Infrared Diode Laser Absorption Research Articles

Near infrared diode laser absorption spectroscopy of acetylene between 6523 and 6587 cm−1

Spectroscopic parameters of the ν1+ν3∑u+-0∑g+ bands of acetylene near 1.525μm have been revised at room temperature. In the spectral region ranging from 6523 to 6587cm−1, ten lines of C2H2 have been analyzed using a high resolution tunable diode laser absorption spectrometer. These transitions are well appropriate to the monitoring of acetylene by laser absorption spectroscopy. The Voigt model is used to fit the molecular absorption lineshape and to provide accurate spectroscopic line parameters. Our data have been found in agreement well with those given in the latest HITRAN database.

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.

Infrared diode laser spectroscopy of H2O–CO2 and D2O–CO2 complexes in the v2 bend region of water

The rovibrational spectrum of H2O–CO2 and D2O–CO2 weakly bound complexes has been measured in the v2 bend region of the water monomer by infrared diode laser absorption spectroscopy. Rotational and centrifugal distortion constants for different internal rotation states in the excited vibrational state have been determined. The band-origin of the spectrum is blue-shifted from that of the water monomer by 0.739cm−1 for H2O–CO2 and 0.083cm−1 for D2O–CO2, respectively. The tunneling splitting is reduced upon vibrational excitation by 0.00698(31)cm−1 for H2O–CO2 and 0.00044(17)cm−1 for D2O–CO2, respectively.

Conformer specific dissociation dynamics of iodocyclohexane studied by velocity map imaging

The photodissociation dynamics of iodocyclohexane has been studied using velocity map imaging following excitation at many wavelengths within its A-band (230 ≤ λ ≤ 305 nm). This molecule exists in two conformations (axial and equatorial), and one aim of the present experiment was to explore the extent to which conformer-specific fragmentation dynamics could be distinguished. Ground (I) and spin-orbit excited (I∗) state iodine atom products were monitored by 2 + 1 resonance enhanced multiphoton ionization, and total kinetic energy release (TKER) spectra and angular distributions derived from analysis of images recorded at all wavelengths studied. TKER spectra obtained at the longer excitation wavelengths show two distinct components, which can be attributed to the two conformers and the different ways in which these partition the excess energy upon C-I bond fission. Companion calculations based on a simple impulsive model suggest that dissociation of the equatorial (axial) conformer preferentially yields vibrationally (rotationally) excited cyclohexyl co-fragments. Both I and I∗ products are detected at the longest parent absorption wavelength (λ ∼ 305 nm), and both sets of products show recoil anisotropy parameters, β > 1, implying prompt dissociation following excitation via a transition whose dipole moment is aligned parallel to the C-I bond. The quantum yield for forming I∗ products, Φ(I∗), has been determined by time resolved infrared diode laser absorption methods to be 0.14 ± 0.02 (at λ = 248 nm) and 0.22 ± 0.05 (at λ = 266 nm). Electronic structure calculations indicate that the bulk of the A-band absorption is associated with transition to the 4A(') state, and that the (majority) I atom products arise via non-adiabatic transfer from the 4A(') potential energy surface (PES) via conical intersection(s) with one or more PESs correlating with ground state products.

Reinvestigation of the Branching Ratio of the CN + O2 Reaction

The reaction of the CN radical with O(2) was studied using infrared diode laser absorption spectroscopy. Detection of NO and secondary N(2)O products was used to directly measure the product branching ratio. After consideration of possible secondary chemistry and comparison to kinetic modeling simulations, the branching ratio of the CN + O(2) reaction into the NO + CO channel was determined to be phi (NO + CO) = 0.20 +/- 0.02, with little or no temperature dependence over the range 296-475 K.

Acetone photolysis at 248 nm revisited: pressure dependence of the CO and CO2 quantum yields

Pressure dependent CO and CO2 quantum yields in the laser pulse photolysis of acetone at 248 nm and T = 298 K have been measured directly using quantitative infrared diode laser absorption. The experiments cover the pressure range from 50 to 900 mbar. It is found that the quantum yields show a significant dependence on total pressure, with Phi(CO) decreasing from around 0.5 at 20 mbar to approximately 0.3 at 900 mbar. The corresponding CO2 yields as observed when O2 exists in the reaction mixture, exhibit exactly the opposite behaviour. For the sum of both a value of 1.05(-0.05)(+0.02) independent of pressure is obtained, showing that the sum of (Phi(CO) + Phi(CO2)) is a measure for the primary quantum yield in the photolysis of acetone. In addition, CO quantum yields and corresponding pressure dependences were measured in experiments using different bath gases including He, Ar, Kr, SF6, and O2 as third body colliders. The theoretical framework in which we discuss these data is based on our previous findings that the pressure dependence of the CO yield is a consequence of a stepwise fragmentation mechanism during which acetone decomposes initially into methyl and a vibrationally 'hot' acetyl radical, with the latter being able to decompose promptly into methyl plus CO. The pressure dependence of the CO yield then originates from the second step and is modelled quantitatively via statistical dynamical calculations using a combination of RRKM theory with a time-dependent master equation (ME) approach. From a comparison of experiment with theory the amount of excess energy in the vibrationally hot acetyl radicals (E* approximately 65 kJ mol(-1)) as well as the characteristic collision parameters for interaction of acetyl with the different bath gases were derived. Values of 90, 280, 310, 545, 550 and 1800 cm(-1) for the average energy transferred per downward collision for the bath gases He, Ar, Kr, O2, N2, and SF6, respectively, are obtained. The calculations also considered different models for the energy transfer kernel P(E,E') and best fits were obtained with a rho-weighted exponential down model.

