Differential Helmholtz resonant photoacoustic cell for spectroscopy and gas analysis with room-temperature diode lasers

The results of theoretical and experimental studies and the design of a multi-purpose differential Helmholtz resonant photoacoustic detector (DHRD) and its applications to high-resolution spectroscopy of molecular gases and gas analysis with a room-temperature diode laser in the near-IR region are summarized. The series of experiments and numerical analysis of the DHRD sensitivity were performed for both types (single-pass and multi-pass) of DHRDs within a wide pressure range 0.1–101 kPa, including the regime of a gas flowing through a DHRD cell. The hardware and electronic arrangement of DHRDs for diode laser spectrometers and gas analyzers providing a limiting absorption sensitivity better than 10^-7 Wm^-1 are described. The results of measurements of spectral line parameters of H₂O near 800 and 1390 nm and CH₄ near 1650 nm (intensities, line broadening and shifting by atomic and molecular gases) are presented and discussed. The problems and the ways of perfection of the methodology and accuracy of DHRD techniques with tunable diode lasers of near-IR and visible spectral ranges are discussed. Received: 1 April 2002 / Revised version: 20 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +7/382-225-8026, E-mail: kvan@asd.iao.ru     

Frequency measurement of I2 lines in the NIR using Ca and CH4 optical frequency standards

2 lines have been carried out using difference-frequency mixing of the emission of CH₄ and Ca frequency standards at 88 THz (or 3.4 μm) and 456 THz (or 657 nm), respectively. A power of ≈10 pW for the mixing product at 815 nm was obtained using a critically phase-matched KTP crystal and input powers of the order of mW. The relative uncertainty of the measurement was a few times 10^-11, limited by the frequency instability of the laser source locked to I₂. Received: 3 December 1997/Revised version: 16 February 1998     

Near-infrared cavity enhanced absorption spectroscopy of hot water and OH in an oven and in flames

A compact diode laser operating around 1.5 μm was used to measure cavity enhanced absorption spectra of hot water molecules and OH radicals in radiative environments under atmospheric conditions. Spectra of air were measured in an oven at temperatures ranging from 300 K to 1500 K. These spectra contained rovibrational lines from water and OH. The water spectra were compared to simulations from the HITRAN and HITEMP databases. Furthermore, spectra were recorded in the flame of a flat methane/air burner and in an oxyacetylene flame produced by a welding torch. The results show that cavity enhanced absorption spectroscopy provides a sensitive method for rapid monitoring of species in radiative environments. Received: 22 February 2001 / Revised version: 23 April 2001 / Published online: 7 June 2001     

Size-selected, supported clusters: the interaction of carbon monoxide with nickel clusters

We report on the size-dependent chemical reactivity of nickel clusters with up to 30 atoms. Monodispersed Ni₃₀ clusters show a higher reactivity for CO dissociation than Ni₁₁ and Ni₂₀. Under our experimental conditions the smallest nickel clusters (Ni_x, x<4) produce nickelcarbonyl complexes. These results demonstrate that such small clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but also essentially because of the distinctive properties of different cluster sizes. In addition thermal desorption spectroscopy of CO shows that on average four molecules are weakly adsorbed per Ni₁₁ at saturation coverage. Using an isotopic mixture of ¹²CO and ¹³CO, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the four COs. A semi-classical model of interacting dipoles is applied to correlate the observed vibrational frequency shifts with the arrangement of the COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters. Received: 23 March 1998/Accepted: 25 August 1998     

On the accurate determination of pressure- induced line shifts in the 2ν<Subscript>3</Subscript> band of H<Subscript>2</Subscript>O at 1320 nm

A simple technique is demonstrated for the accurate determination of pressure-induced line shifts of water in air. High- and low-pressure water samples are simultaneously probed on selected overtone transitions at 1.32 μm using a current-modulated distributed-feedback diode laser and harmonic detection. The resultant profiles yield an average line shift of -293±30 MHz/atm for the 3,3,0 (002)2,2,1 (000)transition at 227251 GHz and -134±7 MHz/atm for the 3,2,1 (002)2,1,2 (000) transition at 227027 GHz. Comparisons are made between first- and second-harmonic detection, and wavelength- and frequency-modulation regimes. The effect of modulation broadening on the returned line shifts is quantified. Received: 12 August 2002 / Revised version: 21 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-01865/275410, E-mail: gus.hancock@chemistry.ox.ac.uk     

