James S. Keller
DOUBLE-RESONANT EXCITATIONS OF GAS-PHASE MOLECULES
Excited states play a critical role in many interesting photophysical, photochemical, and
photobiological processes. Our goal is to understand the structure and dynamics of
photoexcited molecules. To achieve this goal, we employ a variety of nonlinear optical
methods to execute a flexible scheme for the control of reactant geometry in a
photochemical process. At present, this study involves gas-phase molecules that are
synthesized in our laboratory.
We investigate the multidimensional nature of chemical reactions on excited state
surfaces by effecting double-resonant excitations of gas-phase molecules. We find that
the fate of an electronically excited molecule is quite sensitive to its nuclear geometry,
especially in regions where reactive surfaces cross and states can mix with one another.
Thus, the double-resonance in these studies is a vibrational excitation followed by an
electronic excitation. Our experiments probe new regions of reactive surfaces by
pumping molecules to these surfaces via selected ground state vibrational levels. This
basic approach can be applied to a wide variety of molecular systems, addressing
fundamental problems in the field of chemical reaction dynamics. Two projects within
this research area are now underway at Kenyon.
Vibrationally-mediated photodissociation Allison Ogilvie `02; Sule Sidigu `04
These experiments provide a direct measure of our ability to alter the quantum yields and
product states of photofragments through a series of double-resonant excitations. The
photochemistry of nitrous acid makes it an optimal candidate for these studies; however,
its chemical instability makes HONO a less than optimal target. We have managed to
overcome challenges of preparation, storage, and delivery of this sample and are
beginning to collect two-color grating spectra that investigate the effect of parent O-H
stretch on the photochemical fate of this molecule. The current experiments are designed
to augment earlier data on this system and supplement a manuscript now under revision.
Predissociation studied via depletion grating spectroscopy
Anthony Pellecchia `01
This nonlinear optical technique has been proven to work with nitrogen dioxide, a well-
documented photochemical target. We wish to apply depletion grating methods to
molecules whose photochemistry remains a mystery because of strong coupling between
chemically-reactive excited states. Without giving the details, these experiments allow us
to explore the influence of low-frequency vibrations on predissociation rates via grating
spectroscopy. This setup generates a much stronger signal than the vibrationally-
mediated photodissociation experiment described above. Previous experiments were
performed on chlorine dioxide (the topic of my symposium talk last June). We are now
designing experiments on bromine dioxide, a less studied, but potentially more effective
catalyst for ozone destruction than the chlorine species. The synthesis and storage of this
target molecule is proving to be a challenge, however. Experiments on carbon disulfide
(commercially available) may soon be initiated to confirm and add to preliminary data
taken earlier on this chemical system.
PROBING OPTICAL NON-LINEARITY IN ORGANIC LIQUIDS AND THIN FILMS
Organic nonlinear optical materials (NLO) have attracted attention recently as potentially
fast and efficient components of optical communication and computing systems. NLO
materials have been studied in many forms--crystals, organic glasses, vapor deposited
films, Langmuir-Blodgett structures, and poled polymer films; however, for integrated
electro-optic applications, the thermal stability requirements are demanding. The
difficulties in preparing thermally-stable materials has provided a demand for efficient
screening protocols of prospective NLO candidate chromophores. One very effective
screening procedure is the study by electric field induced second harmonic generation
(EFISH) of the effects of heating on chromophore nonlinearity. This is the project I
envisioned when I arrived at Kenyon and purchased equipment necessary to perform an
EFISH experiment. The arrival this fall of Prof. Frank Peiris (Physics) has provided a
second opportunity for our investigation of nonlinearity in condensed phase materials. A
new collaborative venture between our two research groups is described below.
Electric field induced second harmonic generation (Katie Cook `01; Nick Deifel
This general experimental method can be used to assess the nonlinearity of any
centrosymmetric phase, be it a gas, a centrosymmetric molecular crystal, or an organic
solution. In the experiment, a dc field is applied to the sample and synchronized to the
laser pulse so as to statistically orient the molecules of interest to allow the coherent
emission of second-harmonic signal. The magnitude of this signal can be related to the
second-order polarizability (beta) and varies over several orders of magnitude for simple
organic liquids. The beta values obtained in this experiment can be compared to
approximate quantities obtained from a two-level model. Thus, an intuitive picture of
nonlinear polarization can be drawn, and discrepancies between experiment and theory
can yield a more detailed analysis.
The laser setup and optical layout of this experiment are completed. A prototype sample
cell is under construction and due to be tested this month.
Dispersion of second-order optical nonlinearity in thin films (Nell Burger `04).
Professor Peiris brought to Kenyon a series of II-VI semiconductor film samples and has
the necessary contacts to obtain more of these same materials. While he sets up the
apparatus to explore the linear dispersion properties (through reflectivity measurements
using an ellipsometer), he and I are working with a former summer researcher of mine to
plan experiments to probe the nonlinear dispersion of these materials. There is interest in
the absolute value of the second-order nonlinear coefficient |d| for the zinc-blende
semiconductors in general and in |d| for Prof. Peirisí samples in particular. His samples
are thin films created by molecular beam epitaxy and tailored to exhibit specific bulk
properties. In principle, the large second-order dispersion of these materials could be
optimized in a multi-institutional collaboration with Peirisí former colleagues.