This thesis describes the design, construction, operation, and characterisation of an experimental apparatus that produces a source of internally cold, slow molecules that can be used for studying ion-molecule reactions at low temperatures. The apparatus combines buffer-gas cooling with a bent quadrupole velocity selector to cool both the translational and rotational degrees of freedom of the molecules. A cold cell (6 K) is filled with a buffer gas, such as helium, that exhibits sufficiently high vapour pressure for cryogenic applications. Hot molecules (150 to 300 K) enter the cell and thermalise with the buffer gas through collisions. Molecules are subsequently loaded into an electrostatic quadrupole guide, which acts as a velocity filter; only translationally cold polar molecules are guided around the bend. Using a buffer-gas-cooled source of molecules for electrostatic velocity selection, rather than a 300 K effusive source, yields a rotationally cold sample, with J ≤ 3. This rotational selectivity will enable the dependence of reaction cross sections on the reactant rotational state to be examined. Mass spectrometry is used to characterise cold molecular beams of ND3 and CH3F, while (2+1) REMPI spectra are recorded for the ammonia isotopologues. The peak velocity of guided ND3 is 75.86(0.70) ms-1 for standard conditions in a 6 K helium buffer gas cell (1.0 sccm ND3 flow rate, 0.6 mbar helium inlet pressure, ± 5 kV voltage). This corresponds to a peak kinetic energy of 6.92(0.13) K. (2+1) REMPI spectroscopy of the B1E''(v2'=5) ← X(1) transition enabled the rotational state distribution of guided ammonia molecules to be established. PGOPHER simulations of the experimental spectra suggest a rotational temperature of 10 K for ND3 molecules (from a 6 K helium buffer gas cell). The extent of translational and rotational cooling can be controlled by varying the molecular and buffer gas densities within the cell, by changing the temperature of the buffer gas cell (we can operate at 6 K or 17 K), or by changing the buffer gas. The translational temperature of guided ND3 is similar in a 6 K helium and 17 K neon buffer gas cell (peak kinetic energies of 6.92(0.13) K and 5.90(0.01) K, respectively) because the heavier neon gas has a slightly lower thermal velocity at 17 K than helium does at 6 K. Despite similar translational temperatures, the rotational temperature of guided ND3 is lower for molecules exiting the 6 K helium cell compared to the 17 K neon buffer gas cell (10 K and 15 K, respectively). The 6 K helium and 17 K neon buffer gas cells provide an excellent opportunity to investigate the effect of rotational cooling on branching ratios and reaction rates in low temperature ion-molecule reactions. The buffer gas cell and velocity guide described in this work will be combined with a linear Paul ion trap, to facilitate the study of cold ion-molecule reactions.

The development of the field of the science of ultra-cold matter has opened some exciting possibilities in exploring the quantum-mechanical processes which dominate matter interactions at the sub-microscopic scale. Although methods of cooling atoms are well established, molecular cooling is made difficult by molecules’ additional vibrational and rotational degrees of freedom. It was the goal of the research in this work to approach molecular cooling indirectly, by using broadband shaped-pulse photoassociation for the generation of tightly bound ultracold Rb₂ molecules. The experiments towards this goal conducted by our group included a pumpdecay experiment to observe the generation of ground state singlet or triplet molecules. However, attempts to observe an increase in ground state population have been unsuccessful. A pump-probe study of wavepacket dynamics in the 5s+5p electronic state was conducted in order to determine the appropriate timing for the application of an additional pulse to dump population into the ground state. Although the attempt to observe wavepacket oscillations has been unsuccessful, pump-probe studies have yielded the observation of loosely bound excited state molecules as a result of the photoassociation pulse. These results are promising as a first stage in a fully coherent pump-dump approach to stabilisation into the lowest vibrational ground state. This thesis will provide an introduction and overview to the concerns involved in addressing the problem of molecular cooling and generation. Experimental techniques will be discussed including pulsed laser systems, optical parametric amplifi- cation, and the presentation of an original design for pulse shaping with an acoustooptic modulator. The emphasis of these discussions will be on the principles and operating procedures required for the use of these devices as home-built systems. The thesis will conclude with the results of pump-probe experiments utilising the pulse shaper as a spectral cutting device.

