Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime.

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10-100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Synthesis, High-Resolution Infrared Spectroscopy, and Vibrational Structure of Cubane, C8H8.

Carbon-cage molecules have generated a considerable interest from both experimental and theoretical points of view. We recently performed a high-resolution study of adamantane (C10H16), the smallest hydrocarbon cage belonging to the diamandoid family ( Pirali , O. ; et al. J. Chem. Phys. 2012 , 136 , 024310 ). There exist another family of hydrocarbon cages with additional interesting chemical properties: the so-called platonic hydrocarbons that comprise dodecahedrane (C20H20) and cubane (C8H8). Both possess C-C bond angles that deviate from the tetrahedral angle (109.8°) of the sp(3) hybridized form of carbon. This generates a considerable strain in the molecule. We report a new wide-range high-resolution study of the infrared spectrum of cubane. The sample was synthesized in Bari upon decarboxylation of 1,4-cubanedicarboxylic acid thanks to the improved synthesis of literature. Several spectra have been recorded at the AILES beamline of the SOLEIL synchrotron facility. They cover the 600-3200 cm(-1) region. Besides the three infrared-active fundamentals (ν10, ν11, and ν12), we could record many combination bands, all of them displaying a well-resolved octahedral rotational structure. We present here a preliminary analysis of some of the recorded bands, performed thanks the SPVIEW and XTDS software, based on the tensorial formalism developed in the Dijon group. A comparison with ab initio calculations, allowing to identify some combination bands, is also presented.

High-Resolution Far-Infrared Spectroscopy of N-Substituted Two-Ring Polycyclic Aromatic Hydrocarbons: An Extended Study.

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.

Rotation-vibration interactions in the spectra of polycyclic aromatic hydrocarbons: Quinoline as a test-case species.

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 ν45 and ν44 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.

High resolution measurements supported by electronic structure calculations of two naphthalene derivatives: [1,5]- and [1,6]-naphthyridine--estimation of the zero point inertial defect for planar polycyclic aromatic compounds.

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 (ν38-GS centered at about 483 cm(-1) and ν34-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 (ν22-GS centered at about 166 cm(-1) and ν18-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.