Portable fiber-coupled diode-laser-based sensor for multiple trace gas detection

Tunable narrowband mid-infrared radiation from 3.25 to 4.4 μm is generated by a compact fiber-coupled, difference-frequency-based spectrosopic source. A 20-mW external cavity diode laser (with a tuning range from 814 to 870 nm) and a 50-mW distributed-Bragg-reflector diode-laser-seeded ytterbium-doped fiber amplifier operating at 1083 nm are difference-frequency mixed in a multi-grating, temperature-controlled periodically poled LiNbO₃ crystal. A conversion efficiency of 0.44 mW/(W² cm) (corresponding to a power of ≈3 μW at 3.3 μm) represents the highest conversion efficiency reported for a portable device. Performance characteristics of such a sensor and its application to spectroscopic detection of CO₂, N₂O, H₂CO, HCl, NO₂, and CH₄ will be reported in this work. Received: 14 May / Revised version: 24 June 1999 / Published online: 30 September 1999     

Sum frequency generation at 309 nm using a violet and a near-IR DFB diode laser for detection of OH

We have used a violet diode laser at 404 nm and a distributed feedback diode laser at 1320 nm to produce 0.8 nW of radiation at 309 nm by sum frequency generation in beta-barium borate. The UV radiation was tuned mode-hop-free over 30 GHz and used to detect OH radicals produced in a microwave discharge. By chopping the UV light at 500 Hz, we observed a concentration of 2×10¹² cm^-3 with a signal to noise ratio of 30:1. Received: 16 November 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002     

Diode laser spectroscopy using two modes of an InAsSb/InAsSbP laser near 3.6 μm

A new type of multimode semiconductor laser, based on InAsSb/InAsSbP heterostructures, is described. This continuous laser working in a broad range of temperatures (30–100 K) was tested using a closed-cycle He-cryostat and its quality was demonstrated using the laser spectroscopy of gases absorbing in the 2800 cm^-1 region. Two different laser modes were used to increase the spectral range. The spectral characteristics and tunability of the laser were explored as a function of heat-sink temperature and drive current with the aim of developing its use for high–resolution spectroscopy. The laser has potential applications in the field of chemistry, atmospheric research and the study of the kinetics of reactive species. Received: 18 October 1999 / Revised version: 10 May 2000 / Published online: 16 August 2000     

Pressure broadening and shift of transitions of the first overtone of HCl

High-resolution spectroscopic measurements were made using distributed feedback diode lasers. We measured line strength and pressure-induced broadening and shift for two lines, R(3) and P(4), of the first overtone (2?0) ro-vibrational band, for the two isotopomers H³⁵Cl and H³⁷Cl, according to their natural abundances; measurements were also made in the presence of foreign gases. Comparison was made with available data when possible. Received: 29 February 2000 / Published online: 8 November 2000     

Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm 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 are 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 μm 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. Received: 19 January 2000 / Revised version: 6 March 2000 / Published online: 7 June 2000     

Ultra-sensitive mid-infrared cavity leak-out spectroscopy using a cw optical parametric oscillator

We report a portable, all-solid-state, mid-infrared spectrometer for trace-gas analysis. The light source is a continuous-wave optical parametric oscillator based on PPLN and pumped by a Nd:YAG laser at 1064 nm. The generated single-frequency idler output covers the wavelength region between 2.35 and 3.75 μm. With its narrow line width, this light source is suitable for precise trace-gas analysis with very high sensitivity. Using cavity leak-out spectroscopy we achieved a minimum detectable absorption coefficient of 1.2×10^-9 /cm (integration time: 16 s), corresponding, for example, to a detection limit of 300 parts per trillion ethane. This sensitivity and the compact design make this trace-gas analyzer a promising tool for various in situ environmental and medical applications. Received: 19 September 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-228/733-474, E-mail: frank.kuehnemann@iap.uni-bonn.de     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

Remote measurement of volcanic gases by Fourier transform infrared spectroscopy

Received: 25 March 1998     

In-situ sensing of tropospheric water vapor using an airborne near-IR diode laser hygrometer

12 molecules cm^-3 Hz^-1/2 (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midlatitude tropopause conditions or a mixing ratio of 0.6 ppmv under boundary-layer conditions. The corresponding minimum measurable absorbance is 10^-5 Hz^-1/2. The laser hygrometer was field-demonstrated aboard the NASA P3B research aircraft, during a series of flights spanning several weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled mirror hygrometers. In general, the laser hygrometer performed well; however, under some conditions, it reported water vapor number densities 20% greater than the cryogenic frost-point hygrometer. This difference is currently under study. Received: 31 March 1998/Revised version: 3 June 1998     

