Knowledge Bank

The electronic spectra of large polyatomic molecules can be studied to great advantage in substitutional solid solutions at low temperatures (20$\circ$K). The results of such studies on azulene, naphthalene, naphthalene d-8, anthracene and naphthacene will be presented. The first azulene transition is short axis polarized, the second long-axis polarized, the first and second of naphthalene are long and short, respectively, and the first transitions of anthracene and naphthacene are short axis polarized. Emission from the second excited azulene state to the ground state was observed. All spectra are extremely sharp, and accurate values of vibration frequencies in the combining electronic states can be deduced. The extent of vibrational-electronic interaction was ascertained in each transition. There are many similarities among the vibrational patterns of the absorption and emission spectra of the different molecules. The vibrational structure of the spectra reveal semiquantitatively what changes in electron distribution from the ground state have taken place upon excitation.