Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

and anilino groups attached to the central carbon, which bring about the change in
structure required for transition from the excited state potential energy surface to the
ground electronic state where the phenyl and anilino groups are in a “propeller
orientation” with respect to each other. The attractive interaction between the positivelycharged
MG and the negatively-charged polystyrene sulfate could impose a
thermodynamic restriction on the orientation of MG, and thus restrict the orientations of
the phenyl and anilino groups, which would inhibit the torsional motions necessary for
the energy relaxation process. The rigidity of the polystyrene and silica surfaces may
limit the torsional motions required for relaxation, whereas the absence of a rigid
interface for the air/water and alkane/water interfaces permits a more rapid relaxation.
Although the charged colloidal particle creates a significant electric field that extends into
the water solution, our calculations based on our measurements of the colloid surface
charge density, the known viscoelectric constant of water, and the low electrolyte
concentration, indicate that the viscosity increase in the interfacial water is negligible. We
therefore conclude that the slower decay of excited state MG at the colloid/water
interface is not due to an increased local friction arising from the interfacial the
orientation of molecules at interfaces is
Molecular Orientational Distribution at Interfaces Using Pump-Probe SHG
At the present time the orientation of interfacial molecules is obtained from
measurement of the intensities of two linear polarization combinations. From a
single measurement one cannot obtain both the average orientation and the
orientational distribution. It is typically assumed that the distribution is a delta
function. In any case the polarization measurements can be described by an
orientation parameter Req = � cos� ��g/� cos3 � ��g where the brackets indicate the
average for the equilibrium ground state orientational distribution. To fit our data to
an orientational distribution function, e.g. a Gaussian distribution, requires two
variables. They are the mean orientational angle and a quantity �, which is related
to the full width at half‐maximum. In order to obtain these two parameters requires
two independent measurements. The second measurement is for an equilibrium
distribution that is perturbed by a circularly polarized pump pulse that is incident
normal to the interface. By using this arrangement the equilibrium in plane isotropic
orientational distribution is not perturbed. The perturbed ground state distribution
is directly dependent on the probability of excitation of a molecule whose transition
dipole makes an angle 900 ‐ � with the interface normal, i.e. �� . Ec �2 . Measurements
of two polarization combinations immediately after the pump pulse yields a second
measured parameter
R(t=0) = � cos3� ��g/� cos5 � ��g
It is to be noted that the interfacial molecules, both ground and excited state have
not reoriented during the time of the measurements because the time scale for
reorientation of aromatic molecules at the air/water interface has been shown in
our laboratory to be several hundred picoseconds. The fitting of the two
measurements of the organic dye molecule, coumarin 314, at the air/water
interface, to a Gaussian distribution, yields a mean angle of 550 and a full width at
half maximum of 160 . The method we have used can be applied to other two
variable orientational distributions. For example, using the wobbling in a cone
model to describe orientational diffusion at an interface, we found the tilt angle of
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