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We report on high dispersion spectroscopy of the Ha absorption line of the cool DA white dwarf G 29 -38. This is the star for which a recently detected infrared excess has been suggested to be due to a possible brown dwarf companion by Zuckerman and Becklin (1986, 1987). Three echelle spectra obtained at the Multiple Mirror Telescope and at the Kitt Peak Mayall 4m telescope in 1987 December show no evidence for radial velocity variations larger than -'1.1 ± 8.7 km s -1 and are used to derive a weighted heliocentric radial velocity Vr = 33.7 ± 4.3 km s -1 for the white dwarf. No emission component from the hypothesized secondary star is detected. These negative results do not constitute strong evidence against the companion hypothesis, since the expected orbital velocity of the white dwarf component could be quite small, and the companion's line emission could be too faint to be detected. However, the observation of a sharp absorption line core restricts the possible rotation of the white dwarf to < 40 km s -1 and ensures that any surface magnetic field has a strength < 105 gauss. These results make it unlikely that the DA white dwarf has previously been in a cataclysmic variable accretion phase.

A paucity of literature exists on the Independent System analysis of adjacent, parallel transition dipoles. Applying this theory and certain spectral information semiemperical calculations were made to predict absorption profiles and band intensities. To aid in the assignment of the 7*+7 absorption bands it was necessary to obtain the vacuum ultraviolet spectra of cyclohexane and cyclohexene. Because the spectra of these molecules contained sharp, atomic-like absorption bands a Rydberg series could be fitted to certain absorptions, thus the determination of their ionization potentials. Using Independent System analysis profiles and intensities of 7*+q-- absorption bands ins 'several methyl substituted 1,4- cyclohexadienes and isotetralin were predicted where general agreement was found with observed experimental spectra.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1979. / Includes bibliographical references. / by John Edward Pieszcynski. / Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1979.

In this thesis we investigate the large-scale distribution of Ly alpha forest absorption, the effect of ionizing radiation from QSOs on their surrounding intergalactic medium and the primordial abundance of deuterium. We develop a new technique for detecting structure on Mpc scales in the Ly alpha forest. This technique does not rely on identifying individual absorption lines but is rather based on the statistics of the transmitted flux. We demonstrate that the new method is significantly more sensitive to the presence of large-scale structure in the Ly alpha forest than a two-point correlation function analysis. We apply this method to 2 A resolution spectra of ten QSOs which cover the redshift range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are concentrated within a 1-deg^2 field. We find evidence for large-scale structure in the distribution of Ly alpha forest absorption at the > 99 per cent confidence level. Along the line of sight we find overdense Ly alpha absorption on scales of up to 1200 km s^-1. There is also strong evidence for correlated absorption across line of sight pairs separated by < 3 h^-1 Mpc. For larger separations the cross-correlation signal becomes progressively less significant. Using the same technique and dataset we confirm the existence of the proximity effect. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of J = (3.6^+3.5_-1.3) x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1. This value assumes that QSO redshifts measured from high ionization lines differ from the true systemic redshifts by Delta v = 800 km s^-1. Allowing for known QSO variability we find evidence at a level of 2.1 sigma that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. From the complete sample we find no evidence for the existence of a foreground proximity effect, implying either that J > 20 x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1 or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example where a foreground QSO apparently depletes the absorbing gas in four surrounding lines of sight. We discuss the feasibility of pre-selecting absorption systems from low resolution data for a measurement of the primordial deuterium abundance. We present a new, low resolution spectroscopic survey of 101 high redshift QSOs aimed at identifying candidate D/H systems. We further present an echelle spectrum of a Lyman limit system at z = 2.917. We find that this system is most likely heavily contaminated and does not yield an interesting limit on D/H.

Introduction An atom or ion in a solid is associated with a given lattice site. Translational motion can however occur in solids and such diffusion processes can be studied by nuclear magnetic resonance, tracer diffusion and other methods. In ionic compounds the diffusion is also a mechanism for charge transport and hence there is an associated ionic conductivity. [continued in text ...] NOTE: The Abstract contains many indistinct letters in mathematical formulas and it is impracticable to reproduce it here.

This thesis presents an investigation into the factors which control the gain and amplification properties in conjugated materials. Conjugated polymers and perylene dyes are highly fluorescent, are easy to process into thin films, and exhibit strong amplification over a broad gain bandwidth making them ideal for use in lasers and amplifiers. The stimulated emission created when thin films of the red emitting polymer poly(2-methoxy-5-(2'-ethylhexyloxy)- p-phenylenvinylene) (MEH-PPV) were photoexcited with high energy laser pulses was investigated. This was characterised by a dramatic narrowing of the emission spectrum which has been assigned to amplified spontaneous emission (ASE). The emission was found to have a gaussian profile and the gain coefficient was found to be 4 cm-1.The temperature dependence of the absorption, photoluminescence and ASE of films of MEH-PPV was measured. The effect of film morphology on the photophysical properties was investigated by using films cast from two spinning solvents, chlorobenzene (CB) and tetrahydrofuran (THE). Film morphology was found to greatly affect the temperature dependence. A particularly important property is the spectral position of the ASE and the factors which affect it. By controlling the film thickness close to the cut-off thickness for waveguiding in the polymer film it was shown that the peak position of the ASE could be tuned by 31 nm. Modelling of the waveguide modes in the polymer films was used to explain this effect. The cut-off wavelength for each film was measured and good agreement with the theory was found. In order to investigate ways in which energy transfer could be used to control the emission, two perylene dyes were used as a donor-acceptor pair in a host matrix of poly methymethacralate (PMMA). The position of the ASE was found to depend on the acceptor concentration. Measurements of the photoluminescence quantum yield and time-resolved luminescence measurements showed that the energy transfer coefficient was 5x10(^11)mol(^-1)d(^3)

The work presented in this thesis describes an investigation into the properties and behaviour of a new class of nonlinear optical organic chromophores. This study contributes to the optimisation of nonlinear optical molecules through an improved understanding of the relationships between the molecular nonlinear optical properties and the measured macroscopic quantities. A series of highly dipolar non-linear optical chromophores with absorption typically in the range of 350-500 nm have been synthesised by the reactions of amines with tetracyanoquinodimethane (TCNQ). One of the advantages of these materials is the large molecular figure of merit (μβ where μ is the molecular dipole moment and P is the second order polarisability), which theoretically allows large nonlinear optical coefficients to be obtained. The molecular dipole moments of these chromophores were determined both experimentally and theoretically, and were found to agree. The nonlinear optical properties of these compounds in solution were studied using an electric field induced second harmonic generation (EFISH) technique. The measurements of μβ at 1064 nm and 1907 nm in chloroform and acetone are presented. Moderate μβ values were obtained but β is found to be unexpectedly small in chloroform and shows unusual dispersion characteristics in this solvent compared to acetone. Further concentration investigations revealed features that suggest the presence of aggregates within solution. Optical spectroscopy measurements provide evidence of new species whose presence and conformation were found to be solvent-dependent. The results of this work highlight the need for an entire concentration range to be studied if accurate determination of molecular properties of highly dipolar molecules is required. Guest-host polymer films of these materials have been corona poled using a constant current corona triode. Detailed characterisation studies of the second order nonlinearities using second harmonic generation (SHG) were compared to a less dipolar molecule. These investigations showed that the highly dipolar TCNQ derivatives show severe aggregation within the polymer films. The magnitude of the SHG that can be obtained from such systems is therefore limited by this aggregation.