VUV Absorption Spectra of Gas-Phase Quinoline in the 3.5-10.7 eV Photon Energy Range

The absorption spectrum of quinoline was measured in the gas phase between 3.5 and 10.7 eV using a synchrotron photon source. A large number of sharp and broad spectral features were observed, some of which have plasmon-type collective π-electron modes contributing to their intensities. Eight valence electronic transitions were assigned, considerably extending the number of π-π* transitions previously observed mainly in solution. The principal factor in solution red-shifts is found to be the Lorentz-Lorenz polarizability parameter. Rydberg bands, observed for the first time, are analyzed into eight different series, converging to the D₀ ground and two excited electronic states, namely, D₃ and D₄, of the quinoline cation. The R1 series limit is 8.628 eV for the first ionization energy of quinoline, a value more precise than previously published. This value, combined with cation electronic transition data, provides precise energies, respectively, 10.623 and 11.355 eV, for the D₃ and D₄ states. The valence transition assignments are based on density functional theory (DFT) calculations as well as on earlier Pariser-Parr-Pople (P-P-P) self-consistent field linear combination of atomic orbitals molecular orbital results. The relative quality of the P-P-P and DFT data is discussed. Both are far from spectroscopic accuracy concerning electronic excited states but were nevertheless useful for our assignments. Our time-dependent DFT calculations of quinoline are excellent for its ground-state properties such as geometry, rotational constants, dipole moment, and vibrational frequencies, which agree well with experimental observations. Vibrational components of the valence and Rydberg transitions mainly involve C-H bend and C═C and C═N stretch modes. Astrophysical applications of the vacuum UV absorption of quinoline are briefly discussed.