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Computational Spectroscopy of Polycyclic Aromatic Hydrocarbons In Support of Laboratory AstrophysicsPolycyclic aromatic hydrocarbons (PAHs) are strong candidates for the molecular carriers of the unidentified infrared bands (UIR) and the diffuse interstellar bands (DIBs). In order to test the PAH hypothesis, we have systematically measured the vibronic spectra of a number of jet-cooled neutral and ionized PAHs in the near ultraviolet (UV) to visible spectral ranges using the cavity ring-down spectroscopy. To support this experimental effort, we have carried out theoretical studies of the spectra obtained in our measurements. Ab initio and (time-dependent) density.functiona1 theory calculations are performed to obtain the geometries, energetics, vibrational frequencies, transition dipole moments, and normal coordinates of these PAH molecules. Franck-Condon (FC) calculations and/or vibronic calculations are then performed using the calculated normal coordinates and vibrational frequencies to simulate the vibronic spectra. It is found that vibronic interactions in these conjugated pi electron systems are often strong enough to cause significant deviations from the Born-Oppenheimer (BO) approximation. For vibronic transitions that are well described by the BO approximation, the vibronic band profiles are simulated by calculating the rotational structure of the vibronic transitions. Vibronic oscillator strength factors are calculated in the frame of the FC approximation from the electronic transition dipole moments and the FC factors. This computational effort together with our experimental measurements provides, for the first time, powerful tools for comparison with space-based data and, hence, a powerful approach to understand the spectroscopy of interstellar PAH analogs and the nature of the UIR and DIBs.
Document ID
20060015654
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Tan, Xiaofeng
(NASA Ames Research Center Moffett Field, CA, United States)
Salama, Farid
(NASA Ames Research Center Moffett Field, CA, United States)
Date Acquired
August 23, 2013
Publication Date
January 1, 2006
Subject Category
Astrophysics
Meeting Information
Meeting: NASA Laboratory Astrophysics Workshop
Location: Las Vegas, NV
Country: United States
Start Date: February 14, 2006
End Date: February 16, 2006
Funding Number(s)
PROJECT: RTOP 188-01-15
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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