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Charge transfer processes of atomic hydrogen Rydberg states near surfaces
Full text linkWhen approaching a metal surface, the electronic structure of Rydberg atoms or molecules is perturbed by the surface potential and at close enough distances resonant ionisation of the Rydberg electron into the conduction band of the surface can occur. It is possible to interfere in this process and steer the ionisation distance by making use of the polarisability of the Rydberg orbital in the presence of electric fields. The resulting ions from the surface can extracted via electric fields and subsequently detected via well established ion detection schemes. The question of how this charge-transfer process is affected by different properties of the surface (both electronic and structural) represents the main aspect of the work presented in this thesis. At first, the charge transfer of atomic hydrogen Rydberg atoms with a flat gold metal surface is investigated. While such a surface might appear homogeneous, stray fields are present in its vicinity due to local variations in the surface work function. The surface ionisation process as a function of applied electric field is therefore measured experimentally and the results are compared with classical Monte-Carlo simulations (which include stray field effects). This way the possibility to utilize Rydberg states as a probe of the magnitude of such stray fields is demonstrated. To investigate the effect the surface structure can have on the ionisation process, the interaction of Rydberg atoms with surfaces covered by nanoparticles is investigated. Surface ionisation is measured at a 5 nm nanoparticle monolayer surface and it is shown that population transfer between surface- and vacuum-oriented Rydberg states occurs. In addition, results are presented, which suggest a dependence of the ionisation process on the relative size of Rydberg orbital and nanoparticle. Furthermore, charge transfer between a Rydberg state and discrete electronic states at the surface vacuum interface are investigated by performing experiments with a Cu(100) band-gap semiconductor surface. By analysing surface ionisation as a function of collisional velocity ionisation rates can be determined and are subsequently compared with theoretical predictions. The potential of identifying resonant ionisation is thereby demonstrated. Last, a new method to produce 2s atomic hydrogen via mixing of the 2s and 2p state in an electric field is proposed and first experimental results are presented, thus demonstrating viability of the idea. The experiments presented in this thesis represent the most in depth analysis of the charge-transfer process between atomic hydrogen Rydberg states and a range of different surfaces to date. As such, they demonstrate the potential of utilizing the unique properties of Rydberg states and their applicability as surface probes. In addition, these results pave the way for further experiments involving thin films or the phenomenon of quantum reflectivity
Rydberg ionisation into confined and discrete systems
Full text linkThe energy levels of a hydrogen Rydberg atom approaching a metallic structure are perturbed by the image-charge interaction with the surface. At small atom-surface separations surface ionisation of the Rydberg electron can occur, whereby the electron is transferred to a metal-localised state. In previous studies investigating surface ionisation at bulk metallic surfaces, this state has been part of a conduction band; however this thesis focuses on metallic and structured surfaces where the Rydberg electron transfers into a discrete image-state or hybrid 'well-image state'. The surface ionisation of hydrogen Rydberg atoms at a Cu(100) projected band-gap surface is investigated experimentally and theoretically. Experimentally, the surface ionisation of an incident beam of hydrogen Rydberg atoms is measured by extraction of the resulting ions. Resonance-enhanced charge transfer is seen for hydrogen Rydberg states that are degenerate with copper-localised image-states. A wavepacket propagation study shows that for on-resonance states the maximum in the surface-ionisation probability is shifted away from the surface by decreasing the collisional velocity. The discrete hybrid 'well-image states' localised along the surface normal of a thin-film change energy with thin-film thickness. The interaction of hydrogen Rydberg atoms with iron thin films deposited on an insulating substrate is investigated. The preference for electron penetration along the surface normal is seen by the resonance-enhancement of charge transfer at energies where the Rydberg state and well-image state are degenerate. By changing the thickness of the thin film, by in situ depositions, the energies of the well-image state are altered and the Rydberg n-values at which resonances occur, change. At a thickness of 30-monolayer the energetic spacings between the well-image states and the Rydberg states become comparable, and the single well-image state resolution is lost. A wavepacket-propagation study investigates the interaction of a nanoparticle and low-n hydrogen Rydberg atoms. The nanoparticle has a fully confined potential which at small radii yields well-spaced, fully discrete well-image states. Resonance-enhanced charge transfer occurs when the Rydberg state and the nanoparticle well-image state energy levels are degenerate. However, when there is poor energy matching between the nanoparticle well-image state and the Rydberg atom, no charge transfer is seen i.e. surface ionisation does not occur. Overall, the work presented here demonstrates the capability of Rydberg-surface studies to identify discrete, high-lying energy levels at specific surfaces
Cold reactive collisions between neutral molecules and trapped ions
Full text linkThis thesis describes the development and demonstration of techniques for the study of
ion-neutral reactive collisions at low temperatures conducted using two unique experimental
apparatuses comprising radio-frequency Paul traps. One apparatus combines
a Stark decelerator with a Paul trap; the other apparatus features a buffer gas cell
interfaced with a Paul trap through an electrostatic quadrupole guide.
