The Lamb-dip spectrum of the J + 1 ← J (J = 0, 1, 3-8) transitions of HCN: The nuclear hyperfine structure due to H, C, and N

The pure rotational J + 1 ← J transitions, with J = 0, 1, 3-8, of H¹³CN have been observed in the millimeter- and submillimeter-wave region using the Lamb-dip technique to resolve the hyperfine structure due to H, ¹³C, and ¹⁴N. The present observations allow us to provide for the first time the spin-rotation constant of ¹³C and the spin-spin interaction constant S₁₂ (between H and ¹³C) as well as to remarkably improve the quadrupole coupling and spin-rotation constants of ¹⁴N. In addition, a good empirical estimation of C_I(H), based on ab initio calculations, has also been provided. Furthermore, our frequencies together with previous data permit to determine the most accurate ground state rotational parameters known up to now.     

Accurate rotational spectroscopy of sulfur dioxide, SO2, in its ground vibrational and first excited bending states, v2 = 0, 1, up to 2 THz

Approximately 150 pure rotational transitions each have been recorded for SO₂, v₂ = 0 and 1, in selected frequency regions up to 2 THz. The J and K_a quantum numbers reach very high values: 92 and 23, respectively, for the ground vibrational state and 81 and 21, respectively, for the first excited bending state. The highest levels accessed are almost 3000 cm⁻¹ above ground. The relative experimental uncertainties Δν/ν are about 10⁻⁸ for several medium to strong, isolated lines, and generally better than 2.5 × 10⁻⁷. Improved spectroscopic parameters have been obtained for both states, particularly for the excited bending state. In fact, the accuracies with which the energy levels of the v₂ = 1 state are known depend essentially only on the accuracy with which the vibrational spacing is known from infrared spectroscopy.     

The Lamb-dip spectrum of the J = 1 ← 0 transition of DF: Crossing resonances and hyperfine components

The Lamb-dip technique has been applied to the observation of the J = 1 ← 0 transition of DF: for the first time, the hyperfine structure due to D and F have been resolved by using microwave spectroscopy. The high accuracy of this technique allows us to provide hyperfine parameters that are in very good agreement with those obtained from molecular beam experiment. In addition, our frequencies together with the unresolved ones up to J″ value of 47 allow us to provide the most accurate ground state rotational constants of DF known at the moment. Furthermore, due to the presence of a relevant number of strong crossing resonances, the J = 1 ← 0 transition of DF can be considered an illustrative case to show how they modify the shape of Lamb-dip spectra.     

Rotational levels of the (0 0 0) and (0 1 0) states of D2O from hot emission spectra in the 320-860 cm region

The far-infrared emission spectra of deuterated water vapour were measured at different temperatures (1370, 1520, and 1950 K) in the range 320-860 cm⁻¹ at a resolution of 0.0055 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 1150 new measured lines for the D₂¹⁶O molecule corresponding to transitions between highly excited rotational levels of the (0 0 0) and (0 1 0) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max=26 and for the (0 0 0) ← (0 0 0) band, J_max=25 and for the (0 1 0) ← (0 1 0) band, and J_max=26 and for the (0 1 0) ← (0 0 0) band. The estimated accuracy of the measured line positions is 0.0005 cm⁻¹. To our knowledge no experimentally measured rotational transitions for D₂¹⁶O within an excited vibrational state have been available in the literature so far. An extended set of experimental rotational energy levels for (0 0 0) and (0 1 0) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.0012 cm⁻¹ for 692 rotational levels of the (0 0 0) state and 0.0010 cm⁻¹ for 639 rotational levels of the (0 1 0) vibrational state. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surface [J. Chem. Phys. 106 (1997) 4618] for the (0 0 0) and (0 1 0) states is discussed.     

High-precision frequency measurements of the ν1 + ν3 combination band of C2H2 in the 1.5 μm region

A pair of 1.5 μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to transitions of ¹²C₂H₂ and ¹³C₂H₂, using cavity-enhanced Doppler-free saturation absorption spectroscopy. The absolute frequencies of 41 lines of the ν₁ + ν₃ band of ¹²C₂H₂, covering the spectral region 1520-1545 nm, have been measured by use of a passive optical frequency comb generator, referenced to ¹³C₂H₂ transitions of known frequency. The mean experimental uncertainties (coverage factor k = 1) of the frequency values are 3.0 kHz (type A) and 10 kHz (type B). Improved values of the band origin ν₀, rotational constants B′ and B″, and centrifugal distortion coefficients D′, D″, H′, and H″ are presented.     

