High-resolution infrared measurements on HSOH: Analysis of the OH fundamental vibrational mode

High-resolution infrared measurements of the OH-stretching mode of oxadisulfane, HSOH, at 3625 cm⁻¹ have been recorded using a Bruker IFS 120 HR Fourier transform spectrometer. More than 1300 lines have been assigned to the ν(OH) fundamental vibration mode, which is a hybrid band showing a c-type perpendicular band and an a-type parallel band spectrum of an asymmetric rotor molecule. The splitting due to the torsional-tunneling has not been observed in this band. The band center position at 3625.59260(20) cm⁻¹ as well as rotational and centrifugal distortion constants for the ν(OH) vibrational excited state have been obtained from a least-squares fit analysis of a semirigid rotor. In addition the α_OH experimental vibration-rotation correction terms of the OH-stretching mode have been derived and compared to values used in an earlier semi-empirical calculation of the HSOH structure. All data are in very good agreement with high level ab initio calculations and confirm the assignment of an earlier matrix isolation spectrum at 3608 cm⁻¹ to the ν(OH) fundamental mode.     

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49-1.30 μm)

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by differential absorption spectroscopy between 6717 and 7351 cm⁻¹ (1.49-1.36 μm) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4% and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the ν₂+4ν₄ band near 6765 cm⁻¹. The reliability of the method based on the association of lines with coinciding centers in the 81 K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the ν₂+2ν₃ band near 7510 cm⁻¹. The line list for the whole icosad (6717-7655 cm⁻¹) consists of 12 865 transitions at 81 K.     

Infrared emission studies of the AΣ-XΠ electronic transition of the SiC radical

The gas phase infrared emission spectrum of the A³Σ⁻-X³Π electronic transition of SiC has been observed using a high resolution Fourier transform spectrometer. Three bands ν′ − ν″ = 0-1, 0-0, and 1-0 have been observed in the 2770, 3723, and 4578 cm⁻¹ regions, where the 0-1 and 0-0 bands were observed for the first time. The SiC radical was generated by a dc discharge in a flowing mixture of hexamethyl disilane [(CH₃)₆Si₂] and He. A total of 1074 rotational transitions assigned to the 0-1, 0-0, and 1-0 bands have been combined in a simultaneous analysis with previously reported pure rotational data to determine the molecular constants for SiC in the two electronic states. The principal equilibrium molecular constants for the A³Σ⁻ state are: B_e = 0.6181195(18) cm⁻¹, α_e = 0.0051921(20) cm⁻¹, r_e = 1.8020884(26) Å, and T_e = 3773.31(17) cm⁻¹, with one standard deviation given in parentheses. The effect of a perturbation was recognized between the ν = 4 level of X³Π and the ν = 0 level of A³Σ, and the analysis was carried out to determine the interaction parameter between the two states.     

CW-cavity ring down spectroscopy of the ozone molecule in the 6625-6830 cm region

The absorption spectrum of ozone, ¹⁶O₃, has been recorded by CW-cavity ring down spectroscopy in the 6625-6830 cm⁻¹ region. The typical sensitivity of these recordings (α_min ∼ 3 × 10⁻¹⁰ cm⁻¹) allows observing very weak transitions with intensity down to 2 × 10⁻²⁸ cm/molecule. 483 and 299 transitions have been assigned to the 2ν₁ + 3ν₂ + 3ν₃A-type band and to the 2ν₁ + 4ν₂ + 2ν₃B-type band, respectively, which are the highest frequency bands of ozone recorded so far under high resolution. Rovibrational transitions with J and K_a values up to 46 and 12, respectively, could be assigned. Despite well-known difficulties to correctly reproduce the energy levels not far from the dissociation limit, it was possible to determine the parameters of an effective Hamiltonian which includes six vibrational states, four of them being dark states. The line positions analysis led to an rms deviation of 8.5 × 10⁻³ cm⁻¹ while the experimental line intensities could be satisfactorily reproduced. Additional experiments in the 5970-6021 cm⁻¹ region allows detecting the (233) ← (010) hot band reaching the same upper state as the preceding cold band. From the effective parameters of the (233) state just determined and those of the (010) level available in the literature, 329 transitions could be assigned and used for a further refinement of the rovibrational parameters of the effective Hamiltonian leading to a value of 7.6 × 10⁻³ cm⁻¹ for the global rms deviation. The complete list of the experimentally determined rovibrational energy levels of the (233), (242), and (520) states is given. The determined effective Hamiltonian and transition moment operators allowed calculating a line list (intensity cut off of 10⁻²⁸ cm/molecule at 296 K), available as Supplementary material for the 6590-6860 and 5916-6021 cm⁻¹ regions. The integrated band strength values are 1.75 × 10⁻²⁴ and 4.78 × 10⁻²⁵ cm/molecule at 296 K for the 2ν₁ + 3ν₂ + 3ν₃A-type band and to the 2ν₁ + 4ν₂ + 2ν₃B-type band, respectively, while the band intensity value of the (233) ← (010) is estimated to be 1.03 × 10⁻²⁴ cm/molecule.     

