Laser Photoacoustic Spectroscopy Research Articles

External cavity quantum cascade laser for quartz tuning fork photoacoustic spectroscopy of broad absorption features

We demonstrate mid-infrared spectroscopy of large molecules with broad absorption features using a tunable external cavity quantum cascade laser. Absorption spectra for two different Freons are measured over the range 1130-1185 cm(-1) with 0.2 cm(-1) resolution via laser photoacoustic spectroscopy with quartz tuning forks as acoustic transducers. The measured spectra are in excellent agreement with published reference absorption spectra.

High-sensitivity detection of TNT

We report high-sensitivity detection of 2,4,6-trinitrotoluene (TNT) by using laser photoacoustic spectroscopy where the laser radiation is obtained from a continuous-wave room temperature high-power quantum cascade laser in an external grating cavity geometry. The external grating cavity quantum cascade laser is continuously tunable over approximately 400 nm around 7.3 microm and produces a maximum continuous-wave power of approximately 200 mW. The IR spectroscopic signature of TNT is sufficiently different from that of nitroglycerine so that unambiguous detection of TNT without false positives from traces of nitroglycerine is possible. We also report the results of spectroscopy of acetylene in the 7.3-microm region to demonstrate continuous tunability of the IR source.

Study of Gas Exchange in Insects by Sensitive Laser Photoacoustic Spectroscopy

Although quantifying gas exchange in small insect species is of great biological interest, the progress in this field of research is hampered by the inability of most gas detectors to monitor the low emission rates from these insects. Recently, laser based photoacoustic spectroscopy (LPAS) has emerged as a highly sensitive technique that allows gases to be detected at sub‐ppb level. In this work, LPAS was used to study gas exchange in two different insect species. Firstly, loss of water by the minute Western Flower thrips (Frankliniella occidentalis, average weight 50 µg) was recorded by means of CO‐laser PAS. Water loss by a single thrips could be on‐line recorded with a practical detection limit of 30 ppb. Insects released H2O in rather regular patterns, possibly due to discontinuous respiration. These data represent on‐line H2O loss recordings of the smallest insects measured hitherto. In a second experiment, the inert tracer gas SF6 was used to determine the tracheal volume of Attacus atlas pupae. Insects were loaded with SF6 and subsequent release of this gas was monitored using CO2‐laser PAS. Based on the amount of released SF6, one has determined the tracheal volume of Attacus atlas pupae. The value obtained (170±53 µL/g) is in good correspondence with data obtained by other techniques. In addition, with this technique, respiration patterns can be recorded; the results show a good correspondence with the simultaneous recordings of CO2 release using a commercial CO2 analyser.

CO2 Laser Photoacoustic Spectroscopy Applied to Environmental Analysis in the Metropolitan Area of Mexico City

Measurements of atmospheric ethene were performed in the Metropolitan Area of Mexico City by means of a 12C16O2 laser‐based photoacoustic spectrometer. Two types of experimental campaigns were conducted. In the first type, performed in November 1999 and March 2000, air samples were simultaneously collected in canisters during the morning (06:00 to 09:00 h) in Xalostoc (industrial site), La Merced (commercial site), and Pedregal (residential site). The highest ethene levels were usually detected in Xalostoc, followed by those in La Merced and Pedregal. In the second type of campaign, carried out in March 2002, real‐time ethene measurements were performed at the facilities of our institute (CINVESTAV) by continuously drawing ambient air to the photoacoustic spectrometer. Typically, the ethene concentration exhibited two broad bands daily, reaching up to 104 ppbV at around 07:10 h. Ethene levels were also studied as a function of nitrogen oxide data obtained from a government network for atmospheric monitoring, and a linear correlation between ethene and NOx concentrations was found. Finally, the ethene levels in Mexico City were compared with those measured in other cities worldwide.

Influence of Intramolecular Hydrogen Bond Strength on OH-Stretching Overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.

