An ion-atom merged beams setup at the Cryogenic Storage Ring.

We describe a merged beams experiment to study ion-neutral collisions at the Cryogenic Storage Ring of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We produce fast beams of neutral atoms in their ground term at kinetic energies between 10 and 300 keV by laser photodetachment of negative ions. The neutral atoms are injected along one of the straight sections of the storage ring, where they can react with stored molecular ions. Several dedicated detectors have been installed to detect charged reaction products of various product-to-reactant mass ranges. The relative collision energy can be tuned by changing the kinetic energy of the neutral beam in an independent drift tube. We give a detailed description of the setup and its capabilities, and present proof-of-principle measurements on the reaction of neutral C atoms with D2 + ions.

Radiative Rotational Lifetimes and State-Resolved Relative Detachment Cross Sections from Photodetachment Thermometry of Molecular Anions in a Cryogenic Storage Ring.

Photodetachment thermometry on a beam of OH^{-} in a cryogenic storage ring cooled to below 10 K is carried out using two-dimensional frequency- and time-dependent photodetachment spectroscopy over 20 min of ion storage. In equilibrium with the low-level blackbody field, we find an effective radiative temperature near 15 K with about 90% of all ions in the rotational ground state. We measure the J=1 natural lifetime (about 193 s) and determine the OH^{-} rotational transition dipole moment with 1.5% uncertainty. We also measure rotationally dependent relative near-threshold photodetachment cross sections for photodetachment thermometry.

The cryogenic storage ring CSR.

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed, and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 ± 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion), and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm(-3) is derived, equivalent to a room-temperature pressure below 10(-14) mbar. Fast atomic, molecular, and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

Photodissociation of an Internally Cold Beam of CH

We have studied the photodissociation of CH^{+} in the Cryogenic Storage Ring at ambient temperatures below 10 K. Owing to the extremely high vacuum of the cryogenic environment, we were able to store CH^{+} beams with a kinetic energy of ∼60 keV for several minutes. Using a pulsed laser, we observed Feshbach-type near-threshold photodissociation resonances for the rotational levels J=0-2 of CH^{+}, exclusively. In comparison to updated, state-of-the-art calculations, we find excellent agreement in the relative intensities of the resonances for a given J, and we can extract time-dependent level populations. Thus, we can monitor the spontaneous relaxation of CH^{+} to its lowest rotational states and demonstrate the preparation of an internally cold beam of molecular ions.

Polarization and interference effects in ionization of Li by ion impact.

We present initial-state selective fully differential cross sections for ionization of lithium by 24 MeV O8+ impact. The data for ionization from the 2s and 2p states look qualitatively different from each other and from 1s ionization of He. For ionization from the 2p state, to which in our study the m(L)=-1 substate predominantly contributes, we observe orientational dichroism and for 2s ionization pronounced interference which we trace back to the nodal structure of the initial-state wave function.

Role of projectile coherence in close heavy ion-atom collisions.

We have measured fully differential cross sections for single ionization and transfer ionization (TI) in 16 MeV O(7+)+He collisions. The impact parameters mostly contributing to single ionization are about an order of magnitude larger than for TI. Therefore, the projectile beam was much more coherent for the latter compared to the former process. The measured data suggest that, as a result, TI is significantly affected by interference effects which are not present in single ionization.

Ion-lithium collision dynamics studied with a laser-cooled in-ring target.

We present a novel experimental tool allowing for kinematically complete studies of break-up processes of laser-cooled atoms. This apparatus, the 'MOTReMi,' is a combination of a magneto-optical trap (MOT) and a reaction microscope (ReMi). Operated in an ion-storage ring, the new setup enables us to study the dynamics in swift ion-atom collisions on an unprecedented level of precision and detail. In the inaugural experiment on collisions with 1.5 MeV/amu O(8+)-Li the pure ionization of the valence electron as well as the ionization-excitation of the lithium target was investigated.

Structure and stability of the negative hydrogen molecular ion.

We present the results of a Coulomb explosion experiment that allows for the imaging of the rovibrational wave function of the metastable H2- ion. Our measurements confirm the predicted large internuclear separation of 6 a.u., and they show that the ion decays by autodetachment rather than by spontaneous dissociation. Imaging of the resulting H2 products reveals a large angular momentum of J = 25 ± 2, quantifying the rotation that leads to the metastability of this most fundamental molecular anion.

Hot water molecules from dissociative recombination of D3O+ with cold electrons.

Individual product channels in the dissociative recombination of deuterated hydronium ions and cold electrons are studied in an ion storage ring by velocity imaging using spatial and mass-sensitive detection of the neutral reaction fragments. Initial and final molecular excitation are analyzed, finding the outgoing water molecules to carry internal excitation of more than 3 eV in 90% of the recombination events. Initial rotation is found to be substantial and in three-body breakup strongly asymmetric energy repartition among the deuterium products is enhanced for hot parent ions.

A cryogenic electrostatic trap for long-time storage of keV ion beams.

We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures <15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.

Ultraviolet and visible light photodissociation of H3+ in an ion storage ring.

