No description available.

No description available.

No description available.

This thesis reports quasi steady state photoinduced absorption measurements from three conjugated polymers: polypyridine (PPy), polyfluorene (PFO) and the emeraldine base (EB) form of polyaniline. The aim of these experiments was to determine the nature of the photoexcited states existing in these materials in the millisecond time domain, as this has important consequences for the operation of real devices manufactured using these materials. The results from the photoinduced absorption experiments are closely compared with published results from pulse radiolysis experiments. In all cases there is very good correspondence between the two data sets, which has enabled the photoexcited states to be assigned with a high degree of confidence. Quasi steady-state photoinduced absorption involves the measurement of the change in absorption of a material in response to optical excitation with a laser beam. The changes in absorption are small, so a dedicated instrument was developed and optimised for each different sample. Lock-in techniques were used to recover the small signals from the samples. The samples involved were thin films of the polymer spin coated onto sapphire substrates in the cases of PPy and EB. Solution state experiments were conducted on EB. The experiments on PFO were conducted on aligned and unaligned thin films provided by Sony. In the case of the aligned PFO samples, the photoinduced absorption spectrometer was modified to enable polarisation-sensitive data collection. In PPy, both triplet excitons and polarons have been shown to be long-lived photoexcitations, with photoinduced absorption features at 2.29 eV (triplet excitontransition), 1.5 eV and 0.8 eV (polaron transitions). In PFO, the one observed photoinduced band at 1.52 eV is assigned to a triplet exciton. Two photoinduced absorption bands are observed in EB, at 1.4 eV and 0.8 eV. These are assigned to aself-trapped CT singlet exciton and triplet exciton, respectively.

Magic-angle spinning NMR has been used for the first time to investigate two new classes of molecular sieve-type materials. Firstly, various substituted high-silica (ZSM-5-type) molecular sieves have been studied. The substituting elements (B, Fe, Cr, Ti, La.) are (with the exception of boron) unobservable by NMR and so the effects of their substitution are observed indirectly by (^29)Si NMR. Calcination (thermal treatment in air) affects the (^29)Si NMR spectra of several of the samples. Substitution of paramagnetic elements (Fe, Cr) affects the (^29)Si NMR spectra causing substantial line broadening and the appearance of spinning sidebands. The use of other spectroscopic methods of investigation combined with NMR measurements yields a considerable amount of information concerning the environment(s) of the substituting elements. Secondly, several members of a new class of crystalline microporous molecular sieves based on three-dimensional frameworks of phosphorus and aluminium have been studied. Heteroatom substitution (Si, B) into molecular sieves possessing novel and zeolite-related structures has been investigated and the general mode of silicon substitution has been elucidated. Again, calcination produces significant changes in the NMR spectra of these samples. Ordering of the aluminium and phosphorus framework tetrahedra is normally one of direct alternation in which case unique resonances are observed in the (^27)Al and (^31)P NMR spectra. For several framework structures, crystallographically in equivalent sites exist, and (^31)P NMR (also (^29)Si NMR) is particularly sensitive to this phenomenon. (^31)P CP NMR provides information on the interaction of the organic template with the framework and on the possible sources of proton reservoir necessary for the CP experiment.

Many attempts have been made to reduce fluorescence backgrounds in Raman spectra. A critical appraisal of fluoresence rejection techniques reveals that while many techniques are available which improve the Raman/fluorescence ratio (R/F), very few actually increase the spectral signal/noise (R/N), which is the most important parameter. Temporal-resolution of Raman and fluorescence photons was investigated in this laboratory, using a picosecond-laser system and gated photon detection. Two detection methods were evaluated. The first, an intensified diode array detector (DAD), could be gated "on" for periods of ca. 5 ns, at rates of up to 5kHz. This gave a 5-fold increase in R/F, but a slight reduction in R/N, for a fluorescor with τ(_f) ̴̱ 1O.5 ns. The R/N degradation arose as a result of the low laser output intensity at kHz pulse rates, rather than inefficiency in fluorescence rejection. The second method used a continuously-operated photomultiplie tube (PMT), and time-correlated photon counting with ca. 1 ns timing-resolution. This yielded R/F and R/N improvements of ca. 15 and 3 respectively (τ(_f) ̴̱ 12 ns).Although efficient fluorescence rejection was obtained with each system, the corresponding R/N enhancements were not practically significant. However, the development of theoretical models describing the performance of each system has identified modifications which should give valuable improvements. These include the use of a laser with MW peak powers at kHz pulse rates (DAD system), and use of a microchannel-plate PMT with 50 ps timing resolution. When these (and other) modifications are made, significant R/N enhancements (ca. 7 and 13 (DAD and PMT systems respectively)) are expected, thus enabling the study of the majority of "real world" samples. In addition, the limiting theoretical and practical performance of time-resolved rejection is considered, and several hitherto unreported aspects of the behaviour of the laser and detection systems are discussed. Other techniques were also evaluated, in particular utilising the differing Raman and fluorescence response to variations in laser intensity. While the non-linear fluorescence responseto intensity variations of cw lasers has been previously exploited, simple calculations indicate that the use of high-powered pulsed sources could allow discrimination at ca. 100-fold lower average powers. However, a satisfactory test of the calculations requires the construction of apparatus not presently available in this laboratory.