Ammonia detection by using quantum-cascade laser photoacoustic spectroscopy

A pulsed quantum-cascade distributed-feedback laser, temperature tunable from -41 degrees C to +31.6 degrees C, and a resonant differential photoacoustic detector are used to measure trace-gas concentrations to as low as 66 parts per 10(9) by volume (ppbv) ammonia at a low laser power of 2 mW. Good agreement between the experimental spectrum and the simulated HITRAN spectrum of NH3 is found in the spectral range between 1046 and 1052 cm(-1). A detection limit of 30 ppbv ammonia at a signal-to-noise ratio of 1 was obtained with the quantum-cascade laser (QCL) photoacoustic (PA) setup. Concentration changes of approximately 50 ppbv were detectable with this compact and versatile QCL-based PA detection system. The performance of the PA detector, characterized by the product of the incident laser power and the minimum detectable absorption coefficient, was 4.7 x 10-9 W cm(-1).

Ethylene and carbon dioxide production by developing strawberries show a correlative pattern that is indicative of ripening climacteric fruit

Laser photoacoustic spectroscopy continuously quantified the ethylene (C2H4) produced by strawberry flowers and fruits developing in planta. C2H4 was first detected as flower buds opened and exhibited diurnal oscillations (to approximately 200 pl flower−1 h−1) before petal abscission. Exogenous application of silver thiosulphate (STS) to detached flowers inhibited petal abscission and flower senescence. In fruit, C2H4 production was maintained at a ‘low level’ (10–60 pl fruit−1 h−1) until fruit expanded when levels increased in a diurnal pattern (to 200 pl fruit−1 h−1). After expansion, C2H4 production declined to a low level until fruit attained the red‐ripe stage for at least 24 h. After this time, C2H4 levels increased linearly (no diurnal fluctuation) to approximately 1 nL fruit−1 h−1. Twenty‐four hours after the re‐initiation of C2H4 production by red fruit, CO2 levels increased approximately three‐fold, indicative of a respiratory climacteric. STS applied to fruits developing in planta and dissected fruit parts ex situ established that C2H4 production is regulated by negative feedback until fruits had expanded. The C2H4 produced by red‐ripe fruit was regulated by positive feedback. Anti‐1‐amino‐cyclopropane‐1‐carboxylic acid oxidase IgG localization identified immunoreactive antigens of 40 and 30 kDa (Mr) within the fruit achenes of expanding and red‐ripe fruit. Analysis of dissected fruit showed that seed C2H4 accounts for 50% the C2H4 that is detectable from ripe fruit.

CH-Stretching Overtone Spectra of cis- and trans-1,3-Pentadiene

The room-temperature vapor-phase overtone spectra of cis- and trans-1,3-pentadiene (piperylene) have been recorded in the 5000-17500 cm(-1) region with the use of conventional and intracavity laser photoacoustic spectroscopy. The presence of five nonequivalent olefinic CH bonds and one methyl group in each molecule complicates assignment of the spectra. We have used a harmonically coupled anharmonic oscillator local mode model with one oscillator for each of the nonequivalent CH bonds to calculate the CH-stretching overtone spectra and thus facilitate assignment of the spectra. Our calculated spectra are in good agreement with the observed spectra. The observed high overtone spectra are distinctively different despite the similarity of the two molecules and the numerous and broad transitions.

Simultaneous detection of ammonia, methane and ethylene at 1.63 μm with diode laser photoacoustic spectroscopy

Spectral investigation around 6115 cm-1 for simultaneous detection of ammonia, methane and ethylene in gas samples is presented. Experimental data on the ν2+ν3+ν4 combination band of ammonia are reported with a resolution of 1.5 GHz. A trace gas analyzer based on a resonant photoacoustic cell and an external cavity diode laser has been used for detection. A data fitting procedure has been developed in order to improve the system sensitivity and to limit the need of a reference cell. The selected spectral region allows a sensitivity of about 60 ppm for ammonia, 6 ppm for methane and 30 ppm for ethylene with 0.3 mW laser power. An application of simultaneous detection of such molecules in a mixture reproducing their typical abundances in real gas samples from biomass gasification is discussed.

Effects of torsion on O–H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.