Observation of hot bands in the infrared spectrum of H2CO

Two hot bands in the infrared spectrum of formaldehyde (H₂CO) have been identified by means of tunable infrared laser spectroscopy using a jet-cooled sample. One band falls in the region 2760-2800 cm⁻¹; it follows a-type selection rules and it has been assigned as the ν₁ + ν₄ − ν₄ hot band. The other band falls in the region 2800-2860 cm⁻¹; it follows b-type selection rules and it has been assigned as the ν₅ + ν₄ − ν₄ hot band. The observations are restricted to low J and K_a levels. It has consequently been possible to ignore the effects of the extensive Coriolis couplings involving these levels in the analysis of the spectra and to model the rotational structure as that of a simple asymmetric top. Least-squares fits of the data have provided values for the band origins: 2774.2706(11) cm⁻¹ for the ν₁ + ν₄ − ν₄ and 2829.2621(8) cm⁻¹ for the ν₅ + ν₄ − ν₄ band. Term values for the upper vibrational levels involved in the transitions have been determined by use of the previously reported term values for the v₄ = 1 level.     

Emission spectroscopy of a new Δ-1Δ system of VO

High-resolution spectra of VO have been reinvestigated in the 12 000-31 000 cm⁻¹ region. VO was produced in a vanadium hollow cathode lamp by discharging 1.5 Torr of Ar and the spectra were recorded using a Fourier transform spectrometer. The oxygen needed to produce VO was present in the system as an impurity. Three new bands observed in the 21 000-22 100 cm⁻¹ region have been attributed to a new ²Δ-1²Δ electronic transition of VO. Two bands, with origins near 21 044 and 22 038 cm⁻¹, have been assigned as the 0-1 and 0-0 bands of the ²Δ_3/2-1²Δ_3/2 sub-band while a weak band with an origin near 21 975 cm⁻¹ has been assigned as the 0-0 band of the corresponding ²Δ_5/2-1²Δ_5/2 sub-band. A rotational analysis of these sub-bands has been obtained and spectroscopic constants have been extracted. The 1²Δ state is known from the previous analyses of the doublet transitions of VO in the near infrared. The present observation has allowed the determination of the vibrational interval ΔG_1/2 and the equilibrium rotational constants for the 1²Δ_3/2 state.     

High-resolution FTIR measurement of the ν4 band of methylene fluoride-d2

A high-resolution (0.002 cm⁻¹) infrared absorption spectrum of methylene fluoride-d₂ (CD₂F₂) of the lowest fundamental mode ν₄ in the region from 460 to 610 cm⁻¹ has been measured on a Bruker IFS 120-HR Fourier transform infrared spectrometer. More than 3500 transitions have been assigned in this B-type band centered at 521.9 cm⁻¹. The data have been combined with upper state pure rotational measurements in a weighted least-squares fit to obtain molecular constants for the upper state resulting in an overall standard deviation of 0.00018 cm⁻¹. Accurate value for the band origin (521.9578036 cm⁻¹) has been obtained and inclusion of transitions with very high J (?60) and K_a (?34) values has resulted in improved precision for sextic centrifugal distortion constants, in particular D_K, H_KJ, and H_K.     

High-resolution infrared analysis of the ν7 band of cis-ethylene-d2 (cis-C2H2D2)

The spectrum of the ν₇ band of cis-ethylene-d₂ (cis-C₂H₂D₂) has been recorded with an unapodized resolution of 0.0063 cm⁻¹ in the 740-950 cm⁻¹ region using a Bruker IFS 125 HR Fourier transform infrared spectrometer. By fitting 2186 infrared transitions of ν₇ with a standard deviation of 0.00060 cm⁻¹ using a Watson’s A-reduced Hamiltonian in the I^r representation, accurate rovibrational constants for ν₇ = 1 state have been derived. The band center of ν₇ has been found to be 842.20957 ± 0.00004 cm⁻¹. In a simultaneous fit of 1331 infrared ground state combination differences from the present ν₇ transitions, together with 22 microwave frequencies, ground state constants have been improved. The rms deviation of the ground state fit was 0.00027 cm⁻¹.     

