Cavity ring-down spectroscopy (CRDS) is a laser-based absorption spectroscopy technique that is starting to find extensive application as a consequence of the very high sensitivity of the method compared with more traditional absorption spectroscopy techniques. We describe the experimental implementation of CRDS and its application to a number of areas of research including laser diagnostics of hostile environments, reaction kinetics and spectroscopy, with particular emphasis on our ongoing studies of the fast (sub-nanosecond) predissociation of electronically excited states of small molecules and radicals.

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied—most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Continuous wave (cw) diode lasers are increasingly being used as light sources in the visible and near-IR regions of the spectrum for cavity ring-down spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS); the latter technique is also widely known as integrated cavity output spectroscopy (ICOS). The very high sensitivities to weak absorptions that are possible with cw CRDS and CEAS, coupled with the quantitative nature of the absorption measurements, are enabling a rapidly expanding range of applications. We review the benefits and practical implementation of these techniques; methods of data analysis for extraction of quantitative absorption data; the sensitivities of cw CRDS and CEAS, and how they might be optimised; and applications of cw CRDS and CEAS in molecular spectroscopy, atmospheric chemistry, plasma and flame chemistry, analytical science, and medical diagnosis via breath analysis. The development of CRDS and CEAS techniques exploiting cw diode lasers and, very recently, high luminosity light-emitting diodes, has stimulated a wealth of high-sensitivity measurements. Highlights include quantitative measurement of various ultra-trace gases such as: NO₃, NO₂ and ethene in ambient air samples; CO₂ isotopologues, ethane and other organic compounds in human breath samples; and excited electronic states of N₂ and O₂ in plasmas and discharges. Exciting developments include wavelength extension into the mid-IR and UV regions, and use of novel locked-cavity techniques to increase data acquisition rates and sensitivities.

This paper presents a summary of the measurements made during the heavily-instrumented Reactive Halogens in the Marine Boundary Layer (RHaMBLe) coastal study in Roscoff on the North West coast of France throughout September 2006. It was clearly demonstrated that iodine-mediated coastal particle formation occurs, driven by daytime low tide emission of molecular iodine, I₂, by macroalgal species fully or partially exposed by the receding waterline. Ultrafine particle concentrations strongly correlate with the rapidly recycled reactive iodine species, IO, produced at high concentrations following photolysis of I₂. The heterogeneous macroalgal I₂ sources lead to variable relative concentrations of iodine species observed by path-integrated and in situ measurement techniques. Apparent particle emission fluxes were associated with an enhanced apparent depositional flux of ozone, consistent with both a direct O₃ deposition to macroalgae and involvement of O₃ in iodine photochemistry and subsequent particle formation below the measurement height. The magnitude of the particle formation events was observed to be greatest at the lowest tides with the highest concentrations of ultrafine particles growing to the largest sizes, probably by the condensation of anthropogenically-formed condensable material. At such sizes the particles should be able to act as cloud condensation nuclei at reasonable atmospheric supersaturations.