Trapping cold, chemically important molecules with electromagnetic fields is a useful technique to study small molecules and their interactions. Traps provide long interaction times, which are needed to precisely examine these low-density molecular samples. However, the trapping fields lead to nonuniform molecular density distributions in these systems. Therefore, it is important to be able to experimentally characterize the spatial density distribution in the trap. Ionizing molecules at different locations in the trap using resonance-enhanced multiphoton ionization (REMPI) and detecting the resulting ions can be used to probe the density distribution even at the low density present in these experiments because of the extremely high efficiency of detection. Until recently, one of the most chemically important molecules, OH, did not have a convenient REMPI scheme identified. Here, we use a newly developed $1+{1}^{\ensuremath{'}}$ REMPI scheme to detect trapped cold OH molecules. We use this capability to measure the trap dynamics of the central density of the cloud and the density distribution. These types of measurements can be used to optimize loading of molecules into traps, as well as to help characterize the energy distribution, which is critical knowledge for interpreting molecular collision experiments.
Trapping cold, chemically important molecules with electromagnetic fields is a useful technique to study small molecules and their interactions. Traps provide long interaction times, which are needed to precisely examine these low-density molecular samples. However, the trapping fields lead to nonuniform molecular density distributions in these systems. Therefore, it is important to be able to experimentally characterize the spatial density distribution in the trap. Ionizing molecules at different locations in the trap using resonance-enhanced multiphoton ionization (REMPI) and detecting the resulting ions can be used to probe the density distribution even at the low density present in these experiments because of the extremely high efficiency of detection. Until recently, one of the most chemically important molecules, OH, did not have a convenient REMPI scheme identified. Here, we use a newly developed 1+1\ensuremath′ REMPI scheme to detect trapped cold OH molecules. We use this capability to measure the trap dynamics of the central density of the cloud and the density distribution. These types of measurements can be used to optimize loading of molecules into traps, as well as to help characterize the energy distribution, which is critical knowledge for interpreting molecular collision experiments.
Samples of cold and ultracold polar molecules have the potential to revolutionize physical chemistry, precision measurement, and few-body quantum physics. This thesis describes experimental results for the production of cold samples of OH radicals by Stark deceleration of a supersonic beam. Since Stark deceleration cannot increase the phase space density of the sample, the initial production stage of the OH molecule is critical. The first set of experiments describes a general methodology for the production of OH beams with maximal phase space density, as well as the subsequent coupling to a Stark decelerator. Additionally, we describe the redesign of our electrostatic trap, optimized for future collision experiments of OH with co-trapped ultracold Rb atoms. The new design resulted in a 15-fold increase in total number of trapped molecules over the previous design when tested with ND3 molecules. The second set of experiments focuses on laser-based detection of OH molecules at the exit of the decelerator based on laser induced fluorescence (LIF) and resonance enhanced multiphoton ionizaton (REMPI). The latter method uses vacuum-ultraviolet light at 118nm produced by third-harmonic generation in Xe/Ar gas mixtures as the ionizing step. The sensitivity of this latter technique is limited by the attainable photon flux of the ionizing radiation at 118nm. We present detailed measurements of the conversion efficiencies as well as absolute photon fluxes. Strategies to overcome these limitations as well as prospects for detection of OH molecules in a trap are discussed.
Trapped molecular ions in pure rovibronic states are desirable in experiments ranging from cold chemistry to searches for physics beyond the Standard Model. Resonance-enhanced multiphoton ionisation (REMPI) can be used to prepare molecular ions in specific internal states with high fidelities. However, in the presence of electric fields, ionisation spectra exhibit frequency shifts and the ionisation thresholds are broadened. For this reason, REMPI studies are normally conducted in low and highly homogeneous electric fields, whereas the operating principle of rf ion traps requires electric fields that vary in space and time. In order to investigate the impact of this on the state-selectivity of REMPI in ion traps, we have simulated the expected broadening of the ionisation threshold under various operating conditions of a typical linear Paul trap. In many cases, the width of the ionisation threshold exceeds the separation between rotational energy levels, preventing state-selective ionisation. Careful choice of the trapping and laser parameters during loading can reduce this broadening, enabling state-selective ionisation in some instances. Where this strategy is not sufficient, the broadening can be reduced further by rapidly switching the trapping voltages off and on again during loading. This has been demonstrated experimentally for a Coulomb crystal of [Formula: see text] ions without descrystallising it.
