Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostatic spacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beam measurements and semiclassical calculations are presented for xenon atom charge-exchange collisions with Xe+ and Xe2+ at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric Xe++Xe exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed Xe2+ interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present Xe2++Xe symmetric charge exchange measurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-energy drift tube measurements and are significantly lower than previous higher energy measurements. A simple model for symmetric two-electron transfer is proposed that is in good agreement with the present measurements. The onset for the asymmetric charge-exchange process, Xe2++Xe→2Xe+, is observed to be at 10 eV. For this process, a cross section of 2.8±0.9 Å2 is measured for a Xe2+ energy of 600 eV.

Electronic spectra are observed for the monosolvated metal cation complexes Ca+–H2O and Ca+–D2O using resonance enhanced photodissociation spectroscopy. The clusters are produced in a laser vaporization/supersonic expansion source and the mass-analyzed product is observed using a time-of-flight mass spectrometer. Both Ca+ and CaOH+ (or CaOD+) dissociation channels are observed on sharp resonances. Transitions from the ground electronic state to two excited electronic states are assigned, with vibrational progressions in the Ca–OH2 stretching mode. Spectroscopic constants are Ca+–H2O: (2) 2B2←X 2A1 (T0=21 464 cm−1, ΔG1/2=357.9 cm−1) and (2) 2B1←X 2A1 (T0=23 273 cm−1, ΔG1/2=335.9 cm−1); and Ca+–D2O: (2) 2B2←X 2A1 (T0=21 447 cm−1, ΔG1/2=350.9 cm−1) and (2) 2B1←X 2A1 (T0=23 261 cm−1, ΔG1/2=324.1 cm−1). These transitions are rotationally resolved, confirming the structure of the complex to be C2v. The Ca+–H2O bond distance is 2.22 Å and the H–O–H bond angle is 106.8° in the ground state. Comparisons with theoretical calculations are also made.

We describe a simple method of tracking oxygen in real-time with injectable, tissue-integrating microsensors. The sensors are small (500 μm × 500 μm × 5 mm), soft, flexible, tissue-like, biocompatible hydrogels that have been shown to overcome the foreign body response for longterm sensing. The sensors are engineered to change luminescence in the presence of oxygen or other analytes and function for months to years in the body. A single injection followed by noninvasive monitoring with a hand-held or wearable Bluetooth optical reader enables intermittent or continuous measurements. Proof of concept for applications in high altitude, exercise physiology, vascular disease, stroke, tumors, and other disease states have been shown in mouse, rat and porcine models. Over 90 sensors have been studied to date in humans. These novel tissueintegrating sensors yield real-time insights in tissue oxygen fluctuations for research and clinical applications.

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Guided-Ion Beam (GIB) measurements of the Ar ϩ ϩ Ar symmetric charge-transfer (SCT) system are presented for ion energies ranging from 0.2 to 300 eV. Two methods are applied to distinguish primary and secondary ions: (i) based on isotopic-labeling, (ii) based on significant laboratory velocity differences. The absolute cross sections measured with these methods are in excellent agreement at energies above 1 eV. The experimental results are compared with semi-classical calculations performed with various published Ar2 ϩ potentials. The calculations including spin-orbit effects lie within 10% of the isotopeselected and attenuation measurements at all investigated ion energies. The present results lie significantly above the simple Rapp and Francis model [1]. Important errors in the latter approach are pointed out and a correct one-electron model is proposed. First measurements of the differential cross section at 0.5 eV collision energy are briefly mentioned.