Photodetachment studies of negative ions at the Cryogenic Storage Ring

In the present work, experimental studies of the photodetachment process of the Si− and Al−4 systems are presented. The measurements were carried out at the CryogenicStorage Ring (CSR) facility located at the Max Planck Institute for Nuclear Physicsin Heidelberg. By storing Si− for up to 2000 s in the T to monitor the decay of long-lived 2D states as well as the weakly bound 2P states. Employing a set of continuous wave and pulsed lasers, the lifetime for the 2P stateswere found to be τ = (22.5 +/- 2.5)s. For the 2D states, a lower limit of τr > 5.7 h (within a confidence level of 90%) was measured. Furthermore, Multi-Configuration Dirac-Hartree-Fock (MCDHF) calculations were performed on the radiatiave lifetimesof the respective states, which show very good agreement with the present measurement.The second part of the studies focuses on the study of spontaneous cooling of stored Al−4 ions at very long time scales. Here, we present measurements that allow us to monitor the cluster cooling rates for up to 30 min and we compare those results to previous measurements at a room temperature device. We find that the cooling time scale is limited by the redistribution of vibrational energy inside the molecule, which we call slow exchange model, and in combination with recurrent fluorescence for energies above the first electronic excited state, they match the observed cooling rates

Experimental Determination of the Dissociative Recombination Rate Coefficient for Rotationally Cold CH⁺ and Its Implications for Diffuse Cloud Chemistry

Observations of CH+ are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH+ chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH+. Using an electron–ion merged-beams experiment at the Cryogenic Storage Ring, we have determined the DR rate coefficient of the CH+ electronic, vibrational, and rotational ground state applicable for different diffuse cloud conditions. Our results reduce the previously unrecognized order-of-magnitude uncertainty in the CH+ DR rate coefficient to ∼20% and are applicable at all temperatures relevant to diffuse clouds, ranging from quiescent gas to gas locally heated by processes such as shocks and turbulence. Based on a simple chemical network, we find that DR can be an important destruction mechanism at temperatures relevant to quiescent gas. As the temperature increases locally, DR can continue to be important up to temperatures of ∼600 K, if there is also a corresponding increase in the electron fraction of the gas. Our new CH+ DR rate-coefficient data will increase the reliability of future studies of diffuse cloud physical properties via CH+ abundance observations

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 D ions

Isochronous mass spectrometry in an electrostatic storage ring

For sensitive studies of molecular ions in electrostatic storage rings, the exact knowledge of the isobaric composition of stored beams from a variety of ion sources is essential. Conventional mass-filtering techniques are often inefficient to resolve the beam components. Here, we report the first isochronous mass spectrometry in an electrostatic storage ring, which offers a high mass resolution of Δm/m −5 even for heavy molecular species with m > 100 u and uncooled ion beams. Mass contaminations can be resolved and identified at relative fractions down to 0.02%

Quantum-state–selective electron recombination studies suggest enhanced abundance of primordial HeH⁺

The epoch of first star formation in the early Universe was dominated by simple atomic and molecular species consisting mainly of two elements: hydrogen and helium. Gaining insight into this constitutive era requires a thorough understanding of molecular reactivity under primordial conditions. We used a cryogenic ion storage ring combined with a merged electron beam to measure state-specific rate coefficients of dissociative recombination, a process by which electrons destroy molecular ions. We found a pronounced decrease of the electron recombination rates for the lowest rotational states of the helium hydride ion (HeH+), compared with previous measurements at room temperature. The reduced destruction of cold HeH+ translates into an enhanced abundance of this primordial molecule at redshifts of first star and galaxy formation

Metastable states of Si− observed in a cryogenic storage ring

We have used the Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics to study long-lived metastable states of the silicon anion. A Si- beam of 58 keV kinetic energy was stored in the ultra-high cryogenic vacuum of the CSR, employing only electrostatic deflection elements. We used laser systems at various wavelengths to infer information on the decay of the metastable anionic states by selective photodetachment. Our results give evidence of an excited anionic state for which we determine the extremely long lower lifetime limit of 5.7 h at 90% confidence level, consistent with theoretical predictions for the ^2D term. Furthermore, we find an average lifetime of τ = (22:2 ± 2:5) s for the weakly bound ^2P states, employing coincidence counting with a pulsed nanosecond laser at 2.45 µm. Using a laser depletion technique, we produce a pure ground term ^4S_3/2 Si- beam, and we quantify the fraction of ions in metastable states in our initial ion sample. We combine our experimental efforts with state-of-the-art Multiconfiguration Dirac- Hartree-Fock calculations for the radiative lifetimes of all metastable levels of Si-. We find these calculations to be in excellent agreement with our measurements, and to improve previous efforts considerably