Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.     

Magnetic quenching of positronium in semiconductors

The positronium-determined characteristics of electron-positron annihilation in semiconductors in the presence of an external magnetic field are calculated. It is assumed that the positronium atoms can be in one of two states, delocalized or localized (at defects), and undergo not only pick-off annihilation but also orthoparaconversion (³S₁ ¹S₀) on free carriers. Numerical calculations were made for a field H=15 kG. The field has very different effects on localized and delocalized positronium, and in the first case the effect depends strongly on the nature of the defect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.11, pp. 75–78, November, 1980.     

Resonances in positron-atom scattering

Theoretical studies of atomic resonances involving positrons will be discussed in this talk. Investigations on resonances in positron-hydrogen scattering below various hydrogen and positronium thresholds are reviewed, as well as resonances in positronalkali and e⁺-He⁺ scattering. Resonance phenomena in other atomic systems involving positrons will also be discussed. These systems include positronium ions Ps⁻, positronium molecules Ps₂, and positronium hydride PsH.     

Positronium quenching reactions and diffusion in solutions

The temperature effect on positronium quenching rates for p-benzoquinone and iodine in several solvents was determined. The quenching rate constantk ₁ follows ak ₁∝1/η relationship better than thek ₁∝T/η one, predicted by ordinary theory. A simple theoretical treatment based on diffusion theory shows that the temperatureT of the solute, positronium, may not be the same as the temperature of the solvent if the positronium is not fully thermalized. This temperature is found to be about twice the room temperature for the solvents investigated. The findings further support the theory that positronium reactions are “diffusion controlled” in liquid solutions. The activation energies of all the positronium reactions studied are either negligible or less than 0.1 eV.     

Analysis of the production and clusterization of iron atoms under pulsed laser photolysis of Fe(CO)5

Atomic-resonance absorption spectroscopy is used to study the production and loss of iron atoms under dissociation of the Fe(CO)₅ vapor in a quartz reactor that is induced by the pulses of the KrF excimer laser. Iron atoms populate the ground state owing to the quenching of the excited states generated in the course of the laser photolysis and are detected using the resonance absorption at a wavelength of 385.99 nm. The effective quenching rates are in good agreement with the known rates of the quenching of metastable iron atoms by the Fe(CO)₅ molecules. It is demonstrated that a loss of iron atoms is related to the recombination with dimer and trimer formation and the secondary atomic reactions with the Fe(CO)₅, CO, and FeCO molecules. The rates of the main elementary reactions responsible for the loss of iron atoms are determined using the comparison of the experimental results and kinetic simulation data.     

Quenching of vacuum ultraviolet fluorescence emission from electron beam excited quasi-molecular xenon

A pressure dependent quenching of the VUV broad-band fluorescence (bandwidth 15 nm centred at λ 173 nm) from high pressure xenon has been measured for pressures from 25 to 215 p.s.i. The experimental results can be explained by collisional de-excitation of the excited Xe₂ molecules by Xe atoms. A quenching cross section of ≈ 10^-17 cm², and molecular radiative lifetime of 16 nsec have been derived. The implications for the design of VUV molecular xenon lasers are discussed.     

The quenching of Pb(6p 2 1 D 2) atoms in collisions with molecules

The collisions of metastable Pb(6p ^{2 1} D ₂) atoms with various molecules were studied by the diagnostics of radiation from a hollow cathode lamp and a laser on lead vapor. Experiments were performed for a gas flow of lead atoms with argon. The Pb(6p ^{2 1} D ₂) states were excited in a gas discharge in the presence of reagent gas molecules. The absolute rate constants for the quenching and chemical reactions of lead atoms in the ground and excited states were determined. The quantum efficiency of chemical reactions was close to one for the N₂O, CH₂Cl₂, SF₆, and CuBr molecules. Long-lived chemical compounds were formed in these reactions.     

Scattering of positronic atoms on optical phonons in ionic crystals. Positron thermalization

The time for positron thermalization in ionic crystals due to the interaction of positronic atoms with polarizational optical oscillations of the lattice is calculated. The positronic atom is considered as a dipole with moment p induced by the internal crystal field. The dipole moment is evaluated from the value of the positronic-atom deformation parameter determined from data on the magnetic quenching of positronium. For an NaCl crystal, p 1.91D. The positronium thermalization time in ionic, crystals (for the example of NaCl) is estimated at 10^–1 sec. After this time, not only ortho positronium but also para positronium reached total thermalization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizida, No. 5, pp. 66–70, May, 1979.     

