H. Cederquist

Absolute cross sections for multi-electron processes in low energy Arq+−Ar collisions: Comparison with theory

Abstract Absolute cross sections have been measured for a variety of multi-electron processes in low-energy collisions of multiply charged argon recoil ions with neutral argon. The cross sections are compared with theoretical estimates based on an extension of the classical barrier model. Comparison is also made with the statistical theory of Muller et al.

Conetrap: A compact electrostatic ion trap

Abstract We have developed a very simple and compact fully electrostatic ion storage device. It consists of only three electrodes: two conical and one central cylindrical. This simple setup works as two focusing electrostatic mirrors forming a resonator-like structure and thereby confining the ions. We present the design and show the first test results demonstrating an ion-storage lifetime in the 100 ms-range at modest vacuum conditions. Storage lifetimes in the order of minutes are foreseen when UHV conditions are introduced.

Electron capture and energy loss in ∼100 keV collisions of atomic and molecular ions on C60

Abstract We have determined absolute attenuation and electron capture cross sections in ∼100 keV collisions between atomic ions of moderate charge states (He + , He 2+ , Ar 2+ , Ar 3+ , Ar 4+ , Xe 2+ , Xe 3+ , Xe 4+ ) and C 60 molecules in the gas phase. The measured cross-sections support previous findings of the charge dynamics within C 60 from electron capture experiments with higher ionic charges. For a non-negligible fraction of the capture events we observed substantial ionic energy loss (≤2 keV), which is ascribed to collisions where the ion passes through the C 60 molecule. Furthermore, we found that a molecular ion (100 keV CO 2 2+ ) that captures an electron from C 60 can lose similar amounts of energy without dissociating. It is concluded that the predominant contribution to the ionic energy loss comes from electron excitation and ionization.

Dissociation and multiple ionization energies for five polycyclic aromatic hydrocarbon molecules

We have performed density functional theory calculations for a range of neutral, singly, and multiply charged polycyclic aromatic hydrocarbons (PAHs), and their fragmentation products for H-, H(+)-, C(2)H(2)-, and C(2)H(2)(+)-emissions. The adiabatic and vertical ionization energies follow linear dependencies as functions of charge state for all five intact PAHs (naphthalene, biphenylene, anthracene, pyrene, and coronene). First estimates of the total ionization and fragmentation cross sections in ion-PAH collisions display markedly different size dependencies for pericondensed and catacondensed PAH species, reflecting differences in their first ionization energies. The dissociation energies show that the PAH(q+)-molecules are thermodynamically stable for q ≤ 2 (naphthalene, biphenylene, and anthracene), q ≤ 3 (pyrene), and q ≤ 4 (coronene). PAHs in charge states above these limits may also survive experimental time scales due to the presence of reaction barriers as deduced from explorations of the potential energy surface regions for H(+)-emissions from all five PAHs and for C(2)H(2)(+)-emission from naphthalene--the smallest PAH.

First storage of ion beams in the Double Electrostatic Ion-Ring Experiment: DESIREE

We report on the first storage of ion beams in the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. We have produced beams of atomic carbon anions and small carbon anion molecules (C(n)(-), n = 1, 2, 3, 4) in a sputter ion source. The ion beams were accelerated to 10 keV kinetic energy and stored in an electrostatic ion storage ring enclosed in a vacuum chamber at 13 K. For 10 keV C2 (-) molecular anions we measure the residual-gas limited beam storage lifetime to be 448 s ± 18 s with two independent detector systems. Using the measured storage lifetimes we estimate that the residual gas pressure is in the 10(-14) mbar range. When high current ion beams are injected, the number of stored particles does not follow a single exponential decay law as would be expected for stored particles lost solely due to electron detachment in collision with the residual-gas. Instead, we observe a faster initial decay rate, which we ascribe to the effect of the space charge of the ion beam on the storage capacity.

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.

Stabilities of multiply charged dimers and clusters of fullerenes

The authors find even-odd variations as functions of r (<or=7) for multiple ionization of van der Waals dimers in slow Xe(30+)+[C60]2([C60C70])-->...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)-->C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n>3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.

Molecular Growth Inside of Polycyclic Aromatic Hydrocarbon Clusters Induced by Ion Collisions

The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest polycyclic aromatic hydrocarbons (PAHs). Intracluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision process-from the ion impact (nuclear scattering) to the formation of new molecular species-reproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturns moon Titan.

The Quartet System of Li I

The doubly-excited system of quarters in Li I have been studied by the beam-foil technique using optical detection in the VUV. The absolute term value of the lowest quartet 1s2s2p 4P0 has been determined to be 57.4113 ± 0.0006 eV which is much more accurate than earlier experimental results. We have also made the first experimental determination of the term energy of the 1s2p3p 4P state by observing its decay to the 1s2s2p 4P0 state by emission of a spectral line at 1680.2 A. In addition, eight new observations of quartet transitions are reported as well as some lifetimes. The most recent experimental results are summarized and compared to calculations.