The sensitive detection of NO by Faraday modulation spectroscopy with a quantum cascade laser

A spectrometer based on a quantum cascade laser and capable of operating at particular wavelengths in the mid-infrared with very high sensitivity for the detection of open-shell molecules has been developed. It exploits magnetic field modulation in the Faraday rotation configuration. The signals for nitric oxide (NO) that may be observed with this instrument have been studied and their dependence on the J and Ω quantum numbers investigated with a simulation program. It is shown that the Q(3/2) transition of NO in the ²Π_3/2 component at 1875.8 cm^?1 would provide the greatest sensitivity for detection. The experimental observation of the R(21/2) transition of the Ω = 1/2 component gives a detection limit of 41 ppb of NO in air at a pressure of 25 mbar. Detection of NO through the Q(3/2) transition would provide a detection limit of 4 ppb at this pressure.     

Time resolved simultaneous detection of 14NO and 15NO via mid-infrared cavity leak-out spectroscopy

We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near λ?=?5 μm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R?=?99.988 %), we observed a noise-equivalent absorption coefficient of 3?×?10^?10 cm^?1Hz^?1/2. This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for ¹⁴NO and 40 ppt for ¹⁵NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The δ¹⁵N value was obtained with a precision of ±1.2‰ in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of ¹⁵NO as a tracer molecule for endogenous biomedical processes.     

Time resolved simultaneous detection of 14NO and 15NO via mid-infrared cavity leak-out spectroscopy

We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R = 99.988 %), we observed a noise-equivalent absorption coefficient of 3 x 10(-10) cm(-1)Hz(-1/2). This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for (14)NO and 40 ppt for (15)NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The delta(15)N value was obtained with a precision of +/-1.2 per thousand in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of (15)NO as a tracer molecule for endogenous biomedical processes.     

Simultaneous online detection of isoprene and isoprene-d2 using infrared photoacoustic spectroscopy

A photoacoustic spectrometer for the simultaneous detection of isoprene and the deuterated species [4,4-²H]-2-methyl-1,3-butadiene (isoprene-d₂) is presented. Using a sealed-off ¹³CO₂ laser a single-component detection limit of 400 ppt (isoprene) and 600 ppt (isoprene-d₂) was achieved. Simultaneous monitoring of both compounds allowed the detection of labelling levels down to 6% (isoprene-d₂ in total isoprene) with a time resolution of 3 min. In emission studies with Eucalyptus globulus, the deuterated precursor [4,4-²H]-1-deoxy-D-xylulose was fed to a leaf through the transpiration stream. Emission of isoprene-d₂ started as early as 10 min after application of the precursor. Received: 3 May 2002 / Revised version: 31 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-228/733474, E-mail: frank.kuehnemann@iap.uni-bonn.de     

Detection of small traces of 15N2 and 14N2 by Faraday LMR spectroscopy of the corresponding isotopomers of nitric oxide

15 N₂ in a gas phase sample is described. The nitrogen is transformed by a microwave discharge into nitric oxide NO. The latter is analyzed by recording both a ¹⁵NO and a ¹⁴NO Faraday LMR signal. The determination of the transformation rate from N₂ into NO is described. The method of measurement and the achieved sensitivity (∼0.1 ppm ¹⁵N₂≈4 nanomoles ¹⁵N₂/litergas) of the spectrometer are discussed. An application in pharmacology, where ¹⁵N is used as a tracer for metabolism is indicated. First experiments with the exhalation of a rat show that the apparatus is useful to give a new quality of results. Received: 30 April 1996/Revised version: 29 July 1996     

Multi-harmonic detection in wavelength modulation spectroscopy systems

Two multi-harmonic detection methods for wavelength modulation spectroscopy (WMS) systems are presented and compared. The two possibilities discussed in this paper are: simultaneous curve fitting of multiple harmonic spectra, and reconstruction of the transmission from harmonic coefficients. The optimum number of harmonics is four and 25 harmonics, respectively. Compared with standard single-harmonic curve fitting, the methods give about a factor of 3 better performance than standard second-harmonic curve fitting. Concluding, multi-harmonic detection is better than single-harmonic detection and should be used if the system bandwidth is high enough to allow for proper detection of the higher harmonics.     

