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An optical cavity comprises a set of mirrors between which light can be reflected a number of times. The selectivity and stability of optical cavities make them extremely useful as frequency references or discri­mi­nators. With light coupled into the cavity, a sample placed inside a cavity will experience a significantly increased interaction length. Hence, they can be used also as amplifiers for sensing purposes. In the field of laser spectroscopy, some of the most sensitive techniques are therefore built upon optical cavities. In this work optical cavities are used to measure properties of gas samples, i.e. absorption, dispersion, and refractivity, with unprecedented precision. The most sensitive detection technique of all, Doppler-broadened noise-immune cavity enhanced optical heterodyne molecular spectrometry (Db NICE-OHMS), has in this work been developed to an ultra-sensitive spectroscopic technique with unprecedented detection sensitivity. By identifying limiting factors, realizing new experimental setups, and deter­mining optimal detection conditions, the sensitivity of the technique has been improved several orders of magnitude, from 8 × 10-11 to 9 × 10-14 cm-1. The pressure interval in which NICE-OHMS can be applied has been extended by deri­vation and verification of dispersions equations for so-called Dicke narrowing and speed dependent broadening effects. The theoretical description of NICE-OHMS has been expanded through the development of a formalism that can be applied to the situations when the cavity absorption cannot be considered to be small, which has expanded the dynamic range of the technique. In order to enable analysis of a large number of molecules at their most sensitive transitions (mainly their funda­mental CH vibrational transitions) NICE-OHMS instrumentation has also been developed for measurements in the mid-infrared (MIR) region. While it has been difficult to realize this in the past due to a lack of optical modulators in the MIR range, the system has been based on an optical para­metric oscillator, which can be modulated in the near-infrared (NIR) range. As the index of refraction can be related to density, it is possible to retrieve gas density from measurements of the index of refraction. Two such instru­men­tations have been realized. The first one is based on a laser locked to a measure­ment cavity whose frequency is measured by compassion with an optical frequency comb. The second one is based on two lasers locked to a dual-cavity (i.e. one reference and one measurement cavity). By these methods changes in gas density down to 1 × 10-9 kg/m3 can be detected. All instrumentations presented in this work have pushed forward the limits of what previously has been considered measurable. The knowledge acquired will be of great use for future ultrasensitive cavity-based detection methods.

Cavity ring-down spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS) are two well-established absorption spectroscopic techniques originally developed for gas-phase samples. Condensed-phase applications of these techniques still remain rare, complicated as they are by additional background losses induced by condensed-phase samples as well as the intracavity components in which the sample is constrained. This thesis is concerned with the development and application of optical-cavity-based techniques in the condensed phase. Polarization-dependent evanescent wave CRDS (EW-CRDS) has been used to study the molecular orientation at the solid/air and solid/liquid interfaces. An increase in average orientation angle with respect to the surface normal has been observed for both methylene blue and coumarin molecules as a function of coverage at the fused silica/air interface. An orientation-angle-dependent photobleaching of pyridin molecules at the fused silica/methanol interface have also been observed. EW-CRDS has also been used to monitor slow in situ photobleaching of thin dye films deposited on the prism surface. The photobleaching dynamics is interpreted as a combination of first- and second-order processes. A significant fraction of this thesis has been devoted to studying magnetic field effects (MFEs) on the kinetics of the radical pair (RP) reactions in solution, in an effort to understand the ability of animals to sense the geomagnetic field. Two novel optical-cavity-based techniques – broadband CEAS (BBCEAS) and CRDS have been developed for this purpose. BBCEAS uses a supercontinuum (SC) source as the cavity light source and a CCD camera as photodetector, enabling simultaneous acquisition of absorption spectrum across the whole visible region (400 – 800 nm). In CRDS, a tunable optical parametric oscillator has been used as the cavity light source. Combined with the switching of external magnetic field (SEMF) method, this technique allows the decay kinetics of the geminate RPs to be monitored, with nanosecond resolution. Both BBCEAS and CRDS provide sensitivity superior to single-pass transient absorption (TA), a technique traditionally used in the MFE studies. A series of photochemical systems have been studied by BBCEAS and CRDS, respectively, among which, the MFEs of drosophila melanogaster cryptochrome has been observed. Importantly, this is the first time an MFE has been observed in an animal cryptochrome, and provides key supporting evidence for the cryptochrome hypothesis of magnetoreception in animals. Besides the optical-cavity-based techniques, a novel fluorescence detection method of MFEs has also been demonstrated. This technique proved ultrahigh sensitivity when applicable.

