The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

Time-resolved infrared diode laser spectroscopy of the ν1 (C-O stretch) band of the CoCO radical

Infrared spectrum of the cobalt carbonyl radical CoCO produced by the 193 nm excimer laser photolysis of cobalt tricarbonyl nitrosyl Co(CO)₃NO was observed by time-resolved diode laser spectroscopy. More than 600 lines were identified as belonging to the ν₁ (C-O stretch) fundamental band, consisting of the Ω=5/2 and 3/2 subbands, and the associated hot bands , , , and . The ²Δ_i electronic ground state of CoCO was experimentally confirmed. The ν₁ band origins are 1974.172582(93) cm⁻¹ and 1973.53178(14) cm⁻¹ for the Ω=5/2 and 3/2 subbands, respectively. The rotational constant in the ground state was determined as B₀=4427.146(50) MHz. The centrifugal distortion constant D₀=1.1243(68) kHz was obtained for the Ω=5/2 substate of the ground state. The equilibrium rotational constant B_e=4435.44(14) MHz was derived, together with the vibration-rotation interaction constants.     

High resolution rovibrational spectroscopy of pyrimidine: Analysis of the B1 modes ν10b and ν4 and B2 mode ν6b

We report the first high resolution rovibrational analysis of the infrared spectrum of pyrimidine (C₄H₄N₂) based on measurements using our Fourier transform spectrometer, the Bruker IFS 125 HR Zürich Prototype (ZP) 2001. Measurements were conducted at room temperature in a White-type cell with effective optical path lengths between 3.2 and 9.6 m and with resolutions ranging from 0.0008 to 0.0018 cm⁻¹ in the region between 600 and 1000 cm⁻¹. The spectrum was analyzed in the ν₄ (), ν_10b () and ν_6b regions of pyrimidine () using an effective Hamiltonian. A total of about 15 000 rovibrational transitions were assigned. The root mean square deviations of the fitted data are in the ranges d_rms = 0.00018-0.00024 cm⁻¹, indicating an excellent agreement of experimental line data with the calculations. The results are discussed briefly in relation to possible extensions to spectra of DNA bases and to intramolecular vibrational redistribution at higher energy. The analysis of the ν_10b and ν₄ bands will also be useful in the interstellar search for pyrimidine in the infrared region.     

Reassignment of the C-Cl stretching frequency in the Ã(A″) state of CBrCl

In a recent theoretical paper, vibrational frequencies were reported for CBrCl in the and à states. These values agree well with our previously published experimental frequencies, except in the case of , where the theoretical value of 968.3 cm⁻¹ is significantly higher than the experimental value of 712.6 cm⁻¹. This discrepancy prompted a re-examination of the assignments for transitions involving ν₁ in the laser induced fluorescence spectrum. If the progression ν₁ + nν₂ has the assignment of n reduced by 1 (the origin is not observed), a new experimental value of results. The close accord with the theoretical value lends weight to this re-assignment. Consequently we propose that the experimental frequency be changed to .     

The overtone spectrum of carbonyl sulfide in the region of the ν1+4ν3 and 5ν3 bands by ICLAS-VECSEL

The high-resolution overtone spectrum of OCS has been recorded in the region of the ν₁+4ν₃ and 5ν₃ bands by intracavity laser absorption spectroscopy based on an optically pumped vertical external cavity surface emitting laser (VECSEL). The extremely weak ν₁+4ν₃ band at was found to be isolated. The 5ν₃ band at is accompanied by two weaker bands at 9933.53 and assigned to the 12⁰4-00⁰0 and 04⁰4-00⁰0 bands, respectively. In addition, the 01¹5-01¹0 hot band was detected together with the extremely weak band heads of the R branch of the 02^0,25-02^0,20 hot bands. Finally, the 5ν₃ band of the ¹⁶O¹²C³⁴S minor isotopomer, present in natural abundance in the sample, was also observed and rotationally analyzed. Effective state parameters could be retrieved by standard band-by-band rotational fitting of the line positions, leading to a typical rms of . The observed line positions were compared to the predictions of the global model described by Rhaibi et al. [J. Mol. Spectrosc. 191 (1998) 32-44]. In general, the agreement is excellent, close to the experimental uncertainty () thus confirming the high predictive ability of this effective Hamiltonian model. Weak but significant deviations up to were, however, identified for two rotational levels of the highly excited 2,16⁰,0 dark state, observed through a local interaction with the 00⁰5 state. In the case of the ¹⁶O¹²C³⁴S isotopomer, the predicted line wavenumbers of the 5ν₃ band were globally overestimated by about . The new data have been included in the corresponding global model, leading to almost unchanged values of the molecular parameters and a statistical agreement with the experiment.     

