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2A06
Molecular photodissociation on metal surfaces using visible light
(RIKEN1, Univ. Ulsan2, Univ. Toyama3, Univ. Illinois at Chicago4) Emiko Kazuma1,
Jaehoon Jung2, Hiromu Ueba3, Michael Trenary4, Yousoo Kim1
Visible-light-induced molecular photodissociation of dimethyl disulfide ((CH3S)2) adsorbed on
Ag(111) and Cu(111) surfaces was investigated by means of scanning tunneling microscopy (STM)
combined with density functional theory (DFT) calculations. The visible-light-induced
photodissociation on metal substrates has long been thought to never occur, either because
visible-light energy is much smaller than the optical energy gap between the frontier electronic states
of the molecule or because the molecular excited states have short lifetimes due to the strong
hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S-S bond in
(CH3S)2 adsorbed on the metal surfaces was dissociated through direct electronic excitation from the
HOMO-derived MO (nS) to the LUMO-derived MO (*SS) by irradiation with visible light.
Visible-light-induced photodissociation becomes possible due to the interfacial electronic structures
constructed by the hybridization between MOs and the metal substrate states. The molecule-metal
hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light
energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which
results in longer excited-state lifetimes.
【INTRODUCTION】 The UV-light photodissociation of O2, Cl2CO and OCS on metal surfaces has
been observed even at low temperatures, although the excited states of the adsorbates tend to relax
rapidly. Two excitation mechanisms, indirect (substrate-mediated) and direct (intra-adsorbate), have
been proposed. In the indirect mechanism, hot electrons generated in a bulk metal by photoabsorption
transiently enter the unoccupied adsorbate states through an inelastic scattering process, which
initiates photochemical processes.1,2 The reaction probability is determined by the density of hot
electrons, and, thus, depends on the photoabsorption of the metal. In contrast, photodissociation via
the direct excitation of the frontier electronic states of the adsorbates has been reported for only a few
kinds of physisorbed3 and chemisorbed4,5 molecules. Notably, previously reported photodissociation
reactions on single-crystalline metal surfaces have been achieved only by excitation with UV-light,
either due to a wider molecular optical gap than the visible-light energy, or due to short lifetimes of the
molecular excited states resulting from the strong hybridization with the metal surfaces.
In this study, we investigated the visible-light-induced photodissociation of (CH3S)2 on Cu(111)
and Ag(111) surfaces by STM combined with DFT calculations and clarified the reaction mechanism.
【METHODS】 The Cu(111) and Ag(111) substrates were cleaned using repeated cycles of Ar +-ion
sputtering and annealing. The (CH3S)2 molecules were deposited on the substrates maintained at <50
K. All measurements were performed with a low-temperature scanning tunneling microscope
(Omicron GmbH) under ultra-high vacuum (below 4.0 × 10-11 Torr) at 5.0 K. The light was p-polarized
and introduced into the STM chamber with an incident angle of 25° to the sample surface. Periodic
DFT calculations were performed using the Vienna Ab-initio Simulation Package code with Grimme’s
DFT-D3BJ functional that accounts for the dispersive interactions.
1. Frischkorn, C.; Wolf, M. Chem. Rev. 2006, 106, 4207. 2. Lindstrom, C. D.; Zhu, X. Y. Chem. Rev. 2006, 106, 4281.
3. Ying, Z. C.; Ho, W. J. Chem. Phys. 1991, 94, 5701. 4. Zhu, X. Y.; Hatch. S. R.; Campion, A.; White, J. M. J. Chem. Phys.
1989, 91, 5011. 5. Zhou, X. –L.; White, J. M. J. Phys. Chem. 1990, 94, 2643.
【RESULTS AND DISCUSSION】 Figure 1 shows the STM images of the (CH3S)2 molecules on
Cu(111) and Ag(111) obtained before and after irradiation with 532 nm (2.3 eV) light. After irradiation,
some molecules had broken into two identical ball-shaped protrusions. The S-S bond of a single
(CH3S)2 molecule on both Cu(111) and Ag(111) is dissociated to produce two CH3S molecules through
vibrational excitation of the S-S stretching mode by injecting tunneling electrons (> 0.36 eV) from the
STM tip. The STM images of dissociated molecules after light irradiation have the same appearance as
CH3S molecules obtained by injecting tunneling electrons into a (CH3S)2 molecule. This indicates that
S-S bond dissociation of (CH3S)2 on the metal surface was induced by light irradiation.
The reaction ratio (N/N0) was measured to obtain quantitative information on the photodissociation
reaction. The reaction ratio follows an exponential function, exp(-kt) (k: rate constant, t: irradiation
time), because the dissociation reaction, (CH3S)2 → 2CH3S, is a first-order reaction. Figure 2 shows
wavelength () dependence of the photodissociation yield (Y) obtained on Cu(111) and Ag(111). The
Y-spectra show the peak at ~450 nm (~2.76 eV) and the threshold at ~670 nm (~1.85 eV) and ~635
nm (~1.95 eV) on Cu(111) and Ag(111), respectively. Therefore, the photodissociation of (CH3S)2 on
the metal surfaces occurs in the visible-light wavelength region, which is much longer than the
absorption tail for a (CH3S)2 solution. In addition, the Y-spectra do not follow the photoabsorption
spectra of the metal substrates. These results imply that the reaction mechanism cannot be simply
explained by either the indirect mechanism or the direct mechanism for physisorbed molecules.
If the photodissociation can be explained by the direct
excitation mechanism for chemisorbed systems, the reaction
profile must depend on the frontier electronic states of the
molecule hybridized with the metal. To verify the mechanism,
the projected density of states (PDOS) and the spatial Figure 1 (a) Structure of a (CH3S)2
molecule, indicating the photodissociation
distribution of MOs for (CH3S)2 adsorbed on the metal substrates of the S-S bond. Topographic STM
images of (CH3S)2 molecules on (b)
were investigated to examine the detailed electronic structures at Cu(111) and (c) Ag(111) observed at ~5 K
(Vsample = 20 mV and Itunnel = 0.2 nA)
the molecule-substrate interfaces. The hybridization between the before and after irradiation with 532 nm
light (5.86 × 1016 photon cm-2 s-1, 10 min).
molecules and the metal substrates reduces the energy gap The scale bars are 0.5 nm.
between the frontier electronic states near EF, HOMO-derived
MO (nS) and the LUMO-derived MO (*SS), thus enabling
photodissociation by visible light. Furthermore, the weak
interactions between the frontier MOs and the metal substrates,
especially in the unoccupied region, extend the lifetimes of the
excited states sufficiently to induce photodissociation. Therefore,
the hybridization between (CH3S)2 and the metal substrate opens Figure 2 The wavelength () dependence
of the photodissociation yield (Y), which is
a novel reaction pathway for the photodissociation by visible the rate constant (k) divided by the number
light of molecules adsorbed on metals.
Published in J. Am. Chem. Soc. 2017, 139, 3115–3121.
of incident photons per second. The
simulated absorption spectra of bulk Cu
and Ag and the absorption spectrum of a
(CH3S)2 solution are also shown.