Search

Polycyclic aromatic hydrocarbons (PAHs) are highly relevant for astrophysics as possible, though controversial, carriers of the unidentified infrared emission bands that are observed in a number of different astronomical objects. In support of radio-astronomical observations, high resolution laboratory spectroscopy has already provided the rotational spectra in the vibrational ground state of several molecules of this type, although the rotational study of their dense infrared (IR) bands has only recently become possible using a limited number of experimental set-ups. To date, all of the rotationally resolved data have concerned unperturbed spectra. We presently report the results of a high resolution study of the three lowest vibrational states of quinoline C9H7N, an N-bearing naphthalene derivative. While the pure rotational ground state spectrum of quinoline is unperturbed, severe complications appear in the spectra of the v45 and v44 vibrational modes (located at about 168 cm-1 and 178 cm-1, respectively). In order to study these effects in detail, we employed three different and complementary experimental techniques: Fourier-transform microwave spectroscopy, millimeter-wave spectroscopy, and Fourier-transform far-infrared spectroscopy with a synchrotron radiation source. Due to the high density of states in the IR spectra of molecules as large as PAHs, perturbations in the rotational spectra of excited states should be ubiquitous. Our study identifies for the first time this effect and provides some insights into an appropriate treatment of such perturbations. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) molecules are suspected to be present in the interstellar medium and to participate to the broad and unresolved emissions features, the so-called unidentified infrared bands. In the laboratory, very few studies report the rotationally resolved structure of such important class of molecules. In the present work, both experimental and theoretical approaches provide the first accurate determination of the rotational energy levels of two diazanaphthalene: [1,5]- and [1,6]-naphthyridine. [1,6]-naphthyridine has been studied at high resolution, in the microwave (MW) region using a Fourier transform microwave spectrometer and in the far-infrared (FIR) region using synchrotron-based Fourier transform spectroscopy. The very accurate set of ground state (GS) constants deduced from the analysis of the MW spectrum allowed the analysis of the most intense modes in the FIR (v38-GS centered at about 483 cm-1 and v34-GS centered at about 842 cm-1). In contrast with [1,6]-naphthyridine, pure rotation spectroscopy of [1,5]-naphthyridine cannot be performed for symmetry reasons so the combined study of the two intense FIR modes (v22-GS centered at about 166 cm-1 and v18-GS centered at about 818 cm-1) provided the GS and the excited states constants. Although the analysis of the very dense rotational patterns for such large molecules remains very challenging, relatively accurate anharmonic density functional theory calculations appeared as a highly relevant supporting tool to the analysis for both molecules. In addition, the good agreement between the experimental and calculated infrared spectrum shows that the present theoretical approach should provide useful data for the astrophysical models. Moreover, inertial defects calculated in the GS (ΔGS) of both molecules exhibit slightly negative values as previously observed for planar species of this molecular family. We adjusted the semi-empirical relations to estimate the zero-point inertial defect (Δ0) of polycyclic aromatic molecules and confirmed the contribution of low frequency out-of-plane vibrational modes to the GS inertial defects of PAHs, which is indeed a key parameter to validate the analysis of such large molecules. [ABSTRACT FROM AUTHOR]

The far infrared spectrum of cis-methyl formate has been recorded on the AILES beamline of the synchrotron SOLEIL using a Fourier transform infrared spectrometer coupled to a long path cell. The very weak fundamental band associated with the methyl-top torsion mode (ν18) was observed. The frequency analysis was performed using the 'rho axis method', and the microwave and millimeter-wave data from the literature. A precise determination of the band origins (ν18A = 132.4303 cm-1 and ν18E = 131.8445 cm-1) and of the barrier height [V3 = 370.7398 (58) cm-1] have been obtained. The intensity of the ν18 fundamental band was determined to be 3.4 × 10-21 cm-1/(molecule cm-2) at 297 K, equally shared among A-A and E-E transitions, thus leading to a dipole moment component μc(3) equal to 0.0483 D. The results were compared with the ab initio calcula-tions of Senent et al. [Astrophys. J. 627, 567 (2005)]. [ABSTRACT FROM AUTHOR]