Fourier transform spectroscopy of new emission systems of NbN in the visible region

The emission spectrum of NbN has been reinvestigated in the 8000-35 000  cm⁻¹ region using a Fourier transform spectrometer and two groups of new bands were observed. The bands observed in the 18 000-20 000 cm⁻¹ region have been assigned to a new ³Π-X³Δ transition. Three bands with R heads near 19 463.8, 19 659.0 and 19 757.0 cm⁻¹ have been assigned as 0-0 bands of the ³Π₂-X³Δ₃, ³Π₁-X³Δ₂ and ³Π_0±-X³Δ₁ subbands, respectively, of this new transition. Three additional ΔΩ = 0 bands have been observed in the 24 000-26 000  cm⁻¹ region. A 0-0 band with an R head near 25 409.9 cm⁻¹ has been assigned as a ΔΩ = 0 transition having X³Δ₂ as its lower state while two additional bands with heads near 25 518.7 and 25 534.8 cm⁻¹ were found to be ΔΩ = 0 bands having X³Δ₁ as the common lower state. Two of these three bands are perhaps subbands of a ³Δ-X³Δ transition. Most of the excited levels are affected by perturbations.     

Fourier transform emission spectroscopy of the FΔ-XΦ system of TiF

The emission spectra of TiF have been reinvestigated in the 4200-15 000 cm⁻¹ region using the Fourier transform spectrometer associated with the National Solar Observatory at Kitt Peak. TiF was formed in a microwave discharge lamp operated with 2.5 Torr of He and a trace of TiF₄ vapor, and the spectra were recorded at a resolution of 0.02 cm⁻¹. The TiF bands observed in the 12 000-14 000 cm⁻¹ region have been assigned to a new transition, F⁴Δ-X⁴Φ. Each band consists of four sub-bands assigned as, ⁴Δ_1/2-⁴Φ_3/2, ⁴Δ_3/2-⁴Φ_5/2, ⁴Δ_5/2-⁴Φ_7/2, and ⁴Δ_7/2-⁴Φ_9/2. A rotational analysis of the 0-1, 0-0, and 1-0 bands has been obtained and spectroscopic constants have been extracted.     

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 FTIR spectroscopy of the 3ν2 band of PH3

High-resolution Fourier transform spectrum of phosphine (PH₃) at room temperature has been recorded in the region of the 3ν₂ band (2730-3100 cm⁻¹) at an apodized resolution of 0.005 cm⁻¹. About 200 vibration-rotation transitions have been least squares fitted with an rms of 0.00039 cm⁻¹ after taking into account the ΔK = ±3 interaction.     

High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: Temperature dependence and importance of the CH3D contribution

The high resolution absorption spectrum of methane in the 1.58 μm transparency window has been recorded at room temperature and at 79 K by CW-Cavity Ring Down Spectroscopy using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers. The achieved sensitivity (α_min ∼ 3 × 10⁻¹⁰ cm⁻¹) has allowed for a detailed characterization of the 6289-6526 cm⁻¹ region which corresponds to the lowest opacity of the transparency window. A list of 6868 and 4555 transitions with intensities as weak as 1 × 10⁻²⁹ cm/molecule was constructed from the recordings at 297 and 79 K, respectively. By comparison with a spectrum of CH₃D recorded separately by Fourier Transform Spectroscopy, 1282 and 640 transitions of monodeuterated methane, CH₃D, in natural abundance in our sample were identified at 297 and 79 K, respectively.The rotational temperature determined from the intensity distribution of the 3ν₂ band of CH₃D (79.3 K) was found in good agreement with the temperature value previously obtained from the Doppler line broadening. The reduction of the rotational congestion by cooling down to 79 K reveals a spectral region near 6300 cm⁻¹ where CH₃D transitions are dominant.The low energy values of the transitions observed both at 79 K and at room temperature were derived from the variation of their line intensities. These transitions with lower energy determination represent 93.9% and 68.4% of the total absorbance in the region, at 79 K and room temperature, respectively. The quality of the obtained empirical low energy values is demonstrated for CH₄ by the marked propensity of the empirical low J values to be close to integers. The line lists at 79 K and room temperature provided as Supplementary Material allow accounting for the temperature dependence of methane absorption between these two temperatures. The investigated region covering the 5ν₄ band of the ¹²CH₄ isotopologue will be valuable for the theoretical treatment of this band which is the lowest energy band of the icosad.     

