy for different ion-implanted Si. It has been found that at lower energies the monovacancies and vacancy clusters both contribute to the S-parameter while, with the increase in positron energy the vacancy clusters are reduced. The S-parameter is found to be dependent on the fluency of the implanted ions.

Defects Introduced by Low Dose Be-Implantation Probed by a Monoenergetic Positron Beam

1992 ◽  
Vol 262 ◽  
Author(s):  
A. Uedono ◽  
Y. Ujihira ◽  
L. Wei ◽  
Y. Tabuki ◽  
S. Tanigawa ◽  
...  
Keyword(s):  
Ion Implantation ◽  
Positron Annihilation ◽  
Low Dose ◽  
Positron Beam ◽  
Doppler Broadening ◽  
Positron Energy ◽  
P Type ◽  
Incident Positron ◽  
Depth Distributions

ABSTRACTVacancy-type defects in 60-keV Be+-implanted GaAs and InP were studied by a monoenergetic positron beam. The depth distributions of vacancy-type defects were obtained from measurements of Doppler broadening profiles of the positron annihilation as a function of incident positron energy. Vacancy-type defects introduced by ion implantation were observed in n-type GaAs. For p-type GaAs, however, this was not the case. This can be attributed to the recombination of vacancy-type defects and pre-existed interstitial-type defects in p-type GaAs. The defects induced by ion implantation in InP were also studied.

Defect Formation by Ion Implantation in Cz-Si Studied by a Monoenergetic Positron Beam

1992 ◽  
Vol 262 ◽  
Author(s):  
A. Uedono ◽  
Y. Ujihira ◽  
L. Wei ◽  
Y. Tabuki ◽  
S. Tanigawa ◽  
...  
Keyword(s):  
Oxygen Vacancy ◽  
Ion Implantation ◽  
Defect Formation ◽  
Positron Beam ◽  
Low Energy ◽  
Doppler Broadening ◽  
Positron Energy ◽  
Annealing Experiments ◽  
Incident Positron ◽  
Ion Implanted

ABSTRACTVacancy-type defects in ion implanted Si were studied by a monoenergetic positron beam. The depth-distributions of the defects were obtained from measurements of Doppler broadening profiles of the positron annihilation as a function of incident positron energy. The results showed that a size of vacany-clusters introduced by 150-keV P+-ion implantation was found to be smaller than that introduced by 2-MeV P+-ion implantation. This was attributed to an overlap of collision cascades in low-energy (150 keV) ion implanted specimens. From isochronal annealing experiments for 80-keV B+- and 150-keV P+-ion implanted specimens, the defected region was removed by 1200 °C annealing, however, for 2-MeV P+-implanted specimen, two-types of oxygen-vacancy complexes were found to coexist even after 1200 °C annealing.

Ortho-Positronium Reemission Yield and Energy in Surfactant-Templated Mesoporous Silica Films

2008 ◽  
Vol 607 ◽  
pp. 30-33 ◽  
Author(s):  
Laszlo Lizkay ◽  
C. Corbel ◽  
P. Perez ◽  
P. Desgardin ◽  
Marie France Barthe ◽  
...  
Keyword(s):  
Mesoporous Silica ◽  
Pore Size ◽  
Positron Annihilation ◽  
Time Of Flight ◽  
Positron Beam ◽  
High Porosity ◽  
Silica Layer ◽  
Silica Films ◽  
Gamma Energy ◽  
Positron Energy

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.

Slow Positron Studies in Polymers

2010 ◽  
Vol 307 ◽  
pp. 85-92
Author(s):  
S.B. Shrivastava ◽  
Aman Deep Acharya ◽  
R. Sharma
Keyword(s):  
Positron Lifetime ◽  
Hole Concentration ◽  
Low Energy ◽  
Positronium Formation ◽  
Positron Energy ◽  
Slow Positron ◽  
Polymeric Surface ◽  
S Parameter ◽  
Free Volume Hole ◽  
Incident Positron

The diffusion trapping model has been applied to slow positron annihilation in He+ irradiated polystyrene and polystyrene – polystyrene bilayers. The S-parameter and the positron lifetime have been calculated as a function of the incident positron energy. The effect of the fluence upon the nature of the S-parameter curve has been discussed. It has been found that a change in fluence affects positronium formation. The transition rate for surface to positronium formation has been found to be dependent upon the fluence and the atomic number of the irradiated ion. The lifetime results show that, at low energy, the o-Ps annihilates mainly at the polymeric surface. The free volume hole concentration is found to decrease at low energy, and becomes constant at higher energies.

Ion Desorption from TiO2(110) by Low Energy Positron Impact

2017 ◽  
Vol 373 ◽  
pp. 324-327 ◽  
Author(s):  
Takayuki Tachibana ◽  
Luca Chiari ◽  
Masaru Nagira ◽  
Takato Hirayama ◽  
Yasuyuki Nagashima
Keyword(s):  
Positron Annihilation ◽  
Electron Impact ◽  
Impact Ionization ◽  
Low Energy ◽  
Surface Ionization ◽  
Positron Impact ◽  
Ion Desorption ◽  
Incident Positron

We have observed positron-stimulated ion desorption from a TiO2(110) surface. H+ and O+ ions were desorbed at incident positron energies above the desorption thresholds for electron impact. However, only O+ ions were detected at energies below those thresholds. These results suggest that by surface ionization positron annihilation as well as by positron impact leads to the O+ ion desorption. By contrast, it is likely that the H+ ions are not desorbed by positron annihilation, but rather by impact ionization.

Vacancy-Solute Binding Energies in Aluminum by Positron Annihilation

2004 ◽  
Vol 445-446 ◽  
pp. 165-167 ◽  
Author(s):  
Hideaki Ohkubo ◽  
Yasuyoshi Nagai ◽  
K. Inoue ◽  
Z. Tang ◽  
Masuyuki Hasegawa
Keyword(s):  
Positron Annihilation ◽  
Binding Energies

Positronium Molecule Formation on Al (111)a

2008 ◽  
Vol 607 ◽  
pp. 22-24
Author(s):  
S.H.M. Deng ◽  
D.B. Cassidy ◽  
A.P. Mills
Keyword(s):  
Single Crystal ◽  
Positron Annihilation ◽  
Positron Beam ◽  
Single Shot ◽  
Beam Density ◽  
Positron Annihilation Lifetime ◽  
Molecule Formation ◽  
Positronium Lifetime ◽  
Positronium Molecule ◽  
Incident Positron

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.

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