Weak Intramolecular Interactions in Ethylene Glycol Identified by Vapor Phase OH−Stretching Overtone Spectroscopy

Vapor phase OH-stretching overtone spectra of ethylene glycol were recorded to investigate weak intramolecular hydrogen bonding. The spectra were recorded with conventional absorption spectroscopy and laser photoacoustic spectroscopy in the first to fourth OH-stretching overtone regions. The room-temperature spectra are dominated by two conformers that show weak intramolecular hydrogen bonding. A less abundant third conformer, with no sign of hydrogen bonding, is also observed. Vapor phase spectra of the ethylene-d(4) glycol isotopomer were also recorded and used to identify an interfering resonance between CH-stretching and OH-stretching states in the fourth overtone. Anharmonic oscillator local mode calculations of the OH-stretching transitions have provided an accurate simulation of the observed spectra. The local mode parameters were calculated with coupled cluster ab initio methods. The calculations facilitate assignment of the different conformers in the spectra and illustrate the effect of the intramolecular hydrogen bonding.

Laser sensors for trace gases in human breath

Ethylene and ethane as biomarkers for lipid peroxidation in humans can be detected in the exhaled breath by means of very sensitive gas sensors based on high-resolution molecular spectroscopy. To this purpose, a laser photoacoustic spectroscopy sensor (PAS) for online ethylene monitoring and a tunable diode laser absorption spectroscopy sensor (TDLAS) for ethane detection were developed at ENEA Frascati Molecular Spectroscopy Laboratory. The sensors have been calibrated by certified mixtures. The main advantages of the breath analysis by spectroscopic technique are the high sensitivity and the absolutely non-invasive character. During an experimental study in cooperation with Umberto I Hospital (Radiology Institute) of Rome, we were able to detect very low concentrations (under 1 ppb) of trace ethylene content in the air exhaled by patients, following X-ray therapy; results are reported in this article.

Inter-comparison of Laser Photoacoustic Spectroscopy and Gas Chromatography Techniques for Measurements of Ethene in the Atmosphere

Laser photoacoustic spectroscopy (LPAS) is highly suitable for the detection of ethene in air due to the overlap between its strongest absorption lines and the wavelengths accessible by high-powered CO2 lasers. Here, we test the ability of LPAS to measure ethene in ambient air by comparing the measurements in urban air with those from a gas chromatography flame-ionization detection (GC-FID) instrument. Over the course of several days, we obtained quantitative agreement between the two measurements. Over this period, the LPAS instrument had a positive offset of 330 +/- 140 pptv (parts-per-trillion by volume) relative to the GC-FID instrument, possibly caused by interference from other species. The detection limit of the LPAS instrument is currently estimated around 1 ppbv and is limited by this offset and the statistical noise in the data. We conclude that LPAS has the potential to provide fast-response measurements of ethene in the atmosphere, with significant advantages over existing techniques when measuring from moving platforms and in the vicinity of emission sources.

CH-stretching Overtone Spectra of a Fast Rotating Methyl Group: 2-CH3 and 2-CHD2 Pyridines

The CH-stretching overtone spectra of the methyl group in gaseous 2-CH(3) and 2-CHD(2) methylpyridines are recorded with conventional Fourier transform near-infrared spectroscopy in the Deltav(CH) = 1-4 regions and by intracavity laser photoacoustic spectroscopy in the Deltav(CH) = 5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model that incorporates, within the adiabatic approximation, the coupling of the anharmonic CH-stretch vibrations described by Morse potentials with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. The molecular vibrations are calculated in terms of redundant internal coordinates in an unambiguous canonical form. A simultaneous analysis of different isotopic derivatives is thus achieved. The Fermi resonance coupling parameters are those previously determined for toluene. The technique of diabatic rotations is used to disentangle the multiple avoided crossings occurring along the internal rotation coordinate theta in the calculated spectra, which become rapidly very dense owing to the low symmetry of the system. This simulation is successful in reproducing the experimental spectra. In addition, the transferrability of the Fermi resonance coupling parameters between two parent molecules is demonstrated.

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10 12 molecules cm −3 with a signal-to-noise ratio (SNR) of 1.

