Single photon infrared emission spectroscopy of candidate interstellar PAHs.

摘要

The Unidentified Infrared Bands (UIRs) are a family of intense infrared (3–14μm) emission features emanating from regions of space where interstellar dust interacts with UV radiation. Arguments based on band positions and relative band intensities, have led to the hypothesis that the spectral carriers are polycyclic aromatic hydrocarbons (PAHs) and their ions.; Our laboratory's recent development of the SPIRES (Single Photon Infrared Emission Spectroscopy) technique has allowed us to monitor infrared emission from gas-phase, noninteracting, highly excited PAHs. Combining UV laser-induced desorption/excitation with a novel infrared detector capable of detecting single IR photons as long as 28μm, produced spectra which are directly comparable to astrophysical data. Initial results disproved the prevailing hypothesis that neutral PAHs are the primary contributors to the UIR bands.; In Chapter 1, I present an introduction to the UIR phenomenon and the development of the PAH hypothesis, focussing on details that are most readily addressable with the SPIRES technique. Chapter 2 is an overview of the SPIRES spectrometer, recent advances resulting in improved sensitivity, and details on the design and construction of a cryogenic PAH ion beam system.; In Chapter 3, I present new SPIRES spectra that support the hypothesis that partially hydrogenated neutral PAHs (H_n-PAHs) can be responsible for certain weak UIR features in the 3–4μm region, while making little contribution to other UIR features. I then present new results for amino-substituted PAHs and PAHs containing five-membered rings. The latter show promising trends with respect to relative band intensities, where the C=C stretch features in the 6–9μm region are significantly more intense than in the spectra of purely benzenoid PAHs.; The culmination of the SPIRES work to date is the first successful acquisition of the IR emission spectrum of gaseous pyrene cation. While not an excellent match to the astrophysical spectra, the relative band intensities are supportive of the connection between PAH cations and the UIRs. These results, therefore, generally support the conclusion the PAH cations are a major contributor to the UIR bands, although the specific details of molecular size, geometry, and functional groups remain to be understood.