Spectroscopy and Dynamics of the Dipole-Bound States of ortho-, meta-, and para-Methylphenolate Anions.

A photodetachment and photoelectron spectroscopic study by employing a cryogenically cooled ion trap combined with a velocity-map imaging setup has been carried out to unravel the vibrational structures and autodetachment dynamics of the dipole-bound states (DBSs) of o-, m-, and p-methylphenolate anions (o-, m-, and p-CH₃PhO^-). The electron binding energy of the DBS increases monotonically with the increase of the neutral dipole moment to give respective values of 66 ± 15, 123 ± 18, or 154 ± 14 cm^-1 for the o-, m-, or p-isomer. The different electron-donating effects of the methyl moieties in the three geometrically different isomers seem to be reflected in the experiment. Mode-specific DBS dynamics of the o-, m-, and p-CH₃PhO^- complexes have been interrogated by using picosecond time-resolved photoelectron velocity-map imaging spectroscopy. Autodetachment lifetimes of the DBS vibrational Feshbach resonances have been measured and discussed quantitatively using Fermi's golden rule, especially in comparison with those of the phenoxide anion to get insights into the methyl substitution effect on the electron binding dynamics of the metastable DBS.