Cavity Ringdown Spectroscopy (CRDS)

Cavity ringdown spectroscopy (CRDS) is a sensitive technique for measuring the absorbance of a substance. It was originally developed as a tool for determining the reflectivity of mirrows, and has since been widely employed in measuring weak spectroscopic transitions and concentrations of trace gases. Early work with CRDS was limited to the visible region of the spectrum because the high-reflectivity mirrors required by the technique were not available in other regions of the electromagnetic spectrum. Recently, though, high-reflectivity mirrors have become available in the mid-infrared area, enabling high-sensitivity and high-resolution studies of weak vibrational bands, as well as vibrational bands of molecular ions that can only be produced at very low concentrations.

Experimental Technique

An overview of the ringdown experimental technique.

CRDS requires a stable optical cavity, formed with two highly reflective (R > 99.98%) concave mirrors. In the continuous-wave (cw) version of the technique that we use, a cw laser is directed into the cavity, and the length of the cavity is dithered with a piezoelectric crystal. As the length of the cavity changes, the frequency of light that is resonant with the cavity also changes. When the cavity becomes resonant with the laser, light begins to enter the cavity. Its intensity builds up as it reflects back and forth, but also leaks out at a rate that depends on the losses of the mirrors, the intracavity light intensity, and the presence of any molecules that absorb light at that frequency.

The equations used to derive the absorption cross section from the ringdown time constant measurement.

The light leaking out of the cavity is directed onto a detector, and when the signal reaches a predetermined threshold value, the laser beam is diverted away from the cavity by using an acousto-optic modulator. The intracavity light decays exponentially (or, "rings down") with a characteristic time constant τ, which is related to the product of the absorption cross-section and number density (Nσ, or effective absorbance, α), the reflectivity of the mirrors (R), and the distance between the mirrors (L). When an absorbing species is present, the intracavity intensity decays more rapidly owing to the absorption, and τ decreases. After measuring τ for several ringdown events at one frequency, we then tune the laser to a new frequency, measure the new time constant, and repeat until we have covered the spectral region of interest.

Advantages of cw-CRDS

Our CRDS system has a minimum detectable absorption coefficient of ~2x10^-9cm^-1. This is equivalent to being able to measure an absorbance of <5 x 10^-7, which is several orders of magnitude more sensitive than a typical laboratory UV/Vis or FTIR spectrometer. Part of the reason for this increased sensitivity is that because we measure a time constant and not an actual intensity, the measurement is immune to intensity fluctuations in the light source. This is unlike typical absorbance measurements, in which intensity fluctuations are usually the dominant source of uncertainty in the absorbance measurement. As a direct absorption technique, CRDS is generally applicable to any molecule, as it does not depends on any "special" molecular properties. Finally, because this technique is gas-phase, there is no need to account for solvent shifts in the measurements, meaning that our measurements directly probe structure and intramolecular dynamics, and can be easily used to search for molecules in space, or as benchmark data for improving computational chemistry methods.

High-Repetition-Rate CRDS

We are currently working on implementing changes to our ringdown system to greatly increase the rate at which we are able to acquire ringdowns. By keeping the cavity at a fixed length and using a feedback loop, it is possible to “lock” the laser to a cavity resonance, so laser light is always being coupled into the cavity (note: the time that the laser is diverted away from the cavity by the AOM is sufficiently small to maintain a frequency lock when the laser is returned to the cavity). To scan to a different wavelength, the piezo-mounted mirror is slightly moved, and the laser wavelength automatically adjusts to stay locked. By using this method, the ringup and ringdown time constants are the primary factors that limit the rate at which ringdown events can be collected. This will allow for tens of thousands of ringdowns to be collected per second as opposed to the 10-100 per second acquired in the normal CRDS technique. By using the same integration time as traditional CRDS, the S/N can be improved by over an order of magnitude.