New dynamical applications of quantum beat spectroscopy (QBS) to molecular dynamics are employed to probe the angular momentum polarization effects in photodissociation and molecular collisions. The magnitude and the dynamical behaviour of angular momentum alignment and orientation, two types of polarization, can be measured via QBS technique on a shot-by-shot basis. The first part of this thesis describes the experimental studies of collisional angular momentum depolarization for the electronically excited state radicals in the presence of the collider partners. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer (RET), and elastic collisions. Experimental results also have a fairly good agreement with the results of quasi-classical trajectory scattering calculations. Chapter 1 provides the brief theories about the application of the QBS technique and collisional depolarization. Chapter 2 describes the method and instrumentation employed in the experiments of this work. In Chapter 3, the QBS technique is used to measure the total elastic plus elastic depolarization rate constants under thermal conditions for NO(A,v=0) in the presence of He, Ar, N2, and O2. In the case of NO(A) with Ar, and particularly with He, collisional depolarization is significantly smaller than RET, reflecting the weak long-range forces in these systems. In the case of NO(A)+N2/O2, collisional depolarization and RET are comparable, reflecting the relatively strong long-range forces in these systems. In Chapter 4, the QBS technique is used to measure the elastic and inelastic depolarization and total RET rate constants for OH(A,v=0) under thermal conditions in the presence of He and Ar, as well as the total depolarization rate constants under superthermal conditions. In the case of OH(A)+He, elastic depolarization is sensitive to the N rotational state, and inelastic depolarization is strongly dependent on the collision energy. In the case of OH(A)+Ar, elastic depolarization is insensitive to N, and inelastic depolarization is less sensitive to the collision energy, reflecting that the relatively strong long-range force in OH(A)+Ar system. The second part of this thesis describes the experimental studies of photodissociation under thermal conditions. Chapter 5 provides a brief introduction about several polarization parameter formalisms used for photodissociation, and the incorporation of the QBS technique to measure these polarization parameters. In this thesis, most polarization parameters of the molecular photofragments are measured using the LIF method, and the QBS technique is used as a complementary tool to probe these polarization parameters. In Chapter 6, rotational orientation in the OH(X,v=0) photofragments from H2O2 photodissociation using circularly polarized light at 193 nm is observed. Although H2O2 can be excited to both the A and B electronic states by 193 nm, the observed orientation is only related to the A state dynamics. A proposed mechanism about the coupling between a polarized photon and the H2O2 parent rotation is simulated, and the good agreement between the experimental and simulation results further confirms the validity of this mechanism. In Chapter 7, rotational orientation in the NO(X,v) photofragments from NO2 photodissociation using circularly polarized light at 306 nm (v=0,1,2) and at 355 nm (v=0,1) is observed. Two possible mechanisms, the parent molecular rotation and the coherent effect between multiple electronic states, are discussed. NOCl is photodissociated using circularly polarized light at 306 nm, and NO(X,v) rotational distributions (v=0,1) and rotational orientation (v=0) are measured. For the case of NOCl, the generation of orientation is attributed to the coherent effect.

Electronic circuits comprising organic semiconductor thin-films are part of promising technologies for a renewable power generation and an energy-efficient information technology. Whereas TV and mobile phone applications of organic light emitting diodes (OLEDs) got ready for the market awhile ago, organic photovoltaics still lack in power conversion efficiencies, especially in relation to their current fabrication costs. A major reason for the low efficiencies are losses due to the large number of charge carrier traps in organic semiconductors as compared to silicon. It is the aim of this thesis to identify and quantify charge carrier traps in vacuum-deposited organic semiconductor thin-films and comprehend the reasons for the trap formation. For that, the techniques impedance spectroscopy (IS), thermally stimulated currents (TSC), and photoelectron spectroscopy are utilized. In order to assess the absolute energy of charge carrier traps, the charge carrier transport levels are computed for various hole transport materials such as MeO-TPD, pentacene, and ZnPc. Unlike inorganics, organic semiconductors possess in first-order approximation Gaussian distributed densities of states and temperaturedependent transport levels. The latter shift by up to 300 meV towards the energy gap-mid when changing from room temperature to 10 K as it is done for TSC examinations. The frequency-dependent capacitance response of charge carrier traps in organic Schottky diodes of pentacene and ZnPc are studied via impedance spectroscopy. In undoped systems, deep traps with depths of approx. 