A CAVITY RINGDOWN SPECTROSCOPY MERCURY CONTINUOUS EMISSION MONITOR Page: 4 of 13
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Section 2: Experimental
The experimental technique that will be used for this project is Cavity Ring-Down spectroscopy (CRD).
CRD is a sensitive absorption technique that was first developed by O'Keefe and Deacon in 1988.1 This
technique injected a pulse of light into a stable optical cavity formed by two highly reflecting mirrors.
The light reflects back and forth in the cavity giving extremely long effective pathlengths. Using mirrors
with a reflectivity of 99.99% and a 1 meter long cavity it is possible to achieve an effective pathlength of
10 kilometers. As the light reflects back and forth in the cavity a small amount of light is transmitted
through the end mirror of the cavity to a photon detector such as a photomultiplier tube. The light exiting
the cavity decays exponentially with time at a rate determined by round trip loss mechanisms within the
cavity. The measured time constant for the exponential decay of light is called the "ring-down time" of
the cavity. The dominant loss mechanism for an empty cavity is the mirror transmission. However if a
sample species, which absorbs light at a particular wavelength, is placed within the cavity the ring-down
time will decrease from that of the empty cavity at that particular wavelength. The ring-down time is
c[(1-R)+l a , + blc] (1)
where, le is the cavity length, c is the speed of light, R is the reflectivity of the mirrors, aS is the absorption
coefficient of the sample species of interest, is is the pathlength through the sample, and ab is the cross
section for various background losses. Background losses, which are all include in the last term of the
denominator of eq. 1, can include scattering losses due to Mie or Rayleigh scattering or absorptions due to
other components in the sample gas stream. Once the empty cavity losses and any other background
losses have been determined, CRD spectroscopy provides an absolute measure of the concentration of the
absorbing sample of interest within the cavity. This self-calibrating feature differentiates CRD from other
highly sensitive laser-based methods such as laser induced fluorescence (LIF) or resonantly enhanced
multiphoton ionization (REMPI).2
The laser source that will be used for the project is a pulsed Alexandrite laser. This is a solid-state laser
that runs at 50 Hz, will produce pulse energies of >0.5 mJ/pulse at 254 nm, and has a fundamental laser
linewidth of about 10 GHz. A diode seed laser is used with the system to bring the fundamental linewidth
down to approximately 60 MHz, which results in a laser linewidth of about 180 MHz at 254 nm. This
laser pulse will be spatially filtered and mode matched to the ring-down cavity that will be used.
Preliminary results indicate a cavity length of 56 cm is optimum for specially coated plano-concave
mirrors with a 6 m radius of curvature. The actual absorption cell that will be used for the mercury
detection will be placed between the highly reflecting mirrors.
The design of the CRD absorption cell will have an inlet for the sample containing mercury and exit for a
continuous flow. The design is such that a low flow of inert gas can be sent over the face of the highly
reflecting mirrors to insure no degradation of the mirror surface, due to deposited contaminants, and
hence a decrease in the ring-down time resulting in a decrease of the sensitivity of the CRD instrument. A
diagram of the absorption cavity is shown in Fig. 1. From the figure it can be seen that the flow of sample
flue gas will enter the cavity near the end and be exhausted from the center of the cavity to maintain the
gas flow away from the mirror surfaces.
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Christopher C. Carter, Ph.D. A CAVITY RINGDOWN SPECTROSCOPY MERCURY CONTINUOUS EMISSION MONITOR, report, October 1, 2002; Pittsburgh, Pennsylvania. (digital.library.unt.edu/ark:/67531/metadc742672/m1/4/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.