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We applied two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy to investigate the molecular properties of the selected rotamers of o-dimethoxybenzene (ODMB) and m-dimethoxybenzene (MDMB). The present experimental results show that only one stable configuration is involved in the photoexcitation and ionization processes of ODMB, as predicted by theoretical calculations. The adiabatic ionization energy (IE) of ODMB is measured to be 61 617 ± 5 cm(-1). In the case of MDMB, both our experimental and calculated results suggest that there are three stable rotamers coexisting in the sample. The adiabatic IEs of rotamers a, b, and c are determined to be 63 523 ± 5, 64 491 ± 5, and 63 758 ± 5 cm(-1). Analysis on the MATI spectra shows that most of the active cation vibrations of these isomeric species result from in-plane ring motions. In addition, different orientations of the two OCH(3) groups have little effect on these vibrations.

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We have applied two-color resonant two-photon mass analyzed threshold ionization spectroscopy to investigate some ionic properties of N-alkylanilines. The respective adiabatic ionization energies of N-methylaniline (NMA), N-ethylaniline (NEA), and N,N-dimethylaniline (DMA) are determined to be 59 822, 59 204, and 58 018 cm−1 with an uncertainty of about 5 cm−1. This indicates that the longer alkyl chain gives rise to a larger redshift in the IE due to a stronger interaction between the alkyl group and the nitrogen atom in the ionic state. Because the alkyl group gives rise to an increase in the electron density around the nitrogen atom of the neutral species, the IE of DMA is lower than that of NMA. In addition, the N-alkyl substitution also influences the frequency of the internal motion of the cations. However, the frequency variation is dependent upon the vibrational pattern and the extent of the coupling between the N-alkyl group and ring vibrations.

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