Pure Rotational Transitions Research Articles

Millimeter-wave and infrared spectroscopy of thiazole (c-C3H3NS) in its ground state and lowest-energy vibrationally excited states (ν18, ν17, and ν13)

The rotational spectrum of thiazole (c-C3H3NS, Cs, μa = 1.3 D, μb = 0.97 D) from 130 to 375 GHz and high-resolution infrared spectrum from 300 to 1300 cm−1 were obtained and analyzed. Nearly 4700 new, pure-rotational transitions were measured for the ground vibrational state, as well as over 1400 transitions for each of its three lowest-energy vibrationally excited states (ν18, ν17, and ν13). A total of over 5500 rotationally resolved, infrared transitions were measured for fundamentals ν17 and ν13, which form a Coriolis-coupled dyad. Transitions for each of these vibrational states of thiazole were fit to a sextic, A-reduced Hamiltonian in the Ir representation. Computed CCSD(T) values of the spectroscopic constants of thiazole show remarkable agreement with the experimentally determined values. Deuterium enrichment of thiazole allowed for the observation of the ground state and first fundamental (ν18) for three isotopic thiazoles. The direct impact on ν18 by isotopic substitution at C2 and C5 is observed. The combined rotational and high-resolution IR data allowed for an excellent fit of the Coriolis-coupled dyad of ν17 and ν13, which includes more than 60 nominal interstate transitions. Some of the measured resonant transitions are displaced by more than 30 GHz. The least-squares fit yielded six Coriolis coupling terms (Ga, GaJ, GaK, Gb, GbJ, and GbK) and very accurate measurements of the ν17 and ν13 band origins, 603.8511589 (17) cm−1 and 611.2214645 (18) cm−1, respectively. Collectively, these data provide reliable spectroscopic constants that can be used to predict the rotational spectra of thiazole up to 360 GHz.

High-Resolution Infrared Spectroscopy of DC3N in the Stretching Region

The perspectives opened by modern ground-based infrared facilities and the forthcoming James Webb Telescope mission have brought a great attention to the ro-vibrational spectra of simple interstellar molecules. In this view, and because of the lack of accurate spectroscopic data, we have investigated the infrared spectrum of deuterated cyanoacetylene (DC3N), a relevant astrochemical species. The ν1, ν2, and ν3 fundamentals as well as their hot-bands were observed in the stretching region (1,500–3,500 cm−1) by means of a Fourier transform infrared spectrometer. Supplementary measurements were performed at millimeter-wavelengths (243–295 GHz) with a frequency-modulation spectrometer equipped with a furnace, that allowed to probe pure rotational transitions in the investigated stretching states. Furthermore, since HC3N is observed as by-product in our spectra and suffers from the same deficiency of accurate infrared data, its ro-vibrational features have been analyzed as well. The combined analysis of both rotational and ro-vibrational data allowed us to determine precise spectroscopic constants that can be used to model the infrared spectra of DC3N and HC3N. The importance of accurate molecular data for the correct modeling of proto-planetary disks and exoplanetary atmospheres is then discussed.

Rotational and vibrational spectroscopy of 1-cyanoadamantane and 1-isocyanoadamantane

Because of their high stability, the presence of diamond-type molecules has long been suspected in the interstellar medium, a hypothesis supported by the extraction of diamond nanocrystal from some meteorites. We report the rotational and vibrational investigation of two polar derivatives of adamantane (C10H16), 1-cyanoadamantane (C10H15–CN) and 1-isocyanoadamantane (C10H15–NC), using room temperature gas phase absorption spectroscopy. Pure rotational spectra have been recorded at millimeter wavelengths (75–220 GHz) while vibrational spectra were obtained in the far- and mid-infrared domains (50–3500 cm−1). Quantum chemical calculations have been performed on these two C3v rotors to support the spectral analysis enabling the assignment, for both species, of more than 7000 pure rotational transitions in the ground (A1 symmetry) and first vibrationally excited (E symmetry) states, and of most of the infrared active bands. The pure rotational lines were fit to their experimental accuracy using a symmetric-top Hamiltonian. Our study provides all necessary information for an active search of these species in space.

