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The THz rotational lines of CH+ and its isotopologues have been observed in the range of 0.9–2.6 THz with JPL frequency multiplication chains. They were analyzed together with the known A 1 Π - X 1 Σ + band system data by using the Dunham formulation and the conventional vibration–rotation energy formula. The Λ -doubling in 1 Π states was reexamined, and the doubling or shifts were found to be different for the e- and f-parity levels. In this investigation, the e- and f-parity states were treated as two separate states. The major molecular constants were determined accurately for these two states separately, and were compared with the values obtained previously.

Over twenty-five years ago, Herb Pickett introduced his quantum-mechanical fitting programs to the spectroscopic community. The utility and flexibility of the software has enabled a whole generation of spectroscopists to analyze both simple and complex spectra without having to write and compile their own code. Last year Stewart Novick provided a primer for the coming generation of users. This follow-on work will serve as a guide to intermediate and advanced usage of the software. It is meant to be used in concert with the online documentation as well as the spectral line catalog archive.

Rotational spectra of methyl cyanide were recorded newly and were analyzed together with existing spectra to extend the global model of low-lying vibrational states and their interactions to υ 4 = 1 at 920 cm−1. The rotational spectra cover large portions of the 36–1439 GHz region and reach quantum numbers J and K of 79 and 16, respectively. Information on the K level structure of CH3CN is obtained from IR spectra. A spectrum of 2 ν 8 around 717 cm−1, analyzed in our previous study, covered also the ν 4 band. The assignments in this band cover 880–952 cm−1, attaining quantum numbers J and K of 61 and 13, respectively. The most important interaction of υ 4 = 1 appears to be with υ 8 = 3 , Δ K = 0 , Δ l = + 3 , a previously characterized anharmonic resonance. We report new analyses of interactions with Δ K = - 2 and Δ l = + 1 , with Δ K = - 4 and Δ l = - 1 , and with Δ K = - 6 and Δ l = - 3 ; these four types of interactions connect all l substates of υ 8 = 3 in energy to υ 4 = 1 . A known Δ K = - 2 , Δ l = + 1 interaction with υ 7 = 1 was also analyzed, and investigations of the Δ K = + 1 , Δ l = - 2 and Δ K = + 3 , Δ l = 0 resonances with υ 8 = 2 were improved, as were interactions between successive states with υ 8 ⩽ 3 , mainly through new υ 8 ⩽ 2 rotational data. A preliminary single state analysis of the υ 4 = υ 8 = 1 state was carried out based on rotational transition frequencies and on ν 4 + ν 8 - ν 8 hot band data. A considerable fraction of the K levels was reproduced within uncertainties in its entirety or in part, despite obvious widespread perturbations in υ 4 = υ 8 = 1 . In addition to the interstellar detection of rotational transitions of methyl cyanide from within all vibrational states up to υ 4 = 1 , we report the tentative detection of υ 4 = υ 8 = 1 toward the main hot molecular core of the protocluster Sagittarius B2(N) employing the Atacama Large Millimeter/submillimeter Array.

Terahertz absorption spectroscopy was employed to measure the ground state pure rotational transitions of the water isotopologue HD 18 O . A total of 105 pure rotational transitions were observed in the 0.5–5.0 THz region with ∼ 100 kHz accuracy for the first time. The observed positions were fit to experimental accuracy using the Euler series expansion of the asymmetric-top Hamiltonian together with the literature Microwave, Far-IR and IR data in the ground state and ν 2 . The new measurements and predictions reported here support the analysis of astronomical observations by high-resolution spectroscopic telescopes such as SOFIA and ALMA where laboratory rest frequencies with uncertainties of 1 MHz or less are required for proper analysis of velocity resolved astrophysical data.

