In
this laboratory, we study the energetic
and structural factors that affect or control
chemical properties of molecules in gaseous
phase as well as adsorbed on surfaces. New
laser spectroscopic techniques have been
developed or applied to probe the structure
and dynamics of molecules excited with specified
energy and orientation in unique environments.
Specifically, our current activities involve:
Energy
Transfer and Reaction of Highly Vibrationally
Excited Molecules Intramolecular isomerization
and collisional deactivation of molecules
excited with 10,000-50,000 cm -1 vibrational
energy are investigated. The excitation
is prepared by electronic excitation followed
by internal conversion, or by the stimulated
emission pumping technique. IR emission
spectra from the excited molecules are recorded
by a newly developed nanosecond time-resolved
FTIR technique to reveal the energy and
structural evolution of the molecules following
excitation.
Structure,
Spectroscopy and Dynamics of Transient Radicals
A new approach for detecting previously
unknown vibrational modes of transient radicals
has been demostrated. Photodissocaition
of precursors produces the desired transient
radical with high vibrational excitation.
IR emission from these excited species is
then detected by nanosecond time-resolved
FTIR emission spectroscopy. The vibrational
bands, with rotational resolution, allow
the determination of the radical structure.
Time-resolved IR emission spectra also enable
the deduction of energy relaxation and reaction
of the excited radicals.
Dynamics
and Photochemistry of Molecules Adsorbed
on Surfaces The presence of a surface
provides several important factors affecting
the chemical properties of a molecule. In
addition to rapid quenching of molecular
excitation, the surface may change reaction
energetics and alter the reaction channels,
provide new excitation channels through
substrate electrons, and align molecular
adsorbates to facilitate a particular reaction.
All these effects have been identified in
our study of laser-induced polymerization
of formaldehyde on silver. These unique
aspects of surface photochemistry and explored
in several different molecular systems on
metal and oxide surfaces.
Nonlinear
Optical Probe of Surfaces/Interfaces and
Ultrathin Films Nonlinear optical
techniques based on second harmonic generation
and transient grating scattering are developed
for probing the structure, kinetics and
dynamics of a variety of systems involving
a surface or interface: ultrathin molecular
films, the solid-liquid interface in colloids,
and metal or semiconductor surfaces. For
example the glass transition temperature
of ice has been determined by second harmonic
Raleigh scattering, the surfactant adsorption
onto microparticles in colloids can be quantitatively
characterized by second harmonic generation,
ultrafast carrier dynamics at a silicon
surface are revealed by transient grating
scattering. |
