State-resolved reactivity and bond-selective control of methane on Ni(111).
Abstract: State-resolved reactivity is particularly useful to uncover the
energy flow details in gas-surface reactions. Here, we combine supersonic molecular beam
techniques and laser excitation to characterize the mode- and bond-specific reactivity of
methane on a Ni(111) surface. Our results reveal important details about the effect of
vibrational mode and symmetry on the reactivity of this ... read morebenchmark gas-surface reaction. We
first introduce a theoretical model of the state-resolved reactivity of methane on a
Ni(111) surface. This model provides a theoretical approach to understanding the reactivity
of selected vibrational states. More importantly, it can provide an accurate approach to
measure the reactivity of methane in vibrational ground state. This approach relies on a
detailed knowledge of the vibrational structure of methane, its vibrational cooling
dynamics in a supersonic expansion, and understanding of how individual vibrational states
contribute to the overall reactivity. In Chapter IV, we introduce a new scheme to measure
the state-resolved reactivity based on the `King and Wells' method. With the new detection
scheme, we are able to measure reactivity with and without laser excitation simultaneously.
In addition, we are able to probe real-time reactivity during the reaction. This method
will not only largely reduce the experimental time, but also extend our measurements of
state-resolved reactivity to a wider range, which is impossible to measure by post-dose
methods. In Chapter V, we explore the state-resolved reactivity of
CH4 prepared in a combined bending state
(ν2 + ν4) on Ni(111). Our
results provide the first comparison between a bend and stretch vibrational state in the
same polyad. By this comparison, we confirm the trend that stretching vibrations have
better efficiency in promoting reactivity than bending vibrations. In addition, comparing
with the 3ν4 state, we provide experimental evidence
supporting the theoretical calculation that the ν2 bend state
is less efficient than the ν4 bend state in promoting
CH4 activation on Ni(111). In the last chapter, we explore the bond-
and mode-selectivity of CH2D2 dissociation on
Ni(111). Unlike CH4 on Ni(100), our results show that different C-H
vibrations bring almost identical enhancement to the overall reactivity of
CH2D2 on Ni(111). Through comparisons, we
also find surface temperature is more important in a energy-starving region than
vibrational modes or symmetries in promoting
CH2D2 activation. In addition, we compare our
results with the overtone stretching excitation of
CH2D2 in gas-phase and gas-surface reactions.
The results show that ν1 and ν1
C-H vibrations are less localized than their overtone states in
CH2D2 molecules. Our results further uncover
details of the role that vibrations play in the bond-selective dissociation of
CH2D2 on Ni(111).
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Chemistry.
Advisor: Arthur Utz.
Committee: Charles Sykes, Mary Shultz, and Ricardo Metz.
Keywords: Physical chemistry, Chemistry, and Physics.read less