O. Pirali

Performance of the AILES THz‐Infrared beamline at SOLEIL for High resolution spectroscopy

The new THz beamline (AILES) located at the third generation Synchrotron Radiation source SOLEIL is now operating for applications in a wide variety of research themes. In particular, this source with its adapted optics allows high resolution spectroscopic measurements of molecules in the entire infrared and THz range. This presentation focuses on the performances concerning flux, spectral range and stability for molecular spectroscopy. Thanks to these performances, the coupling of synchrotron radiation from a highly stable third generation source with high resolution FTIR spectrometer and with a long path cell (150 m or more) can be particularly advantageous. This fact is related to the optics of the beamline permitting the entire source to be used without aperture stop (entrance iris), even for measurements at highest‐resolution of ∼0.1 μeV (10−3u2009cm−1).

TOWARD A UNIQUE NITROGEN ISOTOPIC RATIO IN COMETARY ICES

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar systems origin. We unambiguously identified emission lines in comets due to the 15NH2 radical produced by the photodissociation of 15NH3. Analysis of our data has permitted us to measure the 14N/15N isotopic ratio in comets for a molecule carrying the amine (-NH) functional group. This ratio, within the error, appears similar to that measured in comets in the HCN molecule and the CN radical, and lower than the protosolar value, suggesting that N2 and NH3 result from the separation of nitrogen into two distinct reservoirs in the solar nebula. This ratio also appears similar to that measured in Titans atmospheric N2, supporting the hypothesis that, if the latter is representative of its primordial value in NH3, these bodies were assembled from building blocks sharing a common formation location.

Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium

Although they are relatively different in band shape, infrared features around 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 and in the absorption spectra of various dense clouds have both been attributed to diamondoid molecules/particles. This assignment is based mainly on infrared spectra of hydrogenated diamond thin films and of diamond nanocrystals of known average size. Here we present an analysis of the astrophysical implications of recently reported solid-state 2.5-12.5 μm spectra of individual diamondoid molecules, up to the size of hexamantane (C26H30). These spectra provide the first experimental measurements of the infrared frequencies of this class of molecules. In addition, laboratory gas-phase infrared emission spectra of the three smallest members of the diamondoid family are reported, as well as theoretical spectra for some larger species. The present data set allows us to relate spectral signatures to the molecular size and structure. The spectra of tetrahedral diamondoids are found to be qualitatively different from those of lower symmetry species, which possibly explains the differences between the astrophysical emission and absorption spectra. Interestingly, the 3.53 μm band is clearly observed in the spectra of these small molecular diamondoids, whereas previous studies on nanodiamond particles found this band only for species larger than ≈50 nm. Our results support the assignment of the 3.43 and 3.53 μm emission features in HD 97048 and Elias 1 to diamondoids of a few nanometers in size as well as the suggestion that smaller diamondoid molecules contribute to the 3.47 μm interstellar absorption band.

Submillimeter-wave and far-infrared spectroscopy of high-J transitions of the ground and ν2=1 states of ammonia

