Cold ion–polar-molecule reactions studied with a combined Stark-velocity-filter–ion-trap apparatus

Abstract

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${}_{2}$CO, and CH${}_{3}$CN, were performed using time-of-flight (TOF) measurements. We confirmed the generation of slow ND${}_{3}$, H${}_{2}$CO, and CH${}_{3}$CN 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 $n={10}^{4}$ 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${}_{2}$H${}^{+}$ ions and velocity-selected CH${}_{3}$CN 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${}_{3}$CN+N${}_{2}$H${}^{+}$ $\ensuremath{\rightarrow}$ CH${}_{3}$CNH${}^{+}$+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.