INFRARED SPECTROSCOPY OF GAS-PHASE POLYCYCLIC AROMATIC HYDROCARBON CATIONS IN THE 10-50 {mu}m SPECTRAL RANGE
- 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, Vol. 741, Issue 2; Other Information: DOI: 10.1088/0004-637X/741/2/74; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION SPECTROSCOPY
ANTHRACENE
ASTROPHYSICS
CATIONS
DENSITY FUNCTIONAL METHOD
FREE ELECTRON LASERS
INFRARED SPECTRA
IONIZATION
PENTACENE
POLYCYCLIC AROMATIC HYDROCARBONS
TETRACENE
TIME-OF-FLIGHT MASS SPECTROMETERS
AROMATICS
CALCULATION METHODS
CHARGED PARTICLES
CONDENSED AROMATICS
DYNAMIC MASS SPECTROMETERS
HYDROCARBONS
IONS
LASERS
MASS SPECTROMETERS
MEASURING INSTRUMENTS
ORGANIC COMPOUNDS
PHYSICS
SPECTRA
SPECTROMETERS
SPECTROSCOPY
TIME-OF-FLIGHT SPECTROMETERS
VARIATIONAL METHODS