The analysis of combination and overtone states of BF3 from 1650 to 4600 cm

High-resolution (0.0015-0.0035 cm⁻¹) infrared spectra of isotopically enriched ¹¹BF₃ have been examined in detail. The analysis of the combination and overtone states within the region of study, from 1650 to 4600 cm⁻¹, led to the assignment of over 25,000 transitions. The major perturbations were due to the Fermi resonances between states possessing one quantum of v₃ and three quanta of v₄. With corrections through the quadratic rotational terms, the equilibrium B_e and C_e values have been determined; 0.3462679(7) cm⁻¹ and 0.1731151(6) cm⁻¹, respectively. An improved set of equilibrium rotational constants for ¹⁰BF₃, consistent with this analysis of ¹¹BF₃ are also given. The averaged equilibrium values for both isotopomers lead to a B-F bond distance of r_e = 130.704 ± 0.005 pm. All of the quadratic anharmonic constants, with the exception of x³³ were independently determined from experiment. For the first time for BF₃, a normal force field analysis was performed that utilized the experimentally determined, fundamental harmonic vibrational frequencies.     

Coherent Raman spectra of the ν1 mode of BF3 and BF3

High resolution (0.001 cm⁻¹) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν₁ symmetric stretching mode of the planar symmetric D_3h molecules ¹⁰BF₃ and ¹¹BF₃. Simulations of the spectra were done using ν₁ rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D_3h molecule recently studied. In the harmonic approximation, the ¹⁰BF₃ and ¹¹BF₃ν₁ Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν₁ stretching frequency for ¹¹BF₃ is found to be 0.198 cm⁻¹higher than for the lighter ¹⁰BF₃ isotopomer. This counterintuitive result is reproduced almost exactly (0.200 cm⁻¹) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and x_ij anharmonicity constants. The ab initio computations also predict to within 1% the ΔB, ΔC changes in the rotational constants in going from the ground state to the v₁ = 1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra.     

The high-resolution, jet-cooled infrared spectrum of pentafluoroethane

The jet-cooled spectrum of pentafluoroethane (C₂HF₅) has been recorded between 1100 and 1325 cm⁻¹ at a resolution of 0.0022 cm⁻¹. A rotational temperature of approximately 10 K was achieved by expanding 50 Torr of C₂HF₅ in 500 Torr of helium. Transitions belonging to five different fundamental vibrations have been assigned and fit to a Watson Hamiltonian: the ν₃ band at 1309.880494(189) cm⁻¹, ν₄ at 1200.734645(67) cm⁻¹, ν₅ at 1142.78147(33) cm⁻¹, ν₁₃ at 1223.334098(115) cm⁻¹, and ν₁₄ at 1147.394185(163) cm⁻¹. The fit of the ν₄ band has an rms deviation of 0.000436 cm⁻¹ compared to the uncertainty in the experimental line position of 0.0002 cm⁻¹. Satisfactory fits were achieved for the other four bands (ν₃, ν₅, ν₁₃, ν₁₄) at this cold temperature, with most of the centrifugal distortion constants fixed at the ground state values. Joint fits with previous work were attempted for the ν₄ and ν₁₃, successfully in the former case and unsuccessfully in the latter.     

The high-resolution infrared spectrum of isoxazole vapor between 800 and 1700 cm

The Fourier transform gas-phase IR spectrum of isoxazole, C₃H₃NO, between 550 and 1700 cm⁻¹ was measured with a resolution of ca. 0.003 cm⁻¹. Ten fundamental bands in the region 800-1700 cm⁻¹ have been analyzed by the Watson Hamiltonian model to yield upper state spectroscopic constants. A number of local resonances have been identified in the bands and explained qualitatively, and the unobserved ν₁₄(A″) fundamental band has been located at 897.5(5) cm⁻¹ from its perturbation effects on the neighboring fundamentals.     