Rotational spectroscopy and bound state calculations of deuterated NH3–H2 van der Waals complexes

Pure rotational transitions of the deuterated NH3–H2 weakly bound complexes formed by NH3/ND3 and H2/D2, namely ND3–H2, NH3–D2 and ND3–D2, have been detected. Three different techniques, a millimeter-wave intracavity jet OROTRON spectrometer, molecular beam Fourier transform microwave (FTMW) cavity spectrometer and the broadband chirped pulse (CP) FTMW spectrometer were used to record the spectra in the frequency range from 20 to 140 GHz. The double resonance method was additionally applied to extend the frequency range of the OROTRON spectrometer to lower and of the FTMW cavity spectrometer to higher frequencies. A search for the transitions of deuterated isotopologues and their assignments were based on the recent study of the main isotopic species NH3–H2 [L.A. Surin et al., Astrophys. J., 838, 27 (2017)] and further confirmed by the bound state calculations presented here. The measured line positions including hyperfine splitting due to the 14N nuclear spin of NH3/ND3 and the D nuclear spins of D2 were analysed in order to determine the molecular parameters and structure of the deuterated NH3–H2 complexes. This study provides an effective new probe of the intermolecular interaction between ammonia and dihydrogen, knowledge of which is important for numerical modelling of astrophysical environments.

A prochiral precursor in space? Accurate laboratory characterization of acetylacetylene in the cm-wave region

The most prominent pure rotational transitions of the prochiral, possibly prebiotic molecule acetylacetylene are recorded in the frequency range of 6.5–26.5 GHz for a reliable spectral characterization prerequisite to a possible extraterrestrial observation. Therefore, three different approaches for global fitting of the vibrational ground state are applied to analyze the experimentally determined transition frequencies using the programs SPFIT, XIAM and ERHAM corresponding to different theoretical treatments solving the internal rotation problem.Furthermore, the resulting internal rotation parameter V3 is compared to those other substituted acetyl species, and derivatives of butynes and possible explanations for the observed trends are given. For the substituted acetyl species, the spectra of acetylfluoride, -chloride, -bromide and -iodide are reanalyzed.The obtained rotational constants have been interpreted in terms of characterizing the geometric structure. A correlation between the tunneling parameter ε1 and V3 of several molecular species displaying spectral internal rotation splitting patterns has been found and quantified. This relationship can simplify the analysis of vibrational ground state rotational spectra in finding a suitable starting point for the fitting procedure using the program ERHAM, which is the predestined choice to analyze and predict signal positions up to very high quantum numbers. Being able to predict those signal positions is crucial for a possible comprehensive astronomical identification and subsequent application as an astrophysical probe for the conditions in interstellar molecular clouds.

The rotationally resolved infrared spectrum of TiO and its isotopologues

In this study, we present the ro-vibrationally resolved gas-phase spectrum of the diatomic molecule TiO around 1000 cm−1. Molecules were produced in a laser ablation source by vaporizing a pure titanium sample in the atmosphere of gaseous nitrous oxide. Adiabatically expanded gas, containing TiO, formed a supersonic jet and was probed perpendicularly to its propagation by infrared radiation from quantum cascade lasers. Fundamental bands of 46-50TiO and vibrational hotbands of 48TiO are identified and analyzed. In a mass-independent fitting procedure combining the new infrared data with pure rotational and electronic transitions from the literature, a Dunham-like parameterization is obtained. From the present data set, the multi-isotopic analysis allows to determine the spin-rotation coupling constant γ and the Born–Oppenheimer correction coefficient ΔU10Ti for the first time. The parameter set enables to calculate the Born–Oppenheimer correction coefficients ΔU02Ti and ΔU02O. In addition, the vibrational transition moments for the observed vibrational transitions are reported.

An improved rovibrational linelist of formaldehyde, H[formula omitted]C[formula omitted

Published high-resolution rotation-vibration transitions of H212C16O, the principal isotopologue of methanal, are analyzed using the MARVEL (Measured Active Rotation-Vibration Energy Levels) procedure. The literature results are augmented by new, high-accuracy measurements of pure rotational transitions within the ground, ν3,ν4, and ν6 vibrational states. Of the 16 596 non-redundant transitions processed, which come from 43 sources including the present work, 16 403 could be validated, providing 5029 empirical energy levels of H212C16O with statistically well-defined uncertainties. All the empirical rotational-vibrational energy levels determined are used to improve the accuracy of ExoMol’s AYTY line list for hot formaldehyde. The complete list of collated experimental transitions, the empirical energy levels determined, as well as the extended and improved line list are provided as Supplementary Material.

Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths

Even if on-board mm-wave/THz heterodyne receivers have been developed to measure greenhouse gases (GHGs) atmospheric profiles, rotational spectroscopy rests under-exploited for their monitoring unlike IR rovibrational spectroscopy. The present study deals with the ability of THz spectroscopy using long interaction path-lengths for GHG laboratory investigations. High-resolution THz signatures of non-polar greenhouse molecules may be observed by probing very weak centrifugal distortion induced rotational transitions. To illustrate, new measurements on CH4 and CF4 have been carried out. For CH4, pure rotational transitions, recorded by cw-THz photomixing up to 2.6 THz in a White type cell adjusted to 20 m, have allowed to update the methane line list of atmospheric databases. Concerning CF4, Fabry-Perot THz absorption spectroscopy with a km effective pathlength was required to detect line intensities lower than 10−27 cm−1/(moleccm−2). Contrary to previous synchrotron-based FT-FIR measurements, the tetrahedral splitting of CF4 THz lines is fully resolved. Finally, quantitative measurements of N2O and O3 gas traces have been performed in an atmospheric simulation chamber using a submm-wave amplified multiplier chain coupled to a Chernin type multi-pass cell on a 200 m path-length. The THz monitoring of these two polar GHGs at tropospheric and stratospheric concentrations may be now considered.

Realizing negative index of refraction in an ensemble of ground-state polar molecules with lasers

We propose a coherent optical method for creating negative refractive index (NRI) for a gaseous ensemble of ground-state polar molecules possessing both permanent electric and magnetic moments. Exploiting the pure rotational transition between the two lowest rotational levels of the ground vibrational state one can generate two dressed states of mixed parity using a microwave laser. These dressed states are then used as the two lower states of a Λ-type three-level scheme using two infrared lasers to couple them to another ro-vibrational level in the ground-state manifold. One of the infrared lasers is used as a weak probe and the other as a control field with a fixed detuning. We take into account local-field effects on the dielectric response of the medium at the probe frequency in terms of Clausius–Mossoti relation. We extract magnetic response out of purely dielectric response and calculate the magnetic permeability of the medium in terms of dielectric susceptibility. Our results show that there is a small frequency window near the two-photon resonance where both electric permittivity and magnetic permeability are negative with vanishingly small absorption. The figure of merit for the medium is shown to exceed unity. We interpret our results in terms of the proximity of EIT and quantum interference. We discuss the possible realization of our method using cold polar molecules that are recently experimentally produced.

Pure Rotational Spectrum of Benzophenone Detected by Broadband Microwave Spectrometer in the 2–8 GHz Range

The investigation on microwave spectrum of benzophenone was conducted with a recently constructed broadband chirped-pulse Fourier transform microwave spectrometer with a heating nozzle in the 2–8 GHz range. In this work, 138 b-type pure rotational transitions were assigned to bridge the measuring gap in the microwave region. The rotational constants for benzophenone were accurately determined by a combined microwave data fitting with frequency coverage between 2–14 GHz and have the following values: A = 1692.8892190(119) MHz, B = 412.6446602(43) MHz and C = 353.8745644(43) MHz.

Molecular Parameters of Tert-Butyl Chloride and Its Isotopologues Determined from High-Resolution Rotational Spectroscopy

A broadband chirped-pulse Fourier transform microwave spectrometer was used to detect the rotational spectra of the products of a chemical reaction in the gas phase from 1-18 GHz under the supersonic expansion condition. In natural abundance, pure rotational energy level transitions of tert-butyl chloride and its isotopologues (13C, 37Cl) were observed and assigned. The rotational spectral parameters (rotational constant, quadrupole coupling constant and centrifugal distortion constant) of these isotopologues were determined. The experimental results are in great agreement with the calculated values of quantum chemistry and the spectral parameters in the literature. The accuracy and the capability for chemical detection of our homemade rotational spectrometer were verified by this experiment.

Detection of a Criegee Intermediate with an Unsaturated Hydrocarbon Substituent: Fourier-Transform Microwave Spectroscopy of Methyl Vinyl Ketone Oxide.

Pure rotational transitions of methyl vinyl ketone oxide, or the so called methyl-vinyl Criegee intermediate, were observed by Fourier-transform microwave spectroscopy. Among the four possible conformers of this species, predicted by theory within an energy window of 3 kcal/mol, only the lowest-energy conformer, the syn-trans form, was detected in a discharged jet of a 1,3-diiode-but-2-ene (either in Z- or E-conformation) and O2 mixture diluted in Ar. Thirty pure rotational transitions with internal rotation splitting of the methyl top were observed. The observed frequencies were analyzed by the XIAM program, yielding an internal rotation barrier of 702.8(28) cm-1, which reasonably agrees with a theoretical value of 680 cm-1 determined with CCSD(T)/cc-pVTZ.

FTMW spectroscopy of epichlorohydrin: Detection of three conformers

Pure rotational transitions of three conformers of epichlorohydrin were observed. Among the three possible conformers expected for this species, spectra of the two chlorine isotopologues, 35Cl and 37Cl, have been detected for the two gauche-conformers, while only the 35Cl isotopologue was detected for the cis-conformer which is much higher in energy. Well resolved chlorine nuclear hyperfine splittings were observed for all the conformers and isotopologues, yielding precisely determined molecular constants.

