Peltier Controlled Growth of Thin Ice Films in the Laboratory and Advancing the Methodology of Cavity Enhanced Laser Induced Fluorescence

Abstract

Cavity enhanced laser induced fluorescence (CELIF) is the first technique to combine cavity ring-down (CRDS) and laser induced fluorescence (LIF) spectroscopies in a single beam experiment. It has been shown previously to extend the dynamic range of CRDS to cover six orders of magnitude in total when observing BPEB concentrations seeded in a pulsed molecular beam. This study has extended CELIF to the most general application where a fluorescer or scatterer fills the length of a pulsed CRD experiment. Under these conditions CELIF is found to produce consistently smaller errors than CRD and is competitive with it but does not extend the dynamic range. Observing acetone fluorescence and nitrogen Rayleigh scattering it has been shown how the CRD signal normalises the LIF signal generated and that the normalisation remains linear during changes to the input powers, pressures and detector gains. Furthermore it has been shown it can be used to measure absolute quantum yields of fluorescence using acetone as an example.

A peltier based set-up for cooling the upper surface of a prism for the growth of thin ice films at temperatures of the troposphere and stratosphere has been constructed. A full temperature range of 225-303K was displayed. Testing showed the optimal conditions of ice growth to be a rapid expansion directed at the surface. Ice films 2.5-11.8um thick have been successfully grown at 225.2+-0.2K covering, at maximum, 96% of a 1cm by 3cm stainless steel prism surface. During growth a strong migration over time to an area 0.028+-0.002cm^2 was seen caused by a temperature gradient on the surface, dT~5K from the centre to the outside of the surface along its short side. To monitor this and ice growth, two methods have been successfully installed and tested. A morphological analysis combined with video monitoring can accurately determine areas within 5% and a HeNe laser reflected from the ice is able to monitor surface thicknesses from interference patterns. Together these offer a complete method to characterise an ice film over the duration of an experiment.