A novel ion trap time-of-flight mass spectrometer has been introduced and characterised.
This apparatus is designed to detect the ion products of reactive collisions
at low temperatures (< 10 K). This detection approach affords excellent detection
efficiency and is broadly applicable to all ion-molecule reactions that one could study
within a linear Paul ion trap. A proof-of-principle reaction between xenon ions and
neutral ammonia isotopologues is examined to test the performance of the new detection
set-up.
With the goal of achieving full control over the reaction parameters, the amount of
alignment retained by rotationally and translationally cold ammonia molecules exiting
the electrostatic guide has been examined using (2 + 1) REMPI detection of the
molecules with varying linear polarisation. Simulations suggests how modifications
to the current setup will enable full orientation of the cooled molecules reaching the
detection region to be achieved.
Experiments on cold charge exchange reactions between sympathetically-cooled Xe+
and room temperature ammonia (NH3 and ND3) have been conducted within lasercooled
Ca+ Coulomb crystals in a linear Paul ion trap. There is ongoing work to
ascertain any difference in reactivity between NH3 and ND3 as the neutral reactant
species. Preliminary data are discussed at a qualitative level, with the prospect of
observing any isotope effect in this charge transfer process. Charge exchange reaction
experiments using cold state-selected ammonia are discussed with the goal of
establishing reaction rate constants at different collision energies.</p
Interaction of Rydberg hydrogen atoms with metal surfaces
Full text linkThis thesis presents a theoretical and experimental investigation of the interaction of electronically excited Rydberg hydrogen atoms with metal surfaces and the associated charge-transfer process. As a Rydberg atom approaches a metal surface, the energies of the Rydberg states are perturbed by the surface potential generated by the image charges of the Rydberg electron and core. At small atom-surface separations, the Rydberg atom may be ionised by resonant charge transfer of the Rydberg electron to the continuum of delocalised unoccupied metal states, with which the Rydberg electron is degenerate in energy. Typically, this ‘surface ionisation’ can be measured by extracting the remaining positively charged ion-cores with externally applied electric fields. By applying various levels of theory, from classical to fully time-dependent quantum calculations, this thesis explores various experimentally relevant effects on the charge-transfer process, such as the magnitude and direction of the externally applied electric field, the atom collisional velocity, the presence of local surface stray fields and electronically structured surfaces. The theoretical results give insight into the previous experimental work carried out for the xenon atom and hydrogen molecule, and point out some of the fundamental differences from the hydrogen atom system.Experiments involving Rydberg hydrogen atoms incident on an atomically flat gold surface, a rough machined aluminium surface and a single crystal copper (100) surface are presented, providing for the first time the opportunity to make a quantitative comparison of theory and experiments. The ability to control the critical distance at which charge-transfer occurs is demonstrated by using Rydberg states of varying dimensions and collisional velocities. By changing the collisional angle of the incident Rydberg beam, the effect of Rydberg trajectory is also investigated. By manipulating the polarisation of the Rydberg electron with electric fields, genuine control over the orientation of the electron density distribution in the charge-transfer process is demonstrated. This property was predicted by the theory and should be unique to the hydrogen atom due to its intrinsic symmetry. By reversing the direction of the electric field with respect to the metal surface, electrons rather than positive ions are detected, with ionisation dynamics that appear to be very different, as predicted by quantum calculations. Experiments involving the single crystal Cu(100) surface also suggests possible resonance effects from image states embedded in the projected bandgap which are shown from quantum calculations to play an important role in the surface charge transfer of electronically structured metal substrates.The experimental technique developed in this work provides some exciting future applications to study quantum confinement effects with thin films, nanoparticles and other bandgap surfaces. The ability to control the Rydberg orbital size, electronic energy, collisional velocity and orientation in the charge-transfer process will provide novel ways of probing the surface’s electronic and physical structure, as well as being a valuable feature in offering new opportunities for controlling reactive processes at metallic surfaces
Digital ion trap mass spectrometry for cold ion-molecule chemistry
No full textA promising new approach for studying cold ion-molecule chemical reactions is the combination of laser- or sympathetically-cooled trapped ions and slow-moving molecules from a cold molecule source, such as a quadrupole velocity selector or a Stark decelerator. Previous reaction studies using trapped atomic ions and slow molecules from a quadrupole velocity selector were able to reach average collision energies as low as 1 K. However, the guided molecules had an approximately room temperature rotational energy distribution, so the reactions studied were not truly cold. Thus, a new molecular source for producing translationally and rotationally cold molecules utilizing buffer gas cooling and quadrupole velocity selection was constructed by K. Twyman and characterized for use in cold reaction studies. This new source of cold molecules is referred to as the buffer gas guide. A new ion trap has been designed and built for use with the existing buffer gas guide. The new ion trap apparatus is compact and mechanically compatible with this new guide. It