DAVIS GROUP- MOLECULAR REACTION DYNAMICS
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The objective of our research is to understand,
at the detailed molecular level, the mechanisms of chemical reactions relevant
to catalysis, combustion, and atmospheric/interfacial chemistry on Earth and
elsewhere. Reactive species such as molecular free radicals and
excited (electronically or vibrationally)
closed-shell molecules are prepared in molecular beams using a variety of novel
laser-based methods. Reactions are induced
by single collisions between molecules in the gas phase in crossed molecular
beams, at liquid interfaces, or induced by absorption of a photon. Neutral products from these reactions are ionized
using “soft” single photon ionization using high-intensity vacuum ultraviolet
(VUV; λ < 180 nm) and extreme ultraviolet (XUV; λ < 110 nm) light
produced by frequency upconversion using novel tabletop
laser-based light sources. The ions are then detected using various highly
sensitive mass spectrometric techniques.
For reactions producing atomic products such as H, D or O, Rydberg
tagging time-of-flight spectroscopy is employed. Most of the information is
derived from measurements of product angular and velocity distributions as a
function of collision energy and reactant internal energy. Check out our Publications Page to see
what we have been doing...
the past, many of the experimental advances in the field of molecular reaction
dynamics of neutral species have involved the detection of nascent products
from photodissociation or bimolecular reactions under
single-collision conditions. Such
experiments have typically employed mass spectrometry using electron impact
ionization, or optical methods such as laser induced fluorescence (LIF) or
resonance enhanced multiphoton ionization (REMPI).
While electron impact ionization can be used to detect any neutral species,
detection sensitivity is limited by the small ionization efficiencies
(typically less than 0.01%), and large background signals due to dissociative
ionization. Although LIF and REMPI offer much greater sensitivity, these
methods require that the spectroscopic fingerprints of the detected molecules
be well-known. While the electronic spectroscopies of atomic and diatomic
molecules are in most cases well-understood, the same can be said for only a small number triatomic and larger molecules. These issues have constrained most studies in
the field to simple reactions leading to diatomic or triatomic molecules.
Vacuum ultraviolet and extreme ultraviolet radiation
can be used for “soft” single-photon nonresonant
ionization of molecules. The only
requirement is that the energy of the photon must exceed the ionization energy
of the molecule of interest (see figure on left). Using single-photon ionization, high
detection sensitivity is possible even when the spectroscopy of the molecule or
radical under interrogation is unknown. In
the past, intense sources of such short-wavelength radiation have been largely
restricted to 3rd generation synchrotron light sources only
available at a few locations worldwide. While
the upconversion of visible and UV lasers to the VUV
and XUV using nonlinear optical methods such as 4-wave mixing in inert gases is
well-established, the efficiencies of such techniques are typically too low
(<<1%) to facilitate nonresonant
To greatly expand the range of chemical systems
that can be studied, we have developed new light sources in the VUV and XUV
based on near-triple resonant 4-wave mixing of high-intensity visible and
near-UV radiation produced by tabletop laser sources. This produces intensities in the VUV and XUV
that are orders of magnitude higher than is normally possible. For example, 10 eV
(125 nm) pulsed radiation at the millijoule level can
be used for “soft” photoionization of products from crossed molecular beams
reactions using a rotatable source apparatus. We are now able to study reactions of
polyatomic free radicals (e.g., C6H5 + O2), or
reactions of transition metal atoms having higher ionization energies (Pt + CH4), or ligand exchange reactions of transition
metal complexes (e.g., C5H5CoH2 + C3H6),
reactions of halogen oxide molecules relevant to stratospheric ozone
destruction, or even reactions on Saturn’s moon Titan.
Our ultrabright VUV
source at 130 nm improves our sensitivity in oxygen atom Rydberg tagging TOF
(ORTOF) spectroscopy, a method developed in our laboratory, by at least two
orders of magnitude. In this case, three
input laser beams for VUV generation by four-wave mixing are each tuned near
atomic resonances in the nonlinear medium.
By combining this method with the use of narrowband lasers, we have
reached the point where the first true crossed beam study of the H + O2
→ OH + O reaction, generally considered to be the single most important
reaction in combustion, is possible.
In the photos below, Dan Albert is working on
the Rydberg O atom Machine. Three
tunable dye lasers are used to produce the 130 nm VUV light, and a fourth laser
in foreground is
used to excite the O(3S) to a high-n Rydberg state. The crossed beams machine is at the right
edge of the photo below.
Dan standing in front of the rotatable detector
crossed beams apparatus.
Below is Michael in the laboratory working on
the XUV light source, which provides intense pulsed light with photon energies
up to 13.8 eV.
In an effort to make chemistry and
physics instrumentation available to high school and introductory college
chemistry and physics classes, we have developed simple spectrophotometers that
can be constructed using readily available parts at extremely low cost
($25). One design is constructed using
Lego® blocks, a light emitting diode, and optical elements including
a lens, slide-mounted diffraction grating, and photodiode detector. The photodiode detector is mounted on a
rotatable arm for wavelength selection based on simple laws of
diffraction. The design is extremely
simple, thereby demonstrating basic physical principles (such as diffraction
and absorption of light) that are frequently lost in commercial “black box”
instruments. Performance, measured by
comparison to a commercial spectrometer, is sufficiently quantitative to
facilitate experiments such as the determination of the pKa
of an acid-base indicator! Click here for more information.
Visit our Publications Page
containing a listing of selected publications from members of the group.
Back to Department of Chemistry and
Courses (For more information, go to the
Chemistry Undergraduate Website):
Fall 2013: Chemistry 2150- Honors
Introductory Chemistry- This class is suited for students with strong high
school preparation in Chemistry.
Completion of this class allows students to go on to Honors Organic
Chemistry (Chem 3590) in the Spring
of their Freshman Year. Students should
have taken Calculus in high school or be enrolled in Calculus at Cornell during
Spring 2014: Chemistry
2880- Physical Chemistry 2- This course is primarily designed for students in
the Biological Sciences. Requires 2870 as a prerequisite.
Information for Prospective Graduate
Several different projects are available for graduate students from the
incoming graduate class. At least two openings are available. For
information about our future research plans, contact Floyd Davis at