Far-Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbons: Zero Kinetic Energy Photoelectron Spectroscopy of Pentacene Vaporized from Laser Desorption
arXiv:1210.6856 · doi:10.1088/0004-637X/715/1/485
Abstract
The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3μm 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_22H_14), a five-ring PAH molecule. We report three IR active modes of cationic pentacene at 53.3, 84.8, and 266μm 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.
27 pages, 5 figures, 4 tables; published in ApJ
References in corpus (10)
- Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
- Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
- The Mid-Infrared Spectrum of Star-Forming Galaxies: Global Properties of PAH Emission
- Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
- Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
- Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump
- Mid-Infrared IRS Spectroscopy of NGC 7331: A First Look at the SINGS Legacy
- Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle
- PAHs in Comets: An Overview
- Polycyclic aromatic hydrocarbons with SPICA