uses a linear Paul ion trap with cylindrical electrodes to trap ions. Two optical axes (one axial and one radial) enable efficient cooling of small ion crystals. A field-free time-of-flight tube and ion detection assembly are also incorporated into the apparatus. A new technique for determining the mass and quantity of trapped ions has also been developed, termed digital ion trap mass spectrometry. The new technique uses a digital RF waveform to trap ions before ejecting the ions radially from the trap using an ejection pulse applied to the trap electrodes. The ions are then detected after free flight along a time-of-flight tube. This technique was characterized by ejecting crystals of various sizes and compositions: Ca+ only, Ca+/CaF +, Ca+/CaOH +/CaOD+, and Ca+/NH +3 /NH +4 /H3O+. A linear relationship between the number of ions ejected (determined by comparing experimental and simulated crystal images) and the integral of the time-of-flight peak was observed for Ca+ and Ca+/CaF +. All mass peaks were resolved. Simulations of the trapped ions and their trajectories through the time-of-flight tube were also performed, and excellent agreement between the simulated and experimental mass resolution was observed. Progress towards combining the buffer gas guide with the previously independent ion trap is also presented. It is anticipated that the combined buffer gas guide ion trap apparatus will enable the study of ion-molecule reactions at low temperatures with translationally and rotationally cold molecules. It is anticipated that the new digital ion trap mass spectrometry technique will simplify the study of reactions when multiple product ions whose masses are separated by only 1 AMU are formed. A new ion trap has been designed and built for use with the existing buffer gas guide. The new ion trap apparatus is compact and mechanically compatible with this new guide. It uses a linear Paul ion trap with cylindrical electrodes to trap ions. Two optical axes (one axial and one radial) enable efficient cooling of small ion crystals. A field-free time-of-flight tube and ion detection assembly are also incorporated into the apparatus. A new technique for determining the mass and quantity of trapped ions has also been developed, termed digital ion trap mass spectrometry. The new technique uses a digital RF waveform to trap ions before ejecting the ions radially from the trap using an ejection pulse applied to the trap electrodes. The ions are then detected after free flight along a time-of-flight tube. This technique was characterized by ejecting crystals of various sizes and compositions: Ca+ only, Ca+/CaF+, Ca+/CaOH+/CaOD+, and Ca+/NH+3/NH+4/H3O+. A linear relationship between the number of ions ejected (determined by comparing experimental and simulated crystal images) and the integral of the time-of-flight peak was observed for Ca+ and Ca+/CaF+. All mass peaks were resolved. Simulations of the trapped ions and their trajectories through the time-of-flight tube were also performed, and excellent agreement between the simulated and experimental mass resolution was observed. Progress towards combining the buffer gas guide with the previously independent ion trap is also presented. It is anticipated that the combined buffer gas guide ion trap apparatus will enable the study of ion-molecule reactions at low temperatures with translationally and rotationally cold molecules. It is anticipated that the new digital ion trap mass spectrometry technique will simplify the study of reactions when multiple product ions whose masses are separated by only 1 AMU are formed
Electrostatic extraction of buffer-gas-cooled beams for studying ion-molecule chemistry at low temperatures
Full text linkThis 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Strong inverse kinetic isotope effect observed in ammonia charge exchange reactions
No full textAccompanying data for publication in Nature Communication
Interaction of molecular Rydberg states with metal surfaces
Full text linkThe interaction between high-n Rydberg states of molecular hydrogen and metal surfaces has been investigated for the first time. Rydberg states of hydrogen possessing either 0 or 2 units of rotational angular momentum, defined by the quantum number N+ , and principal quantum numbers in the range n= 17 22 (for the N+= 2 states) and n=41-45 (for the N+= 0 states) are directed at a grazing angle onto a metal surface (gold or aluminium). At a sufficiently close distance ionisation may occur via tunnelling of the Rydberg electron into the vacant metal conduction band. Any ions formed in the vicinity of the metal are extracted by the application of an electric field and information about the distance at which the ions are formed can be inferred from the magnitude of the applied field required for detection. Two novel effects are observed. Firstly, it appears that the rotation of the H2+ core has a significant effect on the ionisation properties of the Rydberg states in a manner akin to rotational autoionisation, such that the rotational energy of the core is given up to the Rydberg electron. Secondly, the surface ionisation profiles do not vary smoothly with applied field suggesting that at certain fields the feasibility of ionisation is either enhanced or reduced. A preliminary discussion of the origin of the structure is presented in terms of the crossings in the Stark map between the N+= 0 and N+= 2 Stark manifolds. The development of a theoretical model, and an associated Fortran program, involving the technique of complex scaling is also reported. The hydrogen molecules are modeled using an atomic hydrogen system which provides a good first approximation to the behaviour of the Rydberg electron for states with n > 5. Energies and linewidths, for states with principal quantum number n= 6 9 interacting with a model surface, are explicitly calculated at a range of surface separations. From this information, predictions of the ionisation behaviour expected for states of higher principal quantum number are presented
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