We have developed a combined Stark-velocity-filter--ion-trap apparatus for the purpose of reaction-rate measurements between cold trapped ions and slow polar molecules under ultrahigh vacuum conditions. The prerequisite steps such as the characterization of velocity-selected polar molecules (PM), namely ND
3, H
2CO, and CH
3CN, were performed using time-of-flight (TOF) measurements. We confirmed the generation of slow ND
3, H
2CO, and CH
3CN molecules having thermal energies of a few Kelvin. Additionally, the number densities of the slow velocity-filtered polar molecules were determined to be in the range of
n=104 to 10
6 cm${}^{\ensuremath{-}3}$ by calibrating the TOF signals. In a first experiment, the Stark velocity filter was connected to a cryogenic linear Paul trap and reaction-rate measurements between laser-cooled Ca
+ Coulomb crystals and velocity-selected polar molecules were carried out. The observed reaction rates are of the order of 10${}^{\ensuremath{-}5}$ s${}^{\ensuremath{-}1}$, which are much slower than typical reaction rates of molecular ion--polar-molecule reactions at low temperatures. The present results confirm that reaction-rate measurements between velocity-selected polar molecules and sympathetically cooled molecular ions cooled by a laser-cooled Ca
+ Coulomb crystal can be performed. Next we measured the reaction rates between sympathetically cooled nonfluorescent stored ion molecules namely N
2H
+ ions and velocity-selected CH
3CN molecules at the average reaction energy of about 3 K. The measured reaction rate of 2.0(2)$\ifmmode\times\else\texttimes\fi{}
10{}^{\ensuremath{-}3}$ s${}^{\ensuremath{-}1}$ is much faster than those of the Ca
++PM reactions. This is strong evidence that the velocity-selected polar molecules undergo reactive collisions. We also confirmed that the present reaction-rate constant of CH
3CN+N
2H
+ $\ensuremath{\rightarrow}$ CH
3CNH
++N
2 is consistent with the estimated values from the room temperature results and the trajectory-scaling formula of Su et al. In the future, the present velocity-filter combined cryogenic trap apparatus will enable us to perform systematic measurements of cold ion--polar-molecule reactions, which are important problems from a fundamental viewpoint and also contribute to astrochemistry.
A Stark velocity filter with a pulse nozzle was applied to ND 3 , NH 3 , CH 3 I, H 2 CO, C 6 H 5 CN and C 6 H 5 Cl at room temperature. The results demonstrate that the Stark velocity filter works successfully for any kind of nonlinear polar molecule. The number of molecules detected after the filter was approximately 10 7 per pulse with 0.5 ms duration time. The slowest component observed had a velocity corresponding to a few K. The velocity distributions observed were compared to the simulation based on the Stark effect on rotational states. It was found that the nature of the rotational energy level structure is important for predicting the velocity distribution. For heavier asymmetric top molecules, it was found that many avoided crossings due to the second-order interactions make it difficult to treat low-field seeking states properly in the simulation. Simulations show that the combination of He buffer gas pre-cooling and the Stark velocity filter works extremely effectively.
Producing high densities of molecules is a fundamental challenge for low-temperature, ion-molecule reaction studies. Traveling-wave Stark decelerators promise to deliver high density beams of cold, polar molecules but require non-trivial control of high-voltage potentials. We have overcome this experimental challenge and demonstrate continuous deceleration of ND3 from 385 to 10 m/s, while driving the decelerator electrodes with a 10 kV amplitude sinewave. In addition, we test an alternative slowing scheme, which increases the time delay between decelerated packets of ND3 and non-decelerated molecules, allowing for better energy resolution of subsequent reaction studies. We characterize this source of neutral, polar molecules suitable for energy-resolved reaction studies with trapped ions at cold translational temperatures. We also propose a combined apparatus consisting of the traveling-wave decelerator and a linear ion trap with a time-of-flight mass spectrometer and discuss to what extent it may achieve cold, energy-resolved, ion-neutral reactions.
A Stark decelerator can slow down polar molecules to very low velocities. When the velocities are very low, the number of cold molecules obtained is very small. In order to obtain a higher quantity of cold molecules, inspired by the work of Reens et al. [Phys. Rev. Res. 2 (2020) 033 095], we propose an alternative method of operating a Stark decelerator. Through the trajectory simulation of OH molecules in the decelerator, we find that the number of cold molecules can be greatly increased by one order of magnitude at both low and high final velocities on a Stark decelerator consisting of around 150 electrodes. This development is due to the improved longitudinal and the transverse focusing property provided by the new switching schemes and the high-voltage configurations on the decelerator unit.
Within this thesis two experiments will be described. In the first experiment a velocity filter was used to obtain a beam of polar molecules (ND3) with a translational temperature of less than 5K. For the first time the dynamics of individual rotational states inside a velocity filter could be investigated directly. Usually a velocity filter defines only a maximumvelocity that can pass the filter. We successfully implemented two different approaches where molecules below a certain minimal velocity are removed from the beam, turning the velocity filter into a bandpass filter. The second and more interesting experiment is the merged beamexperiment. We developed and constructed the first apparatus which successfullymerged two guided beams, one consisting of polar molecules, the other of paramagnetic particles, into an overlapping parallel movement. With this approach we could lower the temperature at which the reaction of metastable Ne* with ND3, NH3 and CH3F has been investigated by 3 orders of magnitude down to the sub-kelvin regime. Especially the stereochemistry of those reactions revealed an unexpected energy-insensitivity at temperatures below 200K. As our setup is very general the investigation of manymore neutral-neutral reactions will be possible, opening a new route into an temperature regime where only a few reactions have been studied before.
We have measured the translational temperature dependence of the reaction rate constant for CH3CN + Ne+ → products at low temperatures. A cold Ne+ ensemble was embedded in Ca+ Coulomb crystals by a sympathetic laser cooling technique, while cold acetonitrile (CH3CN) molecules were produced by two types of Stark velocity filters to widely change the translational temperatures. The measured reaction rate constant gradually increases with the decrease in the translational temperature of the velocity-selected CH3CN molecules from 60 K down to 2 K, and thereby, a steep increase was observed at temperatures lower than 5 K. A comparison between experimental rate constants and the ion–dipole capture rate constants by the Perturbed Rotational State (PRS) theory was performed. The PRS capture rate constant reproduces well the reaction rate constant at a few kelvin but not for temperatures higher than 5 K. The result indicates that the reaction probability is small compared to typical ion–polar molecule reactions at temperatures above 5 K.