| dc.description.abstracteng | Understanding the fundamentals of energy transfer between molecules and surfaces is of
profound importance in modern chemistry. Here, I investigate an important energy transfer
process, electron transfer (ET), which plays a key role in many surface processes such as
photochemistry and light harvesting using dye-sensitized photovoltaics. I probe the dynamics of
ET by studying what happens when electronically excited molecules collide with surfaces. In
particular I studied scattering of CO (a 3 Π 1), referred to as CO*, from clean and adsorbate
covered Au(111). Because the internal energy of CO* exceeds the work function of the Au(111)
surface, CO* quenching at the surface can lead to electron emission and the yield of electron
emission provides a sensitive probe of the energy transfer mechanisms involved.
These studies required the use of a unique, highly versatile molecule-surface scattering apparatus
which was designed and built during the course of this work. The instrument consists of a Stark
decelerator-based molecular beam source for CO*, a cryogenic sample mount, detectors for ions
and electrons, surface preparation equipment and three laser systems used for the preparation and
state-selective detection of scattered molecules.
The electron emission probability,Dz, depends in an interesting way on the initial vibrational state
of the molecule and the coverage of rare gas adsorbates. Dzis 0.13 ± 0.05 for CO* (ν = 0)on
atomically clean Au(111), 0.19 for a mix of vibrationally excited CO* (ν = 1,2,3), and 0.34 for
a mix of CO* (ν = 4,5,6). Surprisingly, scattering CO* in its ground vibrational state from Ar,
Kr, or Xe covered Au(111) increases rather than decreases Dz; Dz is approximately 0.5 for
monolayer coverage of all three gases and approaches unity upon adsorption of additional Ar and
Kr.
Conventionally, metastable quenching is explained in terms of an Auger process. This
mechanism predicts lower values of Dzthan I observe, a reduction in Dzwith coverage of rare gas
adsorbates, and almost no effect of initial vibrational state. Therefore, an alternative mechanism
is proposed in which electron emission proceeds via formation of a short-lived anion. In the
proposed mechanism, an electron transfers from the gold surface to CO* as the molecule
approaches the surface, forming an anion. Subsequently, the anion relaxes to neutral CO on a
femtosecond time scale by auto-detachment. The electron emitted from the molecule either
iii
escapes into vacuum and is detected or is absorbed by the surface. In contrast to the Auger
mechanism, the magnitude and trends in the measured values of Dzcan all be understood in the
context of the anion mediated de-excitation model.
Favorable overlap of the CO* molecular orbitals with the wave functions of the metal are a key
factor in understanding the high electron emission probability. The increase in Dzwith vibrational
excitation arises because the ground state (
2
Π) of CO
—
is resonant with CO* near a surface only
at extended bond lengths. CO* in higher vibrational states spends more time with extended C-O
bond lengths, thereby increasing the efficiency of the first electron transfer step. The increased
efficiency of the first electron step leads to initial charge transfer (ionization) at greater
molecule-surface distances, followed immediately by auto-detachment; the emitted electron,
therefore, experiences weaker image interaction with the surface and has a higher probability of
escaping into vacuum.
The adsorbate induced increase in Dzcan also be understood in terms of the anion mediated deexcitation model. This increase arises due to an increase in the electron reflection probability as
closed-shell noble gases are adsorbed on a metal surface. With increased adsorbate coverage, the
probability that an electron emitted from the molecule toward the surface is scattered back into
vacuum increases, thus increasing the observed electron signal. After some critical adsorbate
coverage, the initial electron transfer step from surface to molecule becomes inefficient and
electron emission decreases with additional adsorbate coverage.
In addition, I performed thermal desorption measurements of Ar, Kr, Xe, N
2, NO, C
2H2and SF
6
from Au(111) in order to characterize the temperature dependence of adsorbate coverage and to
measure desorption activation energies, which are excellent proxies for binding energies in the
low temperature regime. Binding energy scales with adsorbate polarizability, supporting the
conclusion that the surface-adsorbate bonds are dominated by dispersion forces.
Through measurements of δ, I have developed a better understanding of electron transfer
processes at surfaces. Quenching of CO* proceeds by formation of a transient anion and
subsequent auto-detachment. These measurements provide important reference data for
theoretical models describing dispersion forces and electron transfer at surfaces. I hope this work
inspires continued investigations into dynamics at interfaces. | de |
| dc.subject.eng | Surface Science, Electron Transfer, Electron Emission, Metastable Molecules, Electronically Excited Molecules, Cold Molecules, Temperature Programmed Desorption, Surface Scattering | de |