The 2ν5 overtone bands of perchloryl fluoride

The high-resolution infrared spectra of the monoisotopic species F³⁵Cl¹⁶O₃, F³⁷Cl¹⁶O₃, F³⁵Cl¹⁸O₃ and F³⁷Cl¹⁸O₃ have been studied in the region of the 2ν₅ overtones, from 1100 to 1200 cm⁻¹. Both the parallel and the perpendicular components are clearly observed in the spectra, their origins differing by about 0.4 cm⁻¹. In each spectrum about 2000 transitions have been assigned, 35% of them belonging to . The parallel and perpendicular bands in each manifold have been analyzed separately since no evidence of perturbations has been observed. The rovibration parameters of the v₅ = 2, l₅ = 0 and v₅ = 2, l₅=?2 excited states have been obtained. For the four species combining the and band origins with those of the ν₅ fundamentals the harmonic wavenumbers, , and the x₅₅ and g₅₅ anharmonicity constants have also been derived.     

Rovibrational spectroscopy of the Fermi-interacting ν4 = 1 and ν3 = ν6 = 1 levels of DCF3

The high-resolution infrared spectrum of deuterated fluoroform (DCF₃) was studied in the 700 and 1200 cm⁻¹ regions, with the aim of assigning and analyzing the ν₄ CF₃ asymmetric stretching vibration. The Fermi-type anharmonic coupling between the ν₄ = 1 and ν₃ = ν₆ = 1 rovibrational levels, already mentioned in an early work of Ruoff et al. [Spectrochimica Acta Part A 31A (1975) 1099-1100], was studied here for the first time under high resolution. Assignments in the ν₃ + ν₆/ν₄ band system were confirmed and extended by the identification of the ν₃ + ν₆ − ν₆ and ν₄-ν₆ bands in the 700 cm⁻¹ region, the latter being enhanced near the Fermi crossings of the studied levels. Data from both the hot and difference bands were included in the analysis. The close separation of the studied vibrational levels of about 14.8 cm⁻¹ produces a large variety of resonance crossings which involve levels with . Besides the Fermi () and Coriolis () resonances, they were accounted for by inclusion of additional higher-order ( and ) interaction terms between the vibrational states. The least-squares fit of more that 16,000 vibration-rotation transitions provides a quantitative reproduction of data in all bands.     

High-resolution Fourier transform spectrum of H2S in the region of the second hexade

High-resolution Fourier transform infrared spectrum of was recorded and analyzed in the region of the second hexade . More than 1700 transitions were assigned to the 2ν₁ + ν₂, ν₁ + ν₂ + ν₃, ν₁ + 3ν₂, 3ν₂ + ν₃, 5ν₂, and ν₂ + 2ν₃ bands with the maximum value of quantum number J equal to 18, 18, 13, 11, 13, and 9, respectively. The theoretical analysis was fulfilled with a Hamiltonian model which takes into account numerous resonance interactions between all the vibrational states in this polyad. By a least-square fitting, finally 505 upper energy levels were reproduced by 80 parameters with an rms deviation of 0.0019 cm⁻¹.     