The rotationally-resolved absorption spectrum of formaldehyde from 6547 to 6804 cm

The room temperature absorption spectrum of formaldehyde, H₂CO, from 6547 to 6804 cm⁻¹ (1527-1470 nm) is reported with a spectral resolution of 0.001 cm⁻¹. The spectrum was measured using cavity-enhanced absorption spectroscopy (CEAS) and absorption cross-sections were calculated after calibrating the system using known absorption lines of H₂O and CO₂. Several vibrational combination bands occur in this region and give rise to a congested spectrum with over 8000 lines observed. Pressure broadening coefficients in N₂, O₂, and H₂CO are reported for an absorption line at 6780.871 cm⁻¹, and in N₂ for an absorption line at 6684.053 cm⁻¹.     

Intensity measurements of H2O lines in the spectral region 8000-9350 cm

This paper presents new measurements of H₂¹⁶O lines performed on spectra recorded with the GSMA Fourier Transform Spectrometer (FTS). Our experimental conditions allow one to obtain new line intensity measurements from 10⁻²⁵ to 10⁻²¹ cm/molec at 296 K and self-broadening coefficients in the spectral range centered at 8800 cm⁻¹. In the HITRAN database, data reported for this region is taken from the work of Mandin et al. (1988) [8] and [9] and several articles pointed out problems on the line intensities. We present in this paper some intensity comparisons, first with the HITRAN database, and then with the recent article of Tolchenov and Tennyson (2005) [3]. We finish by a comparison on self-broadening coefficients.     

The high-resolution infrared spectrum of pyrrole between 900 and 1500 cm revisited

The Fourier transform gas-phase infrared spectrum of pyrrole, C₄H₅N, has been recorded with a resolution of ca. 0.003 cm⁻¹ in the 900-1500 cm⁻¹ spectral region. Four fundamental bands, ν₈(A₁; 1016.9 cm⁻¹), ν₂₃(B₂; 1049.1 cm⁻¹), ν₇(A₁; 1074.6 cm⁻¹), ν₂₀(B₂; 1424.4 cm⁻¹) and the overtone band 2ν₁₆(A₁; 962.7 cm⁻¹) have been analysed using the Watson model. The ν₈ and 2ν₁₆ bands are unperturbed; the ν₇ and ν₂₃ bands are locally perturbed, while the ν₂₀ band is globally perturbed by weak c-Coriolis resonance. Upper state vibrational term values, and rotational and centrifugal distortion constants, have been obtained from fits using S-reduction and I^r-representation as well as A-reduction and III^r-representation. A set of ground state rotational and centrifugal distortion constants using A-reduction was obtained from a simultaneous fit of ground state combination differences from all five bands and previous microwave and millimetre-wave data.     

Intracavity laser absorption spectroscopy of D2O between 11 400 and 11 900 cm

The weak absorption spectrum of dideuterated water, D₂O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) between 11 400 and 11 900 cm⁻¹. This spectrum is dominated by the 3ν₁ + ν₂ + ν₃ and the ν₁ + ν₂ + 3ν₃ centered at 11 500.25 and 11 816.64 cm⁻¹, respectively. A total of 530 energy levels belonging to eight vibrational states were determined. The rovibrational assignment process of the 840 lines attributed to D₂O was mostly based on the results of new variational calculations consisting in a refinement of the potential energy surface of Shirin et al. [J. Chem. Phys., 120 (2004) 206] on the basis of recent experimental observations, and a dipole moment surface from Schwenke and Partridge [J. Chem. Phys. 113 (2000) 6592]. The overall agreement between these calculations and the observed spectrum is very good both for the line positions and the line intensities.     

The vibrational spectrum of thiazole between 600 and 1400 cm revisited: A combined high-resolution infrared and theoretical study

