Cavity Ring-down Spectroscopy is a direct absorption spectroscopy that has been used extensively to study the spectra of molecules in the gas phase. The principles are relatively simple and rely in the properties of an optical resonator, Figure 1. Pulsed laser radiation enters the cavity through the first mirror, M1, and makes a number of round trips between the mirrors. The round trip time tr is controlled by the cavity length. At each bounce on the second mirror, M2 some light leaks from the cavity and is detected at a photomultiplier. For a 1m cavity the round trip time is 6.7 ns. The intensity in the cavity decays exponentially and is characterised by a ring-down time, tau, the time it takes the intensity to fall to 1/e of its initial value. For mirror reflectivites of 99.99% the ring-down time tau is or order 30 microseconds during which time the pulse has made 5000 round trips, giving a pathlength of 5km.
When a molecule is resonant with the radiation in the cavity, the molecule absorbs the radiation which is seen as an additional loss from the cavity. The absorption changes the ring-down time, tau. Measurement of tau as a function of wavelength produces the spectrum of the molecule. The Minimum Detectable Absorption Loss (MDAL) determines the sensitivity of the experiment. For pulsed laser systems the MDAL under favourable conditions gives detection limits of 10 8 molecules cm-3. For CW implementations of CRDS the detection levels improve by approximately three orders of magnitude.
The e-CRDS technique has been described in detail elsewhere and will be discussed only briefly here. , Two high-reflectivity mirrors (99.95%) are placed opposite one another to form a linear optical cavity into which laser radiation from continuous wave (cw) diode lasers (at 635 nm or 830 nm) is introduced. The bandwidth of each of the lasers is sufficiently large to overlap more than one cavity mode in a free-running configuration with the radiation building up to a maximum intensity, determined by the cavity Q-factor. The laser is switched off at a repetition rate of 6 kHz and the radiation intensity in the cavity decays with a ring-down time, , determined similarly by the Q-factor. Any species that absorb radiation at the wavelength of the laser cause a decrease in the ring-down, , that is related directly to the absorbance of the species. The timescales of the experiment are calibrated by a digital oscilloscope and provide absolute measures of absorbance.
A Dove prism is introduced to the cavity to provide a total internal reflection (TIR) event while preserving the linear path of the radiation. The TIR event generates an evanescent wave at the interface between the prism and rarer medium and molecules present within the evanescent field can remove radiation from the cavity either by absorbance or scatter causing a change in . The evanescent field penetrates into the medium above the prism, falling to 1/e of its initial intensity at a depth determined by the ratio of the refractive indices of the media and the angle of incidence. For water with the current configuration, the penetration depths are 186 nm and 245 nm for the 635 nm and 830 nm cavities respectively, Figure 2.
Development of the technique is on-going in the group.
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M.D. Wheeler, S.M. Newman, A.J. Orr-Ewing ad M.N.R. Ashfold, J.Chem. Soc Faraday Trans. 1998, 94 337-351.