Photoionization-induced π ↔ H site switching dynamics in phenol+–Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy

The ionization-induced π ↔ H site switching reaction in phenol+–Rg (PhOH+–Rg) dimers with Rg = Ar and Kr is traced in real time by picosecond time-resolved infrared (ps-TRIR) spectroscopy. The ps-TRIR spectra show the prompt appearance of the non-vanishing free OH stretching band upon resonant photoionization of the π-bound neutral clusters, and the delayed appearance of the hydrogen-bonded (H-bonded) OH stretching band. This result directly proves that the Rg ligand switches from the π-bound site on the aromatic ring to the H-bonded site at the OH group by ionization. The subsequent H → π back reaction converges the dimer to a π ↔ H equilibrium. This result is in sharp contrast to the single-step π → H forward reaction in the PhOH+–Ar2 trimer with 100% yield. The reaction mechanism and yield strongly depend on intracluster vibrational energy redistribution. A classical rate equation analysis for the time evolutions of the band intensities of the two vibrations results in similar estimates for the time constants of the π → H forward reaction of τ+ = 122 and 73 ps and the H → π back reaction of τ− = 155 and 188 ps for PhOH+–Ar and PhOH+–Kr, respectively. The one order of magnitude slower time constant in comparison to the PhOH+–Ar2 trimer (τ+ = 7 ps) is attributed to the decrease in density of states due to the absence of the second Ar in the dimer. The similar time constants for both PhOH+–Rg dimers are well rationalized by a classical interpretation based on the comparable potential energy surfaces, reaction pathways, and density of states arising from their similar intermolecular vibrational frequencies.