Absolute, high resolution water transpiration rate measurements on single plant leaves via tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm

A new sampling-free and calibration-free multi-channel hygrometer using near infrared (NIR) tunable diode laser absorption spectroscopy (TDLAS) at 1.37 μm was developed and used to determine absolute transpiration rates of single plant leafs. Four 8×6× 4 cm3, fiber-coupled absorption cells are used to simultaneously measure absolute water vapor concentrations with an absolute accuracy of about 5% and a temporal resolution of about 2 s. Two chambers (BOTTOM, TOP) are directly attached to the leaf surface, while two chambers (IN, OUT) analyze the purge gas supplied to the plant leaf and the total outflow of the leaf chambers. The BOTTOM–TOP comparison provided a direct, leaf-side resolved ratio of stomatal conductance and–by taking into account the purge gas flow and the leaf area exposed–leaf side resolved water transpiration rates. The OUT–IN-difference yielded the total leaf transpiration rate with 2 μmol/m2/s resolution. The new multi-point hygrometer was validated by monitoring of the transpiration dynamics of a plant of the species Epipremnum pinnatum (L.) Engl. during diurnal variation of the leaf irradiation. During these experiments the differential H2O concentration resolution between two chambers was determined to be better than 3 ppm at Δt= 2 s (i.e. better than 711 ppb m Hz1/2). This performance was verified by an Allan analysis over a 30 min time period using CH4 as a surrogate absorber and yielded an average optimum optical resolution of 4.9×10-6 for 83 s measurement time, i.e. a CH4 resolution of 892 ppb, which corresponds to the optical resolution needed for a water sensitivity of 454 ppb m Hz1/2.

Development of a spectrometer using a continuous wave distributed feedback quantum cascade laser operating at room temperature for the simultaneous analysis of N_2O and CH_4 in the Earth's atmosphere

We report on the development and performance of a gas sensor based on a distributed feedback quantum cascade laser operating in continuous wave at room temperature for simultaneous measurement of nitrous oxide (N(2)O) and methane (CH(4)) concentrations at ground level. The concentrations of the gases are determined by a long path infrared diode laser absorption spectroscopy. The long-term stability of the instrument is evaluated using the Allan variance technique. A preliminary evaluation of the instrument performance is realized by in situ measurements of N(2)O and CH(4) concentrations at ground level during 1 day. The sensor has also been applied to study the time response of N(2)O concentrations to a fertilizer addition in a soil sample and for the comparison between various types of soils.

Kinetics of the O + HCNO Reaction

The kinetics of the O + HCNO reaction were investigated by a relative rate technique using infrared diode laser absorption spectroscopy. Laser photolysis (355 nm) of NO2 was used to produce O atoms, followed by O atom reactions with CS2, NO2, and HCNO, and infrared detection of OCS product from the O + CS2 reaction. Analysis of the experiment data yields a rate constant of k1= (9.84 +/- 3.52) x 10-12 exp[(-195 +/- 120)/T)] (cm3 molecule-1 s-1) over the temperature range 298-375 K, with a value of k1 = (5.32 +/- 0.40) x 10-12 cm3 molecule-1 s-1 at 298 K. Infrared detection of product species indicates that CO producing channels, probably CO + NO + H, dominate the reaction.

Temperature and Density of CF Radicals in 60 MHz Capacitively Coupled Fluorocarbon Gas Plasma

Both optical emission spectroscopy and infrared diode laser absorption spectroscopy were applied to measurements of rotational temperatures of CF radicals in 60 MHz capacitively coupled CF4 and CF4/Ar plasmas. The rotational temperature of CF radicals in the excited state was determined by fitting the optical emission spectra with the calculation results. The calculation for emission spectra was performed using two different rotational constant value of A=0.76 and 4.48 for the B2Δ state, and then the constant A was evaluated to be 0.76 from the emission intensities. In the CF4 plasma, the CF rotational temperature was increased from 300 to 380 K as the 60 MHz power was increased from 300 to 1500 W, and the radical density was increased from 1.59×1012 to 5.05×1012 cm-3. On the other hand, in the CF4/Ar plasma, the CF rotational temperature was increased by 20 K with the power, and the radical density was constant at 1.1×1012 cm-3. It was found that the rotational temperatures of CF radicals in the excited state were in equilibrium with those in the ground state under the present conditions.