This study reports the results from analyzing a number of high resolution, high signal-to-noise ratio (S/N) spectra in the 2.06-μm spectral region for pure CO2 and mixtures of CO2 in dry air. A multispectrum nonlinear least squares curve fitting technique has been used to retrieve the various spectral line parameters. The dataset includes 27 spectra: ten pure CO2, two 99% 13C-enriched CO2 and fifteen spectra of mixtures of 12C-enriched CO2 in dry air. The spectra were recorded at various gas sample temperatures between 170 and 297 K. The absorption path lengths range from 0.347 to 49 m. The sample pressures for the pure CO2 spectra varied from 1.1 to 594 Torr; for the two 13CO2 spectra the pressures were ∼10 and 146 Torr. For the air-broadened spectra, the pressures of the gas mixtures varied between 200 and 711 Torr with CO2 volume mixing ratios ranging from 0.014% to 0.203%. The multispectrum fitting technique was applied to fit simultaneously all these spectra to retrieve consistent set of line positions, intensities, and line shape parameters including their temperature dependences; for this, the Voigt line shape was modified to include line mixing (via the relaxation matrix formalism) and quadratic speed dependence. The new results are compared to select published values, including recent ab initio calculations. These results are required to retrieve the column averaged dry air mole fraction (XCO2) from space-based observations, such as the Orbiting Carbon Observatory-2 (OCO-2) satellite mission that NASA launched in July 2014.

Atmospheric investigations rely heavily on the availability of accurate spectral information of hydrocarbons. To extend the ethane database we recorded a 0.0028 cm − 1 resolution spectrum of 12 C 2 H 5 D from 650 to 1500 cm − 1 using a Bruker Fourier Transform spectrometer IFS-125HR at the Jet Propulsion Laboratory. The 98% deuterium-enriched sample was contained in a 0.2038 m absorption cell; one spectrum was obtained with the sample cryogenically cooled to 130.5 K and another at room temperature. From the cold data, we retrieved line positions and intensities of 8704 individual absorption features from 770 to 880 cm − 1 using a least squares curve fitting algorithm. From this set of measurements, we assigned 5035 transitions to the v 17 fundamental at 805.342729(27) cm − 1 ; this band is a c-type vibration, with often-resolved A and E components arising from internal rotation. The positions were modeled to a 22 term torsional Hamiltonian using SPFIT to fit the spectrum to a standard deviation of 7 × 10 − 4 cm − 1 (21 MHz). The prediction of the 5035 line intensities at 130.5 K agreed with observed intensities, but a small centrifugal distortion type correction to the transition dipole was needed to model the intensity of high K a R and P transitions. The integrated band intensities of 3.6628 × 10 − 19 cm − 1 /(molecule cm − 2 ) at 296 K in the 770–880 cm − 1 region was obtained. To predict line intensities at different temperatures, the partition function values were determined at nine temperatures between 9.8 and 300 K by summing individual energy levels up to J = 99 and K a = 99 for the six states up through ν 17 at 805 cm − 1 . We found the energy of A and E are inverted as compared to ground state (with the E state lower than the A state) and the splitting, −241.8(10) MHz, lies between the ground state value of +74.167(18) MHz and the first torsional state (ν 18 = 271.1 cm − 1 ) value of −3382.23(34) MHz. The proximity of the energy splitting to the ground state suggests that the ν 17 state has a similar torsional character. The resulting prediction of singly-deuterated ethane absorption at 12.5 μm enables its detection in planetary atmospheres, including those of Titan and exoplanets.

Terahertz absorption spectroscopy was employed to extend the measurements on the pure rotational transitions of N 2 H + , N 2 D + and their 15 N-containing isotopologues in the ground state and first excited vibrational states for the three fundamental vibrational modes. In total, 91 new pure rotational transitions were observed in the range of 0.7–2.7 THz. The observed transition frequencies were fit to experimental accuracy, and the improved molecular parameters were obtained. The new measurements and predictions reported here will support the analysis of high-resolution astronomical observations made with facilities such as SOFIA and ALMA where laboratory rest frequencies with uncertainties of 1 MHz or smaller are required for proper analysis of velocity resolved astrophysical components.

