Studies of Structural Isomers o-, m-, and p-Fluorophenylacetylene by Two-Color Resonant Two-Photon Mass-Analyzed Threshold Ionization Spectroscopy

We report the vibrational spectra of o-fluorophenylacetylene (OFPA), m-fluorophenylacetylene (MFPA), and p-fluorophenylacetylene (PFPA) in the electronically excited S-1 and cationic ground D-0 states. These new data show that the relative location of the fluorine atom with respect to the acetylenic group can influence the transition energy and molecular vibration. The adiabatic ionization energies of these structural isomers follow the order: PFPA < OFPA < MFPA. It is found that the molecular geometries of these molecules in the D-0 state resemble those in the S-1 state. Detailed spectral analysis suggests that the in-plane ring deformation vibrations are slightly harder in the Do state than the corresponding ones in the S-1 state.