Inelastic incoherent neutron scattering (INS) spectra of hydrogen strongly chemisorbed at coverages of greater than one monolayer on Pd black (Drijfhout, Amsterdam) were obtained (77K) without interference from adsorbed or type-C adsorbed hydrogen. The spectra were assigned by comparison with the vibrational spectra of transition-metal hydridocarbonyl compounds and indicated the occupation by H of two-fold bridging and terminal sites, the relative population of the terminal sites increasing at higher hydrogen coverage. INS spectra (ca 20 to 850 cm(^-1); 10, 16 and 77K) of c-C(_3)H(_6) adsorbed in Co(_4)Na(_4)Al(_12)Si(_12)O(_48), and of c-C(_3)H(_6) and c-C(_3)D(_6) in Mn(_3.5)Na(_5)Al(_12)Si(_12)O(_48) zeolite were consistent with C(_3v) symmetry of the M(II)-cyclopropane sorption complexes (M=Co or Mn). INS spectra (320 to 2230 cm(^-1); 77K) of Kadox 25 ZnO and H(_2) adsorbed on ZnO were obtained. The ѵ(_s) and ѵ(_δ) modes of type I Zn-H surface species were observed at 829 and 1708 cm(^-1). Recent assignments in the literature of the ѵ(_δ) mode of type I O-H species and the ѵ(_as) mode of type II Zn-H-Zn species to weak IR bands at 850 and 1475 cm(^-1) respectively were not corroborated by the INS results. The INS spectrum of ZnO contained an intense feature at 1346 cm(^-1) which was unchanged on adsorption, and assigned to bulk impurities. Raman and IR data of ZnO were also discussed. INS spectra (ca 20 to 850 cm(^-1); 10 and 77K) of C(_2)H(_2) and C(_2)D(_2) adsorbed in partially Ni(^++) ion-exchanged zeolite-Y indicated that the admolecules were non-linear (C(_2v) symmetry) and laterally bonded to the surface.. Bands centred at 774, 684, 546 and 75 cm(^-1) in the INS spectra (ca 20 to 850 cm(^-1); 10 and 77K) of H(_2)O adsorbed in Ca(_6)Al(_12)Si(_12)O(_48) zeolite at coverages of one H(_2)O molecule per Ca"*"*" ion or less were assigned to the wagging, twisting and rocking modes and to the doubly degenerate hindered translations parallel to the surface, respectively of H2O adsorbed at Ca"*"^ sites.INS spectra (lOK) of adsorbed by Zn^ ^Na^ 0^12 5^^11 5^48 ^^^^^^ showed bands at 72 and 155 cm""'", assigned to the tunnelling transitions E^^Eg and E^-^B respectively between the sublevels of the ground torsicxial state of a rigid H2 rotor, coifined to a plane parallel to the adsorbed surface, in a six-fold potential well of barrier height 29±3 kJ mol""'".

The aim of this thesis is to calculate the bound state energies of molecular dimers. The problem is formulated for a system consisting of any two diatomic molecules, treated as rigid rotors. Simplifications which arise from symmetry considerations are fully discussed. The de Vogelaere and R-matrix propagator algorithms have been used to solve the resulting systems of coupled second order differential equations. Their numerical convergence properties are compared in test calculations on the Ar-HCl system. The above methods are used to calculate the bound state energies of H(_2)-H(_2), using four separate ab initio potentials. The CI potential of Meyer, Schaefer and Liu (designated "M80") is found to give the best agreement with spectroscopic measurements, though a small shift in the position of the repulsive wall is indicated. The M80 potential is then used in the remaining calculations; these include the evaluation of the energies of resonances and bound states lying above the dissociation limit of the dimer, corresponding to rotationally excited H (_2). The results of these calculations are used to assess the validity of approximations made in the proposed identification of H (_2)-H (_2) features in the far infrared spectra of the Jovian atmosphere. The Born-Oppenheimer approximation permits the use of the M80 potential to calculate the bound states and resonances of D (_2) -D (_2). That some of these resonances have dual Feshbach/shape character is noted. The dimer structure, accompanying the observed near infrared S (_1)(0) and Q(_1)(0) + S(_o)(0) spectra in ortho-deuterium, is modelled by treating the two D(_2) molecules as distinguishable rigid rotors. We conclude that the experiments provide evidence both for rotational splitting of the levels and for internal rotational predissociation. Alternative line assignments to those hitherto made are also suggested. We end with a general discussion in which suggestions for future work are made.

No description available.

No description available.