Emission Spectra of Aromatic Hydrocarbons by Controlled Electron Impact

Abstract

Abstract The emission spectra of benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes, cymene, and naphthalene were investigated under controlled electron-impact excitation. These aromatic hydrocarbons showed a characteristic band in the 250–400 nm region and several bands of such excited fragment species as H and CH. The characteristic bands are similar to those of fluorescence spectra obtained by optical excitation and, thus, were assigned to fluorescence emissions of the parent molecules. The intensity of the fluorescence emission of benzene decreased at higher electron energies, and fragment emissions became dominant above 100 eV. However, its vibrational structure did not change for electron energies of 5–60 eV. This indicates that cascade processes from high-lying states are negligible for benzene. The fluorescence emission of naphthalene shifted to a shorter wavelength for low electron energies. Together with a theoretical estimation based on the Born approximation, cascade effects from higher excited states (S2 and S3) are found to be the major source of the fluorescence emission of naphthalene.