Study of the ν1, ν1 + ν3 and ν2 + ν3 infrared bands of ONCl

A type (ΔK_a = 0) rovibrational lines of the near-prolate asymmetric top ¹⁶O¹⁴N³⁵Cl have been assigned on high resolution Fourier transform spectra: 820 lines of the ν₁ band, centered around 1800 cm⁻¹, 435 lines of the ν₁ + ν₃ band, centered around 2131 cm⁻¹, and 257 lines of the ν₂ + ν₃ band, centered around 925 cm⁻¹. Least-squares calculations have been carried out over these lines, using the A reduced Watson's hamiltonian in I^r representation; r.m.s. standard deviations of 0.0016 cm⁻¹, 0.0016 cm⁻¹ and 0.006 cm⁻¹ have been respectively obtained, making it possible to measure molecular constants of the (001), (101) and (011) vibrational levels of ¹⁶O¹⁴N³⁵Cl.     

Study of the ν2 + ν3 infrared band of ONCl

Two hundred and sixty A type rovibrational lines of the ν₂ + ν₃ vibrational band of ¹⁶O¹⁴N³⁵Cl, around 925 cm⁻¹, have been assigned; a least-squares calculation with a r.m.s. deviation of 0.0006 cm⁻¹ has made it possible to measure several constants of the (011) vibrational level.     

High-resolution FTIR spectroscopy of the ν8 and Coriolis perturbation allowed ν12 bands of ketenimine

High resolution FTIR spectra have been recorded in the region 250-770 cm(-1) using synchrotron radiation and over 2000 transitions to the ν(8) and ν(12) states of the short lived species ketenimine have been assigned. Ground state combination differences combined with published microwave transitions were used to refine the constants for the ground vibrational state. Rotational and centrifugal distortion parameters for the v(8) = 1 and v(12) = 1 levels were determined by co-fitting transitions, and treating a strong a-axis Coriolis interaction. Selection rules for the observed ν(12) transitions indicate that they arise solely from "perturbation allowed" intensity resulting from this Coriolis interaction.     

Rovibrational study of the ν2 + ν±14, ν2 + 3ν±16, ν1 + ν±15, ν±14 + ν±15, ν±14 + ν±5, ν±5 + 3ν±16, ν±5 + 3ν±36 and ν1 + ν2 infrared bands of CH335Cl studied in a high-resolution FTIR spectrum from 4250 to 4600 cm−1

About 6400 lines belonging to the ν₂ + ν^±1₄, ν₂ + 3ν^±1₆, ν₁ + ν^±1₅, ν^±1₄ + ν^±1₅, ν^±1₄ + ν^±₅, ν^±₅ + 3ν^±1₆ and ν^±₅ + 3ν^±3₆ bands have been assigned. An r.m.s. deviation of 0.047 cm⁻¹ has been achieved by a least-squares fit of 1427 lines. For this purpose, a simplified model, taking into account five anharmonic resonances already found in a previous work [Molec. Phys. 70, 849 (1990)] and the well known x-y Coriolis resonance between the ν₂ and ν₅ modes of methyl halides, was used. Although not observed, the ν^±1₄ + ν^±1₅ and ν₁+ ν₂ parallel bands are strongly coupled by Coriolis resonance to ν₂ + ν^±1₄ and ν^±1₅ respectively. A few secondary resonances remain unexplained in several parts of the spectrum.     

Comparison and assessment of procedures for calculating the R(12) line strength of the ν1 + 2 ν2 + ν3 band of CO2

Recently, results for the CO(2) R(12) line strength parameter have been reported, which differ significantly and are inconsistent with respect to quoted uncertainties. We investigate to what extent this inconsistency might be caused by the chosen data analysis methods. To this end, we assess and compare a parametric fitting procedure and a non-parametric approach. We apply the methods to simulated and measured line spectra, and we specify the conditions required for the safe application of the two procedures. For our present data, the corresponding conditions are satisfied for both methods, and consistent results are obtained. However, the simulations reveal that the fitting procedure can show shortcomings when the uncertainty in the wavenumber is large.     

The estimation of ν6 of octahedral XY6 species

The inactive F_2u vibration frequency of octahedral XY₆ type species for which this frequency is known, was estimated from the remaining frequencies using the Urey—Bradley (UBFF) and orbital valency (OVFF) force fields and their modifications. Various modifications of the generalized valency force field (GVFF) were also considered. Estimated values were scattered within a wide range and the average discrepancy varied from 20 (UBFF) to 68% (modified OVFF). The condition ν₆ = ν₅/√2 (Wilson's rule), with an average discrepancy of 15% gave the closest estimate of the assigned values.     

Temperature dependence of nitrogen broadening of ozone ν3 rovibrational transitions

We report N₂-broadening coefficients for rovibrational transitions of the ν₃ band of ozone in the temperature range 180–296 K. We assume a power law dependence of the temperature for these coefficients, with a temperature exponent n. For the 21 transitions we measured, the average value of n is 0.74 and is well within experimental error of the theoretically predicted value of 0.77. The significance of this parameter in modeling atmospheric IR absorbance due to ozone is discussed.     

