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 .     

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.     

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 .     

The overtone spectrum of C2D2 and C2H2 by ICLAS-VECSEL near 1 μm

The intracavity laser absorption spectra (ICLAS) of dideuteroacetylene, C₂D₂, and acetylene, C₂H₂, have been recorded between 1.03 and 0.99 μm with a vertical external cavity surface emitting laser (VECSEL) leading to the observation of seven and six bands, for C₂D₂ and C₂H₂ respectively, most of them newly reported. The strong ν₁+3ν₃ band of C₂D₂ at is found accompanied by the two Π-Π hot bands with v₄=1 and v₅=1 lower state and by the ν₂+3ν₃+2ν₄ band near . This last band results from an intensity transfer from the ν₁+3ν₃ band induced by the 1/244 anharmonic interaction. The ν₁+3ν₃ band of , present in natural abundance in the sample, could also be detected at in full agreement with local mode model predictions. The different bands of both C₂H₂ and C₂D₂ were found mostly unperturbed and the spectroscopic parameters retrieved from the rovibrational analyses agree satisfactorily with the predictions of the respective effective Hamiltonian models.     

The high resolution infrared bands ν1, ν2, ν4 and ν2 + ν5 of FClO3

The high resolution infrared spectrum of mono-isotopic F³⁷Cl¹⁶O₃ has been studied in the regions of ν₁, ν₂, ν₄ and ν₂ + ν₅ bands, centered at 1060.20, 707.16, 1301.71 and 1292.15 cm⁻¹, respectively. The ν₁ and ν₂ parallel bands are unperturbed so their analysis was straightforward and 3355 and 2433 transitions were assigned, respectively. The band origins, the rotational and centrifugal molecular constants in the v₁ = 1 and v₂ = 1 states have been determined, with standard deviation of the fits σ = 0.00019 and 0.00018 cm⁻¹. The ν₄ fundamental is affected by an anharmonic resonance with the ν₂ + ν₅ combination band. The kl > 0 sublevels cross at kl ? 27 because of the opposite values of and . The anharmonic resonance constant  cm⁻¹ has been derived. The Δl = Δk = ±2 and Δl = 0, Δk = ±3 essential resonances have been found to be effective in ν₄, while in ν₂ + ν₅ only the Δl = Δk = ±2 one was active. A total of 5721 transitions have been assigned, 25% of them belonging to ν₂ + ν₅. The rovibrational parameters and the interaction constants of F³⁷Cl¹⁶O₃ have been obtained. The standard deviation of the fit is 0.0006 cm⁻¹, six times the estimated data precision. The equilibrium geometry of perchloryl fluoride has been determined from the A_e and B_e constants of F³⁵Cl¹⁶O₃ and F³⁷Cl¹⁶O₃. Using the A₀ and B₀ constants of all the symmetric species the r₀ geometry has also been derived.     

The high-resolution infrared spectrum of BF3 from 400 to 1650 cm

High-resolution infrared spectra of boron trifluoride, enriched to 99.5 at. % ¹¹B, have been measured from 400 to 1650 cm⁻¹. In that region we have identified and analyzed 16 absorption bands attributed to the three fundamental bands, two combination bands, 10 hot bands, and one difference band. All possible states were accessed in this region through direct transitions either from the ground state or as hot bands from thermally populated levels. The spectral resolution of the measurements varied from 0.0015 to 0.0020 cm⁻¹. An improved set of ground state rotational constants and rovibrational constants for the infrared-active fundamental vibrations have been determined from over 32 000 assigned transitions. This study resulted in the first direct characterization of the infrared-inactive ν₁ state of ¹¹BF₃ leading to values for ν₁, , and of 885.843205(24), 0.000678548(53), and 0.000337564(66) cm⁻¹, respectively. The Fermi resonance perturbation between the E′ states ν₃ and 3ν₄ (l = ±1) was further elucidated by observation of hot band transitions to both the 3ν₄ (l = ±1) and 3ν₄ (l = ±3) states. Several other resonances were also found including the weak rotational interaction, between the state 2ν₂ and the E′ state of ν₁ + ν₄.     

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.     

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.     

High-resolution Fourier transform spectrum of H2S in the region of 8500-8900 cm

High-resolution Fourier transform infrared spectrum of H₂S was recorded and analyzed in the region of the polyad. More than 450 transitions were assigned to the 3ν₁ + ν₂ and 2ν₁ + ν₂ + ν₃ bands with the maximum values of quantum numbers J and K_a equal to 14, 7, and 14, 9 for these two bands, respectively. The theoretical analysis was fulfilled with the Hamiltonian which takes into account strong resonance interactions among the studied vibrational states (3 1 0), (2 1 1), and also “dark” states (0 3 2) and (2 3 0). The rms deviation is 0.0019 cm⁻¹. The intensity borrowing effect in the doublets in the P-branch transitions of the 3ν₁ + ν₂ and 2ν₁ + ν₂ + ν₃ bands is observed and discussed.     