Absolute frequency measurement of a CO2/OsO4 stabilized laser at 28.8 THz

A cw carbon dioxide laser operating on the 10 μm R(0)_I transition (28.832 THz) was frequency stabilized by a servo lock to the saturated absorption dip of the Q(15) transition of ¹⁸⁸OsO₄. The laser frequency was measured with a cesium-clock-based frequency chain. In addition, the absorption line frequencies Q(14) of ¹⁸⁸OsO₄ and sQ(4,3) of ¹⁵NH₃, were measured relative to the frequency of Q(15). Received: 21 August 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

500-Hz two-photon Ramsey fringes with a SF6 beam: towards a new frequency standard in the 30-THz spectral region

A two-photon Ramsey-fringe experiment with a supersonic beam of SF₆ has been performed with an interzone distance of up to 50 cm. Using a He-seeded beam with 50% of SF₆, the two-photon transition P(4)E⁰ in the 2ν₃ band reveals its magnetic hyperfine structure and the periodicity of the fringes is 500 Hz. The strength of the central fringe of the main hyperfine component corresponds to a flux of 10¹⁰–10¹¹useful molecules/s, which is very promising for a new frequency standard in the 30-THz spectral region. Received: 27 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Infrared degenerate four-wave mixing and resonance-enhanced stimulated Raman scattering in molecular gases and free jets

2 H₂), methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) in a cell under equilibrium conditions and cooled in free jet expansions. For methane at room temperature the detection limit was 2×10¹² molecules per cm³ and quantum state, enabling the detection of trace species with a spatial resolution of 1 mm²×30 mm. In an attempt to study transitions in the ν₁+ν₃ and 2ν₂+ν₃ combination bands of CO₂ or N₂O, it was not possible to observe any DFWM signal. Instead a surprisingly strong, backward- and forward-directed emission was found which could not be attributed to the DFWM process. The signal arising from this emission was more than 2 orders of magnitude stronger than the DFWM signals obtained for other molecules. The frequencies of the emitted radiation were found to correlate with the transitions ν₁+ν₃→ν₁ and 2ν₂+ν₃→2ν₂, respectively. Our investigations lead to the conclusion that the emission can be explained by stimulated Raman scattering, resonantly enhanced by transitions to the combination levels ν₁+ν₃ and 2ν₂+ν₃. This process seems to suppress the generation of DFWM signals. Received: 1 October 1996/Revised version: 6 January 1997     

Local field corrections for light absorption by fullerenes

Semi-empirical atom-atom potential energy calculations based on pairwise additive interactions are performed and, after applying the Born-Oppenheimer approximation to separate high frequency vibrational modes from low frequency orientational and translational modes, the infrared vibrational spectra of CO₂ and N₂O monomers trapped in an argon matrix at a temperature of 5 K are determined. It is shown that only a double substitutional site in argon can accommodate N₂O, whereas CO₂ is trapped in two distinct sites, of single and double substitutional types. The model shows that splitting of the degenerate mode occurs for both molecules in the double site. In the ground electronic state, the vibrational frequency shifts due to the matrix and the vibrational transition moments for low-lying levels are determined using the contact transformation method, as used for gas phase calculations. Calculated energy levels compare well with observed ones and the theory also predicts some unobserved levels. Moreover, calculations show no significant changes in the dipole moments of both CO₂ and N₂O trapped molecules. Received 22 March 2000 and Received in final form 10 May 2000     

Extended-cavity grating-tuned operation of mid-infrared InAsSb diode lasers

2 cryostat that permits placement of both the collimation optics and the extended-cavity elements within a vacuum chamber. We have observed tuning of the laser frequency over a 1.44 THz (48 cm^-1) range by rotation of the external grating. This result was obtained without anti-reflection coating of the laser output facet. Received: 8 July 1997/Revised version: 31 July 1997     

Temperature evaluation of UF6 and cluster detection in nozzle expansion using low-resolution infrared absorption spectroscopy

6 and mixtures of UF₆ with argon and nitrogen through a bidimensional nozzle was studied using low-resolution infrared spectroscopy in the ν₃ absorption band region. The experiments were carried out in order to calculate the molecular temperature of the beam and also to verify cluster formation in the expansion. The molecular beam temperature evaluation was based on the measurements of the low-resolution bandwidth, which were compared to simulated spectra results. The temperatures were also evaluated using the measured pressure at the end of the nozzle by a pitot tube. In the conditions where no cluster formation was observed the calculated theoretical temperatures using an equilibrium expansion model are in good agreement with the data obtained through the analysis of the experimental spectra and through the pitot tube pressure measurement. Cluster formation was observed for temperatures below about 120 K. In these conditions the infrared spectra showed shoulders in the region above 630 cm^-1 and a shoulder or band between 616 and 600 cm^-1. Received: 5 January 1998/Accepted: 14 May 1998