Complete and reliable knowledge of the ammonia spectrum is needed to enable the analysis and interpretation of astrophysical and planetary observations. Ammonia has been observed in the interstellar medium up to J=18 and more highly excited transitions are expected to appear in hot exoplanets and brown dwarfs. As a result, there is considerable interest in observing and assigning the high J (rovibrational) spectrum. In this work, numerous spectroscopic techniques were employed to study its high J transitions in the ground and ν(2)=1 states. Measurements were carried out using a frequency multiplied submillimeter spectrometer at Jet Propulsion Laboratory (JPL), a tunable far-infrared spectrometer at University of Toyama, and a high-resolution Bruker IFS 125 Fourier transform spectrometer (FTS) at Synchrotron SOLEIL. Highly excited ammonia was created with a radiofrequency discharge and a dc discharge, which allowed assignments of transitions with J up to 35. One hundred and seventy seven ground state and ν(2)=1 inversion transitions were observed with microwave accuracy in the 0.3-4.7 THz region. Of these, 125 were observed for the first time, including 26 ΔK=3 transitions. Over 2000 far-infrared transitions were assigned to the ground state and ν(2)=1 inversion bands as well as the ν(2) fundamental band. Of these, 1912 were assigned using the FTS data for the first time, including 222 ΔK=3 transitions. The accuracy of these measurements has been estimated to be 0.0003-0.0006u2002cm(-1). A reduced root mean square error of 0.9 was obtained for a global fit of the ground and ν(2)=1 states, which includes the lines assigned in this work and all previously available microwave, terahertz, far-infrared, and mid-infrared data. The new measurements and predictions reported here will support the analyses of astronomical observations by high-resolution spectroscopy telescopes such as Herschel, SOFIA, and ALMA. The comprehensive experimental rovibrational energy levels reported here will permit further refinement of the potential energy surface to improve ammonia ab initio calculations and facilitate assignment of new high-resolution spectra of hot ammonia.

Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

Gas-phase infrared spectra of several ionized nitrogen substituted polycyclic aromatic hydrocarbons (PANHs) have been recorded in the 600–1600 cm −1 region via IR multiple-photon dissociation (IRMPD) spectroscopy. The UV photoionized PANH ions are trapped and isolated in a quadrupole ion trap where they are irradiated with an IR free electron laser. The PANHs were studied in their radical cation (PANH + ) and protonated (H + PANH) forms, and include quinoline, isoquinoline, phenanthridine, benzo[h]quinoline, acridine, and dibenzo[f,h]quinoline. Experimental IRMPD spectra were interpreted with the aid of density functional theory methods. The PANH + IR spectra are found to resemble those of their respective non-nitrogenated PAH cations. The IR spectra of H + PANHs are significantly different owing to the NH inplane bending vibration, which generally couples very well with the aromatic CH bending and CC stretching modes. Implications of the NPAH (+ ,H + ) laboratory spectra are discussed for the astrophysical IR emissions and, in particular, for the band at 6.2 μm.

Gas-Phase Vibrational Spectroscopy and Ab Initio Study of Organophosphorous Compounds: Discrimination between Species and Conformers

Gas phase vibrational spectra of dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), and triethyl phosphate (TEP) have been measured using FTIR spectroscopy. For DMMP, TMP, and TEP, most of the infrared active vibrational modes have been observed in the 50-5000 cm (-1) spectral range, allowing an unambiguous discrimination between the three molecules. The vibrational analysis of the spectra was performed by comparing with MP2 and B3LYP harmonic and anharmonic force field ab initio calculations. The extension to anharmonic calculations provides the best agreement for the mid-infrared and the near-infrared spectra, but they do not improve the harmonic frequency predictions in the far-infrared domain. This part of the vibrational spectra associated with collective and nonlocalized vibrational modes presents the largest frequency differences between the two lowest energy conformers of DMMP and TMP. These two conformers were taken into account in the vibrational assignment of the spectra. Their experimental evidence was obtained by deconvoluting vibrational bands in the mid-infrared and in the far-infrared regions, respectively. For TEP, the conformational landscape appears very complicated at ambient temperature, and a further analysis at low temperature is required to explain the vibrational features of each conformer.

Rotationally resolved infrared spectroscopy of adamantane

We present the first rotationally resolved spectra of adamantane (C(10)H(16)) applying gas-phase Fourier transform infrared (IR) absorption spectroscopy. High-resolution IR spectra are recorded in the 33-4500 cm(-1)range 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 (T(d) 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: ν(30), ν(28), ν(27), ν(26), ν(25), ν(24), and ν(23). 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).

