Tunable Infrared Diode Laser Research Articles

Infrared spectroscopic and modeling studies of H2/CH4 microwave plasma gas phase from low to high pressure and power

InfraRed Tunable Diode Laser Absorption Spectroscopy technique has been implemented in a H2/CH4 Micro-Wave (MW frequency f = 2.45 GHz) plasma reactor dedicated to diamond deposition under high pressure and high power conditions. Parametric studies such as a function of MW power, pressure, and admixtures of methane have been carried out on a wide range of experimental conditions: the pressure up to 270 mbar and the MW power up to 4 kW. These conditions allow high purity Chemical Vapor Deposition diamond deposition at high growth rates. Line integrated absorption measurements have been performed in order to monitor hydrocarbon species, i.e., CH3, CH4, C2H2, C2H4, and C2H6. The densities of the stable detected species were found to vary in the range of 1012–1017 molecules cm−3, while the methyl radical CH3 (precursor of diamond growth under these conditions) measured into the plasma bulk was found up to 1014 molecules cm−3. The experimental densities have been compared to those provided by 1D-radial thermochemical model for low power and low pressure conditions (up to 100 mbar/2 kW). These densities have been axially integrated. Experimental measurements under high pressure and power conditions confirm a strong increase of the degree of dissociation of the precursor, CH4, associated to an increase of the C2H2 density, the most abundant reaction product in the plasma.

HCl yield and chemical kinetics study of the reaction of Cl atoms with CH3I at the 298 K temperature using the infra-red tunable diode laser absorption spectroscopy

Pulsed ArF excimer laser (193nm) – CW infrared(IR) tunable diode laser Herriott type absorption spectroscopic technique has been made for the detection of product hydrochloric acid HCl. Absorption spectroscopic technique is used in the reaction chlorine atoms with methyl iodide (Cl+CH3I) to the study of kinetics on reaction Cl+CH3I and the yield of (HCl). The reaction of Cl+CH3I has been studied with the support of the reaction Cl+C4H10 (100% HCl) at temperature 298K. In the reaction Cl+CH3I, the total pressure of He between 20 and 125Torr at the constant concentration of [CH3I] 7.0×1014moleculecm−3. In the present work, we estimated adduct formation is very important in the reaction Cl+CH3I and reversible processes as well and CH3I molecule photo-dissociated in the methyl [CH3] radical. The secondary chemistry has been studied as CH3+CH3ICl=product, and CH3I+CH3ICl=product2. The system has been modeled theoretically for secondary chemistry in the present work. The calculated and experimentally HCl yield nearly 65% at the concentration 1.00×1014moleculecm−3 of [CH3I] and 24% at the concentration 4.0×1015moleculecm−3 of [CH3I], at constant concentration 4.85×1012moleculecm−3 of [CH3], and at 7.3×1012 moleculecm−3 of [Cl]. The pressure dependent also studied product of HCl at the constant [CH3], [Cl] and [CH3I]. The experimental results are also very good matching with the modelling work at the reaction CH3+CH3ICl=product (k=(2.75±0.35)×10−10s−1) and CH3I+CH3ICl=product2 (k=1.90±0.15)×10−12s−1. The rate coefficients of the reaction CH3+CH3ICl and CH3I+CH3ICl has been made in the present work. The experimental results has been studied by two method (1) phase locked and (2) burst mode.

Methanogenic activity tests by Infrared Tunable Diode Laser Absorption Spectroscopy

Methanogenic activity (MA) tests are commonly carried out to estimate the capability of anaerobic biomass to treat effluents, to evaluate anaerobic activity in bioreactors or natural ecosystems, or to quantify inhibitory effects on methanogenic activity. These activity tests are usually based on the measurement of the volume of biogas produced by volumetric, pressure increase or gas chromatography (GC) methods. In this study, we present an alternative method for non-invasive measurement of methane produced during activity tests in closed vials, based on Infrared Tunable Diode Laser Absorption Spectroscopy (MA-TDLAS). This new method was tested during model acetoclastic and hydrogenotrophic methanogenic activity tests and was compared to a more traditional method based on gas chromatography. From the results obtained, the CH4 detection limit of the method was estimated to 60ppm and the minimum measurable methane production rate was estimated to 1.09.10−3mgl−1h−1, which is below CH4 production rate usually reported in both anaerobic reactors and natural ecosystems. Additionally to sensitivity, the method has several potential interests compared to more traditional methods among which short measurements time allowing the measurement of a large number of MA test vials, non-invasive measurements avoiding leakage or external interferences and similar cost to GC based methods. It is concluded that MA-TDLAS is a promising method that could be of interest not only in the field of anaerobic digestion but also, in the field of environmental ecology where CH4 production rates are usually very low.