Ultraviolet and visible photodissociation of a vibrationally excited H(3)(+) ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H(3)(+) into H(2)(+) + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H(2)(+) + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10(-20) cm(2) for ultraviolet and some 10(-21) cm(2) for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H(3)(+) in low-density environments, these measurements open promising perspectives.

Rotational cooling of HD+ molecular ions by superelastic collisions with electrons.

Merging an HD+ beam with velocity matched electrons in a heavy ion storage ring we observed rapid cooling of the rotational excitations of the HD+ ions by superelastic collisions (SEC) with the electrons. The cooling process is well described using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclei rotation approximation. We verify the DeltaJ=-2 SEC rate coefficients, which are predicted to be dominant as opposed to the DeltaJ=-1 rates and to amount to (1-2)x10;{-6} cm;{3} s;{-1} for initial angular momentum states with J< or =7, to within 30%.

Process evaluation results from a school- and community-linked intervention: the Trial of Activity for Adolescent Girls (TAAG).

Process evaluation is a component of intervention research that evaluates whether interventions are delivered and received as intended. Here, we describe the process evaluation results for the Trial of Activity for Adolescent Girls (TAAG) intervention. The intervention consisted of four synergistic components designed to provide supportive school- and community-linked environments to prevent the decline in physical activity in adolescent girls. Process evaluation results indicate that the intervention components were delivered from intervention staff to teachers with high fidelity (84-97%) to the protocol and with lower fidelity (range: 18-93%) from teachers to students. Physical activity programs for girls, a unique feature of the TAAG intervention, increased from a mean of 10 programs per school to a mean of 16 and 15 in years 1 and 2, respectively, in intervention schools, with no change in control schools. These findings suggest that a multicomponent school- and community-based physical activity intervention can be delivered with fidelity and result in a middle school environment that supports physical activity for girls.

One-dimensional beam ordering of protons in a storage ring.

The ordering of protons has been observed at a new storage ring, S-LSR, at Kyoto University. Abrupt jumps in the momentum spread and the Schottky noise power were observed for protons for the first time at a particle number of approximately 2000, upon applying electron cooling with electron currents of 25, 50, and 100 mA. The transition temperature was 0.17 and 1 meV in the longitudinal and transverse directions, respectively. The transverse temperature of the proton beam was much below that of electrons at the transition, which played an essential role in the ordering of protons.

Storage-ring measurement of the hyperfine induced 47Ti18+(2s2p 3P0 --> 2s2 1S0) transition rate.

The hyperfine induced 2s2p (3)P(0) --> 2s(2) (1)S(0) transition rate A(HFI) in berylliumlike (47)Ti(18+) is measured. Resonant electron-ion recombination in a heavy-ion storage ring is employed to monitor the time dependent population of the (3)P(0) state. The experimental value A(HFI)=0.56(3) s(-1) is almost 60% larger than theoretically predicted.

Rate coefficients and final states for the dissociative recombination of LiH+.

The dissociative recombination of LiH+ ions with low-energy electrons is observed at a storage ring and the final states are analyzed using fragment imaging and field ionization techniques. The rate coefficient is found to be larger than its estimated value used in astrophysical models. Mostly the highest energetically possible Rydberg states of the lithium atom are populated by the reaction, indicating a common trend for molecular recombination via the noncrossing mode.

Influence of magnetic fields on electron-Ion recombination at very low energies

Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F6+ and C6+ ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T( perpendicular) and T( ||). The excess scales as T(-1/2)( perpendicular) and, surprisingly, as T(-1/2)( ||), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.

Inter-ethnic differences in youth tobacco language and cigarette brand preferences.

OBJECTIVE: To describe and understand variations in cigarette brand preferences between adolescents from varying ethnic and gender groups around the US. DESIGN: A qualitative study where adolescents, both smokers and nonsmokers, were interviewed individually in depth. SETTING: Schools and recreation centers in four sites: urban Maryland (Baltimore), urban Texas (Houston), rural Alabama and rural New Mexico. PARTICIPANTS: 121 adolescent volunteers 13-19 years of age, representing African American, white, American Indian and Hispanic ethnic groups, from both genders. RESULTS: Considerable geographic and ethnic variation exists in terminology used by youth to refer to cigarettes and to their use. Clear patterns in brand preference by ethnic group were found that follow patterns of targeted marketing by ethnicity. White teens preferred Marlboro brand cigarettes, while African-American teens who smoke preferred Newports. Hispanic and American Indian teens were more likely to smoke Marlboro or Camel cigarettes. Hispanic teens were most likely to mention low price as a reason for choosing a particular brand or to state that the brand does not matter. Tobacco advertisements targeting ethnic groups and the use of promotional items to encourage teen smoking were also recognized as factors influencing brand preferences. CONCLUSIONS: These findings have implications for the design of intervention programs aimed at curbing teen smoking. When working with teens who already smoke, using youth language to target messages at perceived characteristics of commonly used brands may be more effective and meaningful than talking about cigarette use in general. Another implication of this work is to shed light on what impact an advertising ban would have on teen brand preferences, brand loyalty, and prevalence of smoking.