Identification of four rotamers of m-methoxystyrene by resonant two-photon ionization and mass analyzed threshold ionization spectroscopy

We report the vibronic and cation spectra of four rotamers of m-methoxystyrene, recorded by using the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energies of the S₁← S₀ electronic transition are found to be 32 767, 32 907, 33 222, and 33 281 cm⁻¹, and the corresponding adiabatic ionization energies are 65 391, 64 977, 65 114, and 64 525 cm⁻¹ for these isomeric species. Most of the observed active vibrations in the electronically excited S₁ and cationic ground D₀ states involve in-plane ring deformation and substituent-sensitive bending motions. It is found that the relative orientation of the methoxyl with respect to the vinyl group does not influence the vibrational frequencies of the ring-substituent bending modes. The two dimensional potential energy surface calculations support our experimental finding that the isomerization is restricted in the S₁ and D₀ states