The Fourier transform infrared gas-phase spectrum of thiazole, C₃H₃NS, has been recorded in the 600-1400 cm⁻¹ wavenumber region with a resolution around 0.0030 cm⁻¹. Nine fundamental bands (ν₅(A′) to ν₁₁(A′), ν₁₅(A″), and ν₁₆(A″)) are analysed employing the Watson model. Ground-state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. A detailed analysis of perturbations identified in the ν₁₁(A′) band at 866.5 cm⁻¹ enables a definitive location of the very weak ν₁₀(A′) and ν₁₄(A″) bands at 879.3 and 888.7 cm⁻¹, respectively. The three levels are analysed simultaneously by a model including Coriolis resonance using an ab initio predicted first order c-Coriolis coupling constant; second and higher order Coriolis parameters are determined. Qualitative explanations in terms of Coriolis resonances are given for a number of crossings observed in ν₅(A′), ν₆(A′), and ν₇(A′) at 1383.7, 1325.8, and 1240.5 cm⁻¹, respectively. The rotational constants, anharmonic frequencies, and vibration-rotation constants (alphas, ) calculated by quantum chemical calculations using a cc-pVTZ and TZ2P basis with B3LYP methodology, have been compared with the present experimental data. The rotation constant differences for each vibrational state, from the ground state values, are closer to experiment from the TZ2P calculations relative to those using cc-pVTZ. The values for Δ_J, Δ_JK, Δ_K, δ_J, and δ_K are close to experiment with both basis sets.     

Spectroscopy of XY3Z (C3v) molecules: A tensorial formalism adapted to the O (3) ⊃ C∞v ⊃ C3v group chain

A tensorial formalism adapted to the case of XY₃Z symmetric tops has been developed. We use the O (3) ⊃ C_∞v ⊃ C_3v group chain. All the coupling coefficients and formulas for the computation of the matrix elements are given for this chain. Such relations are also deduced in C_3v group itself.     

The AB2-XA1 electronic transition of NO2: A rovibronic survey covering 14 300-18 000 cm

More than 250 rotationally resolved vibrational bands of the A²B₂-X²A₁ electronic transition of ¹⁵NO₂ have been observed in the 14 300-18 000 cm⁻¹ range. The bands have been recorded in a recently constructed setup designed for high resolution spectroscopy of jet cooled molecules by combining time gated fluorescence spectroscopy and molecular beam techniques. The majority of the observed bands has been rotationally assigned and can be identified as transitions starting from the vibrational ground state or from vibrationally excited (hot band) states. An exceptionally strong band is located at 14 851 cm⁻¹ and studied in more detail as a typical benchmark transition to monitor ¹⁵NO₂ in atmospheric remote sensing experiments. Standard rotational fit routines provide band origins, rotational and spin rotation constants. A subset of 177 vibronic levels of ²B₂ vibronic symmetry has been analyzed in the energy range between 14 300 and 17 250 cm⁻¹, in terms of integrated density and using Next Neighbor Distribution. It is found that the overall statistical properties and polyad structure of ¹⁵NO₂ are comparable to those of ¹⁴NO₂ but that the internal structures of the polyads are completely different. This is a direct consequence of the X²A₁-A²B₂ vibronic mixing.     

High resolution infrared spectra of the ν1-ν4 bands of BiH3, and ab initio calculations of the spectroscopic parameters

The infrared spectrum of short-lived BiH₃ has been studied by Fourier transform technique. The BiH stretching bands ν₁/ν₃ at 1733.2546/1734.4671 cm⁻¹ and the bending fundamentals ν₂/ν₄ at 726.6992/751.2385 cm⁻¹ have been measured with a resolution of 5.5 and 6.6 × 10⁻³ cm⁻¹, respectively. The spectra were analyzed using different reductions of the rovibrational Hamiltonian accounting for the numerous resonance interactions in particular within the strongly Coriolis-coupled bending dyad. About 1150 and 980 transitions belonging to the ν₁/ν₃ and ν₂/ν₄ bands were fitted with an rms deviation of 0.62 and 0.53 × 10⁻³ cm⁻¹, respectively. High-level ab initio calculations at the coupled cluster CCSD(T) level with an energy-consistent small-core pseudopotential and large basis sets were carried out to determine the equilibrium structure, the anharmonic force field, and the associated spectroscopic constants of BiH₃. The theoretical results are in good agreement with the available experimental data.     

FTS improvements and connection with a long White cell. Application: H2O intensity measurements around 1200 cm

Acquisition electronics, data software and optics of the Fourier transform spectrometer of the Groupe de Spectrométrie Moléculaire et Atmosphérique laboratory have been optimized over the past 5 years. Among the improvements brought on to this system is the optical fitting of 50-m White cell. This cell can be used to achieve absorption path lengths as long as 2 km. This new setup allows us to perform spectroscopic analysis of very small absorption lines.     

