Close coupling scattering calculations have been conducted for the para spin modification of H₂-{He, Ne and Ar}. The XC(fit) potential energy surfaces for H₂-Ne and H₂-Ar have been used for calculations for these two systems, while a newly fitted version of the Schaefer and Kohler potential energy surface was used for the H₂-He system. The fitting procedure employs nine modified Lennard-Jones oscillator functions to describe accurately 90% of the original tabulated potential energy surface to better than 12% error. The scattering calculations for H₂-Arfailed at higher energies due to the presence of a previously undocumented potential energy surface turn-over at R less than 1. 0 Angstroms. Manifold-to-manifold v=1 vibrational relaxation calculations for each of these systems are compared with other experimental and theoretical calculations. These comparisons demonstrate a common discrepancy between previous calculations and the current calculations for each system. The current vibrational relaxation rate constants are generally too small when compared to low temperature values of Audibert et al. and Orlikowski, and the high temperature values obtained by Flower et al. and Dove andTeitelbaum. The current calculations indicate the presence of a dramatic up-turn in the low temperature H₂-He rate constants. Other experimental and theoretical treatments do not exhibit this same up-turn, which is puzzling. A set of follow-up calculations featuring a larger basis set (such as the {16,12,10,8} Flower et al. basis set) and a larger manifold of included relaxation pathways are needed to improve these calculations.

The Raman linewidths of the carbon-halogen stretching mode was measured as a function of temperature in ethyl bromide, isopropyl chloride, isopropyl bromide, t-butyl chloride, t-butyl bromide, chlorobenzene, bromobenzene, iodobenzene and o-dichlorobenzene. The vibrational relaxation times showed a very clear trend. Together with earlier work on methyl iodide, these results provide evidence that the vibrational dephasing efficiencies (T^-1_iso) of the carbon-halogen mode vary in the order of Cl > Br > I. Vibrational dephasing times were calculated from the Fischer-Laubereau Isolated Binary Collision Mode. If further work shows this transferability to extend to other types of skeletal modes in molecular systems, this would have significant ramifications on future vibrational lineshape studies.

Die Relaxation der Styrylfarbstoffe DCM und LDS-750 nach Photoanregung in flüssiger Phase wurde mittels zeitaufgelöster optischer Spektroskopie untersucht. Dabei wurde die Breitband-Pump-Probe-Technik angewandt. Zur Charakterisierung der Relaxation von DCM im elektronischen Grundzustand wurde außerdem die Breitband-Dump-Probe-Technik (stimuliertes Emissionspumpen) eingesetzt. Für die beobachtete schnelle Relaxation von DCM wurde eine annähernd lösungsmittelunabhängige Zeitkonstante von 0.23 (( 0.04) ps im elektronisch angeregten und von 0.28 (( 0.07 ps) im elektronischen Grundzustand gefunden. Sie wurde als Konforma tionsänderung mit nur geringer Ladungsverschiebung charakterisiert. Die weitere spektrale Entwicklung wird in polarer Lösungsmittelumgebung vorwiegend von der Solvatation bestimmt. Für das ionische Polymethin LDS-750 wurden nach der Anregung solvensabhängige Kinetiken beobachtet, die sich durch die Annahme dreier möglicher Konformationen im S1 erklären lassen. / Relaxation of the stryryl dyes DCM and LDS-750 after photoexcitation in the liquid phase has been investigated by means of time-resolved optical spectroscopy. For this purpose, the broadband pump-probe technique was used. To characterize the relaxation of DCM in the electronic ground state, additionally the broadband dump-probe technique (stimulated emission pumping) was applied. An approximately solvent-independent time constant was found typical for the observed fast relaxation of DCM, with values of 0.23 ((0.04) ps in the excited and 0.28 (( 0.07 ps) in the electronic ground state. The relaxation was characterized as conformational change with only a small amount of charge transferred. The further spectral evolution in polar solvents was dominated by solvation dynamics.For the ionic polymethine species LDS-750 solvent-dependent kinetics were found after photoexcitation. They could be accounted for by assuming the existence of three different conformers within the S1 state.