We report on the production of cold, state-selected H2+ molecular ions in a linear rf trap. The ions are produced by (3+1) resonance-enhanced multiphoton ionization (REMPI) of H2, and sympathetically cooled by laser-cooled Be+ ions. After demonstrating and characterizing the REMPI process, we use photodissociation by a deep UV laser at 213 nm to verify the high vibrational purity of the produced H2+ ion samples. Moreover, the large difference between the photodissociation efficiencies of ions created in the v=0 and v=1 levels provides a way to detect a $v=0\ensuremath{\rightarrow}1$ transition. These results pave the way towards high-resolution vibrational spectroscopy of H2+ for fundamental metrology applications.
The study of chemical reactions between ions and neutral species at very low energies reveals precise informations about the dynamics of collisions and fine details of intermolecular interactions. Here, we report progress towards the development of next- generation experiments for the investigation of cold ion-neutral reactions. First, we present a new ''dynamic" hybrid ion-atom trap which enables the study of collisions with a superior energy resolution accessing a regime in which quantum scattering resonances may become observable. Second, we discuss and numerically characterize the concept and properties of a hybrid trap for cold neutral molecules and molecular ions which paves the way for the study of ion-molecule reactions in the millikelvin regime.
In recent years there has been much progress in the field of cold and ultracold molecular physics and a variety of experimental techniques for producing cold matter now exist. In particular, the generation of trapped molecular ions at mK temperatures has been achieved by sympathetic-cooling with laser-cooled atomic ions. By implementing schemes to selectively prepare and control the internal quantum state of molecular ions, and developing detection techniques, it will be increasingly possible to study cold state-selected chemical collisions in an ion-trap. Most molecular species produced in a selected rovibrational state have a lifetime of a few seconds, before the population is redistributed across numerous rovibrational states by interaction with the ambient blackbody radiation (BBR). Consequently, the investigation of state-selected reaction dynamics at low temperatures in experiments where long time scales (minutes to hours) are required, is hindered. This thesis looks into developing strategies that maintain state selection in molecular ions, allowing one to observe state-selected reactions in cold environments, in particular the state-selected reaction between C2H+2 and ND3. Examining reactive ion molecule collisions under cold conditions provides insight into fundamental reaction dynamics, which are thermally averaged out at higher temperatures. A theoretical model is used to investigate laser-driven, blackbody-mediated, rotational cooling schemes for several 1Σ and 2Π diatomic species. The rotational cooling is particularly effective for DCl+ and HCl+, for which 92% and >99% (respectively) of the population can be driven into the rovibrational ground state. For the other systems a broadband optical pumping source is found to enhance the population that can be accumulated in the rovibrational ground state by up to 29% more than that achieved when exciting a single transition. The influence of the rotational constant, dipole moments and electronic state of the diatomics on the achievable rotational cooling is also studied. This approach is extended to consider the BBR interaction and rotational cooling of a linear polyatomic ion, C2H+2, which has a 2Π electronic ground state. The (1-0) band of the ν5 cis-bending mode is infrared active and strongly overlaps the 300 K blackbody spectrum. Hence the lifetimes of state-selected rotational levels are found to be short compared to the typical timescale of ion trapping experiments. Laser cooling schemes are proposed that could be experimentally viable, which involves simultaneous pumping of a set of closely spaced Q-branch transitions on the 2Δ5/2-2Π3/2 band together with two 2Σ+– 2Π1/2 lines. It is shown that this should lead to >70% of total population in the lowest rotational level at 300 K and over 99% at 77 K. In order to identify states of the acetylene ion that could be trapped sufficiently long enough for state-selected reactions in the ion trap with decelerated ND3, the theoretical work has been complemented by experimental investigations into the production of C2H+2 in selected states, and ion trapping of the same using sinusoidal and digital trapping voltages. Appropriate (2+1) REMPI (Resonance Enhanced Multiphoton Ionization) schemes are used to produce C2H+2 in different quantum states, with (1+1) Resonance Enhanced Multiphoton Dissociation (REMPD) employed to detect the ion thus produced. The concept of digital ion trapping for ejection onto MCPs is introduced. A comprehensive comparison between sinusoidal and digital trapping fields has been performed with