Absorption spectra and photoionization of polycyclic aromatics in vacuum ultraviolet region

Abstract

The vapor phase absorption spectra of naphthalene, azulene, anthracene, phenanthrene, para-terphenyl and phenothiazine have been investigated in the near and vacuum ultraviolet wavelength regions. Further, the ionization potentials and the ionization cross sections of these compounds have been observed by a photoionization method. In the near ultraviolet wavelength region, the spectra of these compounds generally showed the π-π ∗ transition with maximum intensity, and having vibrational structures. In the vacuum ultraviolet wavelength region, however, a number of sharp and rather intense peaks belonging to Rydberg transitions appeared and were also accompanied with some vibrational satellites. These peaks fitted into several Rydberg series formulae. The adiabatic ionization potentials, IP A, were determined from the convergence limits of these Rydberg series; the values of IP A of naphthalene (I), anthracene (II), phenanthrene (III), and phenothiazine (IV) are 8.13 4, 7.41 0, 7.69 and 7.62 6 eV, respectively. The photoionization threshold values of (I), (II), (III), and (IV), on the other hand, are 8.14, 7.5, 7.75, and 7.7 eV, respectively. The detailed discussion presented for the typical case of azulene was applied for other compounds in connection with the photoionization phenomena observed in their efficiency curves.