Ν3 Combination Band Research Articles

Spectroscopic observation and ab initio calculations of a new isomer of the CS2 trimer

We report spectroscopic observation and theoretical calculations of a new isomer of (CS2)3 as observed in the regions of the ν3 fundamental band of CS2 (6.5 μm) and the ν1 + ν3 combination band (4.5 μm), using tunable laser sources and a pulsed supersonic slit-jet. The previously observed CS2 trimer has a barrel-shaped structure with three equivalent monomers and D3 symmetry. The new isomer consists of a staggered parallel “dimer pair” of equivalent CS2 monomers with a third CS2 monomer sitting “on top”, similar to the known non-cyclic CO2 trimer. This structure has C2 rotational symmetry corresponding to the b inertial axis of the trimer, as proven by observed nuclear spin statistics. Ab initio calculations correctly give the two observed isomer structures and indicate that they lie very close in binding energy.

Experimental observation of the ν1+3ν3 combination bands of 16O14N18O and 18O14N18O in the near infrared spectral region

The first observation of the ν1+3ν3 combination band of the nitrogen dioxide isotopologue 16O14N18O is presented. The band was measured using Fourier-Transform Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy (FT-IBBCEAS) in the region between 5870 cm−1 and 5940 cm−1. To confirm the assignment, the band was simulated using a standard asymmetric top Watson Hamiltonian using extrapolated rotational and centrifugal distortion constants. Furthermore, the first experimental observation of the ν1+3ν3 band of the 18O14N18O isotopologue is also reported. The positions of ro-vibrational lines of the ν1+3ν3 band of the naturally most abundant isotopologue 16O14N16O were used for wavenumber calibration of line positions.

Detection of C2H2 using a segmented HC-PCF gas cell

This article demonstrates a fiber laser system that can detect acetylene gas (C2H2) at parts per million (ppmv) levels. The authors investigated the use of off-resonance laser wavelength to develop a highly sensitive detection system. They have developed a gas cell using segmented Hollow-Core Photonic Crystal fiber (HC-PCF), which reduced the filling and evacuation time and thus greatly improved the response time. The gas cell was filled with the gas sample and placed inside the laser cavity, which formed an intracavity laser absorption spectroscopic system. The laser wavelength was selected using a fiber Bragg grating (FBG). The fiber Bragg grating wavelength was centered between the R (5) and R (4) rotational lines of the acetylene ν1 + ν3 combination band, at 1522.22 nm, and used to probe their absorption.

Experimental observation of the ν2+4ν3 bands of HD16O and HD18O between 14975 and 15275 cm−1

The ν2+4ν3 combination bands of HD16O and HD18O were measured using Fourier transform-incoherent broadband cavity-enhanced absorption spectroscopy (FT-IBBCEAS) with a spectral resolution of 0.08 cm−1. The ro-vibrational lines of these bands were assigned through comparison with ab-initio molecular lines from the Tomsk variational calculation database (http://spectra.iao.ru/). For HD18O and HD16O in total 114 and 141 strong lines were assigned in the region between 14975.3 cm−1 and 15243.3 cm−1 and between 14998.5 cm−1 and 15274.7 cm−1, respectively. While the very satisfactory agreement of line intensities was used for line assignments, a systematic average discrepancy of ~0.305 cm−1 in line positions was identified between the measured lines of HD18O and the theoretically predicted lines from the Tomsk database. Similarly for HD16O, an approximate wavenumber difference of ~0.361 cm−1 was observed. The wavenumber accuracy of the Fourier transform cavity enhanced absorption spectrometer was confirmed on basis of concurrently measured H216O spectra in the region between 15254.2 cm−1 and 15376.9 cm−1 and corroborated the systematic shifts of the ab initio data. A few lines of the ν1+4ν2+2ν3 bands of HD16O and HD18O were also identified. The data are compared and discussed on basis of existing literature data.

Dual-comb laser spectroscopy of CS2 near 4.6 µm

High-precision absorption spectrum of carbon disulfide (CS2) has been recorded by mid-IR dual-comb laser spectroscopy between 2110 and 2200 cm−1 (4.5–4.7 µm) with sample point spacing of 5 × 10−4 cm−1 and absolute accuracy better than 10−5 cm−1. We present about 980 line positions and intensities retrieved for eleven cold and hot bands of the 12C32S2, 32S12C34S, 32S12C33S, and 13C32S2 isotopologues with the majority of the observed bands belonging to the 12C32S2 isotopologue. Direct measurements of line intensities for most of these CS2 bands are reported for the first time. We compare the measured CS2 line parameters with those obtained in the previous works near 2180 cm−1. As of today, the obtained experimental dataset is the most comprehensive and significantly improves the knowledge of the absorption spectrum of carbon disulfide near 4.6 µm dominated by the ν1 + ν3 combination band.

