Charge exchange and excitation cross sections at collisions of alpha particles with be-like impurity oxygen ions in a plasma

The charge exchange and excitation cross sections at collisions of alphas with O⁴⁺(1s ²2s ²) impurity atoms in a hot plasma for striking energies E _c varying from 20 keV to 2 MeV are determined for the first time. The cross sections are calculated using the method of close-coupling equations with 13 singlet four-electron quasi-molecular states taken as a basis. The partial cross sections of charge transfer to the 1s, 2s, and 2p states of a He⁺ ion and for O⁴⁺(1s ²2s ²) → O⁴⁺(1s ²2lnl’) (n = 2, 3) electronic excitation of an oxygen ion are found. The maximal value of the charge exchange total cross section roughly equals 2.2 × 10^?16 cm² at E _c ≈ 0.7 MeV. The excitation total cross section has a maximum of ≈ 7.7 × 10^?16 cm² at E _c ≈ 80 keV for single-electron excitation and ≈6.5 × 10^?16 cm² at E _c ≈ 0.7 MeV for two-electron excitation.     

Complete density matrix measured for Na(32 P) excited by electron impact

Anisotropies observed in collisional deexcitation of laser excited Na atoms are analysed in terms of the density matrix describing the Na(3P) atom after the process Na(3² S)+e ^?(E)→Na(3² P)+e ^?(E?δE). Collision energiesE range from 5.1 to 22.1 eV, and scattering angles fromθ _col=0 to 25°. The results are presented with reference to a coordinate frame in which the angular momentum transferred during collision is parallel to the quantization axis. This choice allows a convenient exploitation of reflection symmetry and offers most direct relation of measurable quantities to the sublevel excitation amplitudes as e.g. computed by close coupling and distorted wave methods. Comparison between experiment and theory provides a sensitive probe of the strengths and shortcomings of the theory when predicting these alignment and orientation parameters. Since we discuss a complete set of data containing all accessible information on the atomic orbital the present measurements allow the detection of a deviation from coherence in the excitation for small scattering anglesθ _col≈5–10° atE=5.1 eV which we attribute to electron exchange.     

Measurement of the KS0 → π+π−γ decay rate

From a study of 70 000 K° decays in the CERN 2m hydrogen bubble chamber, we have estimated the rate for K_S⁰ → π⁺π^?γ for different cuts in the γ momentum. The results are in good agreement with inner bremsstrahlung on K_S⁰ → π⁺π^?. For γ momentum in the K° c.m. greater than 50 MeV/c, we obtain the branching ratio: Γ(K_S → π⁺π^?γ, k>50 MeV/c)/Γ(K_S→π⁺π^?)=(2.8. ± 0.6)×10^?3. This leads to an estimate for the direct γ emission process: Γ(K_S→π⁺π^?γ direct)/Γ(K_S → π⁺π^?)=(0.3 ± 0.6) × 10^?3.     

Excitation of the Yb II transitions terminating on the low-lying odd levels

Excitation of the transitions from the even levels of a singly charged ytterbium ion that terminate on the low-lying odd levels 4f ¹³(² F°)6s ^{2 2} F°, 4f ¹⁴(¹ S)6p ² P°, and 4f ¹³(² F°_7/2)5d6p(³ D)³[3/2]° is experimentally studied by measuring 51 excitation cross sections at an electron energy of 50 eV, and 16 optical excitation functions are determined within the electron energy range 0–200 eV. The largest magnitudes of the measured cross sections exceed 3 × 10^?17 cm².     

Velocity dependence of total Penning ionization cross sections

The velocity dependence of the total Penning ionization cross sections,σ(v), is measured in the thermal relative velocity region, using a time of flight method.σ(v) curves are reported for the collision systems He(2¹ S)/Ar, Kr, Xe, N₂, Hg, He(2³ S)/Ar, Kr, Xe, N₂, Hg, Ne(³ P _{2, 0})/Kr, Hg, and Ar(³ P _{2, 0})/Hg. In a qualitative discussion it is shown that all features of the measuredσ(v) curves may be explained within the frame of the theory of Penning ionization, allowing to extract information on the physical quantities governing the process: on the interaction potentialV(R) and on the transition probabilityW(R). A theoretical calculation for the He(2³ S)/Ar system shows good agreement with our experimentalσ(v) curve. On the basis of the present results earlier data onσ(v), and on absolute cross sections and rate constants obtained at certain relative velocity distributions are discussed.     

