We have studied laser-induced reactions between a Ca+ Coulomb crystal and the polar molecules of NH3 and ND3 at room temperature. The reaction rates of Ca+*(3d2D3/2, 4p2P1/2) + NH3 (ND3) → products have been determined from time variation of the size of fluorescence images of Ca+ Coulomb crystals. The results show that the reaction rates are very slow, and therefore the reaction-rate measurements between sympathetically cooled molecular ions and slow NH3 (ND3) molecules can be performed using a Ca+ Coulomb crystal as coolant.We have studied laser-induced reactions between a Ca+ Coulomb crystal and the polar molecules of NH3 and ND3 at room temperature. The reaction rates of Ca+*(3d2D3/2, 4p2P1/2) + NH3 (ND3) → products have been determined from time variation of the size of fluorescence images of Ca+ Coulomb crystals. The results show that the reaction rates are very slow, and therefore the reaction-rate measurements between sympathetically cooled molecular ions and slow NH3 (ND3) molecules can be performed using a Ca+ Coulomb crystal as coolant.

Journal of Physics Conference SeriesNov 5, 20121 citation

Cold chemistry with electronically excited Ca+ Coulomb crystals

Alexander D. Gingell, Martin T. Bell, James M. Oldham +2 more

Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (<E(coll)>/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (<E(coll)>/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.

The Journal of Chemical PhysicsNov 15, 201044 citations

Sympathetic crystallization of CaH + produced by a laser-induced reaction

Naoki Kimura, K. Okada, Toshinobu Takayanagi +3 more

We investigated sympathetic Coulomb crystallization of CaH ${}^{+}$ ions produced by a laser-induced reaction through an excited state according to $^{40}\mathrm{Ca}$ ${}^{+}$ ( $4p {}^{2}{P}_{1/2}$ ) $+$ ${\mathrm{H}}_{2}$ $\ensuremath{\rightarrow}$ $^{40}\mathrm{Ca}$ H ${}^{+}$ $+$ H. $^{40}\mathrm{Ca}$ H ${}^{+}$ ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca ${}^{+}$ ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH ${}^{+}$ ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm ${}^{3}$ /s was obtained.We investigated sympathetic Coulomb crystallization of CaH${}^{+}$ ions produced by a laser-induced reaction through an excited state according to $^{40}\mathrm{Ca}$${}^{+}$($4p {}^{2}{P}_{1/2}$) $+$ ${\mathrm{H}}_{2}$ $\ensuremath{\rightarrow}$ $^{40}\mathrm{Ca}$H${}^{+}$ $+$ H. $^{40}\mathrm{Ca}$H${}^{+}$ ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca${}^{+}$ ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH${}^{+}$ ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm${}^{3}$/s was obtained.

Physical Review AMar 30, 201139 citations

Acceleration of the chemical reaction of trapped Ca ions with H2O molecules by laser excitation

K. Okada, M. Wada, L.G.J. Boesten +3 more

The reactivity of laser-excited Ca +* ions in 3 2 D J ( J = 3/2, 5/2) states with neutral H 2 O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled 40 Ca + ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca +* (D J ) + H 2 O → products was found to be 1.8(1) × 10 3 s −1 Pa −1 at an ion temperature of T = 3.1(1) × 10 3 K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca + ions.The reactivity of laser-excited Ca +* ions in 3 2 D J ( J = 3/2, 5/2) states with neutral H 2 O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled 40 Ca + ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca +* (D J ) + H 2 O → products was found to be 1.8(1) × 10 3 s −1 Pa −1 at an ion temperature of T = 3.1(1) × 10 3 K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca + ions.

Journal of Physics B Atomic Molecular and Optical PhysicsDec 20, 200226 citations

Ion-molecule chemistry at very low temperatures: cold chemical reactions between Coulomb-crystallized ions and velocity-selected neutral molecules

Martin T. Bell, Alexander D. Gingell, James M. Oldham +2 more

The recent development of a range of techniques for producing cold atoms and molecules at very low translational temperatures T < or = 1 K has provided the opportunity to investigate collisional processes in a new physical regime. We have recently presented a new experimental method to study low-temperature reactive collisions between translationally cold ions and neutral molecules (S. Willitsch et al., Phys. Rev. Lett. 2008, 100, 043203). Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of translationally cold neutral molecules with a facility to produce ordered structures of cold ions (Coulomb crystals) by laser cooling in a linear quadrupole ion trap. The strong localisation of the ions in the trap in combination with the high sensitivity of laser-induced-fluorescence detection enabled us to study chemical reactions on the single-particle level, down to temperatures of T approximately 1 K. In the current paper, we present a detailed characterisation of the scope and limitations of this method based on our study of the reaction between laser-cooled Ca+ ions and velocity-selected CH3F molecules. The properties of our cold-neutrals source and the dependence of the measured rate constant on the shape of the Coulomb crystals, trapping and laser-cooling parameters are discussed. An extension of our technique for the study of low-temperature reactions with sympathetically cooled molecular ions (translational temperature T > 10 mK) is presented and first results on the charge-transfer reaction between OCS+ and ND3 are discussed. Finally, perspectives for further developments of our method are explored.The recent development of a range of techniques for producing cold atoms and molecules at very low translational temperatures T < or = 1 K has provided the opportunity to investigate collisional processes in a new physical regime. We have recently presented a new experimental method to study low-temperature reactive collisions between translationally cold ions and neutral molecules (S. Willitsch et al., Phys. Rev. Lett. 2008, 100, 043203). Our technique relies on the combination of a quadrupole-guide velocity selector for the generation of translationally cold neutral molecules with a facility to produce ordered structures of cold ions (Coulomb crystals) by laser cooling in a linear quadrupole ion trap. The strong localisation of the ions in the trap in combination with the high sensitivity of laser-induced-fluorescence detection enabled us to study chemical reactions on the single-particle level, down to temperatures of T approximately 1 K. In the current paper, we present a detailed characterisation of the scope and limitations of this method based on our study of the reaction between laser-cooled Ca+ ions and velocity-selected CH3F molecules. The properties of our cold-neutrals source and the dependence of the measured rate constant on the shape of the Coulomb crystals, trapping and laser-cooling parameters are discussed. An extension of our technique for the study of low-temperature reactions with sympathetically cooled molecular ions (translational temperature T > 10 mK) is presented and first results on the charge-transfer reaction between OCS+ and ND3 are discussed. Finally, perspectives for further developments of our method are explored.

Faraday DiscussionsJan 1, 200976 citations

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

K. Okada, T Suganuma, Takahiro Furukawa +3 more

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.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.

Physical Review AApr 26, 201324 citations

We investigated sympathetic Coulomb crystallization of CaH

{}^{+}

ions produced by a laser-induced reaction through an excited state according to

^{40}\mathrm{Ca}

{}^{+}

(

4p {}^{2}{P}_{1/2}

)

+

{\mathrm{H}}_{2}

$\ensuremath{\rightarrow}$

^{40}\mathrm{Ca}

{}^{+}

+

^{40}\mathrm{Ca}

{}^{+}

ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca

{}^{+}

ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH

{}^{+}

ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm

{}^{3}

/s was obtained.

{}_{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}

{}^{+}

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}

{}^{+}

$\ensuremath{\rightarrow}$ CH

{}_{3}

CNH

{}^{+}

{}_{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.