We report on the application of an innovative spectroscopic balancing technique to measure isotopologue abundance
quantification. We employ quartz enhanced photoacoustic spectroscopy in a 2f wavelength modulation mode as an
absorption sensing technique and water vapor as a test analyte. Isotope absorption lines with very close lower energy
levels and with the same quantum numbers have been selected to limit the sensitivity to temperature variations and
guarantee identical broadening relaxation properties. A detection sensitivity in measuring the deviation from a standard
sample δ18O of 1.4%o, in 200 sec of integration time was achieved.
An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two
excitation sources. We used quartz enhanced photoacoustic spectroscopy (QEPAS) in a 2f wavelength modulation mode
as an absorption sensing technique and employed a modulation cancelation approach for spectroscopic measurements of
small temperature differences in a gas mixture and detection of broadband absorbers. We demonstrated measurement of
small temperature differences in a C2H2/N2gas mixture with a sensitivity of 30 mK in 17 sec and detection of hydrazine,
a broadband absorbing chemical species, down to concentration of 1 part per million in volume in 1 sec. In both cases we
used near-infrared laser diodes and selected overtone transitions.nuscrip
We report on novel methods employing a modulation cancellation technique which result in a significantly increase in
the sensitivity and accuracy of trace gas detectors. This method can be applied for isotopomer abundance quantification,
temperature measurements and the detection of large molecules.
A continuous wave, thermoelectrically cooled, distributed feedback quantum cascade laser (DFB-QCL) based
sensor platform for the quantitative detection of ammonia (NH3) concentrations present in exhaled human breath is
reported. The NH3 concentration measurements are performed with a 2f wavelength modulation quartz enhanced
photoacoustic spectroscopy (QEPAS) technique, which is very well suited for real time breath analysis, due to the fast
gas exchange inside a compact QEPAS gas cell. An air-cooled DFB-QCL was designed to target the interference-free
NH3 absorption line located at 967.35 cm-1 (λ~10.34 μm). The laser is operated at 17.5 °C, emitting ~ 24 mW of optical
power at the selected wavelength. A 1σ minimum detectable concentration of ammonia for the line-locked NH3 sensor is
~ 6 ppb with 1 sec time resolution. The NH3 sensor, packaged in a 12"x14"x10" housing, is currently installed at a
medical breath research center in Bethlehem, PA and tested as an instrument for non-invasive verification of liver and
kidney disorders based on human breath samples.
A quartz-enhanced photoacoustic spectroscopy (QEPAS) based multi-gas sensor was developed to quantify
concentrations of carbon monoxide (CO), hydrogen cyanide (HCN), hydrogen chloride (HCl), and carbon dioxide (CO2)
in ambient air. The sensor consists of a compact package of dimensions 25cm x 25cm x 10cm and was designed to
operate at atmospheric pressure. The HCN, CO2, and HCl measurement channels are based on cw, C-band
telecommunication-style packaged, fiber-coupled diode lasers, while the CO channel uses a TO can-packaged Sb diode
laser as an excitation source. Moreover, the sensor incorporates rechargeable batteries and can operate on batteries for at
least 8 hours. It can also operate autonomously or interact with another device (such as a computer) via a RS232 serial
port. Trace gas detection limits of 7.74ppm at 4288.29cm-1 for CO, 450ppb at 6539.11 cm-1 for HCN, 1.48ppm at
5739.26 cm-1 for HCl and 97ppm at 6361.25 cm-1 for CO2 for a 1sec average time, were demonstrated.
