Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3065. Adviser: Taekjip Ha. Includes bibliographical references (leaves 101-104) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6455. Adviser: Munir Nayfeh. Includes bibliographical references (leaves 103-109) Available on microfilm from Pro Quest Information and Learning.

Doctor of Philosophy / Department of Physics / Kristan L. Corwin / Portable frequency references are crucial for many practical on-site applications, for example, the Global Position System (GPS) navigation, optical communications, and remote sensing. Fiber laser optical frequency combs are a strong candidate for portable reference systems. However, the conventional way of locking the comb repetition rate, frep, to an RF reference leads to large multiplied RF instabilities in the optical frequency domain. By stabilizing a comb directly to an optical reference, the comb stability can potentially be enhanced by four orders of magnitude. The main goal of this thesis is to develop techniques for directly referencing optical frequency combs to optical references toward an all-fiber geometry. A big challenge for direct fiber comb spectroscopy is the low comb power. With an 89 MHz fiber ring laser, we are able to optically amplify a single comb tooth from nW to mW (by a factor of 10^6) by building multiple filtering and amplification stages, while preserving the comb signal-to-noise ratio. This amplified comb tooth is directly stabilized to an optical transition of acetylene at ~ 1539.4 nm via a saturated absorption technique, while the carrier-envelope offset frequency, f0, is locked to an RF reference. The comb stability is studied by comparing to a single wavelength (or CW) reference at 1532.8 nm. Our result shows a short term instability of 6 x10^(-12) at 100 ms gate time, which is over an order of magnitude better than that of a GPS-disciplined Rb clock. This implies that our optically-referenced comb is a suitable candidate for a high precision portable reference. In addition, the direct comb spectroscopy technique we have developed opens many new possibilities in precision spectroscopy for low power, low repetition rate fiber lasers. For single tooth isolation, a novel cross-VIPA (cross-virtually imaged phase array) spectrometer is proposed, with a high spectral resolution of 730 MHz based on our simulations. In addition, the noise dynamics for a free space Cr:forsterite-laser-based frequency comb are explored, to explain the significant f0 linewidth narrowing with knife insertion into the intracavity beam. A theoretical model is used to interpret this f0 narrowing phenomenon, but some unanswered questions still remain.

<p>Studying the ultrafast dynamics of small molecules can serve as the first step in understanding the dynamics in larger chemically and biologically relevant molecules. To make direct comparisons with existing computational techniques, the photons used in pump-probe spectroscopy must make perturbative transitions between the electronic states of isolated small molecules. In this dissertation experimental investigations of ultrafast dynamics in electronic excitations of neutral ethylene and carbon dioxide are discussed. These experiments are performed using VUV/XUV femtosecond pulses as pump and probe. </p><p> To make photons with sufficient energy for single photon transitions, VUV and XUV light is generated by high harmonic generation (HHG) using a high pulse energy (&ap;30&ndash;40 mJ) Ti:sapphire femtosecond laser. Sufficient flux must be generated to enable splitting of the HHG light into pump and probe arms. The system produces >10¹⁰ photons per shot, corresponding to nearly 10 MW of peak power in the XUV. Using a high flux of high energy photons creates a unique set of challenges when designing a detector capable of performing pump-probe experiments. A velocity map imaging (VMI) detector has been designed to address these challenges, and has become a successful tool facilitating studies into molecular dynamics that were not possible before its implementation. </p><p> The emphasis on using high energy, single photon transitions allowed theoretical calculations to be directly compared to experimental yields for the first time. This comparison resolved a long standing issue in the excited state lifetime of ethylene, and provided a confirmation of the branching ratio between the two nonadiabatic relaxation pathways that return ethylene back to its ground state from the &pi;*. The participation of the 3s Rydberg state has also been measured by collecting the time resolved photoelectron spectrum during the dynamics on ethylene&rsquo;s &pi;* excited state, confirming calculations predicting the effect of the 3s. </p><p> In carbon dioxide the first time resolved measurement in the lowest electronic excitation of carbon dioxide has been performed. A high kinetic energy release channel shows the signature of wavepacket dynamics within the excited state manifold. Deviation from the direct dissociation predicted for the pumped state provides experimental evidence confirming theoretical predictions of nonadiabatic transitions within the lowest lying electronically excited states. </p>

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The assignment and analysis of high resolution spectra of transient species in the gas phase leads to the unambiguous identification of the carrier of the spectra. Such spectra can only be observed provided a detectable steady state concentration, depending on the particular spectroscopic technique being employed, can be generated. The first half of this thesis is concerned with three methods of producing detectable concentrations by utilising (i) microwave discharge, (ii) electrical discharge, (iii) carbon dioxide laser photolysis with an associated photosensitiser, sulphur hexafluoride. The high resolution electron paramagnetic resonance spectra of iodine and fluorine atoms and the sulphur monoxide and nitrogen difluoride radicals serve as examples of species produced by these methods and some characteristics of the laser photolysis technique are described in Chapter IV. The advent of the infrared semiconductor diode laser and also a solid state millimeter wave source (an IMPATT oscillator, in conjunction with the technique of magnetic resonance) has led to the development of more sensitive high resolution spectroscopic techniques. High resolution spectra of the important silicon monohydride radical have been observed using the former technique and the experiments and subsequent analysis are described in Chapter VI. The millimeter wave magnetic resonance spectrometer designed and constructed by the author is described in Chapter VII, together with the millimeter wave magnetic resonance spectrum of the oxygen molecule, the first spectrum to be observed by this new technique.

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