Synchrotron far infrared spectroscopy of the ground, ν(5), and ν(15) states of thiirane

The high-resolution (0.001 cm(-1)) spectrum of thiirane has been recorded at the far-infrared beamline at the Australian synchrotron between 760-400 cm(-1) and 170-10 cm(-1). Ro-vibrational transitions of the highly Coriolis coupled ν(5) (628.1 cm(-1)) and ν(15) (669.7 cm(-1)) fundamentals, as well as pure rotational far-IR transitions have been assigned, and rotational, centrifugal distortion, and Coriolis interaction parameters determined. ν(15) gains the vast majority of its intensity from an interesting Coriolis intensity stealing mechanism, which is also outlined.     

FTIR spectra of the 2ν4, ν4 + ν6 and 2ν6 bands of formaldehyde

High resolution IR spectra of the overtones and the combination band of the ν₄ and ν₆ modes of formaldehyde (2ν₄, ν₄ + ν₆ and 2ν₆) were measured in the region of 2200–2650 cm⁻¹ using FTIR. The combination band ν₄ + ν₆, whose dipole transition is forbidden from molecular symmetry, was observed due to the intensity borrowed from the other bands. The observed frequencies were analysed by a Hamiltonian in which A-type Coriolis interactions and Darling—Dennison interaction were taken into account. The ratio and the relative signs of the transition dipole moments of the overtone bands, μ_2ν4 and μ_2ν6, have been determined by analysing the intensity distribution of the vibration—rotation lines.     

Study of the spectrum of CH379Br between 1520 and 1700 cm−1: The ν3 + ν6 and the ν5 rovibrational bands

The ν₃ + ν₆ band of CH₃ ⁷⁹Br has been directly analyzed for the first time, and an r.m.s. standard deviation of 0.0035 cm⁻¹ was obtained over 394 lines of K″ΔK = 2 up to 12, through a least-squares calculation using an unperturbed model. Nevertheless discrepancies occur on sub-bands with K″ ⩾ 9, which remain not yet understood. In particular it seems difficult to explain them by a Fermi resonance with the ν₅ band, since it has been possible to fit properly around 580 lines of this band, belonging to sub-bands K″ΔK = 7 up to 16, taking only into account the Coriolis resonance with the ν₂ band and the l(2,2) resonance of ν₅.     

Note: Deperturbation of the ν3 band of BeD2

High rotational levels of the 001 (Σ(u)) state of BeD(2) are perturbed by the nearby 03(3)0 (Φ(u)) state. Deperturbation analysis results in an experimental value for the vibrational energy of the 030 level.     

Infrared emission spectrum of HNO: The ν1 band

HNO has been observed in emission from a radiofrequency discharge through a mixture of ammonia and oxygen. The ν₁ (NH stretching) band in the region 2400–3200 cm⁻¹ has been recorded with a Fourier transform spectrometer using an apodized resolution of 0.04 cm⁻¹. The analysis of the band has been extended to higher K values than in earlier work.     

Rotational spectrum of SiC2 in the ν3 excited state

The rotational spectra of SiC₂ in the vibrationally excited states of the ring deformation mode (υ₃ = 1, 2) were observed in the frequency region of 140–400 GHz by using a source-modulated microwave spectrometer combined with a free space absorption cell. SiC₂ was produced in the cell by discharging a mixture of SiH₄, C₂H₂ and CO. Least-squares analysis of the observed spectral lines yielded the rotational constants and the centrifugal distortion constants precisely. Sextic, octic and decatic centrifugal distortion constants were required in the least-squares fit in order to get a good fit of the observed frequencies to those calculated within experimental errors. The inertial defects for the υ₃ = 1 state and the υ₃ = 2 state do not show a linear dependence on the vibrational quantum number. The quartic centrifugal distortion constants, Δ_JK, Δ_K and δ_K, are abnormally large, and show a large change on the vibrational states. These abnormal behaviours are interpreted in terms of a large amplitude motion of the ν₃ mode.     

Temperature dependence of broadening and shifts of methane lines in the ν4 band

We have recorded high-resolution absorption spectra of methane broadened by dry air and by N₂ at temperatures from −63 to 41°C using a Fourier transform spectrometer. These spectra have been analyzed to determine pressure broadening and line-shift coefficients, along with their temperature dependences, for 148 lines in the ν₄ fundamental band of ¹²CH₄. The experimental uncertainties for lines with J″≤10 are generally <2% for the broadening coefficient b⁰_L, 6–12% for its temperature dependence exponent n, 6–20% for the line-shift coefficient δ⁰, and 20–40% for its temperature dependence coefficient δ′; for J″> 10 the experimental uncertainties are somewhat larger. These results, especially for N₂-broadening, are in excellent agreement with other recent measurements. Since the present results cover a wide range of rotational quantum numbers (J″ up to 14), the variation of the temperature dependence of the half-widths and shifts from line to line within the ν₄ band is also examined.     

Absolute line intensities of the ν9 band of propyne at 15.5 μm

A tunable diode laser spectrometer was used to perform measurements of absolute line intensities for 20 transitions in the ν₉-fundamental of propyne centered at 15.5 μm. From these data, the vibrational band-strength S^v_o was significantly determined to be S^v_o = 196.8 ± 6.0 cm⁻² atm⁻¹ at 297 K. These results will be used for further applications of astrophysical interest such as a refined contribution for modelling Titan's atmosphere.     

