Collisional deexcitation of exotic hydrogen atoms in highly excited states

The deexcitation of exotic hydrogen atoms in highly excited states in collisions with hydrogen molecules has been studied using the classical-trajectory Monte Carlo method. The Coulomb transitions with large change of principal quantum number n have been found to be the dominant collisional deexcitation mechanism at high n. The molecular structure of the hydrogen target is shown to be essential for the dominance of transitions with large Δn. The external Auger effect has been studied in the eikonal approximation. The resulting partial wave cross-sections are consistent with unitarity and provide a more reliable input for cascade calculations than the previously used Born approximation. Received 28 May 2002 Published online 15 October 2002 RID="a" ID="a"e-mail: thomas@physik.unizh.ch     

The radiative decay ϕ → γππ in a coupled-channel model and the structure of f0(980)

A coupled-channel model is used to study the nature of the scalar mesons produced in the decay ϕ → γππ. The K molecular picture of f₀(980) is found to be in a good agreement with the recent experimental data from SND and CMD-2. The structure of the light scalar mesons is elucidated by investigating the S-matrix poles and the q spectral density.     

Kinetic energy of π− p-atoms in liquid and gaseous hydrogen

We have measured the Doppler broadening of neutron time-of-flight spectra from the reaction π⁻ p → π⁰ + n in atomic states. From the data, we infer that the kinetic energy distribution of π⁻ p-atoms in liquid and gaseous hydrogen contains discrete ‘high-energy’ components with energies as high as 200 eV attributed to Coulomb de-excitation. In liquid hydrogen, evidence for Coulomb de-excitation transitions with Δn = 2 has been found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Scattering of muonic hydrogen atoms

Our measurement compares the energy dependence of the scattering cross-sections of muonic deuterium and tritium on hydrogen molecules for collisions in the energy range 0.1–45 eV. A time-of-flight method was used to measure the scattering cross-section as a function of the muonic atom beam energy and shows clearly the Ramsauer–Townsend effect. The results are compared with theoretical calculations by using Monte Carlo simulations. The molecular pdμ and ptμ formation creates background processes. We measure the formation rates in solid hydrogen by detecting the 5.5 MeV (pdμ) and 19.8 MeV (ptμ) γ-rays emitted during the subsequent nuclear fusion processes. This revised version was published online in August 2006 with corrections to the Cover Date.     

New effects in low energy scattering of pµ atoms

Strong solid state effects in low energy scattering of pμ atoms in solid hydrogen are reported and analyzed. Such effects have been observed in TRIUMF experiment E742 where muons are stopped in thin frozen (3 K) layers of hydrogen. Emission of low energy pμ atoms from the hydrogen layer into adjacent vacuum was much higher than expected, based on calculations which ignored the solid nature of hydrogen. Monte Carlo simulations, performed using the scattering cross-sections with solid state effects taken into account, show the important role of the coherent elastic Bragg scattering in the diffusion of pμ atoms. For pμ energies lower than the Bragg cut-off limit (∼2 meV) the total scattering cross-section falls by several orders of magnitude, the hydrogen target becomes transparent and the emission of cold pμ atoms takes place. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mass spectrometric sequencing and acylation character analysis of C-terminal anchoring segment from Influenza A hemagglutinin

Influenza A virus hemagglutinin (HA) is a major envelope glycoprotein mediating viral and cell membrane fusion. HA is anchored in the viral envelope by a light HA(2) chain containing one transmembrane domain and a cytoplasmic tail. Three cysteine residues in the C-terminal region, one in the transmembrane domain and two in the cytoplasmic tail, are highly conserved and potentially palmitoylated in all HA subtypes. The HA(2) C- terminal anchoring segments were extracted to organic phase from the bromelain-digested viruses (subviral particles) of three strains: A/X-31 (H3 subtype), A/Puerto Rico/8/34 (H1 subtype) and A/FPV/Weybridge/34 (H7 subtype). Their primary structures were assessed by matrix-assisted laser desorption/ionization time-of-flight time-of- flight mass spectrometry (MALDI-ToF-ToF MS). Trypsin-type protease-cleaved peptides prevailed over bromelain- cleaved ones in the peptide mixtures. All of them included transmembrane domains. Several distinctive features of the C-terminal HA(2) peptides acylation character were discovered by MALDI-ToF MS: 1) the peptides isolated from the viruses, which were digested by bromelain in the absence of beta-mercaptoethanol, were predominantly triply acylated; 2) the peptides were acylated not only by palmitic, but also by stearic acid residues; 3) the palmitate/stearate ratio was different for the three strains studied; 4) the A/FPV/Weybridge/34 strain has a priority to stearate binding. This fatty acid residue was discovered at the first of three conservative cysteine residues located in the transmembrane domain. It was found that presence of thiol reagent during preparation of subviral particles led to the appearence of the C-terminal HA(2) peptides acylated to different degrees. Triply, doubly, mono- and even unacylated peptides were detected. It was demonstrated that the thioester bond in the isolated acylpeptides was extremely sensitive to thiol reagents.     

High-velocity components in exotic atoms,their origin and effects

The evolution of the kinetic energy distribution during the cascade is shown to play an important role for pionic and muonic atoms in hydrogen and deuterium. The kinetics of muon transfer in excited states is studied with a new cascade code and the results are compared with recent experimental data.     

Time-of-flight measurement of resonant molecular formation in muon-catalyzed dt fusion

Preliminary results are reported for an experiment at TRIUMF where a time-of-flight technique was tested for measuring the energy dependence of the rate for muon-catalyzed dt fusion. Muonic tritium atoms were created following transfer of negative muons from muonic protium in a layer of solid hydrogen (protium) containing a small fraction of tritium. The atoms escaped from the solid layer via the Ramsauer-Townsend mechanism, traversed a drift region of 18 mm, and then struck an adjacent layer of deuterium, where the muonic atom could form a molecular system. The time of detection of a fusion product (neutron or alpha) following muon arrival is dependent upon the energy of the muonic tritium atom as it traverses the drift region. By comparison of the time distribution of fusion events with a prediction based on the theoretical energy dependence of the rate, the strength of resonant formation can in principle be determined. The results extracted so far are discussed and the limitations of the method are examined.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

The nuclear fusion in the muonic molecule ddμ and the charge symmetry violation in the low energy dd interaction

The rate λ_f = 1.0 × 10⁹ s^?1 of the nuclear fusion in the ddμ molecule is calculated. It is shown that the ratio of the neutron to the proton yield equals the ratio of the p-wave cross sections of the d(d,n)³ He and d(d,p)t reactions near the dd threshold.