Close coupling scattering calculations have been conducted for the para spin modification of H₂-{He, Ne and Ar}. The XC(fit) potential energy surfaces for H₂-Ne and H₂-Ar have been used for calculations for these two systems, while a newly fitted version of the Schaefer and Kohler potential energy surface was used for the H₂-He system. The fitting procedure employs nine modified Lennard-Jones oscillator functions to describe accurately 90% of the original tabulated potential energy surface to better than 12% error. The scattering calculations for H₂-Arfailed at higher energies due to the presence of a previously undocumented potential energy surface turn-over at R less than 1. 0 Angstroms. Manifold-to-manifold v=1 vibrational relaxation calculations for each of these systems are compared with other experimental and theoretical calculations. These comparisons demonstrate a common discrepancy between previous calculations and the current calculations for each system. The current vibrational relaxation rate constants are generally too small when compared to low temperature values of Audibert et al. and Orlikowski, and the high temperature values obtained by Flower et al. and Dove andTeitelbaum. The current calculations indicate the presence of a dramatic up-turn in the low temperature H₂-He rate constants. Other experimental and theoretical treatments do not exhibit this same up-turn, which is puzzling. A set of follow-up calculations featuring a larger basis set (such as the {16,12,10,8} Flower et al. basis set) and a larger manifold of included relaxation pathways are needed to improve these calculations.

Various modern spectroscopies have been utilized with considerable success in recent years to probe the dynamics of vibrational and reorientational relaxation of molecules in condensed phases. We have studied the temperature dependence of the polarized and depolarized Raman spectra of various modes in the following dihalomethanes: dibromomethane, dichloromethane, dichloromethane-d2, and bromochloromethane. Among other observed trends, we have found the following: Vibrational dephasing times calculated from the bend) and (C-Br stretch) lineshapes are of the same magnitude in CI^B^. The vibrational dephasing time of [C-D(H) stretch] is twice as long in CD2Cl2 as in CH-^C^, and the relaxation time of (C-Cl stretch) is greater in CI^C^ than in CD2CI2. Isotropic relaxation times for all three stretching vibrations are significantly shorter in C^BrCl than in CI^C^ or CI^B^. Application of the Kubo model revealed that derived modulation times are close to equal for equivalent vibrations in the various dihalomethanes. Thus, the more efficient relaxation of the A^ modes in CE^BrCl can be attributed almost entirely to the broader mean squared frequency perturbation of the vibrations in this molecule.

