Ν3 Combination Band Research Articles

Temperature sensing in shock-heated evaporating aerosol using wavelength-modulation absorption spectroscopy of CO2 near 2.7 µm

A tunable diode laser sensor with a detection bandwidth of 40 kHz is developed for measuring the time-varying gas temperature of CO2 during the evaporation of shock-heated hydrocarbon fuel aerosol. Normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f) is used to probe R(28) and P(70) transitions in the ν1 + ν3 combination band of CO2 near 2.7 µm. The fixed-center-wavelength WMS sensor was first validated in a shock tube with non-reactive CO2/Ar gas mixtures, yielding an accuracy of better than 1.5% over the entire range of 650–1500 K. The sensor was then evaluated in a well-controlled aerosol flow cell, demonstrating the potential for precise gas temperature measurement even when aerosol scattering attenuates more than 99% of the incident light. Applications of this sensor for accurate temperature measurement of evaporating n-dodecane aerosol were then performed in an aerosol shock tube. The time-resolved temperature variation due to the evaporation of fuel droplets was accurately captured without using an off-resonant laser to account for the extinction from droplet scattering. Measured temperatures confirmed the accuracy of the gasdynamic model used to calculate the pre- and post-evaporation shock conditions, as needed in shock tube studies on combustion chemistry.

New infrared spectra of the nitrous oxide trimer

Infrared spectra of N(2)O trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by Miller and Pedersen [J. Chem. Phys. 108, 436 (1998)] in the N(2)O nu(1)+nu(3) combination band region ( approximately 3480 cm(-1)) showed the trimer structure to be noncyclic, with three inequivalent N(2)O monomer units which could be thought of as an N(2)O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N(2)O fundamental band regions nu(3) ( approximately 1280 cm(-1)) and nu(1) ( approximately 2230 cm(-1)). In the nu(3) region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the nu(1)+nu(3) region, but in the nu(1) region all three possible trimer bands are observed. Relationships among the various bands are considered with reference to their rotational intensity patterns, their vibrational shifts, and the properties of the related N(2)O dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully (15)N-substituted species in the nu(1) region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer.

CO2 concentration and temperature sensor for combustion gases using diode-laser absorption near 2.7 μm

A new tunable diode-laser sensor based on CO2 absorption near 2.7 μm is developed for high-resolution absorption measurements of CO2 concentration and temperature. The sensor probes the R(28) and P(70) transitions of the ν1+ν3 combination band of CO2 that has stronger absorption line-strengths than the bands near 1.5 μm and 2.0 μm used previously to sense CO2 in combustion gases. The increased absorption strength of transitions in this new wavelength range provides greatly enhanced sensitivity and the potential for accurate measurements in combustion gases with short optical path lengths. Simulated high-temperature spectra are surveyed to find candidate CO2 transitions isolated from water vapor interference. Measurements of line-strength, line position, and collisional broadening parameters are carried out for candidate CO2 transitions in a heated static cell as a function of temperature and compared to literature values. The accuracy of a fixed-wavelength CO2 absorption sensor is determined via measurement of known temperature and CO2 mole fraction in a static cell and shock-tube. Absorption measurements of CO2 are then made in a laboratory flat-flame burner and in ignition experiments of shock-heated n-heptane/O2/argon mixtures to illustrate the potential of this sensor for combustion and reacting-flow applications.

Gas temperature measurements using widely tunable long-wavelength VCSEL

A method for gas temperature measurements with a widely tunable laser diode is presented. The method involves rapidly switching the laser frequency between two distantly spaced absorption lines chosen for optical thermometry. Direct absorption spectroscopy using a single-mode VCSEL was employed to probe the R10 and R22 lines of the 2ν1+2ν20+ν3 combination band of CO2 near 6355.9 and 6363.7 cm-1 sequentially. A specially designed 0.5-m cryogenic gas cell was filled with 10 mbar CO2 at room temperature and cooled to 150 K with liquid N2. The VCSEL was modulated with a 10-kHz ramp superimposed on a 1-kHz square waveform to scan two 0.04 cm-1 intervals sequentially. The gas temperatures obtained with the VCSEL in the 150–300 K range are in a good agreement with those derived from gas pressure ratios. The maximum relative error of temperature measurements using the VCSEL was ± 3%. A compact VCSEL-based sensor can be developed for gas temperature and concentration measurements in the Martian atmosphere. The method proposed can be used for many applications including in situ monitoring of combustion processes.

