Far-Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbons: Zero Kinetic Energy Photoelectron Spectroscopy of Pentacene Vaporized from Laser Desorption

The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 {mu}m are ubiquitously seen in a wide variety of astrophysical environments. They are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, not a single PAH species has yet been identified...

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Bibliographic Details
Published in:The Astrophysical journal 2010-05, Vol.715 (1), p.485-492
Main Authors: Zhang, Jie, Han, Fangyuan, Pei, Linsen, Kong, Wei, Li, Aigen
Format: Article
Language:English
Subjects:
ABSORPTION
ABSORPTION SPECTROSCOPY
ARGON
AROMATICS
ASTRONOMY
ASTROPHYSICS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
CALCULATION METHODS
COLOR
CONDENSED AROMATICS
DENSITY FUNCTIONAL METHOD
Earth, ocean, space
ELECTRON SPECTROSCOPY
ELEMENTS
EMISSION
ENERGY LEVELS
Exact sciences and technology
EXCITED STATES
FLUIDS
FRANCK-CONDON PRINCIPLE
GASES
HYDROCARBONS
INFRARED SPECTRA
INTERMEDIATE STATE
IONIZATION
NONMETALS
OPTICAL PROPERTIES
ORGANIC COMPOUNDS
ORGANOLEPTIC PROPERTIES
PENTACENE
PHOTOELECTRON SPECTROSCOPY
PHOTOIONIZATION
PHYSICAL PROPERTIES
PHYSICS
POLYCYCLIC AROMATIC HYDROCARBONS
RARE GASES
SORPTION
SPECTRA
SPECTROSCOPY
VARIATIONAL METHODS
VIBRATIONAL STATES
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Summary:The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 {mu}m are ubiquitously seen in a wide variety of astrophysical environments. They are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, not a single PAH species has yet been identified in space, as the mid-IR vibrational bands are mostly representative of functional groups and thus do not allow one to fingerprint individual PAH molecules. In contrast, the far-IR (FIR) bands are sensitive to the skeletal characteristics of a molecule, hence they are important for chemical identification of unknown species. With an aim to offer laboratory astrophysical data for the Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy, and similar future space missions, in this work we report neutral and cation FIR spectroscopy of pentacene (C{sub 22}H{sub 14}), a five-ring PAH molecule. We report three IR active modes of cationic pentacene at 53.3, 84.8, and 266 {mu}m that may be detectable by space missions such as the SAFARI instrument on board SPICA. In the experiment, pentacene is vaporized from a laser desorption source and cooled by a supersonic argon beam. We have obtained results from two-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy photoelectron (ZEKE) spectroscopy. Several skeletal vibrational modes of the first electronically excited state of the neutral species and those of the cation are assigned, with the aid of ab initio and density functional calculations. Although ZEKE is governed by the Franck-Condon principle different from direct IR absorption or emission, vibronic coupling in the long ribbon-like molecule results in the observation of a few IR active modes. Within the experimental resolution of {approx}7 cm{sup -1}, the frequency values from our calculation agree with the experiment for the cation, but differ for the electronically excited intermediate state. Consequently, modeling of the intensity distribution is difficult and may require explicit inclusion of vibronic interactions.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/715/1/485