Kinetics of the NCO + HCNO Reaction

The kinetics of the NCO + HCNO reaction were studied using infrared diode laser absorption spectroscopy. The total rate constant was measured to be k(1) = (1.58 +/- 0.20) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K. After detection of products and consideration of secondary chemistry (primarily O + HCNO and CN + HCNO), we conclude that NO + CO + HCN is the major product channel (phi = 0.92 +/- 0.04), with a minor contribution (phi = 0.04 +/- 0.02) from CO2 + HCNN.

Kinetics of the CN + HCNO Reaction

The kinetics of the CN + HCNO reaction were studied using laser-induced fluorescence and infrared diode laser absorption spectroscopy. The total rate constant was measured to be k(T) = (3.95 +/- 0.53) x 10(-11) exp[(287.1 +/- 44.5)/T] cm3 molec(-1) s(-1), over the temperature range 298-388 K, with a value of k1 = (1.04 +/- 0.1) x 10(-10) cm3 molec(-1) s(-1) at 298 K. After detection of products and consideration of secondary chemistry, we conclude that NO + HCCN is the only major product channel.

Low-k SiOCH Film Etching Process and Its Diagnostics Employing Ar/C5F10O/N2 Plasma

We proposed an environmental harmonic etching gas of C5F10O (CF3CF2CF2OCFCF2), and demonstrated the etching of low-k SiOCH films employing a dual-frequency capacitively coupled etching system. Dissociative ionization cross sections for the electron impact ionizations of C5F10O and c-C4F8 gases have been measured by quadrupole mass spectroscopy (QMS). The dissociative ionization cross section of CF3+ from C5F10O gas was much higher than those of other ionic species, and 10 times higher than that of CF3+ from C4F8 gas. CF3+ is effective for increasing the etching rate of SiO2. As a result, the etching rate of SiOCH films using Ar/C5F10O/N2 plasma was about 1000 nm/min, which is much higher than that using Ar/C4F8/N2 plasma. The behaviours of fluorocarbon radicals in Ar/C5F10O/N2 plasma, which were measured by infrared diode laser absorption spectroscopy, were similar to those in Ar/C4F8/N2 plasma. The densities of CF and CF3 radicals were markedly decreased with increasing N2 flow rate. Etching rate was controlled by N2 flow rate. A vertical profile of SiOCH with a high etching rate and less microloading was realized using Ar/C5F10O/N2 plasma chemistry.

Effects of N2 addition on density and temperature of radicals in 60MHz capacitively coupled c-C4F8 gas plasma

Behaviors of electrons and fluorocarbon radicals were investigated in 60MHz capacitively coupled plasma employing mixture gases of c-C4F8, Ar, and N2. Optical emission spectroscopy was applied to measurements of the rotational temperatures of CF radical and N2 molecule, and infrared diode laser absorption spectroscopy was applied to that of the densities of CF and CF2 radicals. The electron density was decreased from 7.3×1010to3.0×1010cm−3, and electron temperature was increased from 3.0to5.1eV with increasing N2 flow rate. The addition of N2 to Ar∕C4F8 plasma decreased the density of CF radical dominantly as compared with CF2 radical. The rotational temperatures of CF radical were estimated from the (1-2) band of BΔ2–XΠ2 electronic transition of CF radicals. It was found that the rotational temperatures of CF radicals were increased with increasing N2 flow rate. The temperatures of radical indicated the different behaviors from the temperatures of N2 molecule. The rotational temperature of CF radical has a good relation with the electron temperature.

Progress of radical measurements in plasmas for semiconductor processing

In semiconductor processes, reactive plasma is the most important technology for etching, deposition and surface modification of thin films. Radicals have played key roles in plasma processing. In order to realize the high performance of semiconductor processing, important molecular radicals have been measured and their behaviour has been clarified using laser spectroscopic methods such as infrared diode laser absorption spectroscopy, laser induced fluorescence spectroscopy, cavity ring down spectroscopy and recently atomic radicals have also been measured using compact vacuum ultraviolet absorption spectroscopy. Quantitative understanding of kinetics of radicals in plasma will be necessary for nano-scaled semiconductor processing. Their progress is reviewed and the future prospects are presented.

