J.U. Andersen

STRAGGLING IN ENERGY LOSS OF ENERGETIC HYDROGEN AND HELIUM IONS

Abstract The straggling in energy loss for protons and alpha particles is discussed, with special emphasis on correlation effects, stemming from the bunching of electrons in atoms and molecules and from charge-state fluctuations. It is shown that for gaseous targets, the correlation effects give rise to an increased straggling relative to that calculated from an electron-gas picture. For an amorphous, uniform solid, the contribution from correlation is much reduced, and the total energy straggling should approximately be given by the Lindhard-Scharff electron-gas expression. The theoretical description is in good agreement with accurate measurements of energy straggling for hydrogen and helium ions on a variety of atomic and molecular gases. Measurements in solids are often obscured by the influence of target non-uniformity and texture. A simple method to check for such effects is discussed and demonstrated by an example.

On the concept of temperature for a small isolated system

The microcanonical temperature is shown to be a useful concept in calculations of the decay of a small isolated system with well defined energy. A simpler and more transparent description is obtained than in Klots’ formulation of finite-heat-bath theory, where the system is represented by a canonical ensemble. As a further illustration of the utility of the microcanonical temperature concept, we discuss a formula derived by Dunbar for the probabilities for excitation of a single oscillator in a collection of harmonic oscillators with well defined total energy. This formula expresses the excitation probabilities in terms of the temperature for a canonical ensemble with mean energy equal to the energy of the system. However, a much improved accuracy is obtained if the canonical temperature and heat capacity are replaced by their microcanonical values. We justify this replacement through a modified derivation, in which the microcanonical temperature appears as the canonical temperature of a fictitious system...

The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (∼106 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (∼102–104 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 103–104 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine re...

Axial channeling radiation from MeV electrons

Abstract Axial channeling radiation from MeV electrons in Si resulting from spontaneous transitions between transverse energy states is studied theoretically and experimentally. Transitions between tightly bound states are well described within the single-string approximation with a thermally averaged Hartree-Fock continuum potential. This applies both for line energies and for absolute intensities as functions of incidence direction for the electron beam. For weakly bound states the extension of the calculations to describe itinerant states in the tight-binding approximation reproduces the band widths obtained from more complete many-beam calculations. For a thick crystal the discrete lines are Lorentzians with widths determined by incoherent scattering, whereas the finite crystal thickness modifies the line shape and the line width in thinner specimens. For tightly bound states the incoherent scattering is due to thermal displacements of atoms, and the formula presented for the scattering probabilities gives satisfactory agreement with observed line widths when correlations of displacements are included.

Z1 scaling for impact-parameter dependence of inner-shell ionization by heavy ions

Abstract The Z 1 dependence of inner-shell ionization has been investigated by measuring the impact-parameter dependence of Cu-K-shell ionization by H, Be, C, and O, at energies corresponding to 2 MeV/amu. Large deviations from a scaling with Z 1 2 are observed. By scaling results for ionization of Cu by protons, it is attempted to test quantitatively an explanation based upon the modification of the K-electron binding and wavefunction, due to the proximity of the projectile to the Cu nucleus. The scaling is successful for close collisions, but significant discrepancies are found at larger impact parameters. From measurements with oxygen at different energies, the deviations are found to depend strongly on projectile velocity, almost disappearing at the lowest bombarding energy. Possible explanations are considered in connection with a discussion of the binding correction, based on adiabatic perturbation theory. Also the origin of the charge-state effect observed for oxygen (≈20% difference between O 8+ and O 6+ ) is discussed.

Covalency effects on implanted119Sn in group IV semiconductors studied by Mössbauer and channeling experiments

Radioactive119mSn has been implanted with an isotope separator in single crystals of germanium, silicon, and diamond. Implantations of low doses (∼1013 atoms/cm2) at room temperature were performed as well as of higher doses at temperatures of about 400°C. The Mössbauer spectra of these sources show mainly one line. This line originates from119mSn on substitutional lattice sites as determined from channeling experiments with 2 MeV He+ ions on the same samples. The observed systematics of the isomer shifts for119Sn is explained on the basis of the average electronic configuration nsZsnpZp characterizing chemical bonding in the host crystals. The Debye-Waller factors measured at room temperature are compared to values calculated in a high temperature approximation which accounts for impurity-host mass difference.

Impact parameter dependence of K-shell ionization.

Abstract The impact-parameter-dependent probability of K-shell ionization by protons has been measured in the energy range 0.5–2.5 MeV for copper, silver, and gold targets. The data are used to test the scaling relations predicted from semiclassical calculations in first Born approximation, with unperturbed projectile trajectory and nonrelativistic hydrogenic electron wave functions. A systematic procedure is described for correcting such calculations for the inadequacy of these approximations, and the correction scheme is tested by comparison partly with experiments, partly with more complete theoretical treatments. The relation between ionization by scattered particles and by particles emitted in nuclear decays or absorbed in compound-nucleus reactions is discussed, and quantitative estimates are derived for the monopole term, which dominates for projectile velocities much lower than the K-shell electron velocity.

Incoherent Scattering of Electrons and Linewidth of Planar-Channeling Radiation

A theoretical description of planar-channeling radiation is given, with special emphasis on the scattering processes contributing to the radiation linewidth. Owing to coherence between scattering in the initial and final states for the radiation process, the contribution to the linewidth from processes with small momentum transfer (e.g., plasmon excitation) is strongly suppressed, and the linewidth is dominated by thermal interband scattering. Also contributions from nonsystematic reflections and from finite crystal thickness are evaluated. Experimentally, planarchanneling radiation has been studied for 4 MeV electrons in nickel, at different target temperatures, and the radiation energies and linewidths confirm the predictions. The influence of correlation of vibrations is revealed by a variation of the linewidth with the angle to a major axis in the plane. The radiation intensity as a function of incidence angle to the plane is strongly modified by multiple interband scattering, and a comparison between measured and calculated intensities serves as a further check on the theoretical treatment.

Lifetimes of C602− and C702− dianions in a storage ring

C60(2-) and C70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200+/-30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.

Photodissociation of protonated amino acids and peptides in an ion storage ring. Determination of Arrhenius parameters in the high-temperature limit

We have measured the time dependence of the fragmentation of protonated amino acids and peptides upon UV excitation in an electrostatic ion storage ring. After absorption of a 266 nm photon, protonated Trp (TrpH+) has a lifetime of 10–20 μs but also a component with a millisecond lifetime is present. The long lifetime may be due to fluorescence, which leads to ions with lower excitation energy, or to the decay of the Trp+˙ radical cation formed after prompt hydrogen loss in the laser interaction region. Only one component with a lifetime of about 10 μs was detected for TyrH+. The lifetime of photoexcited PheH+ is even shorter with an upper limit of a few microseconds. For the singly protonated tripeptides (LysTrpLysH+ and LysTyrLysH+), the decay curves are found to consist of a single component that can be reproduced with an assumption of statistical decay after equilibration of the photon energy among all vibrational modes. The rate constant is expressed in the Arrhenius form in terms of the microcanonical temperature, and the decay rate is obtained by integration over the energy distribution, which has a spread corresponding to the canonical energy distribution at room temperature. The resulting deviation from exponential decay makes it possible to determine the decay parameters from a measurement at a single photon wavelength. Activation energies of Ea = 1.24 ± 0.07 and 1.5 ± 0.4 eV were determined for LysTrpLysH+ and LysTyrLysH+, respectively, with pre-exponential factors of Ad = 1011.1±0.5 and 1012.9±2.6 s−1.