High-resolution infrared spectroscopy of HCF in the CH stretch region.

We present the results from a high-resolution infrared study of jet-cooled singlet monofluorocarbene (HCF) in the CH stretch region near 2600 cm-1. Absorption signals are recorded using near quantum shot noise limited laser absorption methods. The fully resolved absorption spectra of the CH stretch (ν1) fundamental band and a partial progression of transitions of the HCF bend plus CF stretch (ν2 + ν3) combination band are observed and show clear evidence of a strong rovibrational coupling between the ν1Ka ' = 2 and ν2 + ν3Ka ' = 3 manifolds, including the observation of "dark state" transitions. A detailed perturbation analysis of a c-type Coriolis interaction is carried out for these two coupled vibrational states, providing experimental determination of precise rovibrational constants. A combined ground state combination difference fit of the transitions to the ν1 and ν2 + ν3 vibrational states in this study with previously reported LIF Ã(0,0,0) ← X̃(0,0,0) data has been done to increase the accuracy of the ground state rotational constants [M. Kakimoto et al., J. Mol. Spec. 88, 300-310 (1981)]. Moreover, we report, for the first time, hot band (ν1 + ν3 ← ν3) transitions due to vibrationally excited HCF in the CF stretch mode, ν3. The high-resolution results for all vibrational frequencies and rotational constants are in good agreement with and significantly extend the analysis of the rovibrational manifold of HCF. The present ground state and ν3 spectroscopic parameters now permit improved predictions for pure rotational and ν3 fundamental transitions to aid spectral searches for HCF in the laboratory and the interstellar medium.

Observation of saturated absorption spectrum of 12C2H2 at 1536 nm in a 300 μm hollow glass waveguide

A system for saturated absorption spectroscopy is firstly built (to our knowledge) based on a gas-filled hollow glass waveguide with a core diameter of 300 μm. The performance of the system is demonstrated by observing the saturated absorption of the P(17) transition of the (ν1 +ν3) combination band of acetylene around 1536 nm using an erbium-doped fiber amplifier boosted external cavity diode laser. The saturated absorption spectrum is observed and the observed linewidth of the saturation dip varies from 10 to 20 MHz depending on the gas pressure and the optical launching condition into the GW. Its derivative spectrum is obtained using wavelength modulation spectroscopy and the third derivative signal is used as the error signal to lock the external cavity diode laser to the transition center. A frequency stability of ~60 kHz at 1 s integration time was achieved. Our study shows it is possible to build a mid-IR frequency reference using a large core gas-filled hollow GW.

Infrared bands of CS2 dimer and trimer at 4.5 μm.

We report observation of new infrared bands of (CS2)2 and (CS2)3 in the region of the CS2 ν1 + ν3 combination band (at 4.5 µm) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS2)2 is a cross-shaped structure with D2d symmetry and that for (CS2)3 is a barrel-shaped structure with D3 symmetry. The dimer has one doubly degenerate infrared-active band in the ν1 + ν3 region of the CS2 monomer. This band is observed to have a rather small vibrational shift of -0.844 cm-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel bands and one perpendicular band. Much larger vibrational shifts of -8.953 cm-1 for the perpendicular band and -8.845 cm-1 and +16.681 cm-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

Direct absorption spectroscopy baseline fitting for blended absorption features.

A method for determining the gas concentration from blended absorption features using direct absorption spectroscopy is developed. Blended spectra occur when multiple absorption lines are in close proximity in a laser scan and/or when pressures begin to increase. The blended-feature baseline fitting method estimates the baseline by coupling measured data with simulated fractional transmission at the peaks between absorption features. The method is first validated with synthetically created data. The method is then demonstrated using the absorption lines of CO2(P41e,P42f,P34e,P43e) and H2O in the ν1+ν3 combination band (3683.5 cm-1 to 3686.5 cm-1) in a heated static test cell. Results from the fitting method and wavelength modulation spectroscopy measurements agree within 1%. The method is finally applied to measurements of flue gas emissions from a practical combustion system (wood boiler) and compared to measurements from a hand-held gas analyzer. The mean percent difference of the blended-feature baseline fitting and the wavelength modulation spectroscopy is 2.8% and 5.8% for XCO2 and XH2O, respectively, indicating that this method of direct absorption data analysis can be used for spectra with moderate feature blending, which is useful for scan ranges with multiple closely spaced absorption lines and moderate pressures.