The high-resolution FTIR spectrum of the ν6 band of C2H3D

The absorption spectrum of the ν₆ band of C₂H₃D centered near 1125.27674 cm⁻¹ in the 1100-1250 cm⁻¹ region was recorded with an unapodized resolution of 0.0063 cm⁻¹ using a Fourier transform infrared (FTIR) spectrometer. A total of 947 infrared transitions of the A-B hybrid-type band were assigned and fitted to upper-state (ν₆ = 1) rovibrational constants using a Watson’s A-reduced Hamiltonian in the I^r representation up to eighth-order centrifugal distortion terms. The b-type infrared transitions of the band were analyzed for the first time. The root-mean-square deviation of the fit was 0.00062 cm⁻¹. The ground-state rovibrational constants up to eighth-order terms were also obtained by a fit of 617 combination differences from the present infrared measurements, simultaneously with 21 microwave frequencies with a root-mean-square deviation of 0.00055 cm⁻¹. From this work, the upper-state (ν₆ = 1) and ground-state constants of C₂H₃D were derived with the highest accuracy, so far. The a- and b-type transitions of the hybrid ν₆ band were found to be relatively free from local frequency perturbations. The ratio of the a- to b-type vibrational dipole transition moments (μ_a/μ_b) was found to be 1.05 ± 0.10. From the ν₆ = 1 rovibrational constants obtained, the inertial defect Δ₆ was calculated to be 0.3570 ± 0.0008 μŲ.     

The absorption spectrum of phosphine (PH3) between 2.8 and 3.7 μm: Line positions, intensities, and assignments

Over 8000 line positions and intensities of phosphine (PH₃) at 3 μm have been measured at 0.0115 cm⁻¹ resolution with the McMath-Pierce Fourier Transform spectrometer at Kitt Peak. The observed line intensities ranged from 4.13 × 10⁻⁶ to 4.69 × 10⁻² cm⁻² atm⁻¹ at 296 K, for line positions between 2724.477 and 3601.652 cm⁻¹. This region spans eight interacting vibrational states: 3ν₂ (2940.8 cm⁻¹), 2ν₂ + ν₄ (3085.6 cm⁻¹), ν₂ + 2ν₄ (3214.9 cm⁻¹), ν₁ + ν₂ (3307.6 cm⁻¹), ν₂ + ν₃ (3310.5 cm⁻¹), 3ν₄ (∼3345 cm⁻¹), ν₁ + ν₄ (3426.9 cm⁻¹), and ν₃ + ν₄ (3432.9 cm⁻¹). Assignments have been determined for all the bands except 3ν₄ (a weak band in a highly congested area) for a total of 4232 transitions. The total integrated intensity for this region is 5.70 cm⁻² atm⁻¹ near 296 K, and assigned lines account for 79% of the observed absorption. The two strongest bands in the region are ν₁ + ν₄ and ν₃ + ν₄ with band strengths at 296 K of 1.61 and 2.01 cm⁻² atm⁻¹, respectively. An empirical database of PH₃ line parameters (positions, intensities, and assignments) is now available. Lower state energies (corresponding to assignments from this study) and line widths from the literature are included; default values are used for unassigned features.     

Infrared spectra of the polar and nonpolar N2O dimers in the 1280 cm region of the ν3 fundamental

The high-resolution infrared spectrum of the polar N₂O dimer has been observed in the region of the N₂O ν₃ fundamental (∼1280 cm⁻¹) using a tunable diode laser to probe a pulsed supersonic slit jet. About 120 rotational transitions were assigned in terms of an a/b hybrid band of a planar asymmetric top molecule with a slipped parallel structure. The vibrational origin was determined to be 1290.21 cm⁻¹, showing a blue shift of 5.31 cm⁻¹ with respect to the monomer band origin. In addition, the spectrum of the nonpolar isomer at 1279.71 cm⁻¹ has been remeasured and analyzed in improved detail. Small but widespread perturbations are noted in this band, which appear somewhat similar to larger effects observed previously in the ν₁ + ν₃ region for nonpolar (N₂O)₂.     