O progresso nas pesquisas no campo de átomos a baixíssimas temperaturas tem possibilitado o estudo de fenômenos de fotoassociação e blindagem óptica. Estes dois fenômenos são tipicamente produzidos quando um campo de luz externo é incidido em um MOT. Dependendo das dessintonias das frequências dos lasers em relação à ressonância atômica, se podem realizar seletivamente estes processos. Neste trabalho, os estudos são feitos em átomos de sódio e a exploração apresentada faz parte dos estudos que envolvem a formação e detecção de moléculas fotônicas. Uma exploração dos espetros de fotoionização associativa por dois fótons na região de 0 até -30 GHz foi realizada e a identificação dos estados de longo alcance foi feita. Os níveis vibracionais identificados pertencem majoritariamente aos estados 1g, 0g e 0+u, sendo o estado 0g um estado puramente de longo alcance. Um estudo da blindagem óptica em alguns picos do espectro de fotoionização associativa foi realizada com a adição de outro laser azul de frequência ωA2, e se comprovou saturação na blindagem das colisões inelásticas quando se aumenta a potência deste laser. Com estes estudos demonstramos que os potenciais moleculares intermediários são modificados na presença de campos de luz intensos. / The progress of research in the field of atoms in low temperatures has opened the possibility to study phenomena called fotoassociation and optical shielding. These two phenomena are typically produced when an external light field is focused on a MOT. Depending on the detuning of frequencies of the laser with respect to the atomic resonance can be selectively induced these processes. In this work, the studies are made for sodium atoms, and the presented exploration has been a part of the first studies involving the formation and detection of photonic molecules. An exploration of the fotoasociation spectrum of two colors fotoionizative association between 0 and -30 GHz was performed, and a labeling of the states of long range was carried out. These vibrational levels belong mostly to the states 1g, 0g and 0+u , being the state 0g , a purely long range state. A study of optical shielding of some peaks in the fotoasociation spectrum, as identified to be important in forming the photonic molecule. With respect to the blue laser detuning frequency ωA and its intensity was performed with the addition of another blue laser frequency ωA2, and can show a saturation in the shielding for inelastic collisions when increasing the laser intensity. The studies have showed modification in the intermediary potentials in the presence of an intense light field.

Inelastic collisions between dipolar molecules, assumed to be trapped in a static electric field at cold (> 10−3K) temperatures, are investigated and compared with elastic collisions. For molecules with a Λ-doublet energy-level structure, a dipole moment arises because of the existence of two nearly degenerate states of opposite parity, and the collision of two such dipoles can be solved entirely analytically in the energy range of interest. Cross sections and rate constants are found to satisfy simple, universal formulas. In contrast, for molecules in a Σ electronic ground state, the static electric field induces a dipole moment in one of three rotational sublevels. Collisions between two rotor dipoles are calculated numerically; the results scale simply with molecule mass, rotational constant, dipole moment, and field strength. It might be expected that any particles interacting only under the influence of the dipole-dipole interaction would show similar behavior; however, the most important and general result of this research is that at cold temperatures inelastic rate constants and cross sections for dipoles depend strongly upon the internal structure of the molecules. The most prominent difference between the Λ-doublet and rotor molecules is variation of the inelastic cross section with applied field strength. For Λ-doublet dipoles, cross sections decrease with increasing field strength. For rotor dipoles, cross sections increase proportionally with the square of field strength. Furthermore, the rate constants of the two types of molecules depend very differently on the angular orientations of the dipoles in the electric field.

