Abstract
A new cryogenic linear ion trap beamline has been constructed and commissioned, which serves to inject cold molecular and cluster ions into the RIKEN cryogenic electrostatic ring (RICE). Ions are created with an electrospray ion source, and a quadrupole mass filter is used for mass-selection prior to trap injection. The radio frequency octupole ion trap can be continuously loaded with ions and features a fast ion extraction mode to create short ion bunches with tens of μs duration. We report here on the simulations and development of the ion trap beamline and validate performance with the moderately heavy molecular cation methylene blue. Characterization of the novel trap design with additional wedge-shaped electrodes was carried out, which includes the determination of the temporal and spatial shape of the ion bunch and the total number of ions after extraction. Finally, these ion bunches are synchronized with the switching of a pulsed high-voltage acceleration device downstream of the trap, where the ions obtain a kinetic energy of up to 20 keV. The preparation and control of the keV ion beam are demonstrated for the ion injection into RICE.
Highlights
Radio frequency (RF) ion traps1 developed decades ago can keep low-velocity ions in a confined space in gas-phase
We introduce an ion preparation beamline with a cryogenic RF ion trap intended to accumulate and cool molecular ions to their rovibrational ground states and subsequently inject them into our new storage ring RICE
Low kinetic energy ions from the ESI of ≤30 eV are transported by eight separate RF octupole ion guides (OG1-OG8), where OG5 serves as an ion trap with 110 mm length
Summary
Radio frequency (RF) ion traps developed decades ago can keep low-velocity ions in a confined space in gas-phase. Ion-neutral collisions with a buffer gas reduce the ions’ kinetic energy and eventually thermalize the internal degrees of freedom of molecular ions. Low and well-defined internal excitation of the molecular ions will benefit spectroscopic measurements as it simplifies the obtained spectra and improves comparability to theoretical models. Quadrupole mass filters before and behind the trap are used to analyze the initial and final states of the reaction, or the products are identified by a time-of-flight measurement.. Disadvantages are the loss of precise temporal information of the reaction due to the product extraction as well as often continuous energy exchange with the buffer gas during the reaction in the trap. Fast spectroscopic measurements might be accessible via trap in situ detection of the fluorescence light; this method exhibits a low efficiency and requires luminescence emission from the molecule of interest
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