## 6 Conclusions  
In this work, we presented a thermometry technique that uses the linear relationship between the motional temperature of an ensemble of ions trapped in a multipole trap and the variance of the time-of-flight distribution of the extracted ions. This technique provides a fast and reliable method to determine the energy distribution of, in principle, any ionic species confined in an ion trap, and can be used to measure the external temperature ranging from hundreds of Kelvin down to a few Kelvin. In our study, a deviation of the linear behavior was observed for lower temperatures. This is attributed to the existence of multiple potential minima in the trapping potential. These minima become more prominent at lower temperatures and were included in our model to obtain an accurate temperature calibration curve in the Kelvin range for our thermometry method. Additionally, the simulation of the trapping potential was modified to account for surface charge accumulation, thus increasing the accuracy of our model and allowing us to simulate the spatial distribution of the ion ensemble in the trap. This altered potential was determined by using an axial tomography measurement from our trap as well as a Monte-Carlo simulation, which predicted the energy distribution of ions colliding with room-temperature helium buffer gas. We showed that our octupole radio-frequency wire trap can create, with sufficiently higher voltage, nearly the same effective potential as the corresponding ideal hyperbolic or cylindrical rod trap. Further, a Monte-Carlo simulation shows that, after interaction with a room temperature helium buffer gas, the ions thermalize to the same final energy distribution in all the three trap types. A temperature higher than that of the buffer gas is obtained for all simulated traps attributed to the rf micromotion heating. These findings support that a wire trap with its superior optical access, in comparison to a trap utilizing thicker rods, is an ideal instrument for a hybrid trap  
Figure 9: The variance of the TOF distribution, i.e., \(\sigma^{2}\), plotted as a function of the initial temperature of the ions in the trap before extraction onto the detector. The change in the linear slope can be seen in the left inset. It is interpreted as a consequence of the pockets arising at a temperature regime of about 60 K. Furthermore, the temperature from the corresponding TOF trace of the tomography measurements is derived to be 370 K, which is consistent with the Monte-Carlo simulations’ prediction. This TOF trace is shown in the right inset along with the Gaussian fit representing the distribution.  
and comes at little to no disadvantages. The optical access facilitates the laser cooling of rubidium atoms present in the chamber in order to create a dense atomic cloud and overlap it with the trapped ensemble of hydroxyl anions. With the flexibility to trap and characterize any ionic species in the setup, this system provides an excellent platform to study diverse atom-ion interactions.  
Conceptualization, E.E., R.W., and M.W.; formal analysis, M.N.; software, M.N.; investigation, M.N., S.Z.H., and J.T.; writing--original draft preparation, S.Z.H.; writing--review and editing, M.N., S.Z.H., R.W., and M.W.; supervision, R.W. and M.W.; funding acquisition, R.W. and M.W. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the Austrian Science Fund (FWF) through Project No. I3159-N36 and Deutsche Forschungsgemeinschaft (DFG) under Project No. WE/2661/14-1. S.Z.H. acknowledges the support from IMPRS-QD fellowship and HGSFP.  
The authors would like to acknowledge [PERSON], [PERSON] and [PERSON] for their role in implementing the experimental setup and fruitful discussions.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.