GSRB rotation and rovibration intensities for: The bent HOCl molecule, and for: Quantum monodromy in NCNCS

Considerable work has gone into identifying and delineating the effects of quantum monodromy on molecules. Recently this included consideration of transition intensities of the NCNCS molecule using ab initio electric dipole moment functions in the Generalised SemiRigid Bender (GSRB) model [M. Winnewisser et al. PCCP 16, 17373-17407 (2014)]. In that work a discrepancy between the observed and calculated spectra was discovered. The purpose of the present work is to identify the source of that discrepancy - is it due to the physics of monodromy itself or are other issues involved? To investigate this we use the same theoretical approach for the simpler molecule HOCl. This allows us to identify a programming error affecting the b-type rovibrational transitions. We show that the NCNCS spectrum calculated with the corrected GSRB model does reproduce the experimental spectrum for pure rotational transitions within the ν7 large amplitude bending vibrational states, starting from the ground state and progressing through levels at the monodromy energy and beyond, while also reproducing the a-type υ:1←0rovibrational spectrum. However, the discrepancy with regard to the b-type rovibrational transitions remains. We discuss possible reasons for this discrepancy. In the course of this work we obtain a good GSRB model for the bending vibration-rotation levels of the atmospherically important HOCl molecule and its isotopologues. With ab initio calculations of the electric dipole moment function the GSRB calculates the rotation and bending rovibrational spectrum of HOCl in agreement with other determinations and with experiment. Using the physically constrained GSRB model, errors in tabulated transitions (including in the HITRAN database) were easily identified.

Ab initio potential energy surface and microwave spectrum of the NH3-N2 van der Waals complex.

We present a five-dimensional intermolecular potential energy surface (PES) of the NH3-N2 complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH3 symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH3 unit closest to the N2 unit, which is nearly parallel to the NH3 symmetry axis. The intermolecular distance is 7.01 a0, and the binding energy De is 250.6 cm-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH3 combines with (o/p)-N2, were calculated on this intermolecular potential surface. The computed dissociation energies D0 are 144.91 cm-1, 146.50 cm-1, 152.29 cm-1, and 154.64 cm-1 for (o)-NH3-(o)-N2, (o)-NH3-(p)-N2, (p)-NH3-(o)-N2, and (p)-NH3-(p)-N2, respectively. Guided by these calculations, the pure rotational transitions of the NH3-N2 van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole 14N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH3-N2 nuclear spin isomers.

Rotational spectroscopy of rare iron monoxide isotopologues: A mass-independent analysis

We present pure rotational transitions of the rare iron monoxide isotopologues. , and in their lowest spin states . For , the rotational spectrum reveals hyperfine splitting due to the nuclear spin of . We use Dunham-like parameters to analyse the new laboratory data together with data from the literature. In particular, we are able to derive the Born–Oppenheimer breakdown parameters and and the corresponding equilibrium bond length of . With these new accurate molecular parameters line positions on a sub-MHz accuracy level can be calculated, including those of the radioactive isotopologue . The new data allow for an astronomical search for the rare isotopologues using sensitive radio telescopes, like the Atacama Large Millimetre/submillimetre Array.

The H2O Spectrum of the Massive Protostar AFGL 2136 IRS 1 from 2 to 13 μm at High Resolution: Probing the Circumstellar Disk

Abstract We have observed the massive protostar AFGL 2136 IRS 1 in multiple wavelength windows in the near- to mid-infrared at high (∼3 km s−1) spectral resolution using VLT+CRIRES, SOFIA+EXES, and Gemini North+TEXES. There is an abundance of H2O absorption lines from the ν 1 and ν 3 vibrational bands at 2.7 μm, from the ν 2 vibrational band at 6.1 μm, and from pure rotational transitions near 10–13 μm. Analysis of state-specific column densities derived from the resolved absorption features reveals that an isothermal absorbing slab model is incapable of explaining the relative depths of different absorption features. In particular, the strongest absorption features are much weaker than expected, indicating optical depth effects resulting from the absorbing gas being well mixed with the warm dust that serves as the “background” continuum source at all observed wavelengths. The velocity at which the strongest H2O absorption occurs coincides with the velocity centroid along the minor axis of the compact disk in Keplerian rotation recently observed in H2O emission with ALMA. We postulate that the warm regions of this dust disk dominate the continuum emission at near- to mid-infrared wavelengths, and that H2O and several other molecules observed in absorption are probing this disk. Absorption line profiles are not symmetric, possibly indicating that the warm dust in the disk that produces the infrared continuum has a nonuniform distribution similar to the substructure observed in 1.3 mm continuum emission.

