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Firstly, the origin of the electron correlation problem is outlined and some approaches to its solution are discussed. In Part I, the difference between the exact and Hartree Fock (HF) inter-electronic potentials experienced between a pair of electrons, known as the fluctuation potential, is used to investigate the effect of correlation on small atoms. They are analysed in terms of radial and angular components of correlation and the dominance of angular-based correlation for a large nuclear charge is seen. In Part II, a new technique for examining the effects of electron correlation on molecular systems is developed. This is subsequently used to investigate the ground states of the H2 and H+3 molecules in position and momentum-space. By employing a natural orbital analysis, it was found for molecules that correlation could be examined in terms of the redistribution in electronic probability parallel to the bond (z-correlation), axially around the bond (o-correlation) and perpendicular to the bond in all directions (p-correlation). The origins of these components were analysed mathematically and their effects on the two-particle electron density were displayed. In position-space, although z-correlation was found to be the most dominant, all types of correlation were seen to increase the mean inter-electronic separation. In momentum-space, however, o and p-correlation acted to increase the mean inter-electronic momentum whereas z-correlation acted in opposition to this and had the effect of increasing the probability of locating both electrons travelling parallel to the bond in the same direction. This was compared with the work performed on atomic systems and the HeH+ molecular ion. For the electron-deficient, the investigation provided evidence to suggest that there are three distinct 'bonding regions' bent towards the centre of the molecule.

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The work described in this thesis is principally concerned with the analyses at high resolution (∼ 0.05 cm-1) of the infrared gas phase bands of symmetric and asymmetric top molecules. The species studied are isotopic forms of ethane, ethylene and diborane. In the case of diborane the derived rotation, vibration and vibration - rotation constants have been further used to determine an accurate harmonic potential function and the precise molecular structure. Complete analyses have been performed for the nu2, nu 9, nu11 and nu2 fundamentals of CH3CD 3 and 13CH3CD3. Sets of upper state parameters are determined along with an estimate of the ground state centrifugal distortion constant, DK, for each isotopic species. Perturbations in the nu9 band are accounted for in terms of an A1 - E Coriolis interaction with nu3 and in the nu11 band in terms of the combined effects of an A1 - E Coriolis interaction with nu4 and an E(+/- z) ↔ E (= z) interaction with nu10. A small, localised perturbation in is identified as due to higher order rotational resonance with 2nu92. All first and second order Coriolis interaction parameters are determined. High resolution infrared studies of isotopic ethylenes have been undertaken in the region below 2000 cm-1. Accurate vibration and rotation parameters for the fundamentals of C2H4 and C2 D4 are determined. A full upper state rovibration analysis of the nu7 fundamental of C2D4 is achieved, once the various effects of indirect Coriolis interaction with the inactive nu 4 torsion vibration are taken into account. Several localised perturbations are identified and from these the band centre of the inactive nu4 vibration is estimated accurately. Complete analyses of the nu2, nu 12, 2nu7 and 2nu8 bands of H2CCd 2 reported. Localised perturbations are identified and taken into account in the analyses, enabling the perturbing vibrational levels to be located accurately and the interaction parameters to be determined. Ground state rotation and quartic distortion constants are obtained for 11B2H6 and 11B2D6 from the analyses of the nu17 and nu18 type-A and nu14 and nu9 + nu15 type-C bands of 11B2D6 . Sets of upper state parameters are determined for all vibration levels studied and several localised perturbations are observed and identified. The data obtained from these and previous spectroscopic studies of diborane allow a precise determination of the empirical harmonic potential function to be made for the first time. Thirty of the thirty three independent force constants are determined with numerical significance. The physical significance of the values is probably best demonstrated by the close agreement throughout with scaled ab initio force constants from two independent sources. Finally the availability from this and previous work of 12 precisely determined ground state spectroscopic rotation constants for isotopic diboranes, and a physically realistic harmonic potential function, enables the ground state ro substitution rs, zero-point average r z, and equilibrium r structures to be calculated. The rz and re parameters are entirely compatible with, but more accurately determined than, those available from electron diffraction studies.

A Vernier spectrometer is built with a near-infrared mode-locked Er:doped fiber laser, a Fabry-Perot cavity with finesse of 1000, a diffraction grating and a photo detector. The optical cavity provides high sensitivity in absorption detection by enhancing the interaction length of the light with molecular species contained in the cavity. Coupling an optical frequency comb to the cavity provided a broadband spectral bandwidth with high precision to measure the absorption of several molecular species simultaneously. Also, by using the optical cavity as a filter, transmission of some bunch comb lines was achieved. This comb filtering together with a simple grating and a photodiode formed the Vernier detection technique to provide very fast measurements while it kept the setup very simple and compact. The system allows to detect carbon dioxide in the air and water vapor and OH radicals in the flame in a spectrum spanning from 1550 nm to 1590 nm, approximately. The retrieved spectrum has a resolution of 9.3 GHz being acquired in 0.05 s.

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