Infrared diode laser spectroscopy of the Ne–D2O van der Waals complex: Strong Coriolis and angular-radial coupling

Four internal-rotation/vibration bands of the Ne-D(2)O complex have been measured in the v(2) bend region of D(2)O using a tunable infrared diode laser spectrometer to probe a slit supersonic expansion. Three ortho bands are excited from the ground state Σ(0(00)) to the Σ and Π(1(11), υ(2) = 1) internal rotor states and the n = 1, Σ(0(00), υ(2) = 1) stretching-internal rotor combination state. Strong perturbations between the excited vibrational states are evident. The observed spectra are analyzed separately with a three-state J-dependent Coriolis plus J-independent angular-radial coupling model [M. J. Weida and D. J. Nesbitt, J. Chem. Phys. 106, 3078 (1997)] and a three-state Coriolis coupling model [R. C. Cohen and R. J. Saykally, J. Chem. Phys. 95, 7891 (1991)]. The former model works more successfully than the latter. Molecular constants for the ground and excited vibrational states of ortho (20)Ne-D(2)O isotopomer as well as the Coriolis and angular-radial coupling constants are determined accurately. The van der Waals stretching frequency is estimated to be ν(s) = 24.85 cm(-1) in the ground state and decreases to about 20.8 cm(-1) upon vibrational excitation of the D(2)O bend.

Nitrous oxide dimer: An ab initio coupled-cluster study of isomers, interconversions, and infrared fundamental bands, and experimental observation of a new fundamental for the polar isomer

Improved quantum chemistry (coupled-cluster) results are presented for spectroscopic parameters and the potential energy surface for the N(2)O dimer. The calculations produce three isomer structures, of which the two lowest energy forms are those observed experimentally: a nonpolar C(2h)-symmetry planar slipped-antiparallel geometry (with inward-located O atoms) and a higher-energy polar C(s)-symmetry planar slipped-parallel geometry. Harmonic vibrational frequencies and infrared intensities for these isomers are calculated. The low-frequency intermolecular vibrational mode predictions should be useful for future spectroscopic searches, and there is good agreement in the one case where an experimental value is available. The frequency shifts for the high-frequency intramolecular stretching vibrations, relative to the monomer, were calculated and used to help locate a new infrared band of the polar isomer, which corresponds to the weaker out-of-phase combination of the nu(1) antisymmetric stretch of the individual monomers. The new band was observed in the region of the monomer nu(1) fundamental for both ((14)N(2)O)(2) and ((15)N(2)O)(2) using a tunable infrared diode laser to probe a pulsed supersonic jet expansion, and results are presented.

The cyclic CO2 trimer: Observation of a parallel band and determination of an intermolecular out-of-plane torsional frequency

A new parallel (DeltaK=0) band of the cyclic CO(2) trimer is observed at 2364 cm(-1). The trimers are generated in a pulsed supersonic expansion from a slit-jet nozzle and probed with a tunable infrared diode laser. The band is assigned as a combination of an intramolecular CO(2) monomer nu(3) stretch and an intermolecular out-of-plane torsion, giving a torsional frequency of 12-13 cm(-1). The band is surprisingly strong and completely unperturbed, providing a rare and near perfect example for a parallel band of a symmetric top molecule with C(3h) symmetry and zero nuclear spins.

Spectroscopic Studies of OCS-Doped 4He Clusters with 9−72 Helium Atoms: Observation of Broad Oscillations in the Rotational Moment of Inertia

High-resolution spectra of HeN-OCS clusters with N up to 39 in the microwave region and up to 72 in the infrared region were observed with apparatus-limited line widths of about 15 kHz and 0.001 cm(-1), respectively. The cold (approximately 0.2 K) clusters were produced in pulsed supersonic jet expansions of very dilute OCS + He mixtures and probed using a microwave Fourier transform spectrometer or a tunable infrared diode laser spectrometer. Consistent analyses of the microwave and infrared data yield band origins for the carbonyl stretching vibration, together with rotational parameters for the ground and excited vibrational states. The rotational constant, B, passes through a minimum at N = 9 and then rises as the He atoms uncouple from the OCS rotational motion as a result of superfluid effects. There are broad unexpected oscillations in B, with maxima at N = 24 and 47 and minima at N = 36 and 62. The change in B upon vibrational excitation, which is negative for the OCS molecule, converges to positive values for N > 15. These results help to bridge the gap between individual molecules and bulk matter with atom-by-atom resolution over a significant range of cluster sizes.