O progresso nas pesquisas no campo de átomos a baixíssimas temperaturas tem possibilitado o estudo de fenômenos de fotoassociação e blindagem óptica. Estes dois fenômenos são tipicamente produzidos quando um campo de luz externo é incidido em um MOT. Dependendo das dessintonias das frequências dos lasers em relação à ressonância atômica, se podem realizar seletivamente estes processos. Neste trabalho, os estudos são feitos em átomos de sódio e a exploração apresentada faz parte dos estudos que envolvem a formação e detecção de moléculas fotônicas. Uma exploração dos espetros de fotoionização associativa por dois fótons na região de 0 até -30 GHz foi realizada e a identificação dos estados de longo alcance foi feita. Os níveis vibracionais identificados pertencem majoritariamente aos estados 1g, 0g e 0+u, sendo o estado 0g um estado puramente de longo alcance. Um estudo da blindagem óptica em alguns picos do espectro de fotoionização associativa foi realizada com a adição de outro laser azul de frequência ωA2, e se comprovou saturação na blindagem das colisões inelásticas quando se aumenta a potência deste laser. Com estes estudos demonstramos que os potenciais moleculares intermediários são modificados na presença de campos de luz intensos. / The progress of research in the field of atoms in low temperatures has opened the possibility to study phenomena called fotoassociation and optical shielding. These two phenomena are typically produced when an external light field is focused on a MOT. Depending on the detuning of frequencies of the laser with respect to the atomic resonance can be selectively induced these processes. In this work, the studies are made for sodium atoms, and the presented exploration has been a part of the first studies involving the formation and detection of photonic molecules. An exploration of the fotoasociation spectrum of two colors fotoionizative association between 0 and -30 GHz was performed, and a labeling of the states of long range was carried out. These vibrational levels belong mostly to the states 1g, 0g and 0+u , being the state 0g , a purely long range state. A study of optical shielding of some peaks in the fotoasociation spectrum, as identified to be important in forming the photonic molecule. With respect to the blue laser detuning frequency ωA and its intensity was performed with the addition of another blue laser frequency ωA2, and can show a saturation in the shielding for inelastic collisions when increasing the laser intensity. The studies have showed modification in the intermediary potentials in the presence of an intense light field.

No description available.

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 ²Δ_5/2-²Π_3/2 band together with two ²Σ⁺– ²Π_1/2 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.

Neste trabalho apresentamos a construção de um novo sistema experimental para aprisionamento de átomos e moléculas. Trata-se de uma armadilha de dipolo cruzada totalmente óptica, formada por um laser de fibra com 40 W de potência com comprimento de onda em 1064 nm. Nesse sistema foi demonstrado o aprisionamento de átomos Rb e K e moléculas Rb2. Realizamos dois experimentos distintos neste se novo sistema: No primeiro, estudamos a evolução temporal de amostras dos isótopos 85Rb e 87Rb na armadilha de dipolo cruzada. Pudemos medir pela primeira vez o processo de mudança de estrutura hiperfina em átomos sem a presença de luz ressonante, e observamos a fotoassociação dos átomos pelo laser da armadilha de dipolo. E no segundo, desenvolvemos uma técnica que permite o estudo rápido e direto do decaimento no número de moléculas de Rb2 na armadilha. Com essa técnica, observamos também o comportamento dessas moléculas na presença de átomos preparados em um determinado estado hiperfino do estado fundamental. Estes resultados juntamente com um desenvolvimento teórico, nos sugerem novas perspectivas rumo a produção de um sistema que possa produzir e aprisionar moléculas KRb no estado fundamental 1Σ+ (v = 0). / In this work, we present the construction of a new experimental system for trapping atoms and molecules. It is an all optical crossed dipole trap, formed by a fiber laser with 40 W of power at 1064 nm. In this system, we have trapped K and Rb atoms as well as Rb2 molecules. We have carried out two experiments in this new system. In the first, we studied the temporal evolution of atomic samples of different isotopes, 85Rb and 87Rb, in the crossed trap dipole. We were able to measure for the first time the hyperfine-changing collisions of atoms in the absence of resonant light, and we have observed photoassociation of atoms by the dipole trap laser beam. In the other experiment, we have developed a technique that allows us to study the trapped molecule population decay in the trap. It was also observed the molecule population decay in the presence of atoms prepared in a given hyperfine state of the ground state. These results together with a theoretical development may suggest us new perspectives towards the production of a system that can produce and trap KRb molecules in the ground state 1Σ+ (v = 0).