Author Institution: I. Physikalisches Institut, Univ. Koln, 50937 Koln; and MPIfR, 53121 Bonn, Germany; JPL, California Inst. of Technol., Pasadena, CA 91109, USA; LISA, Univ. Paris 12 \& Paris 7 \& CNRS, 94010 Cr{e}teil, France; PNNL, Richland, WA 99352, USA

The pure rotational spectrum of mono-deuterated ethane, CH 3 CH 2 D, has been measured up to 1600 GHz and spectroscopic constants have been fit to 984 transitions in the ground state and 422 transitions in the first torsional excited state ( ν 18 ). Analyses of the ground state data were performed with the programs SPFIT, ERHAM and XIAM and of the first torsional state with SPFIT and ERHAM to extract molecular and spectroscopic constants. A combined fit of both states using ERHAM was used to determine ρ = 0.4344026(68), which in the symmetric limit is the ratio Iα/Iz and a measure of the periodicity of the internal rotation energy with K and the energy differences between the A and E torsional substates, Δ E ( E – A ), of 74.167(18) and −3382.23(34) MHz for the ground and excited states, respectively. Using these energy differences and the overtone transitions Δ v = 2 from Raman measurements in the literature, the coefficients V 3 and V 6 of the potential function of the internal rotation in CH 3 CH 2 D were determined as V 3 = 1004.56(4) cm −1 and V 6 = 7.09(12) cm −1 . This analysis lays the ground work for the assignment of the IR spectrum of CH 3 CH 2 D between (680–880 cm −1 ) which will help quantify isotopic ratios by remote sensing missions.

The rotational spectrum of tertiary-butyl alcohol has been recorded in selected regions between 8 and 500 GHz. Early data from the University of Wisconsin in the 8–40 GHz region have been combined with recent measurements from the University of Bologna and the Jet Propulsion Laboratory in the millimeter and submillimeter wavelength regions. The spectrum was fit over a wide range of J’s and K’s using a common set of parameters for both the A and E states. This paper describes the initial assignment at Wisconsin and the final procedure used to assign and fit the higher rotational states. The resulting molecular constants and their interpretation are discussed.

We present a new global study of the millimeter wave, submillimeter wave and THz spectra of the lowest three torsional states of acetaldehyde (CH3CHO). New measurements have been carried out between 0.05 and 1.62 THz using three different spectrometers in IRA NASU (Ukraine), PhLAM Lille (France), and JPL (USA). The new data involving torsion–rotation transitions with J up to 66 and Ka up to 22 were combined with previously published measurements and fitted using the rho-axis-method torsion–rotation Hamiltonian. The final fit used 109 parameters to give an overall weighted root-mean-square deviation of 0.69 for the dataset consisting of 8748, 6959, and 4524 transitions belonging, respectively, to the ground, first, and second excited torsional states and 1481 Δvt ≠ 0 FIR transitions belonging to the torsional vt = 0 → 1 and 1 → 2 bands of the molecule. This investigation presents more than a twofold expansion in the J quantum number and almost fourfold expansion in the frequency range coverage for the acetaldehyde rotational spectrum.

We present a new global study of the millimeter, submillimeter and far-infrared (FIR) spectra involving the three lowest torsional states of acetone ((CH3)2CO). New microwave measurements have been carried out between 34 and 940 GHz using spectrometers in IRA NASU (Ukraine), and PhLAM Lille (France). The FIR spectrum of acetone has been recorded on the AILES beamline of the SOLEIL synchrotron facility. The new data involving torsion–rotation transitions with J up to 90 and Ka up to 52 were combined with previously published measurements and analyzed using a model developed recently to study the high resolution spectra of molecules with two equivalent methyl rotors and C2v symmetry at equilibrium (PAM_C2v_2tops program). The final fit included 117 parameters to give an overall weighted root-mean-square deviation of 0.85 for the dataset consisting of 29,584 microwave and 1116 FIR line frequencies belonging, respectively, to the three lowest torsional states (ν12,ν17) = (0,0), (1,0), (0,1) and to the observed fundamental band associated with the methyl-top torsion mode (ν12,ν17) = (0,1) ← (0,0). The high values of rotational quantum numbers involved in this study provide an opportunity to test the performance of the PAM_C2v_2tops program approach for the case of highly excited rotational states.