Infrared spectra of seeded hydrogen clusters: (para-H2)N–N2O and (ortho-H2)N–N2O, N=2–13

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.

Frequency stabilization of tunable infrared diode lasers for time-resolved monitoring of transient species

Stabilization of an infrared diode laser on the absorption line of a transient species has been developed in a laser flash photolysis/infrared diode laser absorption experiment used for kinetic studies. The stabilization is achieved by modulating the optical pathlength of a confocal Fabry–Perot interferometer instead of the injection current of the laser. It is therefore of prime interest for studies in chemical kinetics or photochemistry, where signals are acquired at fixed frequency and where the timescale of the reactions prohibits modulation of the signal. This method is demonstrated on the detection of HCO radical following 266-nm laser photolysis of acetaldehyde.

Infrared spectra of seeded hydrogen clusters: (paraH2)N - OCS, (orthoH2)N - OCS, and (HD)N - OCS, N = 2 - 7.

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS.

Energy distributions of CO produced in an acetone-containing discharge

Tunable infrared diode laser absorption (TDLAS) and Fourier transform infrared absorption spectroscopies (FTIR) have been utilized to characterize the translational, rotational and vibrational distributions of CO in an acetone/argon DC plasma at total pressures ranging from 4 to 5 Torr and currents of 0.1–0.3 A. A broad vibrational distribution of CO was observed with gradually decreasing intensities from the fundamental band to v=12←11. When nitrogen was added to the plasma, the distribution is narrower, due to the efficient energy transfer between CO and N 2 molecules. The measured translational temperature in such plasmas ranged from 400–550 K. The rotational distribution can generally be fitted to a Boltzmann distribution within each vibrational level although the rotational temperature is highest for the lowest vibrational quantum number.

Infrared laser spectroscopy of CCO radical in the region of the C–C stretching fundamental

The C–C stretching fundamental ν3 and the difference band 2ν3−ν3 of the CCO radical have been measured in the gas phase using a tunable infrared diode laser spectrometer. The CCO radical was produced using a flowing mixture of carbon suboxide and helium subjected to a hollow-cathode discharge. The spectral region between 1035–1085 cm−1 was probed. Fifty-five and fifty-three rovibrational transitions were measured in the ν3 and 2ν3−ν3 bands, respectively. The analyses of these bands yielded spectroscopic constants for (001) and (002) vibrational states. The band origins for ν3 and 2ν3−ν3 were determined to be 1066.82407(54) and 1061.91084(63) cm−1, respectively. To check the consistency of the data in this work and those in the C–O stretching region, several lines from the ν1+ν3−ν1 band were also recorded. These lines were observed at their predicted frequency.

Diagnostic studies of H2–Ar–N2 microwave plasmas containing methane or methanol using tunable infrared diode laser absorption spectroscopy

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and nine stable molecules, CH4, CH3OH, C2H2, C2H4, C2H6, NH3, HCN, CH2O and C2N2, in H2–Ar–N2 microwave plasmas containing up to 7% of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20% and 97%. The methyl radical concentration was found to be in the range 1012–1013 molecules cm−3. By analysing the temporal development of the molecular concentrations under static conditions it was found that HCN and NH3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol (RF(CH4) = (2–7) × 1015 molecules J−1, RF(CH3OH) = (6–9) × 1015 molecules J−1) and the respective conversion rates to methane, hydrogen cyanide and ammonia (RCmax(CH4) = 1.2 × 1015 molecules J−1, RCmax(HCN) = 1.3 × 1015 molecules J−1, RCmax(NH3) = 1 × 1014 molecules J−1) have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H2–N2–Ar–CH4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed.

