Laser threshold photoelectron spectra of the cis and trans rotational isomers of p-dimethoxybenzene–Arn (n=0,1,2): Observation of the intermolecular van der Waals stretching and bending vibrational modes in the cation

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Abstract
The cis and trans rotational isomers of p‐dimethoxybenzene–Arn (n=0,1,2) have been studied in a supersonic free jet by two‐color laser resonance enhanced multiphoton ionization threshold photoelectron spectroscopy. The two‐color (1+1’) threshold photoelectron spectra recorded via the S1 state of the cis and trans isomers of the 1:1 and 1:2 argon complexes reveal well resolved vibrational structure characteristic of the low frequency bending and stretching van der Waals vibrational modes. In the case of the trans isomer of the 1:2 complex, a very low frequency progression (11 cm−1) in a nontotally symmetric van der Waals bending mode appears in single quanta in the spectrum. The equivalent spectrum recorded for the cis isomer exhibits structure characteristic of van der Waals stretching modes as well as double quanta excitation in both totally symmetric and nontotally symmetric van der Waals bending modes. The observation of single quantum excitation in formally forbidden van der Waals vibrational modes implies the possibility of a change in the overall symmetry of the complex in the ground cationic state when compared to that in the S1 state. The adiabatic ionization energies (Ia) for the cis and trans isomers of p‐dimethoxybenzene–Arn (n=0,1,2) were measured as 60 774±7 (cis; n=0), 60 687±7 (cis; n=1), 60 509±7 (cis; n=2), 60 563±7 (trans; n=0), 60 479±7 (trans; n=1), and 60 295±7 cm−1 (trans; n=2).

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