Analysis of the ν2 :ν3 + ν4 Fermi resonance in CH3CN isotopic species in solution

Infrared bands due to ν₃ + ν₄, ν₂, ν₃ and ν₄ were recorded for the species CH₃CN, ¹³CH₃CN, CH₃¹³CN and CH₃C¹⁵N in the solvents carbon tetrachloride, chloroform, benzene, pyridine and dimethyl sulphoxide. Values of W₂₃₄ were extracted by the modified Winther method, which are slightly less, 11.2–11.9 cm⁻¹, than the value in the gas, 12.15 cm⁻¹. Relative intensity measurements of ν₃ + ν₄ and ν₂ in CCl₄ are compatible with an unperturbed intensity ratio, I_{ν3 + ν4}/ I_ν2 of ∼ 0.03. The fallibility of the infrared intensity method for determining W, and the need for precise frequency data in the Winther method, are stressed.     

Radiative lifetime of CO+ (X2∑+, ν) ions

The radiative relaxation of CO⁺(X²∑⁺, ν) ions produced by ionization of CO and of OCS with 70 eV electrons has been investigated using the monitor ion technique in a triple FT ICR spectrometer. Since the monitor reaction is exothermic for ν ≥ 1, the experimental lifetimes, 170 ms for CO⁺ from CO and 210 ms for CO⁺ from OCS, should correspond to the overall decay rate of ν ≥ 2 with a small contribution of ν = 1.These values being much larger than the theoretical lifetime of ν = 1 (128.5 ms), computer-simulated overall lifetimes for ν ≥ 1 and ν ≥ 2 have been determined using theoretical lifetimes calculated by Rosmus and Werner and a vibrational population deduced from photoelectron spectroscopy of CO with 40.8 eV photons (Gardner and Samson). The computer-simulated lifetimes, 250 ms for ν ≥ 1 and 125 ms for ν ≥ 2, are indeed about twice as large as the theoretical lifetimes for ν = 1 and ν = 2, respectively, indicating that the large experimental lifetimes may be accounted for by radiative cascade from the upper vibrational levels.     

Diode laser i.r. spectrum and rovibrational analysis of CF3Br in the ν2 + ν3 region

The gas phase i.r. spectrum of CF₃Br, with natural isotopic abundance, has been investigated in the ν₂+ ν₃ region near 1120 cm⁻¹ using a tunable diode laser spectrometer. The measurements have been carried out at low temperature (⋍ 200 K) to minimize the effects due to the “hot” band absorptions. The K structure of many P(J) and R(J) manifolds has been resolved and analyzed: the maximum J value reached for individual lines was 62 and 70 for CF₃⁷⁹Br and CF₃⁸¹Br, respectively. The identified transitions have been used in a least-squares fit to the energy expression up to the quartic terms and molecular parameters for the ν₂ + ν₃ combination have been obtained. Residual weak features due to “hot” bands of ν₃ and ν₆ have been assigned; the J structure has been analyzed by means of a polynomial procedure and spectroscopic constants for both the isotopomers have been derived.     

The ν2 bending vibrational structure of the X̃2Σ+ state of MgNC

We have generated MgNC in supersonic free jet expansions and observed the laser induced fluorescence (LIF) of the A?(2)Π-X?(2)Σ(+) transition. We measured the LIF dispersed spectra from the single vibronic levels of the A?(2)Π electronic state of MgNC, following excitation of each ν(2) bending vibronic band observed, i.e., the κ series of the (0,v(2)('),0)-(0,0,0), v(2)(') = 0, 1, 2, 4, and 6 vibronic bands. In the vibrational structure in the dispersed fluorescence spectra measured, the long progression of the ν(2) bending mode in the X?(2)Σ(+) state is identified, e.g., up to v(2)(')=14 in the (0,6,0)-(0,v(2)('),0) spectrum. This enables us to derive the potential curve of the ν(2) bending mode in the X?(2)Σ(+) state. We used two kinds of models to obtain the potential curve; (I) the customary formula expressed in the polynomial series of the (v(2)(')+(d(2)/2)) term and (II) the internal rotation model. The potential curve derived from model (I) indicates the convergence of the bending vibrational levels at about 800 cm(-1) from the vibrationless level of MgNC, which may correspond to the barrier height of the isomerization reaction, MgNC ? MgCN, in the X?(2)Σ(+) state. Model (II) gives a simple picture for the isomerization reaction pathway with a barrier height of about 630 cm(-1) from the vibrationless level of the more stable species, MgNC. This shows that the v(2)(')=8 bending vibrational level of MgNC is already contaminated by the v(2)(')=2 bending vibrational level of the isomer, MgCN, and implies that the isomerization reaction begins at the v(2) (')=8 level. The bending potential surface and the isomerization reaction pathway, MgNC ? MgCN, in the X?(2)Σ(+) state are discussed by comparing the potential derived in this study with the surface obtained by quantum chemical calculation.