INFRARED SPECTROSCOPY OF GAS-PHASE POLYCYCLIC AROMATIC HYDROCARBON CATIONS IN THE 10-50 {mu}m SPECTRAL RANGE

Journal Article · Thu Nov 10 04:00:00 EST 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/741/2/74; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)· OSTI ID:21612716

Bakker, Joost M; Redlich, Britta; Oomens, Jos ^[1]

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1. FOM Institute for Plasma Physics Rijnhuizen, P.O. Box 1207, 3430 BE Nieuwegein (Netherlands)

The gas-phase infrared spectra of four polycyclic aromatic hydrocarbon (PAH) cations have been recorded in the 10-50 {mu}m (or 1000-200 cm{sup -1}) spectral range via IR multiple photon dissociation (IRMPD) spectroscopy. Ionized PAHs are formed by UV laser ionization in an effusive beam and subsequently irradiated with a single pulse of narrowband tunable infrared light produced by the Free-Electron Laser for IntraCavity Experiments FELICE. The ion population is then analyzed in a time-of-flight mass spectrometer. Upon resonance, dissociation is induced so that IR spectra can be recorded by monitoring either the depleted parent ion intensity or the appearance of fragment ions as a function of the wavelength. The intracavity IR fluence enables the recording of IRMPD spectra of strongly bound PAH cations in the hitherto inaccessible far-IR spectral range. Experimental spectra are presented for the radical cations of anthracene, tetracene, pentacene, and coronene. Spectra calculated with density functional theory at the B3LYP/6-311g(2df,2pd) level reproduce IR frequencies reasonably accurately in this spectral range when a uniform scaling factor of 0.94 over the complete 10-50 {mu}m spectral range is employed. We show that even vibrational modes with a calculated IR intensity lower than 1 km mol{sup -1} can be observed. For the catacondensed PAH cations we find CH out-of-plane bending vibrations involving four adjacent CH groups within a few wavenumbers of 733 cm{sup -1}, closely matching the 13.6 {mu}m UIR band. For the larger systems, pentacene and coronene, we observe a continuous structureless background absorption above 400 cm{sup -1} which is attributed to the high density of IR dipole allowed combination modes for these systems.

• OSTI ID: 21612716
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 741; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION SPECTROSCOPY
ANTHRACENE
AROMATICS
ASTROPHYSICS
CALCULATION METHODS
CATIONS
CHARGED PARTICLES
CONDENSED AROMATICS
DENSITY FUNCTIONAL METHOD
DYNAMIC MASS SPECTROMETERS
FREE ELECTRON LASERS
HYDROCARBONS
INFRARED SPECTRA
IONIZATION
IONS
LASERS
MASS SPECTROMETERS
MEASURING INSTRUMENTS
ORGANIC COMPOUNDS
PENTACENE
PHYSICS
POLYCYCLIC AROMATIC HYDROCARBONS
SPECTRA
SPECTROMETERS
SPECTROSCOPY
TETRACENE
TIME-OF-FLIGHT MASS SPECTROMETERS
TIME-OF-FLIGHT SPECTROMETERS
VARIATIONAL METHODS