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The impact-parameter dependences of $L\ensuremath{\alpha}$, $L\ensuremath{\beta}$, and total $L$-shell x-ray production of Pb by 2-, 3-, and 6-MeV proton bombardment have been measured and compared with theoretical calculations based on the semiclassical approximation (SCA) and the binary-encounter approximation (BEA). The SCA satisfactorily reproduces the details of the ionization probabilities but overestimates the results at large impact parameters. The BEA does not predict the appropriate structure observed in the x-ray production probabilities. The BEA results are too low at small impact parameters, but in good agreement with measured values at large impact parameters. The behavior at large impact parameters explains the failure of the SCA in predicting the $L$-shell x-ray production total cross sections (the calculated values are too high), whereas the BEA cross sections are in close agreement with experiment.
We have measured the spontaneous and photo-induced decays of anionic gold clusters, Au$_N^-$, with sizes ranging from $N = 2$ to 13, and 15. After production in a sputter ion source, the size-selected clusters were stored in the cryogenic electrostatic ion-beam storage ring DESIREE and their neutralization decays were measured for storage times between 0.1 and 100 s. The dimer was observed to decay by electron emission in parallel to neutral atom emission at long times, analogously to the behavior of copper and silver dimers, implying a breakdown of the Born-Oppenheimer approximation. Radiative cooling is observed for all cluster sizes except for the dimer. The decay rates of clusters $N=3,6,8-13,15$ show only a single radiative cooling time. For $N=6-13$ the cooling times have a strong odd-even oscillation with an amplitude that decrease with cluster size, and with the even $N$ having the fast cooling. We compare our results with previous measurements of radiative cooling rates of the corresponding cationic gold clusters, Au$_N^+$, which also show an odd-even effect with a similar oscillation amplitude but at orders of magnitude shorter time scales, and out of phase with the anions.
We describe the design of a novel type of storage device currently under construction at Stockholm University, Sweden, using purely electrostatic focussing and deflection elements, in which ion beams of opposite charges are confined under extreme high vacuum cryogenic conditions in separate "rings" and merged over a common straight section. The construction of this double electrostatic ion ring experiment uniquely allows for studies of interactions between cations and anions at low and well-defined internal temperatures and centre-of-mass collision energies down to about 10 K and 10 meV, respectively. Position sensitive multi-hit detector systems have been extensively tested and proven to work in cryogenic environments and these will be used to measure correlations between reaction products in, for example, electron-transfer processes. The technical advantages of using purely electrostatic ion storage devices over magnetic ones are many, but the most relevant are: electrostatic elements which are more compact and easier to construct; remanent fields, hysteresis, and eddy-currents, which are of concern in magnetic devices, are no longer relevant; and electrical fields required to control the orbit of the ions are not only much easier to create and control than the corresponding magnetic fields, they also set no upper mass limit on the ions that can be stored. These technical differences are a boon to new areas of fundamental experimental research, not only in atomic and molecular physics but also in the boundaries of these fields with chemistry and biology. For examples, studies of interactions with internally cold molecular ions will be particular useful for applications in astrophysics, while studies of solvated ionic clusters will be of relevance to aeronomy and biology.
Spontaneous decays of small, hot silver-cluster anions ${\text{Ag}}_{n}^{\ensuremath{-}}$, $n=4--7$, have been studied using one of the rings of the Double ElectroStatic Ion Ring ExpEriment (DESIREE). Observation of these decays over very long time scales is possible due to the very low residual gas pressure ($\ensuremath{\sim}{10}^{\ensuremath{-}14}$) and cryogenic (13 K) operation of DESIREE. The yield of neutral particles from stored beams of ${\text{Ag}}_{6}^{\ensuremath{-}}$ and ${\text{Ag}}_{7}^{\ensuremath{-}}$ anions were measured for 100 milliseconds and were found to follow single power-law behavior with millisecond time-scale exponential cutoffs. The ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$ anions were stored for 60 s and the observed decays show two-component power-law behaviors. We present calculations of the rate constants for electron detachment from and fragmentation of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$. In these calculations, we assume that the internal energy distribution of the clusters are flat and with this we reproduce the early steep parts of the experimentally measured decay curves for ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$, which extends to tens and hundreds of milliseconds, respectively. The fact that the calculations reproduce the early slopes of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$, which differ for the two cases, suggests that it is the changes in fragmentation rates with internal cluster energies of ${\text{Ag}}_{4}^{\ensuremath{-}}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}$ rather than conditions in the ion source that determine this behavior. Comparisons with the measurements strongly suggest that the neutral particles detected in these time domains originate from ${\text{Ag}}_{4}^{\ensuremath{-}}\ensuremath{\rightarrow}{\text{Ag}}_{3}^{\ensuremath{-}}+\text{Ag}$ and ${\text{Ag}}_{5}^{\ensuremath{-}}\ensuremath{\rightarrow}{\text{Ag}}_{3}^{\ensuremath{-}}+{\mathrm{Ag}}_{2}$ fragmentation processes.
We have studied the mutual neutralization reaction of vibronically cold NO+ with O− at a collision energy of ≈0.1eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments. Published by the American Physical Society 2024
A double electrostatic storage ring named DESIREE is under construction at the Manne Siegbahn Laboratory and Stockholm University. The two rings will have the same circumference, 9.2 m, and a common straight section for merged beam experiments with ions of opposite charges. The whole structure will be contained in a single vacuum vessel resulting in a very compact design. In addition to its unique double ring structure it will be possible to cool DESIREE below 20 K using cryogenerators. This will reduce the internal vibrational and rotational excitations of stored molecules. A cold system will also result in excellent vacuum conditions where longer lifetimes of the stored beams can be expected. While the ion optical calculations have entered a final phase much of the work is now devoted to solve many of the mechanical and cryogenic challenges of DESIREE. In order to test the mechanical and cryogenic properties of insulators, vacuum seals, and laser viewports a small test system has been built. The test system will provide valuable information for the final design of DESIREE.
