In this work, the molecules styrene (ST) and phenylacetylene (PA), as well as their argon complexes ST-Ar and PA-Ar, have been investigated with (1 + 1′) resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy (TES). The first adiabatic ionization energies of ST, PA, ST-Ar, and PA-Ar have been measured as 68 267 ±5, 71 175±5, 68 151 ±5, and 71 027±5 cm-1, respectively. For both ST-Ar and PA-Ar, the first photoelectron band shows structure in the lowest frequency van der Waals (vdW) bending mode in the ground ionic state, with νvdwbeing measured as 15 cm-1 in each case. For each molecule excitation to a particular vibrational level of the S1 state followed by ionization, allows structure in that mode to be observed in the threshold photoelectron spectrum. This has been achieved for three modes in both styrène and phenylacetylene. The experimental ionic vibrational frequencies thus obtained, have been compared with those known for the S 0 and S1 states.
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