Photoacoustic measurement of N2O concentrations in ambient air with a pulsed optical parametric oscillator

Photoacoustic spectroscopy, in combination with a pulsed grazing-incidence optical parametric oscillator (GIOPO), was used for sensitive detection of nitrous oxide (N2O) in ambient air. The ν1+ν3 combination band of N2O was excited with the idler beam of the GIOPO in the 2.76 μm–2.91 μm spectral region, where CO2 and water-absorption lines are also present. Three chemical filters filled with KOH, CaCl2, and P2O5 were used to reduce the CO2 and water concentrations to the level of several parts per billion (109) by volume (ppbv). Photoacoustic spectra containing several absorption lines were recorded and the concentration was determined by an integral evaluation method and a fast Fourier transform evaluation method. The photoacoustic signal was calibrated by a standard mixture of 50.6 parts per million by volume (ppmv) of N2O in synthetic air. Values of 311±5 ppbv, 314±5 ppbv, and 316±5 ppbv were found for three ambient samples collected at nearby roads.

High-precision frequency measurements of the ν1 + ν3 combination band of 12C 2H 2 in the 1.5 μm region

A pair of 1.5 μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to transitions of 12C 2H 2 and 13C 2H 2, using cavity-enhanced Doppler-free saturation absorption spectroscopy. The absolute frequencies of 41 lines of the ν 1 + ν 3 band of 12C 2H 2, covering the spectral region 1520–1545 nm, have been measured by use of a passive optical frequency comb generator, referenced to 13C 2H 2 transitions of known frequency. The mean experimental uncertainties (coverage factor k = 1) of the frequency values are 3.0 kHz (type A) and 10 kHz (type B). Improved values of the band origin ν 0, rotational constants B′ and B″, and centrifugal distortion coefficients D′, D″, H′, and H″ are presented.

Investigations of the Structure of H2O Clusters Adsorbed on TiO2 Surfaces by Near-Infrared Absorption Spectroscopy

The overtone and combination bands of the fundamental vibration modes (nu(1), symmetric stretching; nu(2), bending; and nu(3), asymmetric stretching) attributed to the H(2)O molecules adsorbed on a TiO(2) surface could be observed in the near-infrared (NIR) region. Especially, two absorption bands attributed to the combination (nu(2) + nu(3)) and (nu(1) + nu(3)) modes of the H(2)O molecules adsorbed on the TiO(2) surface were observed at around 1940 and 1450 nm, respectively. From detailed investigations on the (nu(2) + nu(3)) combination band, it was found that H(2)O molecules absorbed on a TiO(2) surface aggregate to form clusters due to the high surface tension of H(2)O arising from the intermolecular hydrogen bonds, and the hydrogen-bonded H(2)O in the bulk part of the cluster and the hydrogen-bond-free H(2)O in the outside spherical part of the cluster could be easily distinguished. Furthermore, it was quantitatively confirmed that the relaxation of the surface energy accompanying the adsorption of H(2)O on the TiO(2) surface stabilized the adsorption states of the hydrogen-bonded H(2)O molecules, while on the other hand, the hydrogen-bond-free H(2)O molecules became unstable as compared to the liquid-phase H(2)O molecules.

Thermal tuning and modulation of a DFB fibre laser with a thin-film heater

Results are given for thermal tuning and modulation of a 1556-nm distributed feedback fibre laser by resistive heating of a thin silver film chemically deposited on the fibre. Without reaching the limits of performance, linear tuning is demonstrated at a rate of 1.72 pm/mW up to about 200 pm, and a peak-to-peak modulation of 100 MHz up to modulation frequencies of 60 Hz. The heat flow is analyzed, and the coated fibre is characterized in terms of the static and dynamic wavelength response to the applied electric power. The performance of the scheme is tested by recording part of the ν1+ν3 combination band spectrum of 13C2H2 with thermal modulation and scanning of the fibre laser.

High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm−1 in slit jet expansions and at room temperature

The ν2+2ν3 combination band of CH412 near 7510 cm−1 was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. ν2+2ν3 is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of ν2+2ν3 at 7510.3378±0.0010 cm−1, the integrated band strength G=(1.3±0.2)×10−4 pm2 and the vibrational transition moment |μν|=(1.0±0.1)×10−3 D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F1-component of ν2+2ν3 interacting by a strong Coriolis resonance with the IR-active F2-component appears to be a dominant perturber.

High-Resolution Absorption Spectroscopy of the 3ν1 and 3ν1 + ν3 Bands of Propyne

The 3ν1 and 3ν1 + ν3 bands of propyne have been recorded at Doppler-limited resolution by Fourier transform spectroscopy and intracavity laser absorption spectroscopy, respectively. The two bands show a mostly unperturbed J rotational structure for each individual K subband. However, as a rule the K structure ordering is perturbed in overtone transitions of propyne and different effective parameters associated with each K subband have been determined. From the vibrational energy levels, a value of −6.6 cm−1 has been obtained for the x13 cross anharmonicity in perfect agreement with the origins of the ν1 + ν3 and 2ν1 + ν3 combination bands estimated from the FTIR spectrum. Hot bands from the v9 = 1 and v10 = 1 levels associated with the 3ν1 + ν3 combination band have been partly rotationally analyzed and the retrieved values of x39 and x3,10 are in good agreement with literature values. Finally, the 4ν1 + ν9 − ν9 band centered at 12 636.6 cm−1 has been recorded by ICLAS. The red shift of this hot band relative to 4ν1 and the ΔBv value are discussed in relation to the anharmonic interaction between the 4ν1 and 3ν1 + ν3 + ν5 levels.