The ν12 band of C2D4

The Fourier transform infrared (FTIR) absorption spectrum of the ν₁₂ fundamental band of ethylene-d₄ (C₂D₄) was recorded in the 1017-1137 cm⁻¹ region with an unapodized resolution of 0.0063 cm⁻¹. Upper state (v₁₂ = 1) rovibrational constants consisting of three rotational and five quartic constants were improved by assigning and fitting 2103 infrared transitions using Watson’s A-reduced Hamiltonian in the I^r representation. The band centre of the A-type ν₁₂ band is found to be 1076.98480 ± 0.00002 cm⁻¹. The present analysis covering a wider wavenumber range and higher J and K_c values yielded upper state constants including the band centre which are more accurate than previously reported. The rms deviation of the upper state fit is 0.00045 cm⁻¹. Improved ground state rovibrational constants were also determined from the fit of 1247 ground state combination differences (GSCD) from the presently-assigned infrared transitions of the ν₁₂ band of C₂D₄. The rms deviation of the GSCD fit is 0.00049 cm⁻¹. In the rovibrational analysis, local frequency perturbations were not detected even at high J and K_a values. The calculated inertial defect Δ₁₂ is 0.32551 ± 0.00001 μŲ. The line intensities of the individual transitions in the ν₁₂ band were measured and the band strength of 39.8 ± 2.0 cm⁻² atm⁻¹ was derived for the ν₁₂ band of C₂D₄.     

Intracavity Laser Absorption Spectroscopy of D2O between 12 850 and 13 380 cm

The high resolution absorption spectrum of dideuterated water, D₂O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12 850-13 380 cm⁻¹ spectral region which is the higher energy region reported so far for this water isotopologue. Very high deuterium enrichment was necessary to minimize the HDO absorption lines overlapping the D₂O spectrum. The achieved sensitivity (noise equivalent absorption α_min ∼ 10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths on the order of 5 × 10⁻²⁸ cm/molecule. The spectrum analysis, based on recent variational calculations has provided a set of 422 new rovibrational energy levels belonging to 11 vibrational states, including rotational sublevels for four new vibrational states and one level of the (0 9 1) highly excited bending state. The very weak (1 0 4)-(0 0 0) band at 13 263.902 cm⁻¹, which is the highest D₂¹⁶O band currently observed, could be assigned despite the fact that the HDO absorption in the region is stronger by three orders of magnitude. The list of 996 D₂¹⁶O transitions is provided as Supplementary Material.     

Infrared spectra of carbon monoxide-hydrogen sulfide van der Waals complexes in the C-O stretching region

The CO-stretching vibration-rotation spectra of CO-H₂S, CO-D₂S, and CO-HDS complexes have been studied in the 2150 cm⁻¹ region using a supersonic slit-jet expansion and a tunable diode laser spectrometer. The spectra were analyzed with the help of very recent microwave pure rotational studies of the same complexes. Two bands were assigned for each of the symmetric hydrogen sulfide isotopes, corresponding to the two nuclear spin modifications, para and ortho. The band origins were blue shifted, relative to the free CO molecule, by about 3.8 cm⁻¹ for CO-H₂S and 4.3 cm⁻¹ for CO-D₂S. These are considerably smaller shifts than exhibited by the related CO-water complexes, indicating that the intermolecular forces in CO-H₂S are weaker and more isotropic.     

Experimental low energy values of CH4 transitions near 1.33 μm by absorption spectroscopy at 81 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.36 and 1.30 μm (7351-7655 cm⁻¹) using a cryogenic cell and a series of distributed feed back (DFB) diode lasers. The investigated spectral range corresponds to the high energy part of the icosad dominated by the ν₂+2ν₃ band near 7510 cm⁻¹. The positions and strengths at 81 K of 3473 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10⁻²⁶ cm/molecule, i.e. significantly lower than the intensity cut off of the HITRAN database in the region (4×10⁻²⁵ cm/molecule at 296 K). From the variation of the line strength between 81 and 296 K, the low energy values of 1273 transitions could be determined. They represent 69% and 81% of the absorbance in the region at 296 and 81 K, respectively. The obtained results are discussed in relation with the few rovibrational assignments previously reported in the region.