Self and N2 broadening coefficients of H2S probed by submillimeter spectroscopy: Comparison with IR measurements and semi-classical calculations

High-resolution lines of H2S self-perturbed and perturbed by N2 have been recorded at room temperature using a frequency modulated THz spectrometer at pressures of H2S ranging from 0.1 to 1.0 mbar. A non-linear least squares fitting of these spectra has been performed to retrieve self- and N2-broadening coefficients of pure rotational transitions (mainly in the RQ sub-branch) of H2S in the submillimeter region. We show that the lineshapes are well reproduced by the Speed-Dependent Voigt profile accounting for the speed dependence of relaxation rates which display linear pressure dependencies. Using the Robert and Bonamy formalism, these coefficients were calculated, showing the need to account for all contributions of the intermolecular interaction for N2-broadening. The calculated coefficients are consistent with the measured data and the average accuracy of the line parameters has been estimated to be about 5%. The observed J″ and Ka″ rotational dependences have been analyzed using an empirical model and compared to previous rovibrational studies in the ν2 fundamental band. In our study the self- and the N2-broadening coefficients decreases monotonically with Ka″ and J″, respectively.

Pure rotational coherent anti‐Stokes Raman scattering spectroscopy of nitric oxide: Determination of Raman tensor invariants

AbstractCoherent anti‐Stokes Raman scattering (CARS) measurements of the pure rotational Raman spectrum of nitric oxide were performed. Measurements were performed in a room‐temperature gas cell using a dual‐broadband pure rotational CARS configuration. In this configuration, a broadband dye laser was used to generate the pump and Stokes beams, and the 355‐nm third‐harmonic beam from a Nd:YAG laser was used as the probe beam. The Raman spectrum of NO is of significant theoretical interest because of the spin splitting in the ground electronic level of NO. A detailed model of the pure rotational Raman spectrum of NO was developed based on an irreducible tensor analysis using Hund's case (a) wave functions as basis states for the analysis. The pure rotational Raman polarizability tensor element in the molecular frame was determined for NO based on fitting pure rotational CARS spectra of mixtures of NO and nitrogen (N2). In addition to the pure rotational CARS spectrum featuring 2Π1/2→2Π1/2 and 2Π3/2→2Π3/2 transitions, there is an electronic Raman transition at 121 cm−1 between the spin split 2Π1/2 and 2Π3/2 ground electronic levels that has been the subject of previous theoretical interest, but the value of the tensor invariant that contributes to the intensity of the electronic Raman transitions has not been quantitatively determined. Analysis of our pure rotational CARS spectra indicates that the magnitude of this term is much smaller than indicated in previous work, but the analysis is complicated by the weakness of the electronic Raman transitions compared with pure rotational transitions in the same spectral region. Future experiments to more definitively determine the magnitude and sign of the polarizability tensor element are proposed.

High-resolution infrared spectroscopy of HCF in the CH stretch region.

We present the results from a high-resolution infrared study of jet-cooled singlet monofluorocarbene (HCF) in the CH stretch region near 2600 cm-1. Absorption signals are recorded using near quantum shot noise limited laser absorption methods. The fully resolved absorption spectra of the CH stretch (ν1) fundamental band and a partial progression of transitions of the HCF bend plus CF stretch (ν2 + ν3) combination band are observed and show clear evidence of a strong rovibrational coupling between the ν1Ka ' = 2 and ν2 + ν3Ka ' = 3 manifolds, including the observation of "dark state" transitions. A detailed perturbation analysis of a c-type Coriolis interaction is carried out for these two coupled vibrational states, providing experimental determination of precise rovibrational constants. A combined ground state combination difference fit of the transitions to the ν1 and ν2 + ν3 vibrational states in this study with previously reported LIF Ã(0,0,0) ← X̃(0,0,0) data has been done to increase the accuracy of the ground state rotational constants [M. Kakimoto et al., J. Mol. Spec. 88, 300-310 (1981)]. Moreover, we report, for the first time, hot band (ν1 + ν3 ← ν3) transitions due to vibrationally excited HCF in the CF stretch mode, ν3. The high-resolution results for all vibrational frequencies and rotational constants are in good agreement with and significantly extend the analysis of the rovibrational manifold of HCF. The present ground state and ν3 spectroscopic parameters now permit improved predictions for pure rotational and ν3 fundamental transitions to aid spectral searches for HCF in the laboratory and the interstellar medium.