This thesis presents the results of three experiments which use
lasers to
investigate energy-transfer and charge-transfer dynamics. The
dynamical
processes studied include nanosecond vibrational energy transfer
in
molecules, subpicosecond electron relaxation in semiconductors,
and
subpicosecond initiation of surface bimolecular reactions on a
metal
crystal.
In experiments using time-resolved coherent Raman
spectroscopy to
probe infrared multiphoton excited molecules, we study CO
2-laser excited
SO2 and
SF6. In SO2 we observe direct n1-mode
excitation and distinguish between
this process and excitation of the nearly resonant n2-mode
overtone. In SF6,
we directly observe n3-mode excitation followed by collisional
energy
redistribution to a heat bath of non-pumped modes. Quantitative
modeling
of the SF6 spectra yields excited vibrational
population distributions and
resolves some long-standing inconsistencies between different
previously
published reports.
In an experiment using time-resolved photoelectron spectroscopy,
we
observe the subpicosecond evolution of an optically-excited
nonequilibrium
electron distribution in silicon. We observe an electron
thermalization time of
less than 120 fs, electron equilibration with the lattice in 1 ps, and
an
energy-dependent electron cooling rate consistent with published
calculations of the electron-phonon scattering rate. The results
indicate the
formation, in 1 ps, of a surface space-charge electron layer with an
electron
density two orders of magnitude greater than the bulk electron
density.
In an experiment using 100-fs laser pulses to induce
desorption of O2
and
reaction of O2+CO to form CO2 on a
Pt(111) surface, we present desorption
and reaction data obtained over an absorbed fluence range of 1-
20 mJ/
cm2 at wavelengths of 800, 400, and 266 nm. We
observe a highly nonlinear
desorption and reaction yield fluence dependence; the data are fit
by a power
law model in which the yield is proportional to fluence to the power
p = 5.9
and 3.8 for the 800 nm and 400 nm data respectively. The ratio of
O2 to CO2
desorption is found to be 14:1, 12:1 and 3:1 at 800, 400, and 266
nm
respectively. At 800 nm, the desorption and reaction are
independent of laser
pulsewidth in the range 100 fs to 3.6 ps.
Finally, this thesis describes the design, development and
operation of
new
equipment used for the surface reaction experiment: a state-of-the-
art
amplified femtosecond Ti:sapphire laser, and an ultrahigh-vacuum
surface-
science chamber.
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