Das Überschussproton (H+) in wässriger Umgebung, schwer fassbar in seiner Struktur und seinem lokalen Umfeld, wurde seit mehr als zwei Jahrhunderten intensiv studiert und wird oft in Bezug auf zwei limitierende Strukturen diskutiert, dem Eigen-Kation (H9O4+) und dem Zundel-Kation (H5O2+). Die vorherrschende Hydratisierungsstruktur und der ultraschnelle fluktuierende Charakter des Überschussprotons in Lösung bei Raumtemperatur bleiben jedoch diskutiert. Die vorliegende Dissertation klärt eine dominante Hydratisierungsstruktur von Überschussprotonen in Lösung unter thermischen Gleichgewichtsbedingungen auf. Zundel-Kationen, selektiv präpariert im Lösungsmittel Acetonitril, wurden mittels ultraschneller zweidimensionaler Infrarot (2D-IR) und Zweifarben-Anrege-Abtastspektroskopie untersucht. Intramolekulare Lebensdauern liegen im sub-100 fs Bereich, viel kürzer als in reinem flüssigen Wasser. Das ,,Zundel-Kontinuum" erklärt sich durch Lösungsmittel getriebene Feldfluktuationen, die das Doppelminimumpotential des Protons in H5O2+ auf der Femtosekundenzeitskala modulieren. Zusammen mit stochastische Besetzungen niederfrequenter Moden, führen beide Effekte zu einer starken Frequenzverschiebung der Fundamentalschwingung der Protonbewegung und ihrer Ober- und Kombinationstöne. Der einzigartige Schwingungscharakter der Protontransfermode wurde ausgenutzt, um zeitaufgelöste Daten von Zundel-Kationen in Acetonitril mit denen von Überschussprotonen in flüssigem Wasser zu vergleichen. Die nahezu identische Schwingungsantwort beider Proben in 2D-IR und Zweifarben-Anrege-Abtastexperimenten identifiziert die (H5O2+)-Gruppierung als eine vorherrschende Hydratisierungsstruktur für Protonen in Wasser. / The excess proton (H+) in aqueous environment, elusive in its structure and local surrounding, has been intensively studied for more than two centuries and is often discussed in terms of two limiting structures, the Eigen cation (H9O4+) and the Zundel cation (H5O2+). However, the prevailing solvation structure and ultrafast fluctuating character of the hydrated proton in solution at room temperature is debated. The present thesis elucidates a predominant solvation structure of excess protons in solution under thermal equilibrium conditions. Zundel cations selectively prepared in acetonitrile solution are investigated by ultrafast two-dimensional infrared (2D-IR) and two-color pump-probe spectroscopy. Intramolecular lifetimes are found in the sub-100 fs range, much shorter than for neat liquid water. The "Zundel continuum" is explained by solvent driven field fluctuations that modulate the double minimum proton potential in H5O2+ on the femtosecond time scale. Together with stochastic populations of low frequency modes, both effects lead to strong frequency excursions of the proton transfer fundamental and its overtone and combination tone transitions. Utilizing the distinct vibrational character of the proton transfer mode, time-resolved data of Zundel cations in acetonitrile are compared to those obtained for excess protons in bulk liquid water. The nearly identical vibrational response of both samples found in 2D-IR and two-color pump-probe experiments identifies the H5O2+ moiety as a predominant solvation structure of protons in water.

No description available.

Doctor of Philosophy / Department of Physics / Brett D. Esry / The Efimov effect plays a central role in few-body systems at ultracold temperature and has thus accelerated a lot of studies on its manifestation in the collisional stability of the quantum degenerate gases. Near broad Feshbach resonances, Efimov physics has been studied both theoretically and experimentally through the zero-energy scattering observables. We have extended the theoretical studies of Efimov physics to a much broader extent. In particular, we have investigated the three-body Efimov physics near narrow Feshbach resonances and have also identified the Efimov features beyond the zero temperature limit. We have found, near a narrow Feshbach resonance, the non-trivial contribution from both of the resonance width and the short-range physics to the three-body recombination and vibrational dimer relaxation. Remarkably, the collisional stability of the Feshbach molecules are found to be opposite to that near the broad resonances: an increased stability for molecules made by bosons and a decreased stability for those made by fermions. The universal physics observed near the narrow Feshbach resonances is further found not to be limited to the zero temperature observables. We have found that the general features of Efimov physics and those pertaining to a narrow resonance are manifested in different energy ranges above zero temperature. This opens the opportunity to observe Efimov physics by changing the collisional energy while keeping the atomic interaction fixed. The landscape of the universal Efimov physics is thus delineated in both of the interaction and the energy domain. We have also investigated Efimov physics in heteronuclear four-body systems where the complexity can be reduced by approximations. In particular, we have proposed ways for controllable production of the Efimov tri-atomic molecules by three-body or four-body recombinations involving four atoms. We have also confirmed the existence of four-body Efimov effect in a system of three heavy particles and one light particle, which has resolved a decade-long controversy on this topic. Finally, we have studied the collisional properties of four identical bosons in 1D, which is important to the experiments on the quantum gases confined in the 1D optical lattices.

