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Femtosecond laser-gas-solid interactions
Claudia Wu
Ph.D. Thesis, Harvard University, 2000, 135 pages export citation
This dissertation discusses two sets of experiments.
The first set of experiments
investigates the interaction of femtosecond laser
pulses with silicon in the environment of
a halogen-containing gas. We find that upon
irradiation, sharp spikes are formed on the
surface. Spike formation strongly depends on the laser
and gas conditions. The spikes are
crystalline and contain a high density of both structural
and chemical defects. The spikes
are very strong light absorbers and exhibit absorption
exceeding A > 0.9 for ultraviolet
(0.25 µm) to near-infrared (2.5 µm) wavelengths.
Spiked avalanche photodiodes show a
more than threefold increase in quantum efficiency upon
illumination with 1.06 µm and
1.31 µm radiation. Unlike ordinary silicon, microstructured silicon
luminesces strongly
in the visible. The photoluminescence intensity and
peak wavelength depend on the laser
conditions used to produce the luminescent surfaces.
In the second set of experiments, we study the
oxidation of CO from CO/O2/Pt(111)
using femtosecond laser pulses. We observe a
nonlinear dependence of both the CO oxidation
and O2-desorption yields on the laser fluence. The
yields depend strongly on
the laser wavelength. Nonthermal electrons must play
an important role in the excitation
mechanism leading to desorption of O2 and
production
of CO2. Oxidation of CO
from CO/O2/Pt(111) induced with femtosecond laser
pulses most likely proceeds via the
formation of an intermediate CO3* complex.
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