Optothermal detection is a spectroscopic technique where the energy input into a gas or other media caused
by absorption of optical radiation is measured directly by means of a thermal detector.1-3 A fraction of the
absorbed energy is transported to the thermal detector by heat conduction or molecular diffusion. In this work a
conventional thermal sensor was replaced by a quartz tuning fork (QTF), and the optical power input into the gas
was modulated at the QTF resonant frequency. We call this approach "resonant optothermoacoustic detection",
or ROTADE. The same experimental setup can be used to conduct a closely related technique, quartz enhanced
photoacoustic spectroscopy (QEPAS).4 QEPAS relies on energy transfer from the initially excited molecular
vibrational state to the translational degrees of freedom. In some cases this process is too slow to follow the
modulation required for QEPAS. In other cases, the resonant energy transfer can result in vibrational excitation
of nitrogen, which relaxes very slowly. ROTADE, on the other hand, detects the energy delivered by molecules
even if this energy is still in the form of vibrational excitation. The molecules will then release their energy to the
QTF upon collision with its surface. Experimental investigations of ROTADE and its comparison with QEPAS
were performed in pure CO2 and 0.5% acetylene in N2 using near-infrared diode lasers. A fiber collimator and
a refocusing lens were used to focus the laser to a ≈15 μm diameter waist. Its position was scanned in the QTF
plane using a 3D translation stage with computer-controlled actuators. Different QTFs were used to compare
the effect of modulation frequency on the ROTADE signal.
The ν1+ν3 combination band of uranium hexafluoride (UF6) is targeted to perform analytical enrichment measurements
using laser absorption spectroscopy. A high performance widely tunable EC-QCL sources emitting radiation at 7.74 μm
(1291 cm-1) is employed as an UF6-LAS optical source to measure the unresolved rotational-vibrational spectral
structure of several tens of wavenumbers (cm-1). A preliminary spectroscopic measurement based on a direct laser
absorption spectroscopy of methane (CH4) as an appropriate UF6 analyte simulant, was demonstrated.
With the anticipated retirement of Space Shuttles in the next few years, the re-supplying of short-lifetime sensors on the
International Space Station (ISS) will be logistically more difficult. Carbon Monoxide (CO) is a well-known combustion
product and its absence in a fire and post-fire environment is a reliable indicator for mission specialists that the air
quality is at a safe to breathe level. We report on the development and performance of a prototype compact CO sensor,
based on the PHOTONS platform [1], developed for the ISS based on tunable diode laser absorption spectroscopy
(TDLAS). A CO absorption line at ~4285 cm-1 is targeted using a distributed-feedback (DFB) laser diode operating at
room temperature. A custom designed Herriott multipass cell 16cm long, with an effective path length of 3.7 m is
employed. Mechanical, optical and electronics systems are integrated into a compact package of dimensions measuring
12.4"x 3.4"x 5". Power consumption is less than 1 W, enabling prolonged battery life. A detection limit of 3 ppm is
achieved when performing 40 second long temperature scans. A recent initial test at NASA-JSC was successful. Future
improvements include the reduction of the sampling volume, scan time and an improved CO minimum detection limit.
Recent advances in the development of sensors based on infrared quantum cascade lasers for the detection of trace gas
species is reported. Several selected examples of applications in environmental and industrial process monitoring as well
as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be
described.
There is an increasing need in many chemical sensing applications ranging from industrial process control to environmental science and medical diagnostics for fast, sensitive, and selective gas detection based on laser spectroscopy. The recent availability of novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers as mid-infrared spectroscopic sources address this need. A number of spectroscopic techniques have been demonstrated. For example, the authors have employed QC-DFB lasers for the monitoring and quantification of several trace gases and isotopic species in ambient air at ppmv and ppbv levels by means of direct absorption, wavelength modulation, cavity enhanced and cavity ringdown spectroscopy. In this work, pulsed thermoelectrically cooled QC-DFB lasers operating at ~15.6 μm were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on the repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the gas sensor giving an advantage of room-temperature operation and low cost. Absorption lines of CO2 and H2O were observed in ambient air providing information about the concentration of these species.
KEYWORDS: Molecules, Gases, Energy transfer, Ultrafast phenomena, Krypton, Raman spectroscopy, Process modeling, Xenon, Chemical species, Molecular energy transfer
Time-resolved Raman probing technique is used to investigate the collisional relaxation of high-laying C-H vibrations in CHF3 and CHF2Cl molecules. The relaxation rates both in collisions with parent molecules and a number of buffer gases were measured. The rate values were found to be abnormally fast. The relaxation is faster than single collision in parent gas and requires a few (3 - 8) collisions for nonpolar gases. For polar gases the relaxation is still faster. The conclusion is made that observed ultrafast relaxation of C-H modes is caused by energy transfer from excited `bright' state to nearby `dark' ones. The theoretical model of this process along with the numerical estimations is presented.
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