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Conventional ion imaging techniques utilized grid electrodes to extract and to accelerate ions toward the detector. The disadvantages of grid electrodes caused transmission reduction, severe image distortions and image blur due to the non-point source geometry. All these problems can be solved by the utilization of an open lens electrode assembly. In velocity mapping, the extracting electric field of an open electrostatic lens that projects the ion cloud onto the detector. The major advantage of the combination of ion lens optics and two-dimensional detection is that ions from different positions with the same initial velocity vector would be mapped onto the same position on the detector, which was named ¡§ velocity map imaging .¡¨ The kinetic energy resolutions achievable with this method are not generally considered as being competitive with the best photofragment translational spectroscopy technique. But Ashfold and co-workers have demonstrated that velocity imaging methods can provide dissociation energy with one wavenumber resolution, i.e., it compares favourably with all rival photofragment translational spectroscopy techniques. We construct an ion imaging apparatus and the pressure inside can be maintained at ~ 10-6 Torr with differential pumping when the pulsed nozzle is shut off. The pressure in the source chamber raises from 2.1¡Ñ10-6 to 1.0¡Ñ10-5 Torr and the pressure in the photolysis chamber raises from 2.4¡Ñ10-7 to 3.6¡Ñ10-7 Torr, when the pulsed nozzle is turned on with a stagnation pressure at 3 bar. Because reactive chemicals attack the piezo disk translator and ruin the Viton O-ring, a modified pulsed nozzle and Teflon O-ring are adopted to overcome these problems. This pulsed nozzle is mounted on a three-dimensional translational stage such that the nozzle can be aligned inside vacuum. The homogeneity of the accelerating electric field is crucial to the performance of the ion imaging apparatus. To meet this requirement, parallel electrodes of identical dimensions have been assured in the manufacture of the ion lens assembly.

The focus of this dissertation is the study of the photodissociation dynamics of halogen oxide species (XO, X = Cl, Br, I). These radical species are known to be important in stratospheric and tropospheric ozone depletion cycles. They are also useful benchmark systems for the comparison to current theoretical methods where they provide insight into the dynamics occurring beyond the Franck-Condon region. These systems are studied using velocity map ion imaging, a technique that measures velocity and angular information simultaneously. Photofragment species are state-selectively ionized for detection using 2+1 REMPI (Resonance Enhanced Multi-Photon Ionization). The instrumentation employs a molecular beam of the XO radicals formed using pyrolitic and photolytic methods. The current work involves the measurement of fundamental physical constants of the XO species. The bond dissociation energy of IO is measured. Vibrational level dependent correlated final state branching ratios of the predissociation of the A(^2 II_3/2) state of ClO and BrO are reported, and comparison to theoretical methods is discussed.

Rotational polarisation effects have been investigated in the rotationally inelastic collisions of NO(X) and Ar by means of theoretical and experimental methods. Rotational polarisation describes the correlation between the <strong>k</strong>–<strong>k'</strong>–<strong>j'</strong> vectors, that is the initial and final relative velocities of the colliding partners and the final rotational angular momentum of the diatom, respectively. The simplest types of polarisation are the rotational orientation, or preferred sense of rotation, and the rotational alignment, or preferred plane of rotation. They are quantised by the renormalised polarisation dependent differential cross sections (PDDCSs) In this thesis the theoretical methods included exact quantum mechanical, quasi- classical trajectory and Monte Carlo classical hard shell calculations. Various features of the interaction potential influence differently the polarisation dynamics. The effects of attraction and soft repulsion were elucidated employing a number of differently modified potentials. The rotational alignment is primarily determined by a classical impulsive, or hard shell mechanism at a collision energy of 66 meV. The attractive and soft repulsive forces only perturb this underlying mechanism. On the other hand, the parity dependent oscillations of the open shell alignment moments are due to differences between the quantum mechanical differential cross sections. It has been shown the bigger the well depth compared to the collision energy, the less applicable becomes the classical hard shell model to describe rotational alignment. The quantum mechanical rotational alignment in the collisions of hard shells was also calculated. The classical and quantum mechanical hard shell models predict different rotational alignment. Nevertheless, the classical alignment is a good approximation to the exact quantum mechanical results. The rotational orientation is much more sensitive to the details of the interaction potential. It does not exist in the classical description of hard shell collisions, if the system exhibits certain symmetry properties. The attraction and finite range repulsion break this symmetry and leads to the molecule having a preferred sense of rotation. In general there is non-vanishing rotational orientation in the collisions of a hard shell in the framework of quantum mechanics. This is due to the finite spatial and temporal interaction of the colliding partners. Quantum mechanical interference effects also play an important role in this phenomenon. The rotational alignment was experimentally determined in the collisions of NO(X) and Ar at collision energy of 66meV with a hexapole state selective ion-imaging apparatus. An algorithm was developed based on the Fourier moment analysis to extract rotational polarisation information from the experimental ion images. It is fast and robust and can also be of used to simulate experimental images. This algorithm was used to retrieve the experimental renormalised PDDCSs ion images. The measurements confirmed that a classical, impulsive dynamics is mainly responsible for the rotational alignment in these collisions.