Search for the P11Σ(1660) resonance in the reactions K− + p → Σ± + π∓ in the momentum range 597 to 888 MeVc

The reaction cross sections, differential cross sections and Σ ⁺ polarization for the reactions K^? + p → Σ^± + π^? at 8 momenta in the range 688 to 833 MeV/c were obtained. A partial-wave analysis was performed over the momentum range 597 to 888 MeV/c using the present data together with data in the neighboring momentum regions. A satisfactory solution was obtained with the well-established S₀₁ Λ(1670), D₀₃ Λ(1690) and D₁₃ Σ(1670) resonances. The addition of a P₁₁ resonance with a mass, width, and branching ratio of 1665 MeV/c², 82 MeV/c² and ?0.13 improved the χ² per degree of freedom by an insignificant amount, indicating that the coupling of the p¹¹Σ(1660) to the Σ^±ν^? channels is at best marginal.     

Untersuchung der vier ersten angeregten Zustände des6Li-Kernes durch Elektronenstreuung

Inelastic electron scattering cross sections were measured for energies below 60 MeV and momentum transfersq between 0.2 and 0.6 fm^?1. Ground state radiative widths Γ _γ ⁰ and transition radiiR _tr were deduced. 2.18 MeV: Γ _γ ⁰ (E2)=(4.40±0.34) · 10^?4 eV,R _tr=(4.28±0.39)fm; 3.56MeV: Γ _γ ⁰ (M1)=(8.31±0.36)eV,R _tr=(2.90±0.10)fm; 4.27 MeV: Γ _γ ⁰ (E2)=(5.4±2.8) · 10^?3 eV,R _tr=(3.4±1.2) fm. The excitation of the 5.37 MeV level shows a transverse angular dependence.     

Interaction potentials and energy transfer cross sections for collisions of metastable helium and neon I: He(23 S)+Ne

Differential cross sections have been measured for He(2³ S)+Ne at kinetic energies between 28 and 370 meV. For energies above 90meV the elastic cross sections show Stückelberg oscillations from curve crossings, which lead to the energy exchange process: He(2³ S)+Ne→He(1¹ S)+Ne(2p ⁵ 4s,3d,4p). Differential cross sections for this inelastic process could be measured above 200 meV. A fit to the data gives the potentials for He(2³ S)+Ne and, less accurately, for He+Ne^*. These results offer a simple explanation, why the exothermic pumping process of the infrared lines of the HeNe laser has a threshold of about 80 meV and a small cross section.     

Nonadiabatically Coupled Hyperspherical Equations Applied to the Collisional Spin Flip of Muonic Hydrogen

Nonadiabatically coupled equations in terms of hyperspherical coordinates are solved for the elastic and hyperfine transitions F→F′ of muonic hydrogen isotopes pμ, dμ, and tμ in collisions pμ(F)+p→p+μp(F′), dμ(F)+d→d+μd(F′), and tμ(F)+t→t+μt(F′), as an extension of our previous work on pμ [Igarashi et al., Phys. Rev. A 58 (1998), 1166]. Converged cross sections are obtained for collision energies 10^?3 to 10² eV and for the total orbital angular momentum L=0 and 1 including the spin–spin interactions V _S explicitly in the Hamiltonian, and for L≥2 neglecting V _S because of the large interparticle separations. The inclusion of V _S for L=0 and 1 reproduces the correct hyperfine-splitting energy Δ∈ in the separated-atom limit, and is found to be of vital importance for the calculation of the spin-flip cross sections for collision energies lower than Δ∈.     

Inclusive strange particle production in π−p interactions at 6 GeV/c

The inclusive production of Λ, K_S⁰, Σ^±(1385) and $\text{K}{}^{\mathrm{1\pm }}\text{(892)}$ in π^?p interactions at 6 GeV/c has been studied. The observed cross sections are: σ(Λ)=0.94±0.06 mb, σ(K_S⁰)=0.98±0.06 mb, σ(Σ⁺(1385))=60±7 μb, σ(Σ^?(1385))=90±9 μb, $\text{σ(}\text{K}{}^{\mathrm{1+}}\text{(892))=216±28 μ}\text{b and}\text{σ(}\text{K}{}^{\mathrm{1?}}\text{(892))=41±8 μ}\text{b}$, respectively. The inclusive spectra of these particles are presented as functions of squared transverse momentum and Feynman scaling variable x. The polarization of Λ has also been investigated. It is found from a comparison with higher-energy data that the inclusive cross sections for Σ^±(1385) and the production ratios Σ^±(1385)/Λ in π^?p at 6 GeV/c have not reached the high-energy limiting values.     

