Design and characterization of a cryogenic linear Paul ion trap for ion–neutral reaction studies

Abstract

Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time—facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300 K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion–neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9 K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap—confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295 K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion–neutral reactions under cold, controlled conditions.

Cold Ion–Molecule Reactions in the Extreme Environment of a Coulomb Crystal

“…There are exciting future directions for the field of ion-neutral chemistry in Coulomb crystals. One example is the construction or operation of cryogenic ion traps for studies of ion-neutral reactions. − Such an environment reduces the temperature of the neutral molecules as well as eliminates the excitation of ion rotational states by blackbody radiation by the chamber walls. A cryogenic ion trap thus reduces and allows for more control of the collision energy of chemical reactions in the ion trap.…”

Cold and ultracold molecules in the twenties

“…In the shorter term, it is likely that cryogenically cooled ion traps (e.g. [92]) will prove to be more versatile in producing vibrationally cold molecular ions, or alternatively the greater use of shaped broadband femtosecond laser pulses to achieve optical pumping of vibrational states towards the ground state [93,94].…”

Spatial and Temporal Detection of Ions Ejected from Coulomb Crystals

“…Instead, reactivity studies typically employ either in situ monitoring of crystal evolution through the comparison of fluorescence images with high-level molecular dynamics simulations − or the destructive determination of ion numbers from time-of-flight mass spectrometry (ToF-MS) measurements. − These methods, used both independently and in combination, have facilitated the successful study of a range of reactions in Coulomb crystals. Fluorescence image comparisons rely on assumptions about the identities of the dark (nonfluorescing) ions, as these species cannot be directly observed by a CCD camera.…”