N2-, H2-, and He-induced collisional broadening of the J = 24 ← 23 transition of HC3N located near 218.3 GHz at different temperatures

We report on experimental collisional relaxation of the J = 24 ← 23 line of HC₃¹⁴N, located near 218.3 GHz, induced by nitrogen, hydrogen, and helium. The measurements were carried out at selected temperatures in the 235-350 K range using a video-type spectrometer. The foreign gas broadening parameters and their temperature dependences were determined assuming Voigt lineshape profiles and the usual T⁻ⁿ temperature law. The experimental broadening parameters are compared with results derived using the ATC collisional formalism.     

Lineshape analysis of the J = 3 ← 2 and J = 5 ← 4 rotational transitions of room temperature CO broadened by N2, O2, CO2 and noble gases

Collisional relaxation has been considered for millimeter lines of carbon monoxide at room temperature. Accurate measurements of carbon dioxide- and rare gases-broadened widths have been performed on the J = 3 ← 2 rotational line of ¹²CO by using a video-type spectrometer. Measurements of nitrogen-, oxygen-, and xenon-broadened widths of the J = 5 ← 4 rotational line of ¹³CO were also carried by using a frequency-modulated spectrometer. A lineshape study performed on all the investigated binary systems provide confirmation that Voigt profile is not a suitable model to analyse experimental lines in the millimeter-waves region. On one hand, using this profile in the low pressure range, i.e. in the Doppler regime, the retrieved collisional linewidths do not follow a linear variation with the perturbing gas pressure. On the other hand, regardless of the pressure, lineshapes exhibit a narrowed profile. An accurate analysis of the pressure dependence of relaxation rates show that the Galatry profile is not appropriate and that experimental lineshapes are actually Speed Dependent Voigt profiles. Accurate broadening parameters were retrieved from this profile and compared to previous reported values and predictions calculated from the Robert-Bonamy formalism. Finally a variation of the ratio of relaxation speed dependence to broadening parameters versus relative masses of the collision partners is presented.     

Rotational spectrum of the v11 = 1 and v14 = 1 vibrational states of CH3CCCCH

The pure rotational spectra of the v₁₁ = 1 and v₁₄ = 1 vibrational states of the main isotopic species of methyldiacetylene have been recorded and assigned in the 80-400 GHz frequency range, spanning the quantum numbers 19 ? J ? 95 and 0 ? K ? 15. The present study allows us to provide accurate rotational, centrifugal distortion and vibration-rotation interaction constants. The experimental investigation has been strongly supported by quantum-chemical computations at the second-order Møller-Plesset theory (MP2) in conjunction with a triple-zeta quality basis set.     

Microwave and sub-mmwave study of CH3SiH3 including the perturbation-allowed torsion-vibration difference band (ν12=0,ν6=3)↔(ν12=1,ν6=0)

The vibration-torsion-rotation Hamiltonian in CH₃SiH₃ has been investigated using Fourier transform microwave methods and tunable sideband far-infrared spectroscopy. Four different studies have been carried out. First, the Q-branch of the torsion-vibration difference band (ν₁₂=0,ν₆=3)↔(ν₁₂=1,ν₆=0) has been measured between 17.8 and 26.6 GHz. When three quanta of the torsional mode ν₆ are excited in the ground vibrational state (gs) for (σ=−1) torsional sublevels with K=6, these transitions become allowed through resonant Coriolis-like coupling to the lowest lying degenerate mode ν₁₂ with no quanta of ν₆ excited. Second, direct l-doubling transitions in the state (v₁₂=1, v₆=0) have been observed between 8.3 and 17.5 GHz for both torsional sublevels σ=0 and σ=±1. In the limit that the intervibrational interactions vanish, the σ-splitting between lines of the same J would be difficult to resolve, but frequency differences of more than 1 GHz due to these interactions have been determined. Third, the (J=1←0) spectrum just below 22 GHz has been re-measured with higher resolution for 0?v₆?4 in the gs and for (v₆=0) in ν₁₂. Finally, the (J=45←44) spectrum near 1 THz has been obtained for 0?v₆?2 in the gs. A global data set of 3423 frequencies has been formed by merging the present 123 measurements with the data set used recently in the simultaneous analysis of the ν₁₂ and ν₅ bands by Schroderus et al. [J. Chem Phys. 115 (2001) 1392]. By refining the (gs/ν₁₂/ν₅) Hamiltonian developed in this earlier work in which the torsional motion is grouped with the vibrational degrees of freedom, a good fit to within experimental error has been obtained by varying 45 parameters. A fit of comparable quality has also been obtained using a similar analysis in which the torsional motion is grouped with the rotational degrees of freedom. The values of the molecular constants determined in the two models are compared.     

