The study of ion-molecule reactions plays a vital role in cold chemistry, thus implying the need of well-controlled ion ensembles in a cold environment. The internal and external degrees of freedom of molecular ions, trapped in multipole radio frequency ion traps, can be cooled via collisions with neutral atoms [1]. Usually the coolant undergoes collisions with a thermal shield mounted on a cryostat attaining temperatures of about 4 K. To overcome this lower temperature limit two approaches have been proposed. Firstly, using a laser-cooled buffer-gas localized at the center of the ion cloud [2]. Secondly, sympathetic cooling via a second laser cooled molecular anion like C
2− [3]. This, in addition to its potential role in forming carbon chain anions in the interstellar medium, makes C
2− an important molecule in anion research. In this contribution we present a newly developed 16-pole cryogenic radio frequency wire trap for spectroscopic and kinetic studies of molecular anions. First results of quenching rates of the first excited vibrational level of C
2− due to collisions with H
2 are presented. Furthermore, possible reaction pathways with H
2 are discussed and recent spectroscopic measurements of the second electronic excited state of C
2− are presented. References: [1] R. Wester , J. Phys. B 42, 154001 (2009) [2] B. Höltkemeier et al., Phys. Rev. A 94, 062703, (2016) [3] P. Yzombard et al., Phys. Rev. Lett. 114, 213001 (2015)
Abstract We demonstrate the precise in situ measurement of extreme-high vacuum (XHV) in a cryogenic linear Paul trap operating at 5.7 K, utilizing laser-cooled Be + Coulomb crystals as dual-mode quantum probes. Two independent methods, elastic collision rate monitoring and reaction rate analysis, were employed to determine the vacuum level within the cryogenic linear Paul trap. Based on the collision rate of a single trapped beryllium ion with background gas, which was measured as 1.3(7) × 10 −4 Hz, the pressure under cryogenic conditions was calculated to be 6.0(3.4) × 10 −12 Pa. Alternatively, by leveraging the reaction rate of Be + ions with background gases, a pressure of 8.4(1) × 10 −12 Pa was obtained. The mutual agreement between these two methods validates the reliability of cold-atom-based vacuum metrology at the 10 −12 Pa scale, successfully overcoming the detection limits of conventional vacuum gauges. This approach, which capitalizes on the unique mass properties and photostability of Be + ions, fills a critical gap in XHV metrology and paves the way for the development of chip-trap-integrated in situ vacuum sensing technologies.
The control of the ultracold collisions between neutral atoms is an extensive and successful field of study. The tools developed allow for ultracold chemical reactions to be managed using magnetic fields, light fields and spin-state manipulation of the colliding particles among other methods. The control of chemical reactions in ultracold atom-ion collisions is a young and growing field of research. Recently, the collision energy and the ion electronic state were used to control atom-ion interactions. Here, we demonstrate spin-controlled atom-ion inelastic processes. In our experiment, both spin-exchange and charge-exchange reactions are controlled in an ultracold Rb-Sr
+ mixture by the atomic spin state. We prepare a cloud of atoms in a single hyperfine spin-state. Spin-exchange collisions between atoms and ion subsequently polarize the ion spin. Electron transfer is only allowed for (RbSr)
+ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wavefunction with the singlet molecular manifold and therefore also the reaction rate. We experimentally show that by preparing the atoms in different spin states one can vary the charge-exchange rate in agreement with theoretical predictions.