Thesis (PhD)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: A tunable narrow-bandwidth laser source combined with a supersonic gas jet as sample is wellsuited for obtaining high-resolution spectra of cold isolated molecules and complexes. In the present study such a laser source in the vacuum ultraviolet was applied to the spectroscopic investigation of rare carbon monoxide (CO) isotopomers and CO-noble gas van der Waals complexes in supersonic gas pulses seeded with natural CO gas. Tunable coherent vacuum ultraviolet radiation was generated by two-photon resonant fourwave sum-frequency mixing of two pulsed dye laser beams in a magnesium vapour medium. Laser induced fluorescence excitation spectra of the A(v'=3)-X(v"=0) vibronic band of CO molecules in a noble gas (neon or argon) jet were obtained by measuring the total undispersed fluorescence from the irradiated sample volume in dependence of the excitation wavelength. The dynamics of the flow-cooling process in the supersonic jet were investigated and the experimental parameters optimised using the rotational temperature of 12C160as determined from rotational line intensities. Rotational temperatures as low as 2 ± I K were observed. Spectroscopic detection of the rare 12C170and 12C180isotopomers was facilitated by the low rotational temperature and high spectral resolution. Six rotational lines of 12C170and four of 12C180were detected in the A(v'=3)-X(v"=0) vibronic band. This demonstrates the low detection limit (circa 3 parts per million) obtained in the experiment. The line wavelengths were determined to an accuracy of 0.2 pm using the well-documented 12C160and 13C160lines for calibration. The spectral results on 12C170are, to our knowledge, the first rotationally resolved laboratory measurements published on the A-X band of this isotopomer. Accurate wavelength data of the stable isotopomers of CO is of importance in the interpretation of astrophysical observations of CO in the interstellar medium. The newly determined 12C170wavelengths were successfully applied to a recent problem in astrophysics (Astrophys. J. Lett. 2003). The conditions in a supersonic jet facilitate the study of weakly bound van der Waals complexes, of which CO-noble gas complexes are prototypes. However, there is no experimental data available on the electronic excitation spectra of the CO-noble gas complexes, lying in the vacuum ultraviolet region. In the present experiment evidence of extensive complexation of the CO in the noble gas jet has been found, but in the spectral region around the A(v'=3)-X(v"=O) band of CO no distinct spectral features that could be associated with these complexes were observed. Having considered the existing knowledge on CO and CO-noble gas complexes and experimental studies on the excitation and dissociation dynamics of Iz-noble gas complexes, we regard complex induced inter-system crossing or electronic predissociation as the most likely causes for these observations. The results on the rare CO isotopomers demonstrate the potential of our experimental setup for high-resolution, isotope and state selective spectroscopy in the vacuum ultraviolet with a high sensitivity for fluorescent species. The availability of the now well-characterised experimental setup in our laboratory opens the way for further investigation of molecular or complex species with spectroscopic features in the vacuum ultraviolet region. / AFRIKAANSE OPSOMMING: Vakuum ultraviolet laser spektroskopie van CO molekules in 'n supersoniese gasstraal: 'n Afstembare smal bandwydte laserbron en 'n supersoniese gasstraal as monster is 'n geskikte kombinasie vir hoë-resolusie spektroskopie van geïsoleerde afgekoelde molekules en komplekse. In hierdie studie is so 'n laserbron in die vakuum ultraviolet gebruik in die spektroskopiese ondersoek van skaars koolstofmonoksied (CO) isotopomere en CO-edelgas van der Waals komplekse in supersoniese gaspulse wat 'n klein persentasie natuurlike CO gas bevat. Afstembare koherente vakuum ultraviolet lig is verkry deur twee-foton resonante vier-golf som-frekwensie vermenging van twee gepulseerde kleurstoflaserbundels in 'n magnesiumdamp medium. Laser-geïnduseerde fluoressensie opwekkingspektra van die A(v'=3)-X(v"=0) vibroniese band van die CO molekules in die edelgasstraal (neon of argon) is uitgemeet deur die totale fluoressensie van die beligte gasmonster, sonder golflengteskeiding, te meet as funksie van die opwekkingsgolflengte. Die dinamika van die vloeiverkoelingsproses in die supersoniese gasstraal is ondersoek en die eksperimentele parameters geoptimeer deur gebruik te maak van die rotasionele temperatuur van 12Cl60 soos bepaal uit die intensiteitsverhoudings van die rotasielyne. Rotasionele temperature tot so laag as 2 ± 1 K is waargeneem. Spektroskopiese waarneming van die skaars 12C170 and 12Cl80 isotopomere is moontlik gemaak deur