0.6 eV and densities in the order of 1016...1017 cm−3 are prevailing. For pentacene, the deep trap density is reduced when the material undergoes an additional purification step. Utilizing p-doping, the Fermi level is tuned in a way that deep traps are saturated. Vice versa, the freeze-out of p-doped ZnPc provides further insight into the influence of trap-filling, impurity saturation and reserve on the Fermi level position in organic semiconductors. Furthermore, charge carrier traps are investigated via thermally stimulated currents. It is shown that the trap depths are obtained correctly only if the dispersive transport of the released charge carriers until their extraction is considered. For the first time, the polarity of charge carrier traps in MeO-TPD, ZnPc, and m-MTDATA is identified from TSC’s differences in release time when spacer layers are introduced in the TSC samples. Simultaneously, tiny hole mobilities in the order of 10−13 cm2 Vs−1 are detected for low-temperature thin-films of the hole transporter material Spiro-TTB. It is shown for Spiro-TTB co-evaporated with the acceptor molecule F6-TCNNQ and a p-doped ZnPc:C60 absorber blend that the doping process creates shallow trap levels. Finally, various organic hole transport materials are examined upon their stability in water and oxygen atmosphere during sample fabrication and storage of the organic electronics. In case of pentacene, ZnPc, MeO-TPD, and m-MTDATA, hole traps are already present in unexposed thin-films, which increase in trap density upon oxygen exposure. A global trap level caused by oxygen impurities is found at energies of 4.7...4.8 eV that is detrimental to hole transport in organic semiconductors. / Elektronische Bauelemente aus Dünnschichten organischer Halbleiter sind Teil möglicher Schlüsseltechnologien zur regenerativen Energiegewinnung und energieeffizienten Informationstechnik. Während Fernseh- und Mobilfunkanwendungen organischer Leuchtdioden (OLEDs) bereits vor einiger Zeit Marktreife erlangt haben, ist die organische Photovoltaik (OPV) noch durch zu hohe Fertigungskosten in Relation zu unzureichenden Effizienzen unrentabel. Ein wesentlicher Grund für die niedrigen Wirkungsgrade sind Verluste durch die im Vergleich zu Silizium hohe Zahl an Ladungsträgerfallen in organischen Halbleitern. Ziel dieser Arbeit ist es, mittels Impedanz-Spektroskopie (IS), thermisch stimulierten Strömen (TSC) und Photoelektronenspektroskopie methodenübergreifend Ladungsträgerfallen in vakuumverdampften organischen Dünnschichten zu identifizieren, zu quantifizieren und ihre Ursachen zu ergründen. Um die Energie von Ladungsträgerfallen absolut beziffern zu können, wird zunächst für verschiedene Lochtransportmaterialien wie z.B. MeO-TPD, Pentazen und ZnPc die Transportenergie aus den in erster Ordnung gaußförmigen Zustandsdichten berechnet. Im Gegensatz zu anorganischen Halbleitern ist die Transportenergie in organischen Halbleitern temperaturabhängig. Sie verschiebt sich beim Übergang von Raumtemperatur zu 10 K, wie für TSC Untersuchungen bedeutsam, um bis zu 300 meV in Richtung der Bandlückenmitte. Mittels Impedanz-Spektroskopie wird die frequenzabhängige Kapazitätsantwort von Ladungsträgerfallen in organischen Schottky-Dioden aus Pentazen und ZnPc untersucht. In undotierten Systemen dominieren Defekte mit Tiefen um 0.6 eV, deren Dichte in der Größenordnung von 1016...1017 cm−3 liegt, sich aber im Fall von Pentazen durch einen zusätzlichen Materialaufreinigungsschritt halbieren lässt. Über p-Dotierung wird das Fermi-Level so eingestellt, dass tiefe Fallen abgesättigt werden können. Umgekehrt liefert das Ausfrieren von p-dotiertem ZnPc weitere Belege für den Einfluss von Fallenzuständen, Störstellen-Erschöpfung und Reserve auf das Fermi-Level in dotierten organischen Halbleitern. Im Weiteren werden Ladungsträgerfallen über thermisch stimulierte Ströme untersucht. Es wird gezeigt, dass die Fallentiefen nur dann konsistent bestimmt werden, wenn der dispersive Transport von freigesetzten Ladungsträgern zur Extraktionsstelle berücksichtigt wird. Durch Einführung von ’Abstandshalterschichten’ werden erstmalig über TSC die Polaritäten von Ladungsträgerfallen in MeO-TPD, ZnPc und m-MTDATA per Laufzeitunterschied bestimmt. Gleichzeitig werden geringste Löcherbeweglichkeiten in der Größenordnung von 10−13 cm2 Vs−1 für stark gekühlte Dünnschichten des Lochtransporters Spiro-TTB gemessen. Wie für Spiro-TTB koverdampft mit dem Akzeptormolekül F6-TCNNQ und p-dotierte Mischschichten der Absorbermaterialien ZnPc und C60 gezeigt, erzeugt Dotierung relativ flache Störstellen. Abschließend werden verschiedene organische Lochtransporter-Materialien auf ihre Stabilität in Wasser- und Sauerstoffatmosphären während der Prozessierung und der Lagerung fertiger elektronischer Bauelemente untersucht. Für Pentazen, ZnPc, MeO-TPD und m-MTDATA werden Löcherfallen in intrinsischen Dünnschichten nachgewiesen. Bei Kontakt mit Sauerstoff nimmt deren Defektdichte zu. Es findet sich ein universales Fallenniveau bei rund 4.7...4.8 eV, verursacht durch Sauerstoffverunreinigungen, welches den Lochtransport in organischen Halbleitern limitiert.