TRIPLET TO SINGLET CONVERSION OF POSITRONIUM IN OXYGEN

Using a aerogel sample with introduced oxygen gas as a variable energy positronium source,the triplet to singlet conversion of positronium in the elastic and inelastic collisions between positronium atoms and oxygen molecules has been investigated with a time selecting energy spectrometer.We have found that the conversion cross section σ_0—p is proportional to 1/v for slow positronium in the elastic collisons,where v is the mean velocity of positronium,and for the inelastic processes,the cross sections are estimated to be σ_0—p=2.1×10^－17cm² and 6.6×10^－18cm² respectively at kinetic energies of positronium near two thresholds.     

Positronium interactions with gases in pores of silicagels

Positronium interaction with O₂, NO, Cl₂ molecules in pores of silicagels has been investigated by the methods of lifetime and angular correlation. For silicagel with diameter of pores ≈100 Å it was obtained that the rate constant of positronium conversion on oxygen was (2.4±0.2)10⁷ sec^?1 atm^?1 and on nitrogen oxide (2.6±0.2)10⁷ sec^?1 atm^?1. The rate constant of the positronium chemical reaction with Cl₂ was found to be ≈10^?11 sec^?1 atm^?1. The results of investigation of positronium interactions with oxygen in the gas phase, organic liquids, zeolites, liquid argon, and nitrogen are explained in terms of the formation of an excited complex PsO ₂ ^* . The decay of this complex to Ps and O₂ makes possible positronium conversion, and stabilization of the complex leads to annihilation in the bound state (chemical reaction). Study of positronium interaction with O₂ in silicagels (diameters of pores 100, 30 and 16 Å) confirms the assumption of the complex formation. The rates of chemical reaction and of conversion become equal for pores of diameter of 20 Å. The estimated lifetime of the PsO ₂ ^* complex is τ _k ? 10^?12 sec.     

Induced decay of positronium and grasers

The differential cross section and the total cross section for the stimulated decay of positronium by an incident photon of frequencyΩ is calculated as a function of the dimensionless variableξ=hΩ/mc ². Forξ?1 the total cross section is found to decrease asξ ^?2. We also look at the particular case of positronium in a black-body radiation field. Expressions for the number of induced annihilations per second as functions of the dimensionless ratiomc ²/kT and the number of positronium atoms are obtained. It is found that this rate is proportional to (kT/mc ²)² forkT?mc ² and to (kT/mc ²)ln(kT/mc ²) forkT?mc ². The possibility of utilizing induced two-photon decay of positronium as a γ-ray laser at the wavelength λ _C /2 is examined, where λ _C is the Compton wavelength.     

Nonadiabatic theory of diamagnetic susceptibility of molecules

A nonadiabatic theory of diamagnetic susceptibility of molecules is presented in which the electrons and nuclei are considered to be a united system of charged particles whose motion is simultaneously per-turbed by a magnetic field. It is found that on separating out the translational motion of the molecule as a whole, there is certain freedom in choosing the phase of the wave function. Its optimum choice corresponds to the gauge of the vector potential with which two contributions opposite in sign to the magnetic susceptibility—the first order diamagnetism and the second order paramagnetism—have minimum magnitudes. Expressions for non-adiabatic calculations of the diamagnetic susceptibility of atoms and molecules are derived. The diamagnetic contributions to the energy of the hydrogen, helium, and lithium atoms, the hydrogen molecule, the π^?μ^?π⁺μ⁺ and p ^? K ^? p ⁺ K ⁺ mesomolecules, and the positronium molecule e ^? e ^? e ⁺ e ⁺ are calculated. The nonadiabatic contribution of the nuclear motion to the diamagnetic susceptibility amounts to 0.01–0.1% for ordinary atoms and molecules, is increased by several hundred times on passing to mesomolecules, and reaches 50% for the positronium molecule.     

Broadening of a two-photon resonance in a zinc atom in collisions with CO<Subscript>2</Subscript>, CO,and NO molecules

The broadening of a two-photon resonance is studied experimentally at the 4s¹S₀?6s³S₁ transition in a zinc atom upon absorption of two waves with a small detuning from an intermediate state in collisions with CO₂, CO, and NO molecules. The measured absolute values of broadening cross sections greatly exceed gas-kinetic cross sections and are (9.4±2.4, 6.5±1.6, and 3.9±1.0)×10^{? 14}cm² for CO², CO, and NO, respectively. Anomalously large rate constants and cross sections obtained in experiments are explained by the efficient resonance quenching of the excited states of zinc atoms in collisions with molecules accompanied by transfer of the energy of excited atoms to vibrational-rotational degrees of freedom of molecules.     