O−H Stretch Overtone Excitation in Methyl and Ethyl Hydroperoxides

We use laser photoacoustic spectroscopy to obtain vibrational overtone spectra in the regions of four and five quanta of O−H stretch (4νOH and 5νOH) for gas-phase methyl (MeOOH), ethyl (EtOOH), and tert-butyl hydroperoxide (t-BuOOH). Integrated cross sections for the main peak due to O−H stretch excitation to 4νOH are similar for all three hydroperoxides (∼2 × 10-21 cm2 molecule-1 cm-1); cross sections for excitation to and 5νOH are an order of magnitude smaller. Interpretation of spectral features using a previously reported vibration−torsion model for ROOH molecules, where excitation of torsional motion about the O−O bond accompanies vibrational excitation, yields plausible fits to the spectra. Simulations with the vibration−torsion model and ab initio calculations suggest barriers for torsional motion about the O−O bond to be higher in t-BuOOH than in MeOOH and EtOOH, with a trend of increasing trans torsional barrier height with increasing size of the R group.

Resonance coupling in the fourth OH-stretching overtone spectrum of formic acid.

The room-temperature vibrational overtone spectra of the formic acid isotopomers HCOOH and DCOOH have been recorded in the third and fourth OH-stretching overtone regions with intracavity laser photoacoustic spectroscopy. Resonance coupling between the OH- and CH-stretching vibrations in HCOOH is clearly identified in the fourth overtone region. This is an example of strong coupling across bonds. In the third overtone region, no resonance is observed. Vibrational energies and intensities of the OH- and CH-stretching overtones and combination bands have been calculated with an anharmonic oscillator local mode model. The pure OH-stretching bright state carries almost all the intensity prior to resonance coupling.

Laser photoacoustic spectroscopy: A powerful tool for measurement of trace gases of biological interest at sub-ppb level

A high sensitive photoacoustic system has been developed to monitor trace gases released by biological samples at concentrations of sub-ppb level. We have applied this technique to measure the release of the plant hormone ethylene during seed germination and ripening of climacteric fruits and of the marker ethylene as a result of doping cultures of human cells with heavy metals, of damaging living organisms by ionising radiation(X-ray)and of unleashing lipid peroxidation in lung epithelium following the inhalation of cigarette smoke.

The effect of NH2-inversion tunneling splitting on the NH-stretching overtone spectra of aniline vapour

The room temperature vibrational overtone spectrum of aniline vapour has been recorded by intracavity laser photoacoustic spectroscopy in the 9700 cm−1 to 16 600 cm−1 region. The NH-stretching overtone bands exhibit spectral structure due to the tunneling splitting of the amino group inversion states. We find that this tunneling splitting steadily decreases with increasing NH-stretching vibrational excitation, and at the fourth NH-stretching overtone the splitting is negligible. Wave numbers and intensities of the NH- and CH-stretching vibrational overtones were calculated with a harmonically coupled anharmonic oscillator local mode model to facilitate spectral assignment. The conventional liquid phase overtone spectra of aniline were recorded and the measured absolute intensities are in good agreement with the calculated intensities.

Absolute line intensities in the ν1 + 3ν3 band of 12C2H2 by laser photoacoustic spectroscopy and Fourier transform spectroscopy

Line intensities and self-broadening coefficients in the ν1 + 3ν3 band of 12C2H2 near 0.8 μm at room temperature were measured by means of both laser photoacoustic and Fourier transform spectroscopy. An experimental protocol has been developed to obtain absolute intensities from the photoacoustic measurements. Namely, the spectrometer was calibrated using water vapour line intensities available in Hitran 1996 [L. S. Rothman et al. (1998) J. quant. Spectrosc. Radiat. Transfer, 60, 665–710]. These photoacoustic line intensities were found to be on average 5% higher than corresponding measurements performed using Fourier transform spectroscopy, the accuracy of the latter being estimated to better than 4%. The accuracy of the photoacoustic intensities is discussed. Previous results from the literature [F. Herregodts, D. Hurtmans, J. Vander Auwera, and M. Herman (1999) J. chem. Phys., 111, 7954—7960] are revised.