Spectra of excess molar absorptivity of aqueous solutions of ionic liquids: Universal chromophores for aqueous electrolytes?

We devised a new data analysis methodology for Near Infrared spectroscopic data of aqueous solutions. We first isolate the non-ideal (i.e. real) part of the results as the excess molar absorptivity, εΕ, then define and calculate the excess partial molar absorptivity of solute, εSE, the effect of solute (S) on the excess molar absorptivity, εΕ. By using these, we learned that there are three universal chromophores in the ν2+ν3 combination band of H2O in the εΕ spectra of aqueous solutions of the total nine electrolytes. Furthermore, we learned from the excess partial molar absorptivity, εSE, we identified a specific chromophore out of three that distinguish the specific characteristics of a given solute on H2O. Here we add an evidence for the universality of three bands in εΕ to include two examples of ionic liquids. In addition, using the literature data, we show that the ν1 overtone of H2O in NaCl – H2O and CaCl2 – H2O also have a limited number of universal chromophores at the same wave numbers in their εΕ spectra.

Excess Partial Molar Absorptivity of Aqueous Solutions of KCl, KBr, CsCl and CsBr: Are There Three Universal Chromophores in the Excess Molar Absorptivity of the ν 2 + ν 3 Band of H2O for Aqueous Salt Solutions?

A new analysis methodology of the ν 2 + ν 3 band of H2O developed by us earlier was applied for aqueous solutions of KCl, KBr, CsCl and CsBr. We isolate the effect of inter-molecular interactions among solute ions and solvent molecules on the near infrared spectra as the excess absorptivity, e E, and then take its mole fraction derivative, $$ \varepsilon_{\text{S}}^{\text{E}} $$ (S stands for the solute). $$ \varepsilon_{\text{S}}^{\text{E}} $$ then signifies the effect of solute S on e E, the transition moment of the ν 2 + ν 3 combination band due only to the intermolecular interactions. Together with the earlier similar studies on aqueous NaCl, NaBr, NaI, (CH3)4Cl, and (C2H5)4Cl, we found that e E has apparently three universal bands among all the aqueous salts studied so far; a negative band centered at 4873 cm−1, a positive one at 5123 cm−1, and a negative band at 5263 cm−1. They were attributed to “solid-like”, “liquid-like” and “gas-like” H2O chromophores, respectively, depending on the local and instantaneous completeness of hydrogen bonding. Our recent thermodynamic studies indicated that K+, Cs+, and Cl− ions are “hydration centers” that are hydrated by a number of H2O molecules, but the bulk H2O away from hydration shells is left unperturbed. Br− is a “hydrophile” ion that forms hydrogen bonds directly to the fleetingly and yet permanently existing hydrogen bond network of H2O, but pins down the fluctuations inherent in pure H2O. The distinction between Cl− and Br− are apparent only in the behavior of $$ \varepsilon_{\text{S}}^{\text{E}} $$ at the 5123 cm−1 “liquid-like” chromophore band. The quantitative differences in the hydration numbers among Na+, K+, and Cs+ are not conspicuous in either of these three chromophores. We stress, however, that at the e E level there seem to be three bands at 4873, 5123, and 5263 cm−1 that could be universal among aqueous solutions of electrolyte solutes.

Diode-laser measurements of linestrength and temperature-dependent lineshape parameters for H2O transitions near 1.4 μm using Voigt, Rautian, Galatry, and speed-dependent Voigt profiles

Linestrengths and lineshapes for 7H2O absorption transitions in the ν1+ν3 combination band near 1.4μm were measured at high resolution using tunable diode laser absorption. H2O- and N2-perturbed lineshapes were measured at 300–1300K and 2–760Torr and Voigt, Galatry, Rautian, and quadratic-speed-dependent-Voigt profiles were least-squares fit to measured spectra using a multi-spectrum fitting routine. Transitions with J″ of 6–18 were studied to highlight the importance of collisional resonance and illustrate how the best-fit lineshape parameters varied with J″. The accuracy of the Voigt profile decreased as J″ increased (especially for J=Kc doublets). For transitions in N2, the collisional-broadening temperature exponents varied from ≈0.8 to −0.4 (for the Voigt profile) as J″ increased and this behavior coincided with increasing speed-dependent collisional-broadening coefficients. The quadratic-speed-dependent Voigt profile was found to offer the best combination of accuracy and robust lineshape parameters; however, Rautian and Galatry profiles provided similar performance.

