posted on 2017-08-21, 00:00authored byMatthias Koch, Mark Pagan, Mats Persson, Sylwester Gawinkowski, Jacek Waluk, Takashi Kumagai
Quantum tunneling of hydrogen atoms
(or protons) plays a crucial
role in many chemical and biological reactions. Although tunneling
of a single particle has been examined extensively in various one-dimensional
potentials, many-particle tunneling in high-dimensional potential
energy surfaces remains poorly understood. Here we present a direct
observation of a double hydrogen atom transfer (tautomerization) within
a single porphycene molecule on a Ag(110) surface using a cryogenic
scanning tunneling microscope (STM). The tautomerization rates are
temperature independent below ∼10 K, and a large kinetic isotope
effect (KIE) is observed upon substituting the transferred hydrogen
atoms by deuterium, indicating that the process is governed by tunneling.
The observed KIE for three isotopologues and density functional theory
calculations reveal that a stepwise transfer mechanism is dominant
in the tautomerization. It is also found that the tautomerization
rate is increased by vibrational excitation via an inelastic electron
tunneling process. Moreover, the STM tip can be used to manipulate
the tunneling dynamics through modification of the potential landscape.