Figure 8. Evanescent wave cavity ringdown spectroscopy
Numerous variants of cavity ring-down spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS) are well-established methods for the detection of highly dilute or weakly absorbing gas-phase species. Both methods take advantage of the enormously increased optical path lengths which may be achieved from multiple passes through a sample located within a high-finesse cavity. Extension of the techniques to the condensed phase has been comparatively slow and yet it is here that such methods could help address some important technological problems. We are developing a range of condensed phase variants of both cavity ringdown and cavity enhanced absorption spectroscopy including some utilising evanescent fields and scanning micro-electrochemistry to study dynamical interfaces.
Combining a high power supercontinuum (broad band) light source with a sensitive, fast readout detector permits the recording, with a good signal to noise ratio, of the full visible spectrum (400-700 nm) of an interfacial layer at rates >600 Hz with data accumulation times of only 10 μs. This is sufficient to follow fast kinetics in condensed phases in real time and has been demonstrated in the broad band detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement.
Figure 9: Combining electrochemistry with CEAS
Cavity enhanced absorption spectroscopy (CEAS) has been combined with phase-sensitive detection and employed to study the effects of static magnetic fields on radical recombination reactions. Interest in the behaviour of radical recombination reactions in magnetic fields had grown dramatically over the last decade, encouraged by the role radicals may play in the magnetic sense in animals. The high sensitivity afforded by modulated CEAS detection offers new possibilities such as the measurement of magnetic field effects in real biological systems which have hitherto been largely beyond the detection capabilities of existing techniques.
Figure 10: Radical recombination reactions within the sample cell are probed using CEAS in the presence of an applied magnetic field. A pair of Helmholtz coils (indicated by concentric circles in the schematic and pictured right) used to generate the magnetic field, B = B0 + ΔB(t), surround the sample cell which lies at the centre of an optical cavity formed by mirrors M1 and M2.
An existing folded optical cavity arrangement has been modified to allow the simultaneous EW-CRDS-measurement of interfacial absorbances of both s- and p-polarized light, the ("dichroic") ratio of which has the potential to provide information on the average orientation of adsorbates: via preferential absorption of s-/p-polarized light depending on the orientation of the transition dipole moment.
Figure 11: Flow injection EW-CRDS (upper figure) used to study in situ the kinetics of porphyrin adsorption to the silica-water interface and the interaction of DNA with the resulting functionalized surface (lower figure).