Analysis of the first triad of interacting states (0 2 0), (1 0 0), and (0 0 1) of D2O from hot emission spectra

The far-infrared and middle-infrared emission spectra of deuterated water vapour were measured at temperatures 1370, 1520, and 1940 K in the ranges 320-860 and 1750-3400 cm⁻¹. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 3550 new measured lines for the D₂¹⁶O molecule corresponding to transitions from highly excited rotational levels of the (0 2 0), (1 0 0), and (0 0 1) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J_max = 29 and K_a(max) = 22 for the (0 2 0) state, J_max = 29 and K_a(max) = 25 for the (1 0 0) state, and J_max = 30 and K_a(max) = 23 for the (0 0 1) state. The extended set of 1987 experimental rotational energy levels for the (0 2 0), (1 0 0), and (0 0 1) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.004 cm⁻¹ for 1952 rovibrational levels of all three vibration states. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surfaces of water isotopic species [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618] is discussed. The latter confirms a good consistency of mass-dependent DBOC corrections in the PS potential function with new experimental rovibrational data.     

CO2-broadening coefficients in the ν4 + ν5 band of acetylene

The CO₂-broadening coefficients of 24 P- and R-branch transitions in the ν₄ + ν₅ band of acetylene were measured at room temperature using a diode-laser spectrometer. These lines with J values up to 26, were located in the spectral range 1270 to 1400 cm⁻¹. The collisional broadening coefficients were retrieved by fitting the experimental profiles to the Voigt, Rautian, and Galatry lineshape models. Two experimental values for the narrowing coefficient were determined from the spectra and compared with the theoretical narrowing coefficient. The calculations of these broadenings were also performed in the frame of a semiclassical formalism involving a simple intermolecular potential with an adjustable parameter. The theoretical results are in good agreement with the experimental results and reproduce well the J dependence of the broadening coefficients.     

The ν1 + 3ν2 + 3ν3 and 4ν1 + ν2 + ν3 bands of ozone by CW-cavity ring down spectroscopy between 5900 and 5960 cm

The absorption spectrum of ozone, ¹⁶O₃, has been recorded in the 5903-5960 cm⁻¹ region by high sensitivity CW-cavity ring down spectroscopy (α_min ∼ 5 × 10⁻¹⁰ cm⁻¹). The ν₁ + 3ν₂ + 3ν₃ and 4ν₁ + ν₂ + ν₃ A-type bands centred at 5919.15 and 5947.07 cm⁻¹ were newly observed. A set of 173 and 168 energy levels could be experimentally determined for the (1 3 3) and (4 1 1) states, respectively. Except for a few K_a = 5 levels of the (4 1 1) state, the rotational structure of the two states was found mostly unperturbed. The spectroscopic parameters were determined from a fit of the corresponding line positions by considering the (1 3 3) and (4 1 1) states as isolated. The determined effective Hamiltonian and transition moment operators were used to generate a list of 785 transitions given as Supplementary Material.     

Measurements of NH3 in the ν2 = 1 state by a solid-state, photomixing, THz spectrometer, and a simultaneous analysis of the microwave, terahertz, and infrared transitions between the ground and ν2 inversion-rotation levels

In this paper, we report 30 THz measurements of ammonia in its excited, ν₂ = 1, inversion state. These measurements include transition frequencies and pressure shifts (where significant enough to be observed). Included in the data are two forbidden and three ro-inversion transitions, as well as 18 transitions never directly measured before. The measurements were made with an all-solid-state, diode-laser, difference-frequency spectrometer. Using an innovative laser-frequency locking scheme, this spectrometer provided accurately determined and continuously tuned THz-frequencies without requiring accurate knowledge of the absolute laser frequencies. The details of the spectrometer’s frequency calibration is discussed. A global analysis of NH₃ based on the available ground state, ν₂ = 1 state, and ν₂-band transitions was carried out, and the resulting set of recommended molecular effective-Hamiltonian parameters for ammonia is presented. In addition, calculated center frequencies and intensities for all possible transitions up to J = 20 between rotation, inversion, and vibration levels in the ground and ν₂ = 1 states are included as supplementary material.     