die lae rotasionele temperatuur en die hoë spektrale resolusie. Ses rotasielyne van 12C170 en vier van 12C180 is waargeneem in die A(v'=3)-X(v"=0) vibroniese band. Dit demonstreer die lae deteksielimiet (ongeveer 3 dele per miljoen) wat bereik kon word. Die golflengtes van die lyne is bepaal met 'n akkuraatheid van 0.2 pm deur die bekende lyne van 12C160en 13C160vir kalibrasie te gebruik. Die resultate ten opsigte van 12C170 is sover vasgestel kon word die eerste rotasioneel-opgeloste laboratorium metings van die A-X band van hierdie isotopomeer. Akkurate golflengte data vir die stabiele CO isotopomere is van belang vir die interpretasie van die astrofisiese waarnemings van CO in die interstellêre medium. Die nuwe 12C170 golflengtes is suksesvol aangewend in die oplossing van 'n onlangse interpretasieprobleem in astrofisika (Astrophys. J. Lett. 2003). Die toestande in 'n supersoniese gasstraal maak die bestudering van swak-gebonde van der Waals komplekse moontlik. Hoewel CO-edelgas van der Waals komplekse as prototipes beskou word, is daar geen eksperimentele data beskikbaar oor die elektroniese opwekkingspektra van hierdie spesies, wat in die vakuum ultraviolet gebied lê, nie. In hierdie studie is daar eksperimentele getuienis gevind vir uitgebreide kompleksering van CO in die edelgasstraal, maar in die spektraalgebied rondom die A(v'=3)-X(v"=O) band van CO is geen duidelike spektrale kenmerke wat met hierdie komplekse geassosieer kan word, waargeneem nie. Na oorweging van die bestaande kennins oor CO en CO-edelgas komplekse en eksperimentele studies oor die opwekking en dissosiasie-dinamika van Iz-edelgas komplekse, beskou ons kompleksgeïnduseerde intersisteemoorgange of elektroniese predissosiasie as die waarskynlikste redes vir hierdie waarnemings. Die resultate oor die skaars CO isotopomere toon die potensiaal van ons eksperimentele opstelling vir hoë-resolusie, isotoop- en toestandselektiewe spektroskopie in die vakuum ultraviolet met uitstekende sensitiwiteit vir fluoresserende spesies. Die beskikbaarheid van hierdie nou deeglik gekarakteriseerde eksperimentele opstelling in ons laboratorium maak verdere ondersoek na molekulêre of kompleks-spesies met spektroskopiese kenmerke in die vakuum ultraviolet moontlik.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

This dissertation explores the use of anion photoelectron imaging to interrogate electronic dynamics in small chemical systems with an emphasis on photoelectron angular distributions. Experimental ion generation, mass selection, laser photodetachment and photoelectron imaging were performed in a negative-ion photoelectron imaging spectrometer described in detail. Results for photodetachment from the simplest anion, H⁻, are used to illustrate fundamental principles of quantum mechanics and provide basic insight into the physics behind photoelectron imaging from a pedagogical perspective. This perspective is expanded by introducing imaging results for additional, representative atomic and small molecular anions (O⁻, NH₂⁻ and N₃⁻) obtained at multiple photon energies to address the energy-dependence of photoelectron angular distributions both conceptually and semi-quantitatively in terms of interfering partial photoelectron waves. The effect of solvation on several of these species (H⁻, O⁻, and NH₂⁻) is addressed in photoelectron imaging of several series of cluster anions. The 532 and 355 nm energy spectra for H⁻(NH₃)n and NH₂⁻(NH₃)n (n = 0-5) reveal that these species are accurately described as the core anion solute stabilized electrostatically by n loosely coordinated NH3 molecules. The photoelectron angular distributions for solvated H⁻ deviate strongly from those predicted for unsolvated H⁻ as the electron kinetic energy approaches zero, indicating a shift in the partial-wave balance consistent with both solvation-induced perturbation (and symmetry-breaking) of the H⁻ parent orbital and photoelectron-solvent scattering. The photoelectron energy spectra obtained for the cluster series [O(N₂O)n]⁻ and [NO(N₂O)n]⁻ indicate the presence of multiple structural isomers of the anion cores, the former displaying sharp core-switching at n = 4, the latter isomer coexistence over the entire range studied. The photoelectron angular distributions for detachment from the O⁻(N₂O)n and NO⁻(N₂O)n isomers deviate strongly from those expected for bare O⁻ and NO⁻, respectively, in the region of an anionic shape resonance of N₂O, suggesting resonant photoelectron-solvent scattering. Partial-wave models for two-centered photoelectron interference in photodetachment from dissociating I₂⁻ is presented and discussed in the context of previous results. New time-resolved photoelectron imaging results for I₂⁻, for both parallel and perpendicular pump and probe beam polarizations, are presented and briefly discussed. Finally, new ideas and directions are proposed.