Multispectrum fitting of line parameters for C2H2 in the 3.8-μm spectral region

Using FT spectra (Bruker IFS 120, unapodized FWHM resolution ≈ 0.001 cm⁻¹) of acetylene ¹²C₂H₂, absolute positions and intensities have been measured for about 250 lines between 2600 and 2800 cm⁻¹ in the and cold bands, and in the and hot bands. These measurements improve the accuracy of wavenumbers previously available and lead to individual line intensities for the first time in this spectral region. A multispectrum fitting procedure has been used to retrieve line parameters from five experimental spectra recorded at different pressures. The frequencies of the ν₃ band of ¹²C¹⁶O₂ allowed to perform an absolute wavenumber calibration. The accuracy of the amount of ¹²C₂H₂ in the sample has been checked using the cold band around 2100 cm⁻¹, and has been estimated to be around ±2%. The average absolute accuracy of the line parameters obtained in this work has then been estimated to be ±0.0002 cm⁻¹ for line positions, and ±5% for line intensities. For each studied band, the vibrational transition dipole moment squared value has been determined, as also empirical Herman-Wallis coefficients. A complete line list containing positions and intensities for the five strongest bands around 3.8 μm has been set up for atmospheric applications.     

Diode laser spectroscopy of CO2 at 790 nm

Absorption lines of ¹²C¹⁶O₂ have been examined by using a tunable diode laser spectrometer in the region around (790 nm). The spectrometer sources are commercially available double heterostructure InGaAlAs tunable diode lasers (TDLs) operating in the “free-running” mode, which allowed the detection of the line positions within . The observed carbon dioxide absorption lines belong to the combination overtone 2ν₂+5ν₃ ro-vibrational band with intensities ranging around .     

Fourier transform emission spectroscopy of CoCl in the 500 nm region

Fourier transform emission spectra of CoCl were obtained with a tube furnace-DC discharge source in the 450-500 nm spectral region. In addition to observing two different band systems, which were assigned as [20.7] and [21.3] by Adam et al. [J. Mol. Spectrosc. 212 (2002) 111], we obtained data for additional sub-bands and vibrational levels. In contrast to the previous work, intense Q branches are seen in the [21.3]- bands which indicate that these bands are likely ΔΛ≠0 transitions. The equilibrium rotational constant B_e for the ground state is 0.1801104(28) cm⁻¹, the equilibrium bond length is 2.065122(16) Å and ΔG_1/2 is 430.418(5) cm⁻¹.     

The ν1 and ν4 fundamental bands of H3SiCl investigated with high resolution

The gas phase infrared spectrum of monoisotopic H₃Si³⁷Cl has been reinvestigated in the ν₁/ν₄ region near 2200 cm⁻¹, using a Fourier transform spectrometer, with a nominal resolution of 0.0027 cm⁻¹. The rovibrational analysis confirms, besides the weak Coriolis x, y resonance between the (v₁ = 1) and (v₄ = 1) levels, the existence of two strong local perturbations in the ν₄ band. These are caused by rotational (Δk = Δl = ±1) type resonances with and , respectively. Another local perturbation of the 12 ? KΔK ? 14 subbands of the ν₄ band, probably due to a (Δk = Δl = ±1) interaction with , was detected and analyzed. All these local perturbations have been studied individually using a simple model of two interacting sublevels. Without the transitions involved in the local perturbations, more than 2000 lines of the ν₁/ν₄ band system were used to obtain a complete set of vibration-rotation parameters set for the v₁ = 1 and v₄ = 1 states. By means of a band contour simulation, both the transition moment ratio ∣M₄:M₁∣ = 1.25 and a positive sign of the Coriolis intensity perturbation were determined.The present results, together with the accurate existing data for ν₂, ν₃, ν₅, and ν₆ bands, allowed us to derive the experimental values, A_e = 2.8722945(37) cm⁻¹ and B_e = 0.2182248(22) cm⁻¹, which are compared with those of ab initio calculations.     

