Real-Time Tunneling Dynamics through Adiabatic Potential Energy Surfaces Shaped by a Conical Intersection.

ABSTRACT

Dynamic shaping of the adiabatic tunneling barrier in the S-H bond extension coordinate of several ortho-substituted thiophenols has been found to be mediated by low-frequency out-of-plane vibrational modes, which are parallel to the coupling vector of the branching plane comprising the conical intersection. The S-H predissociation tunneling rate (k) measured when exciting to the S1 zero-point level of 2-methoxythiophenol (44 ps)-1 increases abruptly, to k ≈ (22 ps)-1, at the energy corresponding to excitation of the 152 cm-1 out-of-plane vibrational mode and then falls back to k ≈ (40 ps)-1 when the in-plane mode is excited at 282 cm-1. Similar resonance-like peaks in plots of S1 tunneling rate versus internal energy are observed when exciting the corresponding low-frequency out-of-plane modes in the S1 states of 2-fluorothiophenol and 2-chlorothiophenol. This experiment provides clear-cut evidence for dynamical "shaping" of the lower-lying adiabatic potential energy surfaces by the higher-lying conical intersection seam, which dictates the multidimensional tunneling dynamics.