260,262,263 Spencer T. Olin Laboratory
Department of Chemistry and Chemical Biology
Cornell University
Ithaca, NY 14853
Office: 607-255-0014 
Lab: 607-254-5084
Student Office: 607-255-9823
FAX: 607-255-4137


This is the Davis Group homepage in the Department of Chemistry and Chemical Biology at  Cornell University.

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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...

Our research is currently supported by the  National Science Foundation, and the U.S. Department of Energy.  Over the years, we have also been supported by the ACS Petroleum Research Fund, and the Alfred P. Sloan Foundation

In 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 photoionization methods. 

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 Chemical Biology

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 Fall 2013.

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 Students:

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 hfd1@cornell.edu