We present the first rotationally resolved spectra of adamantane (C10H16) applying gas-phase Fourier transform infrared (IR) absorption spectroscopy. High-resolution IR spectra are recorded in the 33-4500 cm-1range using as source of IR radiation both synchrotron radiation (at the AILES beamline of the SOLEIL synchrotron) as well as a classical globar. Adamantane is a spherical top molecule with tetrahedral symmetry (Td point group) and has no permanent dipole moment in its vibronic ground state. Of the 72 fundamental vibrational modes in adamantane, only 11 are IR active. Here we present rotationally resolved spectra for seven of them: ν30, ν28, ν27, ν26, ν25, ν24, and ν23. The typical rotational structure of spherical tops is observed and analyzed using the STDS software developed in the Dijon group, which provides the first accurate energy levels and rotational constants for seven fundamental modes. Rotational levels with quantum numbers as high as J = 107 have been identified and included in the fit leading to a typical standard deviation of about 10-3 cm-1. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) and their N-substituted derivatives are among the largest species for which gas-phase high-resolution spectroscopy can be performed nowadays. In this paper we report the observation and analysis of spectra from several N-substituted two-ring PAHs, all taken in the "fingerprint" far-infrared region (<850 cm-1). Together with accurate measurements of their pure rotational transitions in the millimeter and submillimeter ranges, these synchrotron-based Fourier transform infrared (FTIR) measurements provide an accurate description of the rotational energy levels in the ground and low-energy excited vibrational states of these species. To complement the experimental data, anharmonic DFT calculations were performed to obtain relatively accurate rotational and vibrational parameters. The calculated results strongly support the rotational analysis and provide a good estimate of the equilibrium structures for each species. Extended measurements, analysis, and calculations are presented here for the far-IR bands of quinoline (C9H7N), isoquinoline (C9H7N), quinoxaline (C8H6N2), quinazoline (C8H6N2), [1,5]-naphthyridine (C8H6N2), [1,6]-naphthyridine (C8H6N2), and indole (C8H7N) molecules. [ABSTRACT FROM AUTHOR]

Six Fourier-transform FIR spectraof the NH2radicalhave been recorded at high resolution (0.001 cm–1) using synchrotron radiation on the AILES beamline at SOLEIL Synchrotron.Three different experimental discharge setups have been used to observe,in absorption, 1009 pure rotational transitions of NH2inthe vibrational ground state (000) and 170 pure rotational transitionswithin the first excited vibrational state (010). These results constitutea significant extension of the observed quantum numbers for thesetwo states. The spectra permitted several couplings to be resolved(asymmetric coupling, spin-rotation coupling, hyperfine structure)for relatively highly excited energy levels. An effective fit hasbeen realized using both standard Watson-S and -A reductions despitean abnormal centrifugal distortion effect for this light hydride. [ABSTRACT FROM AUTHOR]

Context. Hexamethylenetetramine (HMT) appears to be a potential constituent of several objects in space, including comets or Titan's atmosphere and, as an organic residue of ice irradiation in the laboratory, it may be present in the interstellar medium. Aims. We performed a laboratory study of rotationally resolved intense IR bands of HMT to provide accurate line positions and synthetic spectra to be used for potential astronomical detections. Methods. We used synchrotron-based high-resolution Fourier transform infrared spectroscopy to record the experimental data. A formalism and programs dedicated to the assignment, analysis, and simulation of absorption spectra of tetrahedral molecules were used to exploit the spectra. Results. Infrared spectra of gas phase HMT were recorded and accurate wavenumbers and molecular parameters for four intense bands located in the 1000-1500 cm-1 spectral range suitable for astronomical searches were derived. [ABSTRACT FROM AUTHOR]

Context. Hexamethylenetetramine (HMT) appears to be a potential constituent of several objects in space, including comets or Titan's atmosphere and, as an organic residue of ice irradiation in the laboratory, it may be present in the interstellar medium. Aims. We performed a laboratory study of rotationally resolved intense IR bands of HMT to provide accurate line positions and synthetic spectra to be used for potential astronomical detections. Methods. We used synchrotron-based high-resolution Fourier transform infrared spectroscopy to record the experimental data. A formalism and programs dedicated to the assignment, analysis, and simulation of absorption spectra of tetrahedral molecules were used to exploit the spectra. Results. Infrared spectra of gas phase HMT were recorded and accurate wavenumbers and molecular parameters for four intense bands located in the 1000-1500 cm-1 spectral range suitable for astronomical searches were derived. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Physics is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Far infrared (FIR) gas phase absorption spectra of azulene, quinoline and isoquinoline have been recorded using a Fourier transform Bruker IFS125 interferometer at medium resolution (0.5cm−1). Assignments of these weak vibrational bands were performed using density-functional theory calculations carried out at the harmonic and anharmonic levels. Agreement between observed and calculated band positions is better than 5% for azulene and 1% for quinoline and isoquinoline. The relative band intensities are also correctly predicted. Molecular structure dependence of the FIR spectra is discussed based on the comparative study of three selected FIR modes. [Copyright &y& Elsevier]