A new and flexible velocity-map ion imaging apparatus, designed for the study of photodissociation processes and photon-initiated bimolecular reactions in a single molecular beam, has been constructed, developed and characterised. An image Legendre moment fitting analysis was developed to allow recovery of centre-of-mass (CM) angular scattering and kinetic energy release distributions from velocity-map ion images of the products of photon-initiated bimolecular reactions. The Legendre moment analysis methodology has been applied to images of the HCl(v' = 0,j' = 0-6) products of the reactions of Cl(²P_3/2) atoms with ethane and n-butane at collision energies of 0.24 eV and 0.32 eV respectively. The Cl(²P_3/2) reactants were generated by polarised laser photodissociation of Cl₂ at 355 nm. For reaction with ethane, the CM angular scattering distributions show a steady trend from forward scattering at low j' to more isotropic, but backward peaking, scattering at high j'. An impact parameter-based mechanism is proposed to account for the observed dynamics. Abstraction of a hydrogen atom from a primary carbon site in n-butane is seen to produce rotationally very cold HCl products that are forward scattered, whereas H atom abstraction from a secondary carbon site in n-butane yields more isotropically scattered HCl products formed with higher rotational excitation. A peripheral mechanism is proposed to operate for the primary abstraction channel, whilst a more rebound type mechanism is seen to account for the dynamics of the secondary abstraction channel. Around 22% and 30% of the available energy is found in internal modes of the alkyl radical co-products of the Cl + C₂H₆ and Cl + n-C₄H₁₀ reactions respectively. Possible sources of alkyl co-product excitation are discussed in each case. The hydrogen or deuterium atom abstraction reactions of Cl(²P_3/2) with CH₄, CD₄ and CH₃D, have been studied at mean collision energies around 0.3 eV. Chlorine atom reactants were generated by polarised laser photodissociation of Cl₂ at 308 nm. The methyl radical products were detected using (2+1) resonance-enhanced multi-photon ionisation, coupled with velocity-map ion imaging. The laboratory frame speed distributions obtained from the images are in excellent agreement with previous work. The interpretation of the experiments is shown to be very sensitive to assumptions made about the reactant velocity distributions. If these are assumed to be narrow, the data are seen to suggest that a significant fraction of the product signal must arise from the reaction of Cl with vibrationally excited methane reactants. This conclusion is in agreement with previous photon-initiated reaction studies. However, by allowing for the spread in collision energies in the molecular beam, it is shown that it is possible to fit the data sensibly assuming reaction with vibrational ground state methane alone. CM angular scattering distributions thereby derived are presented for all three reactions.

Title: Implementation of ion imaging technique in experiments with free molecules, clusters and nanoparticles Author: Jaroslav Kočišek Department: Department of Surface and Plasma Science Supervisor: Mgr. Michal Fárník, PhD., DSc., Ústav fyzikální chemie J. Hey- rovského, AV ČR, v.v.i. Abstract: The experimental work is focused on implementation of novel tech- niques to study clusters and nanoparticles in molecular beams. A new exper- imental system was tested, which combines the technique of velocity map ion imaging with pulsed molecular beam source. The same method and new mass spectrometer were implemented on the apparatus with molecular beam of size selected clusters. The new methods were used to study environmental effects on photo and electron induced chemistry. The most important results concerns on influence of expansion conditions on the structure of formed neutral clusters of HBr and C2H2. Results of experiments with HNO3 and CF2Cl2 molecules are then crucial for understanding heterogeneous processes in the Stratosphere. Keywords: ion imaging, mass spectrometry, molecular beams, photochemistry, nanoparticles

This dissertation focuses on developing new methods using the scanning electrochemical microscope (SECM) to produce chemical concentration maps of different chemical species on various surfaces. Reactive oxygen species (ROS) and transition metal ion maps were generated, indicating the presence or absence of relative types of chemical species on the surface. Imaging of both species was based on a modified scanning UME tip and monitoring the change in the tip impedance. 4-Nitrobenzenediazonium tetrafluoroborate was used as the main modifier, and resultant nitrophenyl groups on the modified electrodes were electrochemically converted to aniline to yield the two types of modified electrodes. In the presence of ROS, a permanent change in the impedance accompanies reaction of the surface layer with the ROS, and this change can be used to map the localized reactive species. The spot scanning method was introduced over continuous scanning to enhance the sensitivity. This enhanced method generated a more effective method to map ROS compared to the diAC/dxmajor image in the continuous scanning method. Images obtained by this sacrificial method show that alternating current SECM (AC-SECM) can be used to map ROS on a surface. The capacitive change gives direct indication of the concentration of these highly reactive species. Transition metal ions showed a partially reversible adsorption with aniline-modified electrodes. Localized concentrations of buffered copper and nickel divalent cations were generated by pumping through a micro-capillary embedded in a substrate. Copper and nickel ions on these substrates were mapped successfully. A solution of calcium ions was used as the negative control. Biased nickel, copper, and lead wire-embedded substrates were line scanned to validate these results. An aniline-modified electrode was placed away from the metal wire and the time taken for metal ions to reach the electrode tip was measured after a voltage pulse. These data were compared with calculated diffusion times. Both systems were optimized using the medium pH, scan rates, and tip potentials. AC-SECM coupled with modified electrodes showed the capability of mapping both ROS and some transition metal ions semi-quantitatively.

