A simple yet sensitive experimental scheme to record
high-resolution optical absorption and/or optical rotation spectra of molecules
in open air, cells or jets, using ideas from the field of
CRD spectroscopy
is demonstrated. Light from a narrow band cw laser is coupled into a high-finesse
stable optical cavity when accidentally coincident with one of the multitude
of modes of this cavity. While rapidly scanning the narrow band laser,
the time-integrated intensity exiting the cavity is monitored. Under the
conditions that the scanning laser is sufficiently long in resonance with
one of the cavity modes that the light intensity inside the cavity approaches
its limiting value, the time-integrated light intensity exiting the cavity
is in a good approximation proportional to the ring down time $\tau$. Direct
absorption spectra and/or optical rotation spectra can therefore be obtained
by plotting the inverse of the time-integrated intensity versus the laser
frequency. With a single mode laser scanning over typically a 1 cm$^{-1}$
spectral region scanning rates of 5-100 Hz have been employed in this study,
yielding high quality spectra in a matter of seconds. The noise equivalent
detection limit readily approaches 10$^{-3}$ of the baseline intensity,
fully comparable to the best CRD spectra reported to date. As the spectral
information is deduced from the time-integrated signal rather than from
the time-dependence of the signal it is possible to perform these measurements
with relatively low power lasers and cheap detection systems. The techniques
have been demonstrated by recording absorption spectra of oxygen, water, and
ammonia in a cell, oxygen and ammonia in a jet, and magnetic rotation spectra
of oxygen in a cell.
(reference)
Near-infrared Cavity Enhanced Absorption spectroscopy of
hot water and OH in an oven and flames
A compact diode laser operating around 1.5 micrometer is used to measure CEA
spectra of hot water molecules and OH radicals in radiative environments under
atmospheric conditions. Spectra of air are measured in an oven at temperatures
ranging from 300 K to 1500 K. These spectra contain rovibrational lines from
water and OH. The water spectra are compared to simulations from the HITRAN and
HITEMP databases. Furthermore, spectra are recorded in the flame of a flat
methane/air burner and in an oxyacetylene flame produced by a welding torch. The
results show that CEA spectroscopy provides a sensitive method for the rapid
monitoring of species in radiative environments.(reference)
A compact open-path optical ammonia detector is developed. A tunable external
cavity diode laser operating at 1.5 micrometer is used to probe absorptions of
ammonia via the Cavity Enhanced Absorption (CEA) technique. The detector is
tested in a climate chamber. The sensitivity and linearity of this
system is studied for ammonia and water at atmospheric pressure. A
cluster of closely spaced rovibrational overtone and combination band
transitions, observed as one broad absorption feature,
is used for the detection of ammonia. On these molecular transitions a
detection limit of 100 ppb (1 s) is determined. The ammonia measurements are
calibrated independently with a chemiluminescence monitor. Compared to other
optical open-path detection methods in the 1-2 micrometer region, the present
result shows an improved sensitivity for contactless ammonia detection by over
one order of magnitude. Using the same set-up, a detection-limit of 100 ppm
(1 s) is determined for the detection of water at atmospheric pressure.
(reference)
Absorption spectra of rotationally cold ammonia molecules have
been recorded in the 6400--6630 cm-1 region, using the cavity enhanced
absorption technique in combination with a slit nozzle expansion. Two
perpendicular rovibrational bands have been identified; the nu1+nu3
band at 6609 cm-1 , and a band at 6557 cm-1 which is
assigned to a transition into the |l|=2 component of the
nu1+2nu4 state. (reference)
Cavity Enhanced Absorption spectroscopy in the 10 micrometer region using a
waveguide CO2 laser
The cavity enhanced absorption technique is extended into the 10 micrometer region
using a line-tunable continuous wave CO2 laser. Part of the laser beam is
deflected by an acousto optical modulator (AOM), and is used to excite a
mechanically unstable high-finesse optical cavity. In order to assure a stable
and optimal transmittance of light through the cavity, the laser frequency and
the cavity eigenfrequencies are modulated independently. The time-integrated
intensity of the light exiting the cavity, which is inversely proportional to
the cavity losses, is measured using a lock-in detection scheme. An absorption
detection sensitivity of 1.5*10^{-6} cm-1 Hz^{-1/2} is readily
obtained with a rather simple setup.(reference)
Cavity Enhanced
Absorption and Cavity Enhanced Magnetic Rotation Spectroscopy.