Evaluation of Hydrazine Reduction by Cellulose Acetate Filters Using Infrared Tunable Diode Laser Spectroscopy

Cellulose acetate (CA) filters have been investigated to determine their hydrazine (N2H4) breakthrough characteristics using a system based on tunable diode laser absorption spectroscopy (TDIAS). The breakthrough mass loading sorption curves for hydrazine were dependent on both the flow rate and the concentration. In experiments using a 4.5 ppmv hydrazine standard, the amounts of hydrazine retained by the CA filter were 4.25 microg at a flow rate of 2.82 L/min and 65 microg at a flow rate of 0.28 L/min. These loadings are much greater than the 31.5 ng/cigarette of hydrazine reported in smoke for unfiltered cigarettes. Further, CA filters exposed to four and eight puffs of smoke actually made the filter more efficient in retaining hydrazine compared to CA filters that had not been exposed to smoke. Therefore, if hydrazine is present in smoke at the levels reported in unfiltered cigarettes, all of the hydrazine would be trapped by the CA filter, and would be unable to break through during smoking. A unique feature of this analytical method is that the instrument does not require calibration after molecular parameters have been determined, in this case from previously acquired quantitative hydrazine FT-IR reference spectra.

Absorption spectroscopic studies of carbon dioxide conversion in a low pressure glow discharge using tunable infrared diode lasers

The time and spatial dependence of the chemical conversion of CO2 to CO were studied in a closed glow discharge reactor (p = 50 Pa,I = 2-30 mA) consisting of a small plasma zone and anextended stationary afterglow. Tunable infrared diode laser absorption spectroscopy has been applied to determine the absolute ground state concentrations of CO and CO2. After a certain discharge time an equilibrium of the concentrations of both species could be observed. The spatial dependence of the equilibrium CO concentration in the afterglow was found to be varying less than 10%. The feed gas was converted to CO more predominantly between 43% and 60% with increasing discharge current, forming so-called quasi-equilibrium states of the stable reaction products. The formation time of the stable gas composition also decreased with the current. For currents higher than 10 mA the conversion rate of CO2 to CO was estimated to be1.2×1013 molecules J-1. Based on the experimental results, a plasma chemical modelling has been established.

Hydrazine detection limits in the cigarette smoke matrix using infrared tunable diode laser absorption spectroscopy

Infrared absorption lines of hydrazine are broad and typically not baseline resolved, with line strengths ∼100 times weaker than the more widely studied compound ammonia. Hardware and software improvements have been made to a two-color infrared tunable diode laser (IR-TDL) spectrometer in order to improve the limit of detection (LOD) of hydrazine (N 2H 4) in the cigarette smoke matrix. The detection limit in the smoke matrix was improved from 25 parts-per-million-by-volume (ppmv) to 4.2 ppmv using a 100 m pathlength cell with acquisition of background spectra immediately prior to each sample and 100 ms temporal resolution. This study did not detect hydrazine in cigarette smoke in the 964.4–964.9 cm −1 spectral region, after mathematically subtracting the spectral contributions of ethylene, ammonia, carbon dioxide, methanol, acrolein, and acetaldehyde. These compounds are found in cigarette smoke and absorb in this spectral region. The LOD is limited by remaining spectral structure from unidentified smoke species. The pseudo random noise (root mean square) in the improved instrument was 2×10 −4 absorbance units (base e) which is equivalent to a 0.09 ppmv hydrazine gas sample in the multipass cell. This would correspond to a detection limit of 0.44 ppmv of hydrazine, given the dilution of the smoke by a factor of 5 by the sampling system. This is a factor of 10 less than the 4.2 ppmv detection limit for hydrazine in the smoke matrix, and indicates that the detection limit is primarily a result of the complexity of the matrix rather than the random noise of the TDL instrument.

Infrared Line Strengths and Pressure Broadening Coefficients in the ν12 Vibrational Band of Hydrazine Measured by Infrared Tunable Diode Laser Spectroscopy

Infrared line positions, linestrengths, and pressure-broadening coefficients are required to determine absolute trace gas concentrations from high-resolution absorption spectra. We have measured these values for the infrared absorption lines in the ν12 band (antisymmetric wag) of hydrazine between 965.4 and 965.7 cm−1 using a high-resolution tunable diode laser system with 0.0006 cm−1 spectral resolution and an 18-m path length multiple pass absorption cell. A continuous flow of hydrazine diluted to 100–500 parts per million in nitrogen at pressures between 0.1 and 50 Torr was used to prevent decomposition in the sampling cell. The total integrated linestrength from 965.477 to 965.663 cm−1 is (2.20±0.14)×10−20 cm2 molecule−1 cm−1. The strongest line in this region at 965.5557 cm−1 has a strength of 2.8×10−21 cm2 molecule−1 cm−1. The only isolated line in this region, at 965.4905 cm−1, has a broadening coefficient by nitrogen of 0.11±0.01 cm−1 atm−1 at 295 K. These results are compared to the qualitative high resolution spectra determined by other researchers using FTIR spectroscopy, and have been used to construct a HITRAN-format line list that can be used both to spectroscopically distinguish hydrazine from other components and to determine its concentration in a complex gas phase mixture.