Temperature-Dependent, Nitrogen-Perturbed Line Shape Measurements in the ν1 + ν3 Band of Acetylene Using a Diode Laser Referenced to a Frequency Comb

The P(11) line of the ν1 + ν3 combination band of C2H2 was studied using an extended cavity diode laser locked to a frequency comb. Line shapes were measured for acetylene and nitrogen gas mixtures at a series of temperatures between 125 and 296 K and total pressures up to 1 atm. The data were fit to two speed-dependent line shape models and the results were compared. Line shape parameters were determined by simultaneously fitting data for all temperatures and pressures in a single multispectrum analysis. Earlier pure acetylene measurements [Cich et al. Appl. Phys. B 2012, 109, 373-38] were incorporated to account for self-perturbation. The resulting parameters reproduce the observed line shapes for the acetylene-nitrogen system over the range of temperatures and pressures studied with average root-mean-square observed-calculated errors of individual line measurement fits of approximately 0.01% of maximum transmission, close to the experimental signal-to-noise ratios. Errors in the pressure measurements constitute the major systematic errors in these measurements, and a statistical method is developed to quantify their effects on the line shape parameters for the present system.

Near-infrared absorption detection in picolitre liquid volumes using amplified fibre loop ring-down detection

Cavity ring-down spectroscopy is performed using a single-mode fibre loop as an optical cavity. The loop contains absorption cells for either liquids or gases, as well as an erbium-doped fibre amplifier. The fibre amplifier is gain-clamped using a second fibre-loop. Ring-down times can be tuned to permit several hundred roundtrips by adjusting the loss ratio between the two nested loops using a variable optical attenuator. Ring-down measurements were performed either in the time domain or using phase shift measurements.Acetylene gas was detected through a ro-vibrational absorption line of the ν1–ν3 combination band at 1532nm at a limit of detection of 50ppm. Liquid 1-octyne and 1-dodecyne were detected in a 1pL volume at detection limits of ∼20% dilution in dodecane through their C–H stretch vibrational overtone transition.

High-resolution spectroscopy and analysis of the ν2 + ν3 combination band of SF6 in a supersonic jet expansion†

Sulphur hexafluoride is a very strong greenhouse gas whose concentration is increasing in the atmosphere. It is detected through infrared absorption spectroscopy in the strong ν3 fundamental region. Due to the existence of low-lying vibrational states of this molecule, however, many hot bands arise at room temperature and those are still not known. We present here a contribution to the elucidation of this hot band structure, by analysing the ν2 + ν3 combination band. We use a supersonic jet expansion high-resolution spectrum at a rotational temperature of ca. 25 K that was recorded thanks to the Jet-AILES setup at the Source Optimisée de Lumière d’Energie Intermédiaire du LURE (SOLEIL) Synchrotron. The simplified structure of this cold spectrum allowed us to assign 444 rovibrational lines and to fit effective Hamiltonian parameters, leading to a very satisfactory spectrum simulation. The parameters obtained in this way allow to calculate the spectrum of the ν3+ν2−ν2 hot band.

Molecular structure and hydrogen bonding in pure liquid ethylene glycol and ethylene glycol–water mixtures studied using NIR spectroscopy

The molecular structure and hydrogen bonding of ethylene glycol (EG) and EG-water mixtures in the liquid phase were studied by using near-infrared (NIR) spectroscopy. The spectra were evaluated using a two-dimensional (2D) correlation approach, moving-window 2D correlation analysis and chemometric methods. The minor changes for the CH stretching bands indicate that the structures of pure liquid EG and EG-water mixtures are determined by the intermolecular hydrogen bonding through the OH groups. The analysis of the ν2 + ν3 combination band of water reveals that in EG-rich solutions the molecules of water are predominantly bonded with two molecules of EG and this cooperative hydrogen bonding is stronger than that in bulk water. Further increase in the water content leads to formation of small water clusters around OH groups of EG. Comparing results for the binary mixtures of water with different organic solvents one can conclude that the total amount and distribution of the polar groups are the most important factors determining the solubility of water in the organic phase. The distribution of these groups depends on the length and structure of the hydrocarbon chain. Due to high population and relatively uniform distribution of the OH groups of EG water has unlimited solubility in liquid EG.