ABSTRACT A theoretical and experimental study of the vibrational and chemical relaxation rates of diatomic gases, in flows behind shock waves and along nozzles,is made here. ¶ The validity of the conventional relaxation rate models, which are generally used to analyse experiments, is tested by developing a detailed microscopic description of the diatomic relaxation processes. Assuming the diatomic molecules to be represented by the anharmonic Morse Oscillator, the vibrational Master equation, which describes the time variation of each vibrational energy level population, is constructed by allowing one-quantum vibration to translation (V-T) energy exchanges and vibration to vibration (V-V) energy exchanges between the molecules. Dissociation and recombination are allowed to occur from, and to, the uppermost vibrational level. Solving the Master equation, it is found that a number of effects are explained by the inclusion of V-V transitions. In particular it is found that V-V energy exchanges cause the induction time for H2 dissociation to be increased; suggest that the linear rate law, for H2 and Ar mixtures, fails for a H2 mole fraction above 20%; give an acceleration of vibrational excitation as equilibrium is approached for H2 and N2; cause the vibrational temperature to be lower than the value found without V-V transitions for vibrational de-excitation in nozzle flows of H2 and N2, and conversely for recombination of H2 in nozzle flows. The most important result is the demonstration that conventional nozzle flow calculations, with shock-tube-determined dis-sociation and vibrational excitation rates, appear to be valid for the recombining and vibrationally de-excitating flows considered. ¶ The dissociation rates of undiluted nitrogen are measured in the free-piston shock tube DDT, using time-resolved optical interferometry, over a temperature range of 6000-14000K and confirm the strong temperature dependence of the pre-exponential factor observed by Hanson and Baganoff (1972). ¶ The vibrational de-excitation and excitation rates are determined in the small free-piston shock tunnel T2 over temperature ranges of 2000-4000K and 7000-10300K, respectively, by measuring the shock angles and curvatures, from optical interferograms, of flow over an inclined flat plate in the nonequilibrium nozzle flow. The de-excitation rate is found to be within a factor of ten of the excitation rate, while the excitation rate of N2 by collision with N is found to be less than about 50 times the excitation rate of N2 by N2. The dissociation rates of nitrogen, in the flow behind a shock attached to a wedge, are investigated in the large free-piston shock tunnel, using the shock curvature technique. The discrepancy, reported by Kewley and Hornung (1974b), between theory and experiment at the highest enthalpy is found to be resolved by including the measured helium contamination (Crane 1975) in the free-stream. Reasonable agreement is obtained between experimental shock curvatures and calculations using accepted dissociation rates.

Photoinduced electron transfer is a fundamentally interesting process; it occurs everywhere in the natural world. Studies on electron transfer shed light on questions about the interaction between molecules and how the dynamics of these can be utilized to steer the electron transfer processes to achieve a desired goal. The goal may be to get electrons to the electrode of a solar cell, or to make the electrons form an energy rich fuel such as hydrogen, and it may also be an input or output for molecular switches. The importance of electron transfer reactions will be highlighted in this thesis, however, the main motivation is to gain a better understanding of the fundamental processes that affect the rate and direction of the electron transfer. A study of photoinduced electron transfer (ET) in a series of metallophorphyrin/bipyridinium complexes in aqueous solution provided fresh insight concerning the intimate relationship between vibrational relaxation and electron transfer. The forward electron transfer from porphyrin to bipyridinium as well as the following back electron transfer to the ground state could be observed by femtosecond transient absorption spectroscopy. Both the reactant and the product states of the ET processes were vibrationally unrelaxed, in contrary to what is assumed for most expressions of the ET rates. This could be understood from the observation of unrelaxed ground states. The excess energy given by the initial excitation of the porphyrin does not relax completely during the two steps of electron transfer. This is an unusual observation, not reported in the literature prior the studies presented in this thesis. This study also gave the first clear evidence of electronically excited radical pairs formed as products of intramolecular electron transfer. Signs of electronically excited radical pairs were seen in transient spectra, and were further verified by the observation that the rates followed a Marcus normal region behavior for all excitation wavelengths, despite the relatively large excess energy of the second excited state. This thesis also concerns electron transfer in solar cell dyes and mixed valence complexes. In the ruthenium polypyridyl complex Ru(dcb)2(NCS)2, where dcb = 4,4’-dicarboxy-2,2’-bipyridine, inter-ligand electron transfer (ILET) in the 3MLCT state was followed by means of femtosecond transient absorption anisotropy that was probed in the mid-IR region. Unexpectedly, ILET was not observed because electron density was localized on the same bpy during the time-window allowed by the rotational lifetime.