Tunable diode laser absorption studies of hydrocarbons in RF plasmas containing hexamethyldisiloxane

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and three stable molecules, CH4, C2H2 and C2H6, in radio frequency plasmas (f=13.56 MHz) containing hexamethyldisiloxane (HMDSO). The methyl radical concentration and the concentration of the stable hydrocarbons, produced in the plasma, have been measured in pure HMDSO discharges and with admixtures of Ar, while discharge power (P=20–200 W), total gas pressure (p=0.08–0.6 mbar), gas mixture and total gas flow rate (Φ=1–10 sccm) were varied. The methyl radical concentration was found to be in the range of 1013 molecules cm-3, while methane and ethane are the dominant hydrocarbons with concentrations of 1014–1015 mol cm-3. Conversion rates to the measured stable hydrocarbons (RC(CxHy): 2×1012–2×1016 molecules J-1 s-1) could be estimated in dependence on power, flow, mixture and pressure. Under the used experimental conditions a maximum deposition rate of polymer layers of about 400 nm min-1 has been found.

A time resolved laser study of hydrocarbon chemistry in H2-CH4surface wave plasmas

Time resolved tunable infrared diode laser absorption spectroscopyhas been used to detect the methyl radical and four related stable molecules,CH4, C2H2, C2H4 and C2H6, in H2 surface wave plasmas (f = 2.45 GHz, power density≈10-50 W cm-3) containing 10% methane understatic conditions at different pressures (p = 0.1-4 Torr). For the firsttime, the time dependence of the conversion of methane to the methyl radicaland three stable C-2 hydrocarbons was studied in a fixed discharge volumenearly up to a stationary state. The degree of dissociation of the methaneprecursor was found to increase by up to 96% in the stationary state, and themethyl radical concentration was measured to be in the range of1012-1013 molecules cm-3. The concentrations of bothC2H2 and C2H4 produced in the plasma showed amaximum at a distinct time before decreasing. In contrast, the C2H6 concentration was observed to increase with time to a nearly constantvalue between 6×1012 and 2×1014 molecules cm-3 varying with pressure.Based on time resolved concentrations, conversion rates to the measured C-2hydrocarbons (RC(C2Hy) = 1011-1013 molecules J-1) could be estimated in dependence on pressure in a surface wavedischarge. The influence of diffusion on the spatial distribution of thehydrocarbon concentration in the discharge tube was considered. A qualitativemodel has been developed in order to describe the chemical processes and toidentify the main plasma chemical reaction paths.

On the hydrocarbon chemistry in a H2 surface wave dischargecontaining methane

Tunable infrared diode laser absorption spectroscopy has been usedto detect the methyl radical and four stable molecules, CH4, C2H2,C2H4 and C2H6, in a H2 surface wave discharge (f = 2.45 GHz andpower density ≈10-50 W cm-3) containing up to 10% methaneunder different flows (Φ = 22-385 sccm) and pressures (p = 0.1-4 Torr).The degree of dissociation of the methane precursor varied between 20% and85% and the methyl radical concentration was found to be in the range of1012 molecules cm-3. The methyl radical concentration and theconcentrations of the stable C-2 hydrocarbons C2H2, C2H4,C2H6, produced in the plasma, increased with an increasing amount ofadded CH4 as well as with increasing pressure. For the first time,fragmentation rates of methane (RF(CH4) = 1×1015-2.5×1016 molecules J-1) and conversionrates to the measured C-2 hydrocarbons (RC(C2Hy):5×1013-3×1015 molecules J-1) could be estimated withdependence on the flow and pressure in a surface wave discharge. The influenceof diffusion and convection on the spatial distribution of the hydrocarbonconcentration in the discharge tube was considered by a simple model.