The 313 nm band system of SeO2. Part 2: rotational structure

Rotational analyses of the first four bands in the ν₁ progression of the transition of SeO₂ at 312.7 nm are reported. The gas phase sample, which contained selenium isotopes in natural abundance, was formed in a free jet expansion from a heated nozzle source. The rotational constants for the state show a rather erratic dependence on v₁^′, consistent with the identification of small, local perturbations in the rotational structure. The A-rotational constant shows a particularly irregular dependence on v₁^′. The r₀ structure was therefore determined from the B and C values of ⁸⁰SeO₂ to be:

     

Rotational and vibrational structure in the 288 nm band system of the FeCl2 radical

The laser excitation spectrum of the 288 nm band system of FeCl₂, formed in a free-jet expansion, has been recorded at a rotational temperature of approximately 10 K. Vibronic transitions are observed from the ground state to two close-lying excited electronic states that differ in inversion (g, u) parity. Two extensive progressions in the symmetric stretching vibration have been identified, referred to as Progressions A and B. The main features of Progression A, which is based on the band, are allowed transitions to the excited electronic state of ungerade symmetry. Progression B is built on the band and consists of vibronically induced transitions to the gerade excited state. A substantial decrease in the symmetric stretching vibrational wavenumber is observed on excitation . Local perturbations are found to cause relative shifts between the different isotopomers. Several vibronic bands have been recorded and analysed at rotational resolution for the three isotopomers Fe³⁵Cl₂, Fe³⁵Cl³⁷Cl, and Fe³⁷Cl₂ in natural abundance. All bands show perpendicular rotational structure of a linear molecule, and have been unambiguously assigned to a Ω = 5-4 transition, consistent with the inverted ⁵Δ_g ground state predicted by ab initio and DFT calculations. The zero-point averaged FeCl bond length is determined to be in the upper and lower electronic states. The results show that the molecule is linear in both states.     

High-resolution diode laser absorption spectroscopy of the O-H stretch overtone band (2,0,0)←(0,0,0) of the HO2 radical

The O-H stretching overtone (2ν₁) of the HO₂ radical was observed between 6603.2 to by using tunable diode laser absorption spectroscopy (TDLAS). About 1000 lines were observed in this region of which 491 transitions could be definitively assigned to the 2ν₁. The spectrum is observed to be an A/B hybrid band with band features of both a perpendicular and parallel nature. Transitions of the A-type bands with K_a^′=0-3, N^′?16 and transitions of the B-type bands with K_a^′=0,1, N^′?15 were assigned. The origin calculated from the best fit to the present spectrum is at which is ∼ higher than previously reported. The overtone spectrum is observed to be heavily perturbed, possibly by Fermi resonance with energy levels of the nearby (ν₂+5ν₃) state.     

Thermal infrared cross-sections of C2F6 at atmospheric temperatures

Spectral absorption cross-sections, k_ν (), have been measured in the 8.0 and bands of C₂F₆. Temperature and total (N₂-broadening) pressure have been varied to represent the conditions specified in various models of the terrestrial atmosphere so that the absorption cross-sections can be applied directly to the optical remote-sensing of C₂F₆ in the atmosphere. The measured absolute intensities of the 8.0 and the bands are (1.636±0.003)×10⁻¹⁶ and (, respectively.     

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the band and 3047 rotational lines of the band have been assigned. The band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔK_a = ± 1 transitions were observed. The excited state is identified to be the S₁¹A₁(ππ^∗) state. In contrast with this, the band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S₂¹B₂(ππ^∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.     

MRCI studies on the electronic structure of AlN and AlN, and the electron affinity of AlN

Using multireference configuration-interaction methods and double to triple-zeta basis sets with semidiffuse and polarization functions, potential energies and spectroscopic constants for low-lying doublet, and quartet states of AlN⁻ were calculated. has R_e=3.280 bohr and . lies 0.17 eV above the ground state. Using an estimated electron affinity of 2.1 eV for AlN, four states of AlN⁻ are found to be stable, namely , , , and . Comparisons with the isovalent anions BN⁻ (three stable states) and AlP⁻ (seven stable states) are made. Photo-detachment of an electron from the state of AlN⁻ can lead to an accurate determination of the energy difference between the two close-lying lowest states of AlN, and , predicted here to be 0.09 eV apart.     

Diode laser slit-jet spectra and analysis of the ν14 fundamental of 1-chloro-1,1-difluoroethane (HCFC-142b)

High resolution infrared spectra (0.001 cm⁻¹ FWHM) have been measured for mixtures of 1-chloro-1,1-difluoroethane in Ne, expanded in a supersonic planar jet. The ν₁₄ fundamental has been analyzed for both isotopic species, and .