High-resolution infrared spectroscopy of the charge-transfer complex [Ar–N2]+∙: A combined experimental/theoretical study

A combined experimental and theoretical study of the charge-transfer complex [Ar-N(2)](+) is presented. Nearly 50 transitions split by spin-rotation interaction have been observed by means of infrared diode laser absorption spectroscopy in a supersonic planar plasma expansion. The band origin is at 2272.2563(18) cm(-1) and rotational constants in the ground and vibrationally (NN-stretch) excited state amount to 0.128701(8) cm(-1) and 0.128203(8) cm(-1), respectively. The interpretation of the data in terms of a charge switch upon complexation is supported by new ab initio calculations. The best estimate for a linear equilibrium structure yields R(e)(NN)=1.102 A and R(e)(Ar-N)=2.190 A. Predictions for molecular parameters not directly available from the experimental results are presented as well. Furthermore, the electronic structure and Ar-N bonding mechanism of [Ar-N(2)](+) have been analyzed in detail. The Ar-N bond is a textbook example of a classical 2-center-3-electron bond.

Falloff curves for the unimolecular decomposition of two acyl radicals: RCO (+M) → R + CO (+M) by pulsed laser photolysis coupled to time‐resolved infrared diode laser absorption

AbstractThe rate constants kd for the unimolecular decomposition of two acyl (RĊO) radi‐ cals: 2,2‐dimethylpropionyl (CH3)3CĊO and 2‐methylpropionyl (CH3)2CHĊO (pivaloyl and isobutyryl, respectively in what follows) have been directly measured via the time‐resolved kinetics of CO formation as a function of pressure and temperature. The acyl radicals are generated by H‐atom abstraction of the aldehydic hydrogen from the corresponding substituted propanals using pulsed laser photolysis of aldehyde/Cl2/N2 mixtures. The buildup of CO concentration is monitored by time‐resolved infrared absorption at ≅2177 cm‐1. The extent of the chain reactions triggered by the reactions of Cl2 with alkyl and acyl radicals (thus generating further Cl atoms after the initial laser pulse) has been limited by adding variable known concentrations of O2, a radical scavenger which is sequestering all radicals to form less reactive RO2 radicals. Besides kd, we have also determined for the first time the rate constants of the reactions of the two acyl radicals with O2; since the latter do not exhibit any clear trend with either temperature or pressure, we thus adopt the average of all experimental values: i.e. for pivaloyl : k = (3.5 ± 1.0) × 10−12 cm3 molecule−1 s−1, and for isobutyryl : k = (3.2 ± 1.0) × 10−12 cm3 molecule−1 s−1. Our values of kd, which are in the falloff range, are compared to those obtained from theoretical calculations using the Troe formalism 5. For pivaloyl at 298 K, we obtain the following limiting rate constants : k0 = 2.1 × 10−13 cm3 molecule−1 s−1, k∞ = 4.3 × 105 s−1, FC = 0.515, which are at variance with the theoretical predictions of Tomas et al. (Phys Chem Chem Phys 2000, 2, 1165–1174). For isobutyryl, our experimental data are entirely consistent with the predictions of Tomas et al. Extrapolation of our Troe parameters to tropospheric conditions (298 K, atmospheric pressure) shows, in agreement with previous authors, that for both radicals the reaction with O2 is several orders of magnitude faster than the unimolecular decomposition. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 611–624, 2005

Pressure dependence for the CO quantum yield in the photolysis of acetone at 248 nm: A combined experimental and theoretical study

The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20-900 mbar (N2) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a significant dependence on total pressure with Phi(CO) decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH3CO) radical and its unimolecular fragmentation as well as the application of statistical (RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via(2a)) of the internally excited primary CH3CO* photolysis product with an excess energy of approximately 62.8 kJ mol(-1). Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step (1): CH3COCH3+hv--> CH3CO*+ CH3, step (2a): CH3CO*--> CH3+ CO or step (2b) CH3CO*-(+M)--> CH3CO. Formation of CO via a direct and/or concerted channel CH3COCH3+hv--> 2CH(3)+ CO (1') is considered to be unimportant.

Infrared diode laser absorption spectroscopy of C2H2 and C2H6 in capacitively coupled methane RF discharges

Low-temperature RF discharges with methane as feed gas are widely used for the deposition of hydrogenated films. The film properties depend strongly on the chemical composition and therefore two of the main stable products in this kind of discharge, namely ethane (C2H6) and acetylene (C2H2), have been measured for the understanding of the reaction kinetics in the plasma. An absorption spectrometer has been built up for the investigation of the concentrations of these as a function of the input power and the flow rate. The time scales for reaching steady state after the discharge is switched on and the depletion time scale after the plasma is switched off have been determined. Assuming the recombination of CH3 molecules to be the only production mechanism for C2H6 and using a simplified rate equation, the measured densities of C2H6 can be reproduced very well by analytical fitting curves.

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.