光化学反应中氩气对激发态锂原子猝灭速率研究

本文对光化学法分离中氩气对2P激发态锂原子的碰撞猝灭速率进行研究.在弱激光作用下,通过对不同氩气压强下的锂原子蒸气吸收光谱和激发态锂原子的荧光发射光谱进行测量,得到氩气对2P激发态锂原子的猝灭速率常数为(12.29±0.92)×10-18m3·s-1.实验结果表明:在光化学锂同位素分离研究中,氩气对锂原子2P激发态的猝灭速率远小于2P激发态锂原子的自发辐射速率,碰撞猝灭效应对分离选择性的影响非常小,可以忽略不计.     

One-loop corrections of orders (Zα)6m1/m2 and (Zα)7 to the fine structure of muonium

Corrections of orders (Zα)⁶ m ₁/m ₂ and (Zα ⁷ from one-loop two-photon amplitudes to the energy spectra of hydrogen-like atoms are calculated by expanding the relevant integrand in a Taylor series. A method for averaging the resulting quasipotential in the d-dimensional coordinate representation is formulated. Numerical values are obtained for the corresponding contributions to the fine structure of the muonium, hydrogen, and positronium atoms. __________ Translated from Yadernaya Fizika, Vol. 66, No. 9, 2003, pp. 1767–1777. Original Russian Text Copyright ? 2003 by Martynenko, Faustov.     

Positron-lithium scattering with the inclusion of positronium formation

Detailed studies have been made of elastic scattering and positronium formation in low energy collisions of positrons with lithium atoms for the two partial wavesl=0,1. For this system, as for all alkali atoms, the positronium formation channel is open even at zero positron energy. A two-channel version of the Kohn variational method is used with trial functions containing many variational parameters, and reasonably well converged results are obtained. The s-wave positronium formation cross section is infinite at zero positron energy but it then falls rapidly to become several orders of magnitude smaller than the elastic scattering cross section which has a maximum value of approximately 100 ₀ ² at a positron energy of 0.5 eV. For p-wave scattering the positronium formation cross section rises to a value of approximately 10 ₀ ² at an energy of 0.1 eV, with the elastic scattering cross section rising to a maximum of approximately 60 ₀ ² just below the first excitation threshold at 1.84 eV.     

Evidence,by metastable quenching spectroscopy,that CO adsorbs on K/Ni(111) near potassium and lies flat or tilts when heated

By using metastable quenching spectroscopy (MQS) we show that if a K/Ni(111) surface with low K coverage (θ_K = 0.1 and 0.12) is exposed to low doses of CO (0.3 L), the CO molecules are adsorbed near the potassium atoms. Heating from 118 to 298 K results in spectral changes that indicate that the CO molecules closest to the potassium atoms tilt strongly or lie flat on the surface.     

Estimates of axion signal from positronium decay

Numerical values for axion decay channel branching ratio in positronium decays are presented. From the ³S₁ state the maximum is of the order of 10^?6. If the axion mass were to be in the vicinity of the positronium mass, the best way of observing the signal would be the decay from the 2¹P₁ state. The feasibility of attaining this by optical pumping is indicated.     

Kinetics of energy deactivation in the O2(1Δ)-I medium

The kinetics of energy-exchange and quenching processes in the O₂(¹Δ)-I medium plays an important role in the operation of the chemical oxygen-iodine laser. However, up to now, many of the key processes in this medium are not fully understood. In this work, the flow tube technique is used to determine the rate of excitation energy deactivation in the O₂(¹Δ)-I medium in the presence of small concentrations of water and carbon dioxide. The concentrations of electronically excited species are measured with spectrometers calibrated in absolute spectral responsivity. The effective rate constant for the deactivation of O₂(¹Δ) molecules in processes involving iodine atoms is determined. The results are compared to those obtained in previous works.     

Positrons — an alternative probe to electron scattering

Interaction of positrons with atoms and molecules differs from electron interaction due to the difference in polarity of the charge. This makes positrons an alternative tool to study atomic and molecular structure. Recent measurements of the total cross-sections for positron scattering at low energies on He, Ar, H₂, N₂, C₆H₆, C₆H₁₂, C₆H₇N carried out at Trento University [Karwasz et al., Acta Phys. Pol. 127, 666 (2005)] are discussed and compared to electron scattering results. All measured total cross-sections exhibit an increase with decreasing positron energy in the limit of zero energy; H₂, N₂, Ar, show regions of constant cross-section which are a few eV-wide, characteristic of scattering on a hard-sphere potential. Helium shows two resonant structures much below the positronium formation threshold. They may be attributed to virtual positronium formation. In conclusion, positron scattering is complementary to electron scattering. The total cross-sections do not show Ramsuaer minima but constant values, and new resonances appear.