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This thesis is concerned with coherent effects in high resolution mid-infrared gas phase spectroscopy using quantum cascade lasers (QCLs). An introductory chapter explains the importance of QCLs as radiation sources in the mid-infrared region of the spectrum and goes on to detail their development and structure. A discussion of coherent effects in spectroscopy follows, leading into the second chapter which discusses the theories relevant to the experimental sections of the thesis. In chapter 2 the theory underpinning direct and velocity selective, Doppler-free spectroscopy is discussed and a density matrix formalism is followed to derive the equations of motion that govern coherent excitation effects in two-level systems. In the final part of the chapter this treatment is extended to three-level systems. The equations derived in this chapter form the basis of quantitative interpretations of the phenomena observed in experimental data and presented in the remainder of the thesis. In chapter 3 the characterisation of a high power, narrow linewidth QCL is carried out. This laser is then used to perform direct and sub-Doppler resolution spectroscopy on NO, demonstrating non-linear absorption at high laser intensities and providing a measurement of the laser linewidth in the limit of slow frequency tuning. As the slow tuning rate increases, evidence of coherent transient effects is presented and density matrix theory used to model this behaviour. The data presented include the first observations of asymmetric Lamb dips and the onset of rapid passage oscillations from a Lamb dip. Pump-probe experiments on NO, utilising two cw QCLs are presented in chapter 4. The high level of velocity selection afforded by QCL excitation leads to coherent transient signals at far lower probe scan rates than previously reported. The effect of altering both the scan rate and the gas pressure and the importance of hyperfine structure are presented. A radio frequency noise source applied to one of the lasers is shown to broaden the laser linewidth, leading to rapid dephasing. A two-colour polarisation spectroscopy experiment is also presented which allows the measurement of both the absorption and the Doppler-free dispersion signals and the three-level density matrix formalism presented at the end of chapter 2 used to model the non-linear response of the system. The final chapter details the use of an acousto-optic modulator to create a pulse of mid-IR light using a cw QCL and the application of this to time resolved pump-probe spectroscopy. This capability suggests the prospect of achieving coherent population transfer by stimulated Raman adiabatic passage (STIRAP) using two such pulses. Simulations based on a simple three-level model and including Zeeman coherences are presented, which take the measured properties of the lasers used in this thesis as inputs to predict the potential population transfer achievable in NO as well as providing useful information about the angular momentum polarisation of the excited molecules. An experimental realisation of STIRAP would require the lasers to be stabilised, and so the final part of the chapter details experimental attempts to achieve stabilisation of an external cavity QCL, and suggests future avenues for improved implementation.

The aim of this thesis is a study of NV centres in crystalline and nanocrystalline diamond by laser spectroscopy methods. In the theoretical part we discuss the laser spectroscopy methods, the studied material - diamond and the NV colour centres. In the experimental part we discuss the influence of nanoparticle size on luminescence spectra. We measure the luminescence of samples at room and also at low temperatures depending on the intensity and wavelength of the excitation. We study the photo-conversion of negatively charged state of NV centres to the neutral in detail. We make the time resolved measurements of the luminescence on streak camera for characterization the dynamical properties of the studied samples. The result is the comparison of lifetimes of the states in NV centres in selected samples.

Centros de cor e íons de metal de transição em cristais iônicos tem mostrado um grande potencial como candidatos a meios laser ativos sintonizáveis. Para determinar se tais materiais são apropriados ou não para operar como meios laser, seus parâmetros espectroscópicos fundamentais tem de ser conhecidos. No presente trabalho, nos descrevemos um metodo espectroscópico que permite determinar o diagrama de níveis de energia da espécie investigada, o tempo de decaimento, a eficiência quântica e a secção de choque de emissão. O tempo de decaimento foi medido usando-se um bombeamento senoidal de excitação e detecção sensível a fase; obtendo-se 1,6ms para o kcl:TL POT.0 (1), 17ns para o LIf:F IND.2, 28ns para o LIf:F2 POT.+ E 4ns para o LIf:F3 POT.+. Usando-se uma técnica de absorção rotulada foi possível determinar absorções do estado fundamental e do primeiro estado excitado da espécie investigada mesmo com a interferência de outros centros. Técnica utilizada em kcl:tl onde níveis de energia do centro de cor de TL POT.0 (1) foram identificados vários outros agregados de tl devido a preparação do cristal, que perturbam o centro TL POT.0 (1) também foram identificados. Vários outros agregados de T1, devido a preparação do cristal, que perturbam o centro TL POT.0 (1), também foram observados. / Color centers and transition metal ions in ionic crystals have shown a qreat potential as candidates for tunable laser sources. In order to determine whether or not such materials are suitable to operate as laser media, their fundamental spectroscopic parameters have to be known. In the present work we describe a spectroscopic method that can determine the energy level diagram of the species under investigation, the decay time, quantum efficiency and emission cross section. Firstly, the decay time was measured by using a sinosoidal pumping excitation and phase sensitive detection. Using a specially home designed cryostat we were able to measure the decay time in the temperature range of 77K to 310K, and therefore, the temperature dependence of the quantum efficiency. This technique was applied to the T1°(1):KCl, F2:LiF, F+3:LiF and F+2:LiF, where decay times of 1.6s, 17ns, 4ns and 28ns were measure, respectively. Secondly, by using a pump-probe technique we were able to determine the excited-state absorption of the studied species, even under the interference of other centers. This is particularly important in color centers due to the presence of several other species during the centers formation. Besides, this technique can identify excited state absorptions that can eliminate laser medium candidates. This technique was used in KCl:T1 crystals where laser active Tl° (1) color center energy levels were identified. Besides, several other Tl aggregates due to the crystal preparation that perturb the Tl° (1) center were also observed.