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No description available.

No description available.

No description available.

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No description available.

This thesis describes the analysis of dijet production in photoproduction and DIS, using the ZEUS detector at HERA. At ZEUS, positrons and protons were collided with a centre of mass energy of /i = 300 GeV during the 1996-1997 running period and y/s = 318 GeV thereafter. Dijet production is studied for the all-flavours case and for the case when a charm quark is demanded. In the all-flavours analysis, the dependence of dijet production on the virtuality of the exchanged photon, Q2, was studied by measuring dijet cross sections in the range 0.0 < Q2 < 2000 GeV2, using data from the 1996-1997 running period corresponding to an integrated luminosity of 38.6 pb_1. Dijet cross sections were measured in the photon-proton centre-of-mass frame for jets with transverse energy E1 > 7.5 and 6.5 GeV and pseudorapidity in the range 3.0 < rfet < 0.0. The variable x BS, a measure of the fractional photon momentum entering the hard process, was used to enhance the sensitivity of the measurements to the photon structure. The Q2 dependence of the ratio of low to high x BS events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low x BS contribution relative to that of high x BS. Leading-logarithmic parton-shower Monte-Carlo predictions which use a partonic structure for the photon, falling smoothly with increasing Q2 provide a qualitative description of the data. The all-flavours dijet analysis was repeated with the demand that a D*(2010) meson was identified in the decay channel D* -> D0tts > (Kit)its with pr{D*) > 3.0 GeV and t7(D*) < 1.5. The charm data were collected from the running period 1996-2000, corresponding to an integrated luminosity of 120.4 pb-1, and cover the range of photon virtualities 0.0 < Q2 < 5000 GeV2. The measurements were compared to next-to-leading-order QCD predictions which were generally found to give a good description of the data. Leading order pQCD models with parton shower simulation which include a parameterisation of the virtual photon were found to give a good description of the DIS data. Such models were found to be consistent with the photoproduction data only when the non-perturbative vector meson like component of the resolved photon is removed. When no parameterisation of the hadronic component of the virtual photon was used DGLAP evolution was found to lie below the data, and CCFM evolution generally closer to the data.

A new technique to investigate the phenomenon of two-photon emission in atomic nuclei has been developed and an experiment performed at the Chalmers University of Technology, Gothenburg. The long-lived first excited state in 7²GE was populated by the inelastic scattering of a pulsed neutron flux.

In the present thesis clusters of nucleic acid bases and water are used as model systems of cellular DNA to investigate UV induced radiation damage in the gas phase. A new experimental system has been designed and commissioned to perform rnultiphoton ionization experiments on hydrogen bonded clusters of the nucleic acid bases: adenine, thymine and uracil, as well as on the related chromciphores: 5-fluorouracil and hypoxanthine. As the result of pulsed nanosecond laser irradiation in the wavelength range of 220-230 nm. cluster, monomer and fragment ions were detected using a time-of-flight mass spectrometer. Possible multiphoton ionization pathways of clustered and isolated molecules including the role played by short (singlet) and long-lived (triplet) excited states as well as excited state tautomeric transitions are discussed. Signal intensities as a function of laser pulse fluence were measured. The slope of a logarithmic plot of the signal intensity versus fluence yields the so-called 'photon order' and provides information on the number of photons leading to the production of cluster, monomer and fragment ions. The results indicate two-photon ionization for uracil and thymine in dry molecular beams (i.e. photon orders equal to 1.9±0.2 and 1.4±O.1, respectively). An interesting result in the form of photon order equal to 3.0±0.5 has been measured for adenine in a dry molecular beam irradiated with an unfocused laser beam. This suggests a three photon ionization process, however further experimental work is required in order to exclude factors related to stability of the laser beam spatial profile. Hydrated cluster ions with up to 7 water molecules attached to a single uracil and 4 water molecules attached to adenine were unambiguously identified after adding water vapour to the molecular beam source. A maximum of 7 water molecules attached to adenine-uracil base pairs were detected. Possible ionization pathways of such hydrated clusters mediated by excited states tautomeric transitions are discussed. On the basis of photon orders measured under hydrated conditions, it is proposed that sequential MPI processes with 2, 3, and 4 photon absorption lead to production of cluster ions, cluster fragment ions (including protonated monomers), and molecular fragment ions, respectively.