The absorption spectra of neutral pentacene (C 22 H 14 ) and its radical cation (C 22 H 14 + ) and anion (C 22 H 14 -) isolated in solid Ne, Ar, and Kr have been measured from the ultraviolet to the near-infrared. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing molecular anion production in matrices doped with alkali metal (Na or K). TD-DFT calculations were carried out to assist in the assignments for the measured spectral features of the pentacene ions.

The absorption spectra of neutral pentacene (C 22 H 14 ) and its radical cation (C 22 H 14 + ) and anion (C 22 H 14 -) isolated in solid Ne, Ar, and Kr have been measured from the ultraviolet to the near-infrared. The associated vibronic band systems and their spectroscopic assignments are discussed together with the physical and chemical conditions governing molecular anion production in matrices doped with alkali metal (Na or K). TD-DFT calculations were carried out to assist in the assignments for the measured spectral features of the pentacene ions.

Abstract. Polycyclic Aromatic Hydrocarbons (PAHs) are thought to be the carriers of the ubiquitous infrared emission bands (UIBs). Data from the Infrared Space Observatory (ISO) have provided new insights into the size distribution and the structure of interstellar PAH molecules pointing to a trend towards larger-size PAHs. The mid-infrared spectra of galactic and extragalactic sources have also indicated the presence of 5-ring structures and PAH structures with attached side groups. This paper reports for the first time the laboratory measurement of the UV-Vis-NIR absorption spectra of a representative set of large PAHs that have also been selected for a long duration exposure experiment on the International Space Station ISS. PAHs with sizes up to 600 amu, including 5-ring species and PAHs containing heteroatoms, have been synthesized and their spectra measured using matrix isolation spectroscopy. The spectra of the neutral species and the associated cations and anions measured in this work are also compared to astronomical spectra of Diffuse Interstellar Bands (DIBs).

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It has been demonstrated that sufficient numbers of mass-selected cations can be directly deposited into growing inert gas matrices at low temperatures for spectroscopic analysis. However, the mechanism that must exist to maintain electrical neutrality in the matrix is not understood. We report here the direct observation of counterions in matrices formed during the codeposition of mass-selected cations and neon. The addition of carbon dioxide to the neon matrix gas during deposition of a mass-selected cation beam results in detectable quantities of CO2 •-. The direct measurement of negative charges, as positive ions impinge on a cold target, provides insights into the mechanism through which charge neutrality is maintained in this experiment. Negative charge formation is strongly dependent on the temperature of the metallic surfaces surrounding the matrix window and the presence of condensable gases on these surfaces.

Sufficient quantities of mass-selected cations have been isolated in inert matrices for vibrational spectroscopic observation for the first time. When 15-25-nA beams of CF3+ from CF3C1, CFsBr, or CF3H are co-deposited for 10-25 h with neon or argon at 5 K, the antisymmetric stretching vibration (u3) of the cation can be observed by FTIR spectroscopy. The mechanism by which the matrix maintains the necessary approximate neutrality is not certain; however, both positive and negative charges are detected when the matrices containing massselected cations are allowed to warm.

Vibrational spectroscopic characterization of neon matrices in which mass-selected CS2 •+ and CS•+ were deposited reveals absorptions due to (ν3) CS2 •+ (1207.1 cm-1) and (ν3) CS2 •- (1159.4 cm-1). The results are compared to previous CO2 •+ studies from this laboratory [Godbout et al. J. Phys. Chem. 1996, 100, 2892]. We also report controlled annealing studies of the matrices in which clustering of ionic species with neutral molecules is observed.

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