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SEARCH FOR THE 1.67 MU-M PAH EMISSION BAND - MORE UPPER LIMITS

In an attempt to further resolve the origin and excitation mechanism of the unidentified infrared emission bands, we have obtained high resolution spectroscopy in the H-band for three objects with strong 3.28 and 3.4 mu m emission features. In the scenario of highly vibrational excited, isolated, polycyclic aromatic hydrocarbons a broad band resonance at about 1.67 mu m with an intensity of I-3.4/I-1.67 less than or equal to 6 has been predicted, that should be detectable for our three objects: the Orion bar at position 4, He2-77 and the Red Rectangle. However, this prediction has not been confirmed by us. Our upper limits show that the intensity of the second harmonic should be at least 48 times weaker than the intensity of the 3.4 mu m C-H stretching mode. We conclude that both the hot band theory and the explanation that the excitation of the 3.4 mu m feature is due to molecular side groups is questionable. More laboratory data such as measurements of the first overtone emission of PAH structures like cyclopropylidene (C3H2) and phenanthereneare (C10H14) are needed.