Rovibrational spectroscopy of the ν2=1, ν3=1, and ν2=ν3=1 levels of CDF3

The high-resolution FTIR spectra of deuterofluoroform (CDF3) at essentially Doppler-limited resolution of 2.5×10−3cm−1 were studied in the regions of the ν2 (1111cm−1) and ν3 (694cm−1) fundamental and the ν2+ν3 (1802cm−1) combination bands. The analysis of the two fundamental bands performed simultaneously with available submillimeter wave data leads to a significant improvement of spectroscopic constants with respect to those from previous studies with lower resolution. The assignments in the ν2+ν3 combination band were checked with the assistance of the (ν2+ν3)−ν2 and (ν2+ν3)−ν3 hot bands, which all were assigned for the first time. The rovibrational analyses were done in the approximation of isolated vibrational levels, reproducing the experimental data within the experimental accuracy in case of the ν2=1 and ν3=1 levels, but leaving some local perturbations in the ν2=ν3=1 level that cannot yet be quantitatively explained.

The vibrational spectra of CO2+, (CO2)2+, CO2−, and (CO2)2− trapped in solid neon

When a Ne:CO2 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned to CO2+, (CO2)2+, CO2−, and (CO2)2−. The lowest (μ 2∑u+) Renner component of the bending fundamental of CO2+ trapped in a neon matrix appears near the gas-phase band center, but other Renner components are undetectable. Absorptions of a photolabile product correspond to the recently identified CO-stretching fundamentals of (CO2)2+. Weak infrared absorptions at 1253.8 and 714.2 cm−1 are assigned to ν1 and ν2 of CO2−, respectively, and a moderately intense absorption at 2894.7 cm−1 is assigned to the ν1+ν3 combination band of that product. As in other recent argon- and neon-matrix studies, two weak infrared absorptions can be assigned to the two infrared-active OCO-stretching fundamentals of the D2d structure of (CO2)2−. Detailed isotopic substitution studies support all of these assignments. A weak absorption near the CO2 bending fundamental, for which isotopic substitution data are incomplete, may be contributed either by a second fundamental of (CO2)2− (D2d) with b2 symmetry or by a weakly interacting (CO2)n⋅⋅CO2− complex. Such ion–molecule complexes contribute other absorptions near ν3 of CO2 and of CO2−.

Reactions of Laser-Ablated Iridium Atoms with O2. Infrared Spectra and DFT Calculations for Iridium Dioxide and Peroxo Iridium(VI) Dioxide in Solid Argon

Laser-ablated iridium atoms react with O2 to give the linear dioxide OIrO and the side-bound dioxygen adduct (O2)IrO2 as primary reaction products, with ν3 absorptions at 930.0 and 875.0 cm-1, respectively. The (ν1 + ν3) combination bands for these species were observed, confirming the assignments of the ν1 and ν3 modes for (O2)IrO2 and providing an estimate for the ν1 mode in OIrO. The symmetric and antisymmetric stretching modes of the Ir(O2) ring in (O2)IrO2 were also observed. Density functional calculations support these assignments and show that the (O2)IrO2 species is best formulated as a peroxide with iridium in the +6 oxidation state, providing the first example of an iridium(VI) oxo complex.

The Far Infrared Spectrum of the NO Dimer

The far-infrared spectrum of the NO dimer in the gas phase has been observed for the first time using a long-path (180 m) cooled (99 K) absorption cell and a Bomem Fourier transform spectrometer. Four weak vibration–rotation bands were detected in the 120–450 cm−1region and assigned to intermolecular vibrations of thecis–planar ON–NO complex. The strongest is a typebband at 239.36 cm−1, which is assigned as the ν2(symmetric bending) fundamental. A second typebband at 134.50 cm−1, partly obscured by2Π3/2←2Π1/2transitions of the NO monomer, is assigned as the ν3(intermolecular stretch) fundamental. A third typebband at 351.38 cm−1can then be assigned as the ν2+ ν3combination band. Finally, a very weak typeaband at 429.14 cm−1is assigned as the ν6(antisymmetric bending) fundamental. We report here detailed analyses of the ν2and ν2+ ν3bands, which yielded upper state rotational parameters, centrifugal distortion parameters, and band origins. Somewhat less extensive analyses for ν3and ν6give their rotational parameters and band origins. These results resolve the mystery of the true values of the intermolecular vibrational frequencies of the NO dimer, which turn out to be rather different from previous determinations based on condensed phase spectra.