Rovibrational states of ClHCl− isotopologues up to high J: a joint theoretical and spectroscopic investigation

Explicitly correlated coupled cluster theory at the CCSD(T*)-F12b level (T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys., 2007, 127, 221106) and two precise spectroscopic parameters (K. Kawaguchi, J. Chem. Phys., 1988, 88, 4186) were used to construct an accurate near-equilibrium analytical potential energy function (PEF) for the highly anharmonic centrosymmetric hydrogen-bonded complex ClHCl(-) (Re = 3.1153 Å). From variational calculations with that PEF, a large number of rovibrational energies of different isotopologues up to high values of the rotational quantum number J was obtained. Theory helped with the assignment of lines observed by IR diode laser spectroscopy in the ν1 + ν3 combination band of (35)ClH(35)Cl(-) and (37)ClH(35)Cl(-) and enabled us to elucidate rather subtle patterns of rovibrational interactions. Furthermore, transition dipole moments were predicted and analysed as well as unusual isotopic effects.

Temperature-dependent pressure broadened line shape measurements in the ν 1+ν 3 band of acetylene using a diode laser referenced to a frequency comb

Using an extended cavity diode laser referenced to a femtosecond frequency comb, the P(11) absorption line in the ν 1+ν 3 combination band of the most abundant isotopologue of pure acetylene was studied at temperatures of 296, 240, 200, 175, 165, 160, 155, and 150 K to determine pressure-dependent line shape parameters at these temperatures. The laser emission profile, the instrumental resolution, is a Lorentz function characterized by a half width at half the maximum emission (HWHM) of 8.3×10−6 cm−1 (or 250 kHz) for these measurements. Six collision models were tested in fitting the experimental data: Voigt, speed-dependent Voigt, Rautian–Sobel’man, Galatry, and two Rautian–Galatry hybrid models (with and without speed-dependence). Only the speed-dependent Voigt model was able to fit the data to the experimental noise level at all temperatures and for pressures between 3 and nearly 360 torr. The variations of the speed-dependent Voigt profile line shape parameters with temperature were also characterized, and this model accurately reproduces the observations over their entire range of temperature and pressure.

Basis set selection for the calculation of the IR fundamental intensities for 1,1-C2H2F2 and F2CO

The theoretical calculation of the CC, CO and the CF2 asymmetric stretching infrared intensities of the isoelectronic 1,1-C2H2F2 and F2CO molecules has resulted in large errors varying from 41.0 to 60.5 km mol−1 or 13.5% to 28.0% of their experimental intensities when using extended 6-311++G(3d,3p) basis set wave-functions at the Moller–Plesset 2 level. For this reason new theoretical intensity results are reported using wave-functions calculated with a large variety of basis sets at both the MP2 and QCISD levels. Accurate intensities were obtained with wave-functions formed from basis sets with polarization functions but without diffuse functions, 6-31G(d,p), 6-31G(2d,2p), 6-31G(3d,3p), 6-311G(d,p) and 6-311G(3d,3p) at both electron correlation levels. Best results for these bands were obtained with the 6-31G(2d,2p) wave-function at the QCISD level providing errors less than 10 km mol−1 and 3.4% of the experimental values. The rms errors for all the 1,1-C2H2F2 and F2CO intensities at this level are 12.3 and 6.5 km mol−1, respectively. These wave functions at both electron correlation levels also resulted in accurate values for the fundamental intensities of trans-C2H2F2 and cis-C2H2F2 with rms errors of less than 10 km mol−1. Theoretical calculations of the infrared intensities of the 1,1-C2HDF2 isotopomer indicate that the experimental determination of the CH symmetric stretching intensity of 1,1-C2H2F2 is probably over-estimated owing to severe band overlap and/or Fermi resonance with the ν2 + ν3 combination band.

Absolute frequency measurement of a water-stabilized diode laser at 1.384 μm by means of a fiber frequency comb

We report on the absolute frequency measurement of an extended-cavity diode laser stabilized against a Doppler-free vibration–rotation transition of the H218O isotopologue near 1.384 μm. Absolute determination is performed by comparing the water-stabilized laser frequency with respect to a GPS-disciplined Rb microwave standard by means of a self-referenced fiber-based optical frequency comb. The line center frequency of the 22,1→22,0 transition of the H218O ν1+ν3 combination band is found to be 216 519 045 955(13) kHz.