respect to trap depth and stability regions. Programs have been developed to calculate the stability regions for different ions under varying experimental conditions. The trap depth has been derived for both digital and sinusoidal trapping fields. It is observed that as τ increases, the trap depth of a digital trap increases. For τ = 0.293, the trap depth and stability diagram for both sinusoidal and digital trapping fields would be equivalent. The trap depth at which the sinusoidal trap operates experimentally in our research group is ˜1.36 eV. In contrast, the experimental parameters at which the digital trap operates generates a trap depth of 1.21 eV. Ca+ Coulomb crystals have been formed, stably trapped and stored for extended periods of time in both sinusoidally and digitally time-varying trapping fields. The sympathetic cooling of a diverse range of ions into Ca+ Coulomb crystals is demonstrated, again using both sinusoidal and digital trapping fields. Mass spectrometric detection of ionic reaction products using a novel ejection scheme has been developed, where ejection is achieved by switching off the trapping voltage and converting the quadrupole trap into an extractor-repeller pair by providing the ion trap electrodes with appropriate ejection pulses. This technique is developed using a digital trapping voltage rather than the sinusoidal trapping voltage, as ejection with sinusoidal trapping voltages is not clean (resonance circuitry used in the electronics induces ringing after switching off the trapping voltage). Coulomb crystals, both pure Ca+ and multi-component crystals, are ejected from the ion trap and the TOF trace obtained is recorded on an oscilloscope. When the integrated, base-line subtracted TOF peak is plotted against the number of ions in a Ca+ crystal and sympathetically-cooled Ca+ – CaF+ crystal, a linear relationship is obtained. This technique is found to be well mass-resolved, with the signal arising from CaOH+ (57 amu) and CaOD+ (58 amu) resolvable on the TOF trace. This technique would enable one to monitor a reaction in a Coulomb crystal where the reactant and product species are both either lighter or heavier than calcium, such as the reaction between C2H+2 and ND3, something which has not been previously possible. It is, also, potentially a very important technique for reactions with many product channels.
Research in the field of cold polar molecules is progressing rapidly. An array of interesting\ntopics is being developed including precision measurement and fundamental tests, quantum phase transitions, and ultracold chemistry. In particular, dipolar molecules in well-defined quantum states enable exquisite control of their interactions via applied electric fields. The long-range, anisotropic interaction between dipolar molecules leads to new collision dynamics that could be used for novel collective effects, quantum state engineering, and information processing. The focus of this Thesis is the production of cold samples of neutral OH molecules via Stark deceleration and magnetic trapping for novel collision experiments. This novelty results from our combination of cold external beams and trapped target molecules which facilitates observation of dipolar effects as well as low temperature collision resonances. The large permanent electric dipole moment of OH allows us to precisely tune the lab-frame velocity of the molecular packets from ∼ 500 m/s to rest. We have magnetically trapped OH packets at the terminus of our Stark decelerator at a temperature of 70 mK and density of 106 cm−3. With improved understanding of Stark deceleration, we optimize the decelerator efficiency and its coupling to the magnetic trap. Our latest trap design uses permanent ring magnets to create a three-dimensional magnetic quadrupole field. Use of magnetically trapped OH molecules for collision experiments with external beams allow us the unique opportunity of observing both elastic and inelastic collisions. In addition, the trap confinement yields sensitivity to differential elastic cross sections at low collision energies. This open magnetic trap has allowed measurement of collision cross sections between trapped OH and external supersonic beams of He and D2, the latter of which is of astrophysical interest due to the role of H2-OH collisions in pumping interstellar OH megamasers. The combination of trapped OH molecules and temperature tuned beams of He and D2 has facilitated measurement of the lowest-energy D2-OH collision cross sections yet reported. More recently, we report the first observation of electric-field dependent cross sections between two different species of cold polar molecules - OH and ND3 - thereby demonstrating control over molecular scattering in the cold regime. By combining for the first time the production techniques of Stark deceleration and buffer gas cooling, we increase the molecular interaction time by∼ 105 over traditional crossed-beam experiments to gain enhanced sensitivity at the characteristic densities of cold molecule production.