INFRARED-EMISSION SPECTRA OF BENZENE AND NAPHTHALENE - IMPLICATIONS FOR THE INTERSTELLAR POLYCYCLIC AROMATIC HYDROCARBON HYPOTHESIS

Emissions from the fundamental region (approximately 3050 cm-1, 3.3-mu-m) and first overtone (approximately 6000 cm-1, 1.7-mu-m) of the C-H stretching modes in the small aromatic hydrocarbons benzene (C6H6) and naphthalene (C10H8) were observed following ultraviolet laser excitation. These two molecules respectively represent the "prototype" and smallest members of the polycyclic aromatic hydrocarbon (PAH) chemical family, proposed as likely carriers of a set of infrared features widely observed in various dust-containing astronomical sources. Wavelength- and time-resolved emission spectra in the 3050 cm-1 region show contributions from anharmonically shifted DELTA-v = -1 transitions originating in v = 1, 2, and 3 in benzene and v = 1 and 2 in naphthalene, as well as underlying continuum emission. The measured anharmonicities agree with literature values, and relative intensities match those predicted by theory for DELTA-v = -1 transitions. The emission transition frequencies differ significantly from the corresponding absorption frequencies for both molecules, and depend on the vibrational energy of the emitter. Thus, unambiguous identification of the interstellar emitters cannot be established just on the basis of matching the emission frequencies to laboratory absorption spectra. The laboratory emission spectra were used to calculate the spectra expected under conditions prevailing in the interstellar medium (ISM). The overtone emission measurements were used to predict the intensity of a weak 5980 cm-1 overtone feature which should be present in the interstellar spectra according to the PAH hypothesis.