Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638803
Title: Spectroscopic studies with lasers
Author: Shaheen, N.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1986
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Abstract:
This thesis is concerned with some aspects of the phenomenon called the 'Opto-Galvanic Effect'. This effect is produced as a result of transient laser-induced excited atom, electron and ion density variations in excited gaseous media such as an electric discharge or a flame. In this thesis the medium used was a low pressure hollow cathode glow discharge in neon as the main constituent. The underlying theory of the optogalvanic effect is reviewed and discussed and then used for comparison with results obtained in experiments carried out using pulsed tunable laser radiation in neon discharges. An account is given of the design, construction and optimization of 'home-made' nitrogen and tunable dye lasers and the optimization procedure developed for a commercial XeCl excimer laser are described. The nitrogen and XeCl lasers were used to pump the dye lasers which provided the tunable laser beam used for the production of the OG signals. Two hollow cathode discharge tubes and their associated vacuum and gas handling system were designed and built and these are described as well as the electronic equipment used to detect the OGE signals. Experiments were carried out using these two tubes and three commercial hollow cathode tubes containing Pr, Na and rare earths in neon as the buffer gas. The optogalvanic spectrum of neon was observed oscillography and recorded using a boxcar averager and chart recorder. Both positive and negative signals were obtained over the wavelength range 568< λ < 605 nm. Regardless of the mode of illumination, the OGS is always initially negative. Only in the case of a resonant excitation starting from a metastable state does the OGS become positive. The positive OGS is approximately two orders of magnitude larger than the negative OGS. The sign and magnitude of the signals depend mainly on the operating regime of the discharge which determined the response in the OGE. Specifically, operation of the discharge in the abnormal glow regime leads to an enhancement in the negative OG signals. Despite sustained attempts the OG spectra of the metals and rare earths could not be detected with the apparatus. A second series of experiments were carried in order to see if the OGE could provide information about some of the properties of the plasma in a glow discharge and about the secondary ionization processes in a discharge operating in the low current density 'Townsend' regime. In the glow discharges - both normal and abnormal regimes were examined as a function of current and laser intensity - OG signals originating from two resonant transitions were studied. One corresponds to the 1s5-2p2, transition at 588.19 nm and originates from the 1s5 metastable level. The second corresponds to the 2p3-4s1 transition at 591.89 nm and involves levels which have allowed dipole transitions to the ground state and which are relatively highly excited levels. From the investigation caried out on the variation of the OGS with the discharge current we deduced the following: firstly, the metastable relaxation coefficient is approximately zero at zero current, secondly, the relaxation coefficient, in the case of the 588.19 nm transition, shows a tendency towards saturation, at currents higher than 10mA, thirdly in the case of the 591.89 nm transition, the relaxation coefficients are linearly dependent on the current. From the study of the variation in the OGS with the intensity of the laser beam we observed that there is a trend towards saturation. However, the saturation value lies at some value higher than the maximum power - 1.8kW - of the dye laser used. From a study of the temporal characteristic of the OGS obtained under 'Townsend-type' conditions the time scale involved was of the order of millisecond, this implies changes induced in the secondary ionization processes involved in the discharge.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.638803  DOI: Not available
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