Rovibrational spectrum of the Ne-N2O van der Waals complex in the 1285 cm region

The rovibrational spectrum of the Ne-N₂O van der Waals complex has been recorded in the symmetric stretching mode region of the N₂O monomer (∼1285 cm⁻¹) using a tunable diode laser spectrometer in conjunction with an astigmatic multi-pass cell and a pulsed supersonic slit jet. The spectra of both ²⁰Ne-N₂O and ²²Ne-N₂O isotopomers are assigned and analyzed using a Watson S-reduced asymmetric-rotor Hamiltonian. The rotational and centrifugal constants for the excited vibrational state are accurately determined. The band-origin of the spectrum is determined to be ν₀ = 1285.12251(18) cm⁻¹ for ²⁰Ne-N₂O and 1285.12363(27) cm⁻¹ for ²²Ne-N₂O, which shows a blue-shift of 0.21921 cm⁻¹ for ²⁰Ne-N₂O and 0.22033 cm⁻¹ for ²²Ne-N₂O from that of the N₂O monomer, respectively.     

Global analysis of CH3Cl and CH3Cl: simultaneous fit of the lower five polyads (0–2600 cm)

The global analysis of the infrared spectrum of chloromethane involving the ground state and the 13 vibrational states lying up to 2600 cm⁻¹ is reported. This work incorporates and extends to the fifth polyad, the preliminary study of the lower four polyads published by [J. Mol. Spectrosc. 221 (2003) 199]. More than 20 000 transitions (including numerous hot bands) for each isotopomer ¹²CH₃³⁵Cl and ¹²CH₃³⁷Cl have been assigned and fitted with a standard deviation of about 3 × 10⁻⁴ cm⁻¹ close to the experimental precision. A common set of 288 (resp. 303) effective parameters was determined for each isotopomer. Our global model allowed us to reproduce simultaneously and accurately the resonances already encountered in Polyad 3 and the new ones involved in Polyad 5.     

Intracavity laser absorption spectroscopy of HDO between 11 645 and 12 330 cm

The weak absorption spectrum of monodeuterated water, HDO, has been recorded by intracavity laser absorption spectroscopy (ICLAS) between 11 645 and 12 330 cm⁻¹. This spectrum is dominated by the ν₂ + 3ν₃ band of HDO at 11969.76 cm⁻¹. A total of 497 energy levels belonging to 12 vibrational states were determined while only 140 levels were previously reported from a recent investigation by Fourier transform spectroscopy in the same spectral region. The rovibrational identification process of the 1378 lines assigned to the HDO isotopologue was mostly based on the results of the accurate variational calculations of Schwenke and Partridge. The overall agreement between these calculations and the observed spectrum is very good. However, strong discrepancies in the calculated line intensities were evidenced in a few cases corresponding to an intensity transfer to a dark state through local resonance interaction.     

Interactions between vibrational polyads of propyne, H3CCCH: Rotational and rovibrational spectroscopy of the levels around 1000 cm

The study of vibration resonance physics in propyne is based on experimental measurements of about 600 new rotational transitions between 495-590 and 700-760 GHz in excited vibrational levels v₅ = 1, v₈ = 1, v₁₀ = 3 and v₉ = v₁₀ = 1 with vibrational energies around 1000 cm⁻¹. The limits to the assignments and analysis were imposed by as yet unresolved anharmonic resonances with the states of the next higher polyad of levels lying above 1200 cm⁻¹, which affect the rotational states involved in transitions that would be measurable with non-vanishing intensities. Vibration-rotation spectra pertaining to the levels in question were studied in the regions 880-1150 cm⁻¹ (the ν₅ and ν₈ fundamental bands), 550-750 cm⁻¹ (the v₉ = v₁₀ = 1 ← v₁₀ = 1 hot bands) and 250-400 cm⁻¹ (the v₁₀ = 3 ← v₁₀ = 2 “superhot” bands). A simultaneous least-squares fit of both types of data provides their reliable but in the case of accurate rotational data not always fully quantitative reproduction.     

