Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H2, or D2) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH5+ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam-ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H3+ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H4+ or CH6+ via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (microeV) to the dissociation continuum using infrared transitions.
Cooling all degrees of freedom of a molecule, a cluster, or even a nanoparticle which is suspended in a vacuum, is an experimental challenge. Without suitable schemes, cold or ultracold chemical reactions are not feasible. Methods such as laser based preparation of very slow atoms, decelerating molecules to low velocities with electric fields or freezing molecular ions into Coulomb crystals, are generally not suitable to cool the vibrational or rotational motion of molecules. This contribution describes a new method in which a beam of slow atoms or molecules (H, He, H2, or D2) is used for cooling charged particles confined in a multi-electrode rf trap. For reaching sub-K temperatures, the fast part of a cold effusive beam is removed with a shutter before the slow remaining neutrals interact with the ion cloud. The development of a pulsed cold beam source is discussed as well as suitable methods for determining the ion temperature. A challenging application is to prepare internally cold CH5+ for spectroscopy or chemistry. New experimental results for hydrogen abstraction in collisions with slow H atoms are reported at energies of a few meV. For evaluating these measurements and for predicting effective rate coefficients at lower energies, the kinematic conditions of the slow neutral beam-ion trap arrangement have been analyzed in detail. The potential of cooling ions such as protonated methane or H3+ with slow energy selected H atoms is briefly mentioned. An interesting process is the formation of weakly bound ions such as H4+ or CH6+ via radiative or ternary association. Such ions are ideal candidates for preparing the corresponding collision complexes very close (microeV) to the dissociation continuum using infrared transitions.
This thesis describes the combination of two sources of cold atomic or molecular species which can be used to study a wide range of ion-molecule reactions. The challenges in forming these species and in determining the fate of reactive events are explored throughout. Reactions occur in a volume within a radio-frequency ion trap, in which ions have previously been cooled to sub-Kelvin temperatures. Ions are laser-cooled, with migration of ions slowed sufficiently to form a quasi-crystalline spheroidal structure, deemed a Coulomb crystal. Fluorescence emitted as a consequence of laser-cooling is detected; the subsequent fluorescence profiles are used to determine the number of ions in the crystal and, in combination with complementary simulations, the temperature of these ions. Motion imparted by trapping fields can be substantial and simulations are required to accurately determine collision energies. A beam of decelerated molecules is aimed at this stationary ion target. An ammonia seeded molecular beam enters a Stark decelerator, based on the original design of Meijer and co-workers. The decelerator uses time-varying electric fields to remove kinetic energy from the molecules, which exit at speeds down to 35 m/s. A fast-opening shutter and focussing elements are subsequently used to maximise the decelerated flux in the reaction volume while minimising undecelerated molecule transmission. Substantial fluxes of decelerated ammonia are obtained with narrow velocity distributions to provide a suitable source of reactant molecules. Combination of these two techniques permits studies of reactions between atomic ions and decelerated molecules that can be entirely state-specific. Changes in the Coulomb crystal fluorescence profile denote changes in the ion identities, the rate of these changes can be used to obtain rate constants. Determination of rate constants is even possible despite the fact that neither reactant nor product ions are directly observed. This work has studied reactions between sympathetically cooled Xe+ ions and guided ND3 and has obtained data consistent with prior studies. Determination of reactive events is complicated if ion identities can change without affecting the fluorescence profile, or if multiple reaction channels are possible. A range of spectroscopic techniques are discussed and considered in regards to determining rate constants and product identities. Pulsed axial excitation of trapped ions can follow rapid changes in average ion weights and subtle changes for small crystals. Time-of-flight mass spectrometry is also demonstrated using the trapping electrodes and is suitable for discrimination of ions formed within the trap.
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.
This work presents various novel results in the field of experimental and theoretical trapped ion studies. All investigations involve laser-cooled 138Ba+ ions confined in a linear quadrupole ion trap, which serve as coolant for complex molecular ions, as target for collisions with neutral atoms, or as model system in theoretical analyses. The special feature of the apparatus used in this work, is a molecular ion source based on electrospray ionization that allows for the production of gas-phase molecular ions of almost arbitrary species with a maximum mass-to-charge ratio of 2,000 Da. Trapped together with laser-cooled 138Ba+ ions, the molecular ions can be cooled down, kept cold and investigated for many minutes and in principle for up to hours in a nearly collisionless environment.
