Abstract
The resonant two-photon ionization and mass analyzed threshold ionization spectroscopic techniques have been used to study the molecular properties of (1) deuterium-substituted isotopomers of aniline and indole derivatives, (2) rotational isomers of o-dimethoxybenzene and m-dimethoxybenzene, and (3) positional isomers of fluoroindole. These new vibronic and cation spectra provide information about (1) origin of the S1 □ S0 electronic transition, (2) adiabatic ionization energy, (3) active vibrations in the electronically excited S1 and cationic D0 states. The ab initio and density functional theory calculations are also performed to support our experimental findings. The present results on aniline derivatives show that the N-deuteration leads to a small decrease in the frequencies of the characteristic N-inversion motion and some ring vibrations involving the amino group. Comparing the experimental data of 1D-indazole, 1H2D-benzimidazole, 1D2H-benzimidazole, and 1D2D-benzimidazole with those of indzole and benzimidazole, one finds that the H/D substitution mainly takes place on the five-membered ring containing the N atom. Studies of rotational isomers show that o-dimethoxybenzene has only one stable configuration involved in the photo-excitation and ionization processes. In contrast, there are three stable rotational isomers participated in the m-dimethoxybenzene experiments. Analysis on the vibronic and cation spectra shows that most of the active vibrations of these isomeric species in the S1 and D0 states result from in-plane ring vibrations. Moreover, different orientation of the two OCH3 groups has little effect on the observed vibrations. The experimental results of fluoroindole show that the the ionization energies among these position isomers only differ by a few hundreds of wavenumbers. Most of the observed active vibrations of the cations result from the in-plane ring modes. The F substitution effects on the molecular vibration and transition energy somewhat depend on the location of the fluorine atom on the aromatic ring.