Gas-phase detection of discharge-generated DSOD

We report the first spectroscopic detection of perdeuterated 1-oxadisulfane, DSOD, generated in a radio-frequency plasma of D₂S and D₂O. The chain molecule DSOD produces a perpendicular-type spectrum, with well-known spectral features encountered in previous studies of geometrically related molecules, such as compact Q-branches, which are clearly recognizable. The arrangement of the transitions shaping the Q-branches usually provides sufficient proof for a clear-cut detection of a chain molecule such as DSOD. Guided by quantum chemical calculations, we have located the band center of the -branch of DSOD in the frequency region near 466.5 GHz using the Cologne terahertz spectrometer. This -branch displays both b- and c-type spectra, quite analogous to the behavior of the corresponding -branch of HSOH. In addition, we have been able to detect an internal rotational splitting of ∼0.5 MHz for c-type transitions of the -branch, which lends independent support to our present assignment. From the measurements performed on Q-branches, we derive the following differences in rotational constants: A−(B+C)/2=93331.001(15) MHz, and (B−C)/4=172.95923(29) MHz, in excellent agreement with theoretical predictions.     

Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden electronic transition of the cumulene carbene C₃H₂ have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm⁻¹ region is a result of vibronic interaction and is assigned to a combination of a₁ and b₂ vibrations with a frequency around 2250 cm⁻¹. Another vibronic band near 15 810 cm⁻¹ has an unusual rotational structure because the K_a = 0-1 subband is absent. It is assigned to a combination of a₁ and b₁ vibrations, ∼1850 cm⁻¹, which borrow intensity from the near lying state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the ùA₂ state, which were used for the determination of the geometry. The stronger transition of C₃H₂, measured in a neon matrix in the 16 161-24 802 cm⁻¹ range, was not detected. The reason for this is a short lifetime of the state, leading to line broadening.     

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.     

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.     

Substitution structure of cyanogen, NCCN, from high-resolution far infrared spectra

The lowest lying vibrational bands of the gas-phase spectra of cyanogen, NCCN, and four of its isotopomers, , , , and , were recorded with a Fourier transform interferometer. The resolution was limited by the maximum optical path difference (MOPD) attainable with the interferometer to . Rovibrational transitions of the ν₅ () and also the ν₂-ν₅ () band systems were assigned for all five isotopomers. The use of an effective Hamiltonian for linear molecules to fit the data yielded precise spectroscopic vibrational and rotational constants for the vibrational states (v₁v₂v₃v₄v₅) or (v₄v₅)=(00), (01), (02), (03), and (01000). These data include the first rotationally resolved transitions involving (01000). Complete substitution (r_s) structures of cyanogen, based on both single and double isotopic substitution of the parent species, were calculated. The derived structure is r_CC=138.48(17) pm and r_CN=115.66(13) pm. The two r_s structures coincide within the errors due to remaining contributions of zero-point vibrations.     

Methane diode laser overtone spectroscopy at 840 nm

Overtone absorption lines of ¹²CH₄ have been examined by using a tunable diode laser (TDL) spectrometer in the region around (840 nm) where the combination overtone band ν₁+3ν₃ lies. The spectrometer sources are commercially available heterostructure GaAlAs TDLs operating in the “free-running” mode, which allowed the detection of the line positions within . The wavelength modulation spectroscopy (WMS) and the second harmonic detection technique permitted the measurements of minimum absorbances of the order of ?5×10^-6. This allowed to observe the weakest lines of the band with absorption cross-sections of the order of ?2×10^-25 cm²/molecule or /amagat. For some of them self-, air-, He- and H₂-broadening coefficients have been obtained at room temperature.     

Laser excitation spectrum of C3 in the region 26 000-30 700 cm

The vibrational structure of the electronic state of C₃ in the region 26 000-30 775 cm⁻¹ has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born-Oppenheimer components of the state, and by the effects of the double minimum potential in the Q₃ coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν₁ and ν₃ vibrations. For instance, the levels 2 1⁺ 1 and 0 1 ⁺ 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm⁻¹ apart, centered about the expected position of the 2 1⁺ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm⁻¹ (a vibrational energy of over 5000 cm⁻¹) can now be assigned. A vibronic level at 30181.4 cm⁻¹, which has a much lower B′ rotational constant than nearby levels of the state, possibly represents the onset of vibronic perturbations by the electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].