Reactions of Laser-Ablated Cobalt Atoms with O2. Infrared Spectra of Cobalt Oxides in Solid Argon

Laser-ablated Co atoms react with O2 to give the linear dioxide, the monoxide, and the rhombic (CoO)2 dimer molecules. The linear OCoO molecule is characterized by oxygen-18 substitution in strong ν3 (945.4 cm-1) fundamental and weak ν1+ν3 (1717.6 cm-1) combination bands. Annealing to 20−30 K to allow diffusion and reaction of cold cobalt atoms and dioxygen markedly increases weak bands due to the Co(O2) and CoOO complexes and higher species including OOCoO, OOCoO2, (O2)CoO2, and the dimer (CoO2)2. DFT and CASSCF calculations performed on the three CoO2 isomers and rhombic (CoO)2 support these assignments; DFT calculations are also presented for the rhombic (FeO)2 molecule.

Reactions of Laser-Ablated Vanadium Atoms with Dioxygen. Infrared Spectra of VO, VO2, OOVO2, and V2O2 in Solid Argon

Laser-ablated V atoms react with O2 to give primarily the bent VO2 dioxide molecule and a smaller yield of the monoxide VO. The OVO valence angle is estimated as 118 ± 3° from oxygen isotopic ν3 frequencies and 121° from DFT calculations. New absorptions appear on annealing to 35 K that can be assigned to two V2O2 isomers and the superoxo adduct to VO2, namely the OOVO2 molecule. Both VO2 and OOVO2 reveal weak ν1 + ν3 combination bands, which support the assignment of ν1 and ν3 fundamental vibrations.

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

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.

Infrared spectra of the reaction products of laser ablated lead atoms and oxygen molecules in condensing argon and nitrogen

Reactions of laser ablated lead atoms with oxygen molecules in condensing argon and nitrogen streams gave the PbO, OPbO, Pb(O2), OPb(O2), PbOPb, Pb2O2, PbPb(O2), and Pb4O4 molecules. The formation of lead monoxide and linear dioxide molecules is the main difference with thermal evaporation experiments. The linear OPbO isomer was identified from oxygen-18 and lead (natural abundance) isotopic substitution in the strong antisymmetric stretching absorption at 764.8 cm−1; a weak ν1+ν3 combination band was observed at 1423.4 cm−1. Two bands [ν1=728.7 cm−1 (Ar) and 730.6 cm−1 (N2) and ν2=437.3 cm−1 (Ar) and 449.6 cm−1 (N2)] are assigned to the cyclic isomer Pb(O2) based on oxygen isotopic substitution. The position of the O–O stretching fundamental shows that Pb(O2) is a peroxidelike molecule. The secondary reaction Pb(O2)+Pb gives a large yield of cyclic Pb2O2 and allows observation of very strong antisymmetric Pb–O stretching fundamentals and combination bands with the two symmetric stretching fundamentals. Density functional frequency calculations with effective core potentials support assignment of the observed infrared absorptions.

Matrix isolation study of the interaction of excited neon atoms with BF3: Infrared spectra of BF2, BF+2, BF+3, and BF−3

When a Ne:BF3 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit shows prominent absorptions of BF2. All three vibrational fundamentals and the ν1+ν3 combination band of BF2 isolated in solid neon are identified, and a normal coordinate analysis is conducted. In addition, infrared absorptions of the BF+3, BF+2, and BF−3 molecular ions are assigned, and the processes which occur when the solid deposit is exposed to visible and ultraviolet radiation are considered.

The vibrational spectra of molecular ions isolated in solid neon. XI. NO+2, NO−2, and NO−3

When a Ne:NO2 or a Ne:NO:O2 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, infrared absorptions of NO+2, NO−2, and NO−3 appear. Detailed isotopic substitution studies support the assignment of prominent absorptions to ν3 of NO+2 and NO−2 and of weak to moderately intense absorptions to the ν1+ν3 combination band of each of these species. When the contribution of anharmonicity is considered, the positions of the NO+2 absorptions are in satisfactory agreement with the values for the stretching fundamentals obtained in a recent gas-phase study of that species. When the sample is exposed to 240–420 nm mercury-arc radiation, the initially present absorptions of NO−3 trapped in sites with a small residual cation interaction diminish in intensity, and the unsplit ν3(e′) absorption of isolated NO−3 grows. The mechanism responsible for this growth in the absorption of isolated NO−3 is considered.