In this work, the sympathetic cooling of complex molecular ions to translational temperatures below 1 K is demonstrated for different types of molecules covering a mass range from 182 to 12,400 Da. Using the example of the protein cytochrome c, with a mass of about 12,400 Da the most massive molecular species sympathetically cooled in an ion trap so far, the methods for detection of successful trapping and cooling, and temperature determination are presented in detail. In one case, multiply protonated molecules of the protein cytochrome c have been cooled to less than 0.75 K. Furthermore, two methods for measuring photodestruction rates of cold, trapped complex molecular ions have been developed that can, for example, be applied in destructive spectroscopy schemes. Both techniques are demonstrated using the example of singly protonated glycyrrhetinic acid molecules (GAH+) dissociated by an ultraviolet (UV) laser. Measurements of the photodestruction rate were performed at different intensities of the UV laser, and rates as low as 0.05 s-1 and less have been determined, which is only possible due to the long ion storage times that can be achieved with this apparatus. For the UV laser wavelength of 266 nm, a destruction cross section of (1.1±0.1)∙10E-17 cm2 for GAH+ has been determined.
In this work, feasibility studies towards internal cooling of trapped complex molecular ions have been performed. Such an additional internal cooling could be realized via collisions with laser-cooled neutral atoms. To test their suitability, neutral atoms of Li, Na, K, Rb, Cs, and Yb have been collided with trapped laser-cooled and not cooled Ba+ ions as well as with complex molecular ions. For all species charge exchange reactions with Ba+ ions have been observed excluding these species for collisional cooling. While K, Rb, and Cs showed charge exchange reactions with ground state and laser-excited Ba+ ions, the charge exchange rates of Yb and Ba+ were found to be state dependent. Collisions of neutral Yb atoms were found to be reactive and lead to the formation of adducts and fragments, in contrast to the alkali metals for which no such reactions have been observed. In addition, the motion of cooled, trapped ions has been studied theoretically via simulations. When the translational energy of a trapped ion ensemble is sufficiently reduced, a first-order phase transition from a liquid to a solid state occurs with the ion ensemble changing from a disordered ion cloud to an ordered ion crystal. As ions in a linear quadrupole trap arrange in shell structures, a free motion in radial direction is disabled, and ions can only move to neighboring shells when their kinetic energy is high enough to overcome a certain potential barrier. The simulations have shown that intershell diffusion rates in the considered ion crystals increase exponentially when the ion temperatures exceed values that well agree with the predictions of other established models. Ion ensembles of different ion numbers (300, 500, and 1000 138Ba+ ions) and symmetries (prolate and spherical) have been investigated showing a clear tendency for the phase transition temperatures: The higher the ion number, the lower is the transition temperature.
Precision vibrational spectroscopy of the molecular hydrogen ions is of significant interest for determining fundamental constants, for searching for new forces, and for testing quantum electrodynamics calculations. Future experiments can profit from the ability of preparing molecular hydrogen ions at ultralow kinetic energy and in preselected internal states, with respect to vibration, rotation, and spin degrees of freedom. For the homonuclear ions (H2+,D2+), direct laser cooling of the rotational degree of freedom is not feasible. We show by quantum calculations that rotational cooling by cold He buffer gas is an effective approach. For this purpose we have computed the energy-dependent cross sections for rotationally elastic and inelastic collisions, ${\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}N)+\mathrm{He}\phantom{\rule{0.16em}{0ex}}\ensuremath{\rightarrow} {\mathrm{h}}_{2}{}^{+}(v=0,\phantom{\rule{0.16em}{0ex}}{N}^{\ensuremath{'}})+\mathrm{He}$ (where h=H,D), using ab initio coupled-channel calculations. We find that rotational cooling to the lowest rotational state is possible within tens of seconds under experimentally realistic conditions. We furthermore describe possible protocols for the preparation of a single quantum state, where also the spin state is well defined.
Abstract Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.
Ultracold cold molecular ions promise new directions in various studies of fundamental physics, such as precision measurements, ultracold chemistry and quantum information sciences. All these exciting applications require the molecular ion to be prepared at ground state of motional and internal degrees of freedom. It has been proposed that this stringent goal could be achieved through sympa-thetic cooling via collisions with laser-cooled neutral atoms. Three fundamentalissues of this method are addressed in this thesis.First, an analytical model is established to accurately describe collision-induced heating of a single ion in contact with cold neutral atoms. This model reveals that micromotion interruption is the cause of heating, and gives results about steady-state temperature and sympathetic cooling rate verified by Monte-Carlo simulations. It also provides insight into the power-law tails observed in theenergy distribution of the trapped ion.Next, we consider the case of multiple ions, whose inter-particle Coulomb repulsion causes ions in the Coulomb crystal state to spontaneously melt into a gas phase ion cloud, due to the same micromotion interruption mechanism. The analysis of this problem with a plasma model leads to the experimental determination of a quantity central to plasma physics, Coulomb Logarithm, in anion trap.Finally, we demonstrate a molecular ion spectroscopy technique through the example of trap-depletion photodissociation of BaCl + . Although not sensitive to rotational structure, this method already reveals much about the fundamental quantum physics in the photodissociation process. The measured cross-section results paves the road toward state-selective spectroscopy currently going on in our lab.