Infrared Single Photon Counting Fluorescence Spectroscopy (IRSPCES) of UV Excited Naphthalene: The Mechanism of Interstellar Infrared Emission Bands

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Ohio State University

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1. L. J. <not clear>


Polycyclic aromatic hydrocarbons (PAH's) are hypothesized to be the carriers of the ubiquitous diffuse intersellar bands (DIB's) and the unidentified infrared bands (UIR's) $^{1, 2, 3, 4}$ which are found in interstellar dust clouds that are irradiated by starlight. The UIR's occur principally at 3.3, 3.4, 6.2, 7.7, 8.6 and $11.3 \mu m$. The mechanism for the IR emission is postulated to be UV excitation of electronic transitions followed by intermolecular energy transfer and subsequent IR fluorescence. A new experiment has been designed in order to test this hypothesis. A 5\% mixture of naphtalene in argoin is expanded through a pulsed nozzle into a vaccuum where it is intersected with either 193 nm or 248 nm radiation 20 nozzle diameters downstream from the orifice. IR fluorescence is collected with an f/7 lens and dispersed by a 0.5m monochromator. A blocked impurity band $detector^{2}$ is used to detect the radiation. This solid state detector employes a gain mechanism to produce a measurable discrere current pulse for single incident infrared photons. The fast time response of the detector and following photon-counting electronics allow observation times shorter than the time between gas kinetic collisions following the UV excitation pulse. Thus, collision free spectra are observed.
Author Institution: Department of Chemistry, University of California at Ber***ky; Department of Chemistry, Ma{\AA}y University; Inst<not clear>, Ma{\AA}y University; JILA, University of Colifornia