The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations

Using synchrotron radiation, we performed the rotationally resolved Fourier transform infrared absorption spectroscopy of three bands of naphthalene C10H8, namely ν(46)-0 (centered at 782 cm(-1), 12.7 μm), ν(47)-0 (centered at 474 cm(-1), 21 μm), and ν(48)-0 (centered at 167 cm(-1), 60 μm). The intense CH bending out of plane ν(46)-0 band was recorded under supersonic jet-cooled conditions using a molecular beam (the Jet-AILES apparatus) and the low frequency ν(47)-0 and ν(48)-0 bands were measured at room temperature in a long absorption path cell. The simultaneous rotational analysis of these bands permitted us to refine the ground state (GS) and ν(46) rotational spectroscopic constants and to provide the first sets of constants for the ν(47) and ν(48) modes. The experimental rotational constants were then used as reference data to calibrate theoretical models in order to provide new insights into the accuracy of anharmonic calculations. The B97-1 functional associated with the cc-pVTZ and ANO-RCC basis sets gave a consistent set of results, for rotational constants and fundamental frequencies. The data presented here pave the way for the search of naphthalene through its far-infrared spectrum in different objects of the interstellar medium.

Global analysis of the high temperature infrared emission spectrum of 12CH4 in the dyad (ν2/ν4) region

We report new assignments of vibration-rotation line positions of methane ((12)CH4) in the so-called dyad (ν2/ν4) region (1100-1500 cm(-1)), and the resulting update of the vibration-rotation effective model of methane, previously reported by Nikitin et al. [Phys. Chem. Chem. Phys. 15, 10071 (2013)], up to and including the tetradecad. High resolution (0.01 cm(-1)) emission spectra of methane have been recorded up to about 1400 K using the high-enthalpy source developed at Institut de Physique de Rennes associated with the Fourier transform spectrometer of the SOLEIL synchrotron facility (AILES beamline). Analysis of these spectra allowed extending rotational assignments in the well-known cold band (dyad-ground state (GS)) and related hot bands in the pentad-dyad system (3000 cm(-1)) up to Jmax = 30 and 29, respectively. In addition, 8512 new transitions belonging to the octad-pentad (up to J = 28) and tetradecad-octad (up to J = 21) hot band systems were successfully identified. As a result, the MeCaSDa database of methane was significantly improved. The line positions assigned in this work, together with the information available in the literature, were fitted using 1096 effective parameters with a dimensionless standard deviation σ = 2.09. The root mean square deviations dRMS are 3.60 × 10(-3) cm(-1) for dyad-GS cold band, 4.47 ×10(-3) cm(-1) for the pentad-dyad, 5.43 × 10(-3) cm(-1) for the octad-pentad, and 4.70 × 10(-3) cm(-1) for the tetradecad-octad hot bands. The resulting new line list will contribute to improve opacity and radiative transfer models for hot atmospheres, such as those of hot-Jupiter type exoplanets.

Photoionization of cold gas phase coronene and its clusters: Autoionization resonances in monomer, dimer, and trimer and electronic structure of monomer cation

Polycyclic aromatic hydrocarbons (PAHs) are key species encountered in a large variety of environments such as the Interstellar Medium (ISM) and in combustion media. Their UV spectroscopy and photodynamics in neutral and cationic forms are important to investigate in order to learn about their structure, formation mechanisms, and reactivity. Here, we report an experimental photoelectron-photoion coincidence study of a prototypical PAH molecule, coronene, and its small clusters, in a molecular beam using the vacuum ultraviolet (VUV) photons provided by the SOLEIL synchrotron facility. Mass-selected high resolution threshold photoelectron (TPES) and total ion yield spectra were obtained and analyzed in detail. Intense series of autoionizing resonances have been characterized as originating from the monomer, dimer, and trimer neutral species, which may be used as spectral fingerprints for their detection in the ISM by VUV absorption spectroscopy. Finally, a full description of the electronic structure of the monomer cation was made and discussed in detail in relation to previous spectroscopic optical absorption data. Tentative vibrational assignments in the near-threshold TPES spectrum of the monomer have been made with the support of a theoretical approach based on density functional theory.