Absolute cross sections of electron attachment to molecular clusters: Part I. Formation of (CO2) N −

A procedure of determining absolute cross section σ^? of electron attachment to (CO₂)_N clusters at pair collisions in crossed beams is suggested. The cross section is measured as a function of energy (E = 0.1–50 eV) and of cluster mean size N in a beam $(\bar N = 2 - 4000 molecules)$ . It is found that, even at $\bar N > 200$ and E ≤ 3 eV, σ^? is equal to, or larger than, 7 × 10^?13 cm², i.e., by more than one order of magnitude exceeds the maximal cross section of CO₂ ionization by electron impact. The dependences σ^? $(\bar N,E)$ have two wide continua at E ≤ 5.2 eV and E ≥ 6.9 eV, which correlate well with known functions of CO₂ electron-impact-induced excitation. These continua are attributed largely to formation of (CO₂) _N ^? ions during electron thermalization and solvation in the clusters. At E → 0, the polarization capture of an incident electron by the cluster leads to a sharp increase in cross section σ^?(E). From the dependences σ^? $(\bar N,E)$ measured, the thermalization and sovation probabilities for electrons with E ≤ 0.8 eV and the rate of electron energy loss in the cluster are found.     

Inclusive single and double diffractive dissociations inK + p interactions at 32 GeV/c

We study single and double inclusive diffractive production in a 32 GeV/c K ⁺ p experiment in MIRABELLE at the Serpukhov accelerator. From reactionsK ⁺ p→K ⁺+X ⁺ andK ⁺ p→X ⁺+p we determine the total proton and kaon single diffractive dissociation cross sections \(\sigma (p\xrightarrow{{K^ + }}p_D ) = 0.90 \pm 0.12 mb\) and \(\sigma (K^ + \xrightarrow{p}K_D^ + ) = 0\) . 90±0.17 mb, respectively. In either case the only notable contributions come from dissociations into 1 and 3 charged particles. Kaon dissociation exhibits a pronounced slope-mass correlation. The search for double diffractive production in reactionsK ⁺ p→(K ⁺π^?π⁺)+X ⁺ andK ⁺ p→(pπ ⁺ π ^?)+X ⁺ leads in either case to an estimated total double diffractive cross section σ(K ⁺p→K _D ⁺ p_D) of ?220 μb. The double dissociation differential cross section exhibits a large slope of ?10GeV^?2 in the nearthreshold mass region, rapidly decreasing to ?4 GeV^?2 with increasing excitation mass. At our c.m. energy \((\sqrt s \simeq 8 GeV)\) the ratio σ_{inel dif}/σ_el is 0.85±0.10, the total diffractive cross section σ_dif≡σ_el+σ_{inel dif}=4.41±0.24 mb and the fraction σ_dif(K ⁺ p)/σ_tot(K ⁺ p) is 25±2%. TheK ⁺ andK ^? diffractive excitation mass spectra, differential cross sections and total diffractive cross sections are very similar for both single and double dissociations.     

Coincidence studies with polarisation analysis of the photon emission induced inK-inert gas collisions

The linear and circular polarisations of theK(4² P→4² S) transition detected in delayed coincidence with the inelastically scattered potassium atom are presented forK- He, Ne, Ar, Kr and Xe. For all systems the Stokes vector measurements have been performed as a function of the potassium scattering angle at a beam energy of 10³ eV. The coherence properties of the excitation process are studied with the help of the measured degree of polarisation. For He, Ne and Xe the excitation amplitudes (e.g. the parametersλ andχ) are deduced from the measured Stokes vector. For Ar and Kr the collisionally excited state is not a pure state, and it is concluded that both single excitation of theK atom and simultaneous excitation of both collision partners contribute toK(4² P) excitation. The results are compared with the predictions of the semiclassical treatment of the excitation process.     

Two-nucleon production of hyperons and the search for strange dibaryons

The double charge exchange reaction³He(K^?,π ⁺)Xn was studied at 870 MeV/c. In the X missing mass range below the sigma-nucleon production threshold (2130 MeV/c²), events were detected which can be attributed to the two-nucleon process pp(K^?,π ⁺)λn. This reaction and mass range also offers good prospects for finding theI=1/2,l=1 (¹ P₁) spin-singlet dibaryon D_s suggested as the lowest massS=?1 dibaryon in the MIT Bag Model. Although the existence of the D_s is not ruled out by the present data, there is no need to invoke such an object to account for the observed events below σ production threshold. We show that the cross section level for these events is compatible with a dominant two-nucleon mechanism K^?p→π ⁰λ,π ⁰p →π ⁺n. We also offer an interpretation of the recent (K^?,K⁺) data on nuclear targets from Iijimaet al., which display a broad peak centered around a K⁺ momentum of 600 MeV/c. We find that the two-nucleon mechanism K^?N →πY,πN→K⁺Y produces cross sections which are at least an order of magnitude smaller than those observed, and we suggest that the one nucleon process K^?p →Φλ, followed by the decayΦ → K⁺K^?, accounts for the data.     

Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 2³ S ₁ metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn ^?5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 2³ S?n ³ P series was verified. It is shown that the cross sections for a triplet level excitation from the 2³ S ₁ metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 10³ for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.     

Investigation of photoionization and photodissociation of an oxygen molecule by the method of coupled differential equations

The photoionization and photodissociation of an oxygen molecule are investigated theoretically by the method of coupled differential equations. The molecular orbitals of the core are calculated in the MO LCAO approximation. The molecular orbitals of a photoelectron in the discrete and continuous spectra are determined by the single-center method. The wave functions of vibrational motion of the oxygen molecule are obtained within the diabatic approach. The described method is used for calculating the predissociation and autoionization widths of the 2σ _u ^?1 (c ⁴Σ _u ^? )n/σ_g, v Rydberg states of the oxygen molecule. The total cross sections of the resonant photoionization and neutral predissociation calculated for the oxygen molecule in the excitation energy range 20.6–24.8 eV are in good agreement with the available experimental data.     

Amplitude and natural-parity-exchange analysis of K±p → (Kπ)±p data at 10 GeV/c

High statistics data for the reactions K^±p → K_S⁰π^±p at 10 GeV/c are analysed. The K¹(1^?), K¹(2⁺), and K¹(3^?) resonance parameters and production cross sections are calculated. The Kπ production amplitudes are determined as a function of t and the produced Kπ mass. Isoscalar natural-parity-exchange (NPE) is dominant. The t dependence of the K^± NPE amplitudes have a cross-over at t = ?0.3 (GeV/c)² for both K¹(890) and K¹(1420) production, being more pronounced for the K¹(1420). Natural-parity-exchange interference effects are isolated. The NPE amplitudes are decomposed into pomeron-, f-, and ω-exchange contributions. S-wave Kπ production is found to be consistent with the Kπ partial-wave analyses of charge-exchange reactions.     

First observation of the production and decay of the Σc+

An event with the decay chain Σ_c⁺ → Λ_c⁺ + π⁰, Λ_c⁺ → K^? + p + π⁺, has been observed in an exposure of BEBC, equipped with a track sensitive target, to the wide band neutrino beam from the SPS at CERN. The event has a unique three constraint kinematic fit to the ΔS = ?ΔQ reaction v + p → μ^? + p + K^? + π⁺ + π⁺ + π⁰ with both gammas from the π⁰ decay detected. The proton and other final state particles are identified. The masses are M(Λ_c⁺) = 2290 ± 3 MeV/c², M(Σ_c⁺) = 2457 ± 4 MeV/c² and M(Σ_c⁺) ? M(Λ_c⁺) = 168 ± 3 MeV/c². Including other data one obtains M (Σ_c⁺⁺) ? M(Σ_c⁺) = 0 ± 4 MeV/c².     

φ meson production fromp\bar p collisions at\sqrt s = 1.8 TeV

Fermilab experiment E735 located at the CO intersection region of the $\sqrt s = 1.8$ TeV $p\bar p$ collider analysed over 900 Φ→K ⁺ K ^? events. Measured were the transverse momentum spectrum, the correlation between the average transverse momentum <pt> and the charged particle multiphcityN _c , as well as the probability of Φ production per charged track,N _Φ /N _c , versusN _c . We have also made an estinate of the total inclusive cross section for Φ mesons, $\sigma (p\bar p \to \phi X) = 7.3 \pm 2.2 mb$ .     

Vibrationally inelastic scattering of H∼ ions from H2, N2, O2 and CO2

State-resolved measurements are presented for vibrational excitation of H₂, N₂, O₂ and CO₂ by H^? impact in the collision energy rangeE _cm=20–180 eV and for scattering in the forward direction (0±0.5°). The data obtained from the measurements are the relative intensities and differential cross sections for vibrational excitation up toν′=4, the transition probabilitiesP _0→ν′ and the vibrational energy transferΔE _vib. For the systems H^?-H₂, N₂, O₂ the vibrational inelasticity increases in the order H₂-N₂-O₂. The mechanism for vibrational excitation in these systems is due to transient charge transfer from the H^? ion into antibonding orbitals of the target molecule which provides a bond stretching force during the collision. For H₂ and N₂, the results are compared with corresponding measurements for H⁺ scattering where the interaction mechanism is quite different. In the case of CO₂, vibrational excitation in forward scattering is caused primarily by the long-range dipole interaction. The spectra are very similar for H^?-CO₂ and H⁺-CO₂. Finer details are attributed to the influence of transient charge transfer and valence interactions.