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This thesis describes the development and application of condensed-phase cavity-based spectroscopic techniques - namely cavity ring-down spectroscopy (CRDS); cavity enhanced absorption spectroscopy (CEAS); broadband cavity enhanced absorption spectroscopy (BBCEAS) and evanescent wave (EW) variants of all three. The recently-developed cavity technique of EW-broadband cavity enhanced absorption spectroscopy (EW-BBCEAS) has been used—in combination with a supercontinuum source (SC) and a sensitive, fast readout CCD detector—to record of the full visible spectrum (400–700 nm) of a silica-liquid interfacial layer (with an effective thickness ca. 1 µm), at rapid acquisition rates (> 600 Hz) that are sufficient to follow fast kinetics in the condensed phase, in real time. The sensitivity achieved (Amin= 3.9 x 10-5) is comparable with previous EW-CRDS and EW-CEAS studies, but the spectral region accessed in this broadband variant is much larger. The study of liquid|air interfaces using EW cavity-based techniques is also illustrated for the first time. The first application of BBCEAS to the analysis of microfluidic samples, flowing through a microfluidic chip, is illustrated. Proof-of-principle experiments are presented, demonstrating the technique’s ability to provide full visible broadband spectral measurements of flowing microfluidic droplets, with both high detection sensitivity (αmin < 10-2 cm-1) and excellent spatial and temporal resolution: an SC light source and sensitive, fast readout CCD allowed measurement repetition rates of 273 Hz, whilst probing a very small sample volume (ca. 90 nL). A significant portion of this thesis is devoted to demonstrating the powerful capabilities of CEAS, CRDS and BBCEAS in monitoring radical recombination reactions and associated magnetic field effects (MFEs) in solution. The efficacy of CEAS as a high-sensitivity MFE detection method has been established in a proof-of-principle study, using narrow band CEAS in combination with phase-sensitive detection: MFE-induced absorbance changes of ca. 10-6 could be detected using the modulated CEAS technique and the data are shown to be superior to those obtained using conventional transient absorption (TA) methods typically employed for MFE measurements. The powerful capabilities of CRDS in monitoring radical recombination reactions and associated MFEs are also demonstrated. In particular, a pump-probe CRDS variant allows not only high sensitivity (Amin on the order 10-6), but also sub-microsecond time-resolution. Combined, these features represent significant advantages over TA. Finally, SC-BBCEAS is used to measure full visible spectra of photoinduced reactions and their MFEs. The applicability of this approach to in vitro MFE studies of Drosophila cryptochrome is demonstrated—the results mark the first in vitro observation of a magnetic field response in an animal cryptochrome, a key result supporting the hypothesis that cryptochromes are involved in the magnetic sense in animals.

A range of spectroscopic and computational techniques have been applied to the study of three metal cluster systems: vanadium monoxide, the Au2 molecule, and RhnN2O+ clusters. A new instrument has been built for spectroscopy experiments on metal clusters, consisting of a laser ablation cluster source and a linear time-of-flight mass spectrometer. The instrument was characterised using nitric oxide spectroscopy and applied to the electronic REMPI spectrum of vanadium oxide in the visible region. The rotational constants and band origins of several known states have been determined, and the observation of a new spin-forbidden transition has been used to connect the energies of the quartet and doublet manifolds of VO. A new 3 2Π state was also observed and characterised. The photoionisation and photodissociation of Au2 were then studied at 157 nm, and between 35500 and 37200 cm-1 with another new instrument recently constructed in the group. Excited and ground state Au photofragments were produced in both spectral regions, and have been detected and assigned using velocity map imaging. The 157 nm photodissociation produced gold atom products in the seven highest-energy accessible channels in a single-photon dissociation process. The complex near-UV spectrum involved two-photon excitation to two 0g+ excited states close to their dissociation thresholds, followed by predissociation to thirteen different Au product channels. The branching ratios for dissociation into each of these channels varied across the spectrum as different dissociation limits and curve crossings occurred. The mid- and far-infrared multiple-photon dissociation spectra of RhnN2O+ clusters have been recorded using the argon-tagging action spectroscopy technique and free electron laser radiation. The results have been compared to density functional theory calculations to deduce the nature of the binding and the likely low-lying electronic and geometrical structures. The N2O was found to be molecularly adsorbed on the surface of the cluster but, upon infrared heating of the complex via the N2O vibrational modes, was observed to undergo a reaction, producing N2 and cluster oxides RhnO+. The reaction is believed to be thermal and mode-independent, but the efficiency of the surface reaction does vary with cluster size, with the n=5 cluster showing no detectable reaction.