Measurements of the 3ν 3 band of 14N15N16O and 15N14N16O using continuous-wave cavity ring-down spectroscopy

The absorption spectra of the 3ν₃ band of nitrous oxide isotopologues, ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O, have been measured using diode laser cavity ring-down spectroscopy in 6400–6463 and 6465–6532 cm^-1, respectively. Spectroscopic parameters and the rotational line intensities of the bands have been determined. We have applied this spectroscopic technique to the measurements of the absolute isotope ratio of those isotopologues using the absolute line intensities. PACS 32.10.Bi; 33.20.Vq; 33.70.-w; 42.55.Px; 42.62.Fi     

A differential detection scheme for Faraday rotation spectroscopy with a color center laser

A differential detection scheme to reduce source noise and to improve sensitivity based upon the Faraday effect has been developed. This method is particularly useful in connection with low-power, high-noise laser sources and multipass arrangements where a degradation of polarization quality is substantial. The theoretical background of the differential method is developed. It is tested with magnetic rotation signals from transitions of stable species with low modulation sensitivity and signals from free radicals.     

Investigation of passive metal electrodes using modulation spectroscopy

There are many uncertainties left about the structure of semiconducting passive layers on metal electrodes. In this report an optic-electric method is described which makes it possible to determine the change of the layer structure in relation to the electrode potential. By modulating the electrode potential this method results in changing the oxidation degree in connection with the optical properties of the surface layer. Measuring the modulated reflectance of a passive nickel electrode yields new information about the structure of the layer in the potential region between the first and the second passivity. Further investigations of iron and titanium electrodes are described.     

Simultaneous antigen detection using multiplex dyes

To detect several antigens simultaneously, antibodies directed against different antigens were immobilized on a quartz surface. The antigens were tagged with multiplex, dyes, which show different fluorescence lifetimes but similar excitation and emission spectra. The antigens were detected by recognizing the characteristic fluorescence lifetime. Furthermore, the effect of labeling of the dye on the antigen molecules was examined.     

Hydrogen doppler spectroscopy using 15N ions

The energy spread of atomic and molecular ion beams from the 4 MV Dynamitron tandem accelerator at the Ruhr-Universität Bochum has been studied and in part minimized. Using the E_R = 6.40 MeV narrow resonance in ¹H(¹⁵N,!%)¹²C with an ¹⁵N energy spread of 4.55 keV, the Doppler broadening for several hydrogen-bearing gases was found to be in good agreement with expectation: e.g. for NH₃ gas a rotational-vibrational Doppler width of 10.41 - 0.25 keV was observed (theory = 10.4 keV). Studies of the vibrational Doppler widths of H-bonds on a Si 𘜄¢ surface were performed using a 4?%-ray detection system together with UHV-chambers for sample preparation, transport, and analysis. The results showed that further improvements in the experimental set-ups are needed for such investigations.     

Application of a broadband blue laser diode to trace NO2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450?nm. A detection limit of 18?ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7?mW at a 1?s integration time, which corresponds to a 1?σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1)?W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.     

Band structure investigations of GaN films using modulation spectroscopy

The paper presents investigation results concerning band structure of gallium nitride and position of intrinsic and associate defect levels. Main optical characteristics (transmission, reflection and luminescence) were measured in both ordinary and λ-modulation mode for epitaxy-grown GaN films, allowing to determine valence band splitting caused by spin-orbital interaction (48 meV) and crystalline field (10 meV). Analysis of photoluminescence spectra made it possible to identify main recombination mechanisms involving donor and acceptor levels formed by intrinsic point defects , and their associates.     

FLaser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the 87Rb atom with D2 line transition F = 2 →F＇ = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro-optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.     

Laser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the87 Rb atom with D2 line transition F = 2 → F = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro–optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.