Frequency measurement of pure rotational transitions in the v2 = 1 state of H2O

Frequencies of pure rotational transitions in the v₂ = 1 vibrationally excited state of H₂¹⁶O were measured with a tunable far-infrared spectrometer in the frequency range of 0.5-5 THz. Molecular parameters of Watson’s A-reduced Hamiltonian have been obtained to reproduce the observed frequencies.     

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.     

The bΣ(b0) → XΣ(X10,X21) and aΔ(a2) → X21 transitions of AsBr

Emission spectra of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺,X₂1) and a¹Δ(a2) → X₂1 transitions of AsBr have been measured in the near-infrared spectral region with a Fourier-transform spectrometer. The arsenic bromide radicals were generated in fast-flow systems by reaction of arsenic vapor (As_x) with bromine and were excited by microwave-discharged oxygen. The most prominent features in the spectrum are the Δv = +1,0,−1, and −2 band sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₁0⁺) transition in the range 11 700-12 700 cm⁻¹. With lower intensities, the Δv = 0 and −1 sequences of the b¹Σ⁺(b0⁺) → X³Σ⁻(X₂1) sub-system show up in the same range. Further to the red, between 6000 and 6700 cm⁻¹, the Δv = 0, +1, and −1 sequences of the hitherto unknown a¹Δ(a2) → X₂1 transition are observed. Analyses of medium- and high-resolution spectra have yielded improved molecular constants for the X₁0⁺, X₂1, and b0⁺ states and first values of the electronic energy and the vibrational constants of the a2 state.     

Line mixing and speed dependence in CO2 at 6227.9 cm: Constrained multispectrum analysis of intensities and line shapes in the 30013 ← 00001 band

Line position, intensity and line shape parameters (Lorentz widths, pressure shifts, line mixing, speed dependence) are reported for transitions of the 30013 ← 00001 band of ¹⁶O¹²C¹⁶O (ν₀ = 6227.9 cm⁻¹). The results are determined from 26 high-resolution, high signal-to-noise ratio spectra recorded at room temperature with the McMath-Pierce Fourier transform spectrometer. To minimize the systematic errors of the retrieved parameters, we constrained the multispectrum nonlinear least squares retrieval technique to use quantum mechanical expressions for the rovibrational energies and intensities rather than retrieving the individual positions and intensities line by line. Self- and air-broadened Lorentz width and pressure-induced shift, speed dependence and line mixing (off-diagonal relaxation matrix elements) coefficients were adjusted individually. Errors were further reduced by simultaneously fitting the interfering absorptions from the weak 30012 ← 00001 band of ¹⁶O¹³C¹⁶O as well as the weak hot bands 31113 ← 01101, 32213 ← 02201, 40014 ← 10002 and 40013 ← 10001 of ¹⁶O¹²C¹⁶O in this spectral window. This study complements our previous work on line mixing and speed dependence in the 30012 ← 00001 band (ν₀ = 6347.8 cm⁻¹) [V.M. Devi, D.C. Benner, L.R. Brown, C.E. Miller, R.A. Toth, J. Mol. Spectrosc. 242 (2007) 90-117] and provides key data needed to improve atmospheric remote sensing of CO₂.     

Doppler-limited rotational spectrum of the NH radical in the 2 THz region

The Doppler-limited rotational spectrum of the NH radical in its electronic (X) and vibrational ground state has been measured using the frequency stabilized Cologne side-band spectrometer in the frequency region near 2 THz. The nitrogen ¹⁴N nuclear hyperfine patterns have been observed accompanying the resolved fine (J^′←J″) structure of the N=2←1 rotational transition. The observed peak frequencies were analyzed in detail together with the previously measured hyperfine frequencies of the N=1←0 rotational transition and with combination differences obtained from the high-resolution electronic spectra to derive precise rotational, centrifugal distortion, fine, and hyperfine parameters. In the numerical analysis the essential attention has been paid to partly resolved and unresolved hyperfine structures. The peak positions of the partly or fully overlapped lines were analyzed with the help of a profile simulation with estimated half-widths and calculated relative intensities and in this manner the least square fit of the unresolved and partly resolved lines was significantly improved. The NH radical is an extremely important species in nitrogen chemical reaction networks in the interstellar medium and atmospheric chemistry.     

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.     

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.