Cavity Ring-down Spectroscopy

Cavity-ringdown laser absorption spectroscopy (CRLAS) is a complementary technique to laser-induced fluorescence (LIF) because it is suited to the detection of species difficult to detect by LIF. Singlet methylene is such a species. A combustion intermediate that cannot be trapped because of its high reactivity, singlet methylene also cannot be measured by molecular beam spectrometry because its ionization potential is so close to its ground triplet state. Neither can it be measured by LIF becauseits fluorescence is rapidly quenched. However, singlet methylene has accessible transitions in the visible region of the spectrum (620 nm), where good mirrors can be easily produced. Since CRLAS is absorption based, it is not limited by the quenching effects that render LIF useless. In Andy McIlroy's setup in the Flame Chemistry and Diagnostics Laboratory, pulsed laser light is injected into a cavity formed by a pair of highly reflective mirrors. The trapped light reflects back and forth in the cavity, and a constant fraction is transmitted through the exit mirror for each round trip. The intensity envelope of these pulses exhibits a simple exponential decay, wherein the time to reach 1/e of the initial pulse intensity is called the "cavity-ringdown" time. Ideally, the ringdown time depends only on the mirror reflectivities, cavity dimensions, and the sample absorption between the mirrors.