We report the pure rotation spectrum of HN2+ as measured by Fourier transform (FT) absorption spectroscopy in the 20-40 cm−1 spectral range. The cations are produced in a liquid nitrogen cooled hollow cathode discharge cell developed on the AILES beamline of synchrotron SOLEIL. The setup was optimized by recording rotation-vibration spectra of H3+ (with the ν2 band centered at about 2521 cm−1), HN2+ (with the ν1 band centered at about 3234 cm−1), and HCO+ (with the ν1 band centered at about 3089 cm−1). Many rotation-vibration lines have been assigned for each ion and five pure rotational transitions have been detected for HN2+. These results demonstrate the feasibility of recording far-infrared spectra of cationic species using FT broad band spectroscopy associated with the bright synchrotron radiation continuum as an alternative to laser-based frequency tunable techniques. [ABSTRACT FROM AUTHOR]

Abstract: Pure rotational transitions of OH and SH radicals have been recorded in the THz spectral range using cw-THz and synchrotron-based FT-FIR techniques. Line lists on these radicals have been completed in the three and two lowest vibrational states for OH and SH, respectively. Furthermore, the hyperfine structure of OH and SH has been observed for the first time using infrared IR FT-spectroscopy, and at frequencies higher than 1THz, respectively. A combined fit has been made for each of these radicals including v =0, 1 and 2 for OH and v =0 and 1 for SH. [Copyright &y& Elsevier]

Abstract: The far infrared spectrum of S-(-)-perillaldehyde, a monoterpene containing an aldehyde functional group, has been recorded in the gas phase using FTIR spectroscopy. The vibration signature of the three most populated rotamers has been observed and identified in the 30–650cm−1 range. The vibration assignment was based on the scaled B3LYP/cc-pVDZ harmonic force field of Partal Ureña et al., 2008 . Anharmonic contributions calculated at the HF/6–31+G⁎ level were found negligible. [Copyright &y& Elsevier]

CF 4 , or tetrafluoromethane, is a chemically inert and strongly absorbing greenhouse gas, mainly of anthropogenic origin. In order to monitor and reduce its atmospheric emissions and concentration, it is thus necessary to obtain an accurate model of its infrared absorption. Such models allow opacity calculations for radiative transfer atmospheric models. In the present work, we perform a global analysis (divided into two distinct fitting schemes) of 17 rovibrational bands of CF 4 . This gives a reliable model of many of its lower rovibrational levels and allows the calculation of the infrared absorption in the strongly absorbing ν 3 region (1283 cm − 1 / 7.8 μm), including the main hot band, namely ν 3 + ν 2 − ν 2 as well as ν 3 + ν 1 − ν 1 ; we could also extrapolate the ν 3 + ν 4 − ν 4 absorption. This represents almost 92% of the absorption at room temperature in this spectral region. A new accurate value of the C–F bond length is evaluated to r e = 1.314860 ( 21 ) Å. The present results have been used to update the HITRAN, GEISA and TFMeCaSDa (VAMDC) databases. [ABSTRACT FROM AUTHOR]

The fundamental vibrational bending mode ν 2 of disulfur monoxide, S 2 O, and the associated hot band 2 ν 2 - ν 2 have been observed at high spectral resolution for the first time at the SOLEIL synchrotron facility using Fourier-transform far-infrared spectroscopy. This transient species has been produced in a radio-frequency discharge by flowing SO 2 over elemental sulfur. The spectroscopic analysis has been performed using the newly developed Automated Spectral Assignment Procedure (ASAP) which has enabled the accurate determination of more than 3500 energy levels of the v 2 = 1 and 2 vibrational states. The procedure provides a fast and convenient way to analyze large data sets in a straightforward manner, if one of the two vibrational states involved in the transition is accurately known from prior work. In addition to the high-resolution synchrotron study, pure rotational spectra of S 2 O in the v 2 = 1 and 2 vibrational states were observed in the frequency range 250–500 GHz by absorption spectroscopy in a long-path absorption cell. From these combined measurements, extensive molecular parameter sets have been determined, including full sets of sextic and two octic centrifugal distortion terms. Highly accurate band centers (to better than 10 - 5 cm −1 ) have been derived for both vibrational bands. [ABSTRACT FROM AUTHOR]