Effects of potassium on the reaction pathway of C3H7 species over Mo2C/Mo (1 0 0)

The adsorption and surface reactions of propyl iodide on clean and potassium-modified Mo₂C/Mo(1 0 0) surfaces have been investigated by thermal desorption spectroscopy (TPD), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) in the 100-1200 K temperature range. This work is strongly related to the better understanding of the catalytic effect of Mo₂C in the conversion of hydrocarbons. Potassium was found to be an effective promoter: it induced the rupture of C-I bond in the adsorbed C₃H₇I even at 100 K. The extent of C-I bond scission varied approximately linearly with the concentration of K coverage at the adsorption temperature of 100 K. As revealed by HREELS and TPD measurements the primary products of the dissociation are C₃H₇ and I. The former one was stabilized by potassium and underwent dehydrogenation and hydrogenation to give propene and propane. The desorption of both compounds is reaction-limited process. A fraction of propyl groups was converted into di-σ-bonded propene, which was stable up to ∼380 K. The coupling reaction of propyl species was also facilitated by potassium and resulted in the formation of hexane and hexene with T_p ∼ 230-250 K. Hydrogen was released with T_p = 390 K, indicative of a desorption limited process. The effect of potassium was explained by the extended electron donation to adsorbed propyl iodide in one hand, and by the direct interaction between potassium and I on the other hand. This was reflected by the shift of the desorption of potassium from the coadsorbed layer at and above 1.0 ML to higher temperature, and by the coincidal T_p values (∼700 K) of potassium and iodine. The formation of KI was also supported by the appearance of a loss feature at 650 cm⁻¹ in the HREEL spectra attributed to a phonon mode of KI.     

The ν1 and ν3 stretching fundamental bands of PD3

The infrared (IR) spectrum of PD₃ has been recorded in the 1580–1800 cm⁻¹ range at a resolution of 0.0027 cm⁻¹. About 2400 rovibrational transitions with J^′=K^′22 have been measured and assigned to the ν₁ (A₁) and ν₃ (E) stretching fundamentals. These include 506 “perturbation-allowed” transitions with selection rules Δ(k−l)=±3. Splittings of the K^′′=3 lines have been observed. Effects of strong perturbations are evident in the spectrum. Therefore the rovibrational Hamiltonian adopted for the analysis explicitly takes into account the Coriolis and k-type interactions between the v₁=1 and v₃=1 states, and includes also several essential resonances within these states. The rotational structure in the v₁=1 and v₃=1 vibrational states up to J^′=K^′=18 was reproduced by fitting simultaneously all experimental data. Thirty-four parameters reproduced 1950 transitions retained in the final cycle with a standard deviation of the fit equal to 4.9 × 10⁻⁴ cm⁻¹ (about the precision of the experimental measurements).     

Intracavity laser absorption spectroscopy of HDO between 12 145 and 13 160 cm

The high resolution absorption spectrum of monodeuterated water, HDO, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12 145-13 160 cm⁻¹ region. The achieved sensitivity (noise equivalent absorption on the order of α_min ∼ 10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths as weak as 10⁻²⁷ cm/molecule which is about 50 times lower than the weakest line intensities previously detected in the considered region.The rovibrational assignment of the 1179 lines attributed to the HDO isotopologue was based on the results of the variational calculations of Schwenke and Partridge as well as the recent calculations based on a new HDO potential energy surface refined from the fitting to the available experimental data. The overall agreement between these new calculations and the observed spectrum is very good, the rms deviation of the differences between the calculated and observed energy values being 0.05 cm⁻¹. A set of 304 new experimental HDO energy levels was obtained. In particular, band origins for the (1 2 2), (2 0 2), and (3 1 1) vibrational states, at 12 568.190, 12 644.652, and 12 919.938 cm⁻¹, respectively, and their rotational sublevels are derived for the first time. A detailed HDO database of 1337 transitions was constructed and is provided as Supplementary Material.