Neste trabalho, estudamos a fotoassociação de átomos de rubídio no regime de curto alcance. Realizamos espectroscopia por perda de átomos em uma armadilha magneto-óptica de 85Rb usando um laser de fibra de alta potência, o qual possuia largura de linha da ordem de 1MHz e 50W de potência no intervalo de 1060nm a 1070nm. Dois níveis vibracionais do potencial excitado 0u+ foram observados (v=137 e v=138). Além disso, medimos o tempo de vida de uma armadilha óptica de dipolo cruzada. Como esperado, o tempo de vida é menor quando o laser está sintonizado na ressonância. Um modelo teórico prevê a distribuição dessas moléculas nos níveis vibracionais do estado eletrônico fundamental após o processo de fotoassociação. Os resultados, nos sugerem perspectivas para a produção de moléculas de Rb2 no estado vibracional fundamental. / In this work, we studied short-range photoassociation of rubidium atoms. We realize trap-loss spectroscopy in a magneto-optical trap of 85Rb using high power fiber laser, which had around 1MHz linewidth and 50W power at 1060nm to 1070nm interval. Two vibrational levels of the 0u+ excited potential were observed (v=137 e v=138). Besides that, we measured the lifetime of a crossed optical dipole trapped. As expected, the lifetime is shorter when the laser is tuned on resonance. A theoretical model predicts the molecular distribution in the vibrational levels of electronic ground state. The results suggest us perspectives to produce Rb2 molecules in the ground vibrational state.

Neste trabalho apresentamos a construção de um novo sistema experimental para aprisionamento de átomos e moléculas. Trata-se de uma armadilha de dipolo cruzada totalmente óptica, formada por um laser de fibra com 40 W de potência com comprimento de onda em 1064 nm. Nesse sistema foi demonstrado o aprisionamento de átomos Rb e K e moléculas Rb2. Realizamos dois experimentos distintos neste se novo sistema: No primeiro, estudamos a evolução temporal de amostras dos isótopos 85Rb e 87Rb na armadilha de dipolo cruzada. Pudemos medir pela primeira vez o processo de mudança de estrutura hiperfina em átomos sem a presença de luz ressonante, e observamos a fotoassociação dos átomos pelo laser da armadilha de dipolo. E no segundo, desenvolvemos uma técnica que permite o estudo rápido e direto do decaimento no número de moléculas de Rb2 na armadilha. Com essa técnica, observamos também o comportamento dessas moléculas na presença de átomos preparados em um determinado estado hiperfino do estado fundamental. Estes resultados juntamente com um desenvolvimento teórico, nos sugerem novas perspectivas rumo a produção de um sistema que possa produzir e aprisionar moléculas KRb no estado fundamental 1Σ+ (v = 0). / In this work, we present the construction of a new experimental system for trapping atoms and molecules. It is an all optical crossed dipole trap, formed by a fiber laser with 40 W of power at 1064 nm. In this system, we have trapped K and Rb atoms as well as Rb2 molecules. We have carried out two experiments in this new system. In the first, we studied the temporal evolution of atomic samples of different isotopes, 85Rb and 87Rb, in the crossed trap dipole. We were able to measure for the first time the hyperfine-changing collisions of atoms in the absence of resonant light, and we have observed photoassociation of atoms by the dipole trap laser beam. In the other experiment, we have developed a technique that allows us to study the trapped molecule population decay in the trap. It was also observed the molecule population decay in the presence of atoms prepared in a given hyperfine state of the ground state. These results together with a theoretical development may suggest us new perspectives towards the production of a system that can produce and trap KRb molecules in the ground state 1Σ+ (v = 0).