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

Miyazaki, Mitsuhiko; Sakata, Yuri; Schütz, Markus; Dopfer, Otto; Fujii, Masaaki

FG Lasermolekülspektroskopie und Umweltphysik

The ionization-induced pi <-> 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 pi-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 pi-bound site on the aromatic ring to the H-bonded site at the OH group by ionization. The subsequent H <-> pi back reaction converges the dimer to a pi <-> H equilibrium. This result is in sharp contrast to the single-step pi <-> H forward reaction in the PhOH+-Ar-2 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 pi <-> H forward reaction of tau(+) = 122 and 73 ps and the H <-> pi back reaction of tau_ = 155 and 188 ps for PhOH+-Ar and PhOH+-Kr, respectively. The one order of magnitude slower time constant in comparison to the PhOH+-Ar-2 trimer (tau(+) = 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.
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