During my master thesis I became familiar with ion imaging and velocity map imaging (VMI) techniques in experiments with molecular beam. My major contribution was writing a computer program for data acquisition and pre-analysis. The program features initial filtering of the data, improving significantly signal to noise ratio. Non-trivial effort was required to make the program compatible with three different CCD cameras implemented on different experiments in the laboratory. I have also simulated ion trajectories in VMI and wrote a program code to imulate VMI image distortions due to imperfections in apparatus geometry. These programs will be useful in aligning and tuning the apparatus. Finally, I wrote a program for data manipulation and conversion to formats compatible with various data analysis programs available in the laboratory. All the above programs I have tested by measurements of 243 nm photodissociation of HBr molecules.

Few experimental techniques have found such a diverse range of applications as has ion imaging. The field of chemical dynamics is constantly advancing, and new applications of ion imaging are being realised with increasing frequency. This thesis is concerned with the application of a fast pixelated imaging sensor, the Pixel Imaging Mass Spectrometry (PImMS) camera, to ion imaging applications. The experimental possibilities of such a marriage are exceptionally broad in scope, and this thesis is concerned with the development of a selection of velocity-map imaging applications within the field of photoinduced molecular dynamics. The capabilities of the PImMS camera in three-dimensional and slice imaging applications are investigated, in which the product fragment Newton-sphere is temporally stretched along the time-of-flight axis, and time-resolved slices through the product fragment distribution are acquired. Through experimental results following the photodissociation of ethyl iodide (CH₃CH₂I) at around 230 nm, the PImMS camera is demonstrated to be capable of recording well-resolved time slices through the product fragment Newton-sphere in a single experiment, without the requirement to time-gate the acquisition. The various multi-hit capabilities of the device represent a unique and significant advantage over alternative technologies. The details of a new experiment that allows the simultaneous imaging of both photoelectrons and photoions on a single detector for each experimental acquisition cycle using pulsed ion extraction are presented. It is demonstrated that it is possible to maintain a high velocity resolution using this approach through the simultaneous imaging of the photoelectrons and photoions that result from the (3 + 2) resonantly enhanced multi-photon ionisation of Br atoms produced following the photodissociation of Br₂ at 446.41 nm. Pulsed ion extraction represents a substantial simplification in experimental design over conventional photoelectron-photoion coincidence (PEPICO) imaging spectrometers and is an important step towards performing coincidence experiments using a conventional ion imaging apparatus coupled with a fast imaging detector. The performance of the PImMS camera in this application is investigated, and a new method for the determination of the photofragment detection efficiencies based on a statistical fitting of the coincident photoelectron and photoion data is presented. The PImMS camera is applied to laser-induced Coulomb explosion imaging (CEI) of an axially chiral substituted biphenyl molecule. The multi-hit capabilities of the device allow the concurrent detection of individual 2D momentum images of all ionic fragments resulting from the Coulomb explosion of multiple molecules in each acquisition cycle. Correlations between the recoil directions of the fragment ions are determined through a covariance analysis. In combination with the ability to align the molecules in space prior to the Coulomb explosion event, the experimental results demonstrate that it is possible to extract extensive information pertaining to the parent molecular structure and fragmentation dynamics following strong field ionisation. Preliminary simulations of the Coulomb explosion dynamics suggest that such an approach may hold promise for determining elements of molecular structure on a femtosecond timescale, bringing the concept of the `molecular movie' closer to realisation. Finally, the PImMS camera is applied to the imaging of laser-induced torsional motion of axially chiral biphenyl molecules through femtosecond Coulomb explosion imaging. The target molecules are initially aligned in space using a nanosecond laser pulse, and torsional motion induced using a femtosecond 'kick' pulse. Instantaneous measurements of the dihedral angle of the molecules are inferred from the correlated F+ and Br+ ion trajectories following photoinitiated Coulomb explosion at various time delays after the initial kick pulse. The technique is extended to include a second kick pulse, in order to achieve either an increase in the amplitude of the oscillations or to damp the motion, representing a substantial degree of control of the system. Measurements out to long kick-probe delays (200 ps) reveal that the initially prepared torsional wave packet periodically dephases and rephases, in accordance with the predictions of recent theoretical work.