Large amplitude motion in 9-methylanthracene: High-resolution spectroscopy and Ab Initio theoretical calculation

Cavity Ring-Down Spectroscopy of Anthracene, 9-Methylanthracene, and 2-Methylanthracene in Supersonic Expansion

The S₀-S₁ absorption spectra of anthracene (C₁₄H₁₀), 9-methylanthracene (C₁₅H₁₂), and 2-methylanthracene (C₁₅H₁₂) are measured in the ultraviolet region between 330 and 375 nm (26,666 to 30,303 cm^-1) with cavity ring-down spectroscopy in supersonic free-jet expansions of argon. The associated vibronic band systems and their spectroscopic assignments are discussed and compared to previous studies performed using fluorescence excitation and dispersed fluorescence techniques. Density functional theory (DFT) calculations were carried out to study the structures and evaluate the vibrational transitions of the ground and excited states. Time-dependent DFT calculations of the first electronic excited states and Franck-Condon factor calculations were carried out to assist in the assignment of the experimentally measured vibronic bands. The vibronic spectra obtained in absorption agree well with fluorescence excitation spectra in terms of peak positions but exhibit different relative band intensities. We find a very good match between experimentally obtained vibronic line positions and the peak positions of the quantum chemically calculated Franck-Condon excitation lines.