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Femtosecond-laser-induced reactions at surfaces
Richard J. Finlay
Ph.D. Thesis, Harvard University, 1998, 147 pages export citation
We have experimentally studied chemical reactions at the surfaces
of
platinum and silicon using subpicosecond laser pulses to induce
the
reactions. The laser pulses stimulate electrons, which in turn
stimulate
molecules absorbed on the surface. Irradiation of
CO/O2/Pt(111)
with 300-fs
laser pulses yields O2 and CO2. We
measured the yields of O2
and CO2
induced by laser pulses of various wavelengths to determine that
nonthermalized electrons stimulate the adsorbates. When the
fluence in the
laser pulses is high, the effective cross section for desorption is
high
compared to the cross section measured using an arc lamp
source. We
demonstrated that by controlling the fluence in the subpicosecond
laser
pulses we can access either the low or the high cross section
regimes. We
used isotopic labeling to show that the O2 desorption
is molecular,
and to
discover some properties of the pathway to CO2. In
many
simulations of
molecular dynamics following subpicosecond-pulsed laser
excitation, the
substrate is assumed to be static. We discovered, however, that
subpicosecond laser pulses with fluences above 50 µJ/mm
2
induce reaction
between O2 adsorbed on Pt(111) and atoms from
beneath the
surface --
atoms near the surface move. Laser pulses of even higher fluence
ablate the
substrate. We irradiated silicon surfaces with 10 000-µJ/mm
2, 100-
fs pulses
in a chamber filled with halogen gases. We discovered that sharp
spikes
develop on the surface, and deduced some of the elements of a
model for the
spike formation. We also report discovery of an electron-beam
induced
reaction in C6H6/O2/
Pt(111) that yields phenol.
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