Figure 1. Hydroperoxyalkyl radicals—denoted QOOH—are key intermediates in oxidation chemistry in a wide array of fields. They control autoignition at the early stages of combustion, whereas in the atmosphere they enable rapid autooxidation that leads to the formation of secondary organic aerosols. The Combustion Research Facility plays a lead role in detecting these molecules and predicting their impacts.

Technical Details

Oxidation turns hydrocarbons into water and carbon dioxide through an interconnected web of reactions. The hydrocarbon’s molecular structure and the conditions of oxidation, such as temperature, pressure, and composition, profoundly impact phenomena ranging from air quality to engine performance.

In this aspect of our chemical physics program, we focus on low-temperature auto-oxidation. The richness of the chemistry allows us to uncover fundamental physical chemical phenomena while we create quantitative models describing the molecular transformations. Figure 3 shows schematically the major pathways for alkanes, but this scheme is often inaccurate or incomplete for the description of the low-temperature oxidation mechanism of unsaturated or substituted hydrocarbons. We use multiplexed experimental methods to identify and follow the time-evolution of the intermediates in an isomer-resolved manner. We also construct and solve the chemical master equation for these typically multiwell chemical systems. When combined, our theoretical and experimental methods provide a powerful approach to gain fundamental insight into oxidation processes in a wide pressure and temperature range. In addition to understanding fundamental physical chemical concepts, such as structure-reactivity relationships, our research also provides a solid basis for predictive models.

Experimental techniques

  • Low- and high-pressure photolytically initiated multiplexed photoionization mass spectrometry
  • Jet-stirred reactor for thermally initiated multiplexed photoionization mass spectrometry
  • Time-resolved step-scan Fourier spectroscopy for state-resolved studies of exothermic reactions
  • Time-resolved broadband cavity-enhanced absorption spectroscopy

Figure 2. Photoionization mass spectrometry experimentally characterizes key reaction intermediates like ketohydroperoxides, sampled from controlled photolysis or thermal reactors, such as the thermal jet-stirred reactor depicted here. (Source: )

Theoretical techniques

  • RRKM-based master equations for multiwell systems
  • Ab initio molecular dynamics
  • Automated approaches to construct the potential energy surfaces

Figure 3. General scheme for low-temperature hydrocarbon oxidation. (source:

Key Contributions


  • Ruben Van de Vijver, University of Ghent
  • Stephen J. Klippenstein, Argonne National Laboratory
  • Adam Trevitt, University of Wollongong, Australia
  • Brandon Rotavera, University of Georgia
  • Giovanni Meloni, University of San Francisco

PIs: Nils Hansen, David L. Osborn, Leonid Sheps, Craig A. Taatjes, Judit Zádor

When molecules absorb visible or ultraviolet light, they are promoted to an excited state in which the electrons of the molecule occupy different orbitals. This process is critical not only to life on Earth (e.g., photosynthesis), but also to human interactions (e.g., vision). We seek to understand the fundamental details of how this electronic excitation couples to nuclear coordinates (the bond lengths and angles in a molecule) to test and refine quantum mechanical models of nature. CRF researchers employ three main approaches to probe these dynamics:

  • Ultrafast pump-probe spectroscopy to follow short time dynamics
  • Velocity map imaging to probe quantum-state resolved energy deposition
  • Multiplexed photoionization mass spectrometry and PEPICO (to provide a universal view of final states and measure isomer-resolved branching ratios)

The figure below shows an example of the photodissociation of acetylacetone, a molecule that has characteristics of ketones, enols, and polyenes. Its photochemistry is much richer than previously assumed and shows evidence of bond breaking and isomerization on both the excited and ground electronic states.

Figure: Photochemical product channels in the one-photon and multiphoton dissociation of acetylacetone. Source: Sandia National Laboratories.

Key Contributions


  • Balint Sztaray, University of the Pacific
  • Andras Bodi, Paul Scherer Institute, Switzerland
  • Patrick Hemberger, Paul Scherer Institute, Switzerland
  • Scott Kable, University of New South Wales

PIs: Krupa Ramasesha, Laura McCaslin, David Chandler, David Osborn, Lenny Sheps, Judit Zador

A central goal of chemistry is to control the path of a chemical reaction, thereby producing a desired product with minimal waste or impurities. However, the arrangement of atoms in any molecule predisposes it to certain reaction pathways, often leading to undesirable products. By pushing molecules far from their equilibrium state, it may be possible to enhance new reaction pathways and inhibit others. For example, we used ultraviolet light to rearrange the atoms in a common molecule (acetaldehyde) into a different, less stable structure (vinyl alcohol).


Figure 2: Fraction of column-integrated formic acid triggered by
photo-isomerization of acetaldehyde to vinyl alcohol in
GEOS-Chem 3D chemical transport model. Source: Sandia National Laboratories.

In this work, the oxidation product of vinyl alcohol (formic acid) was the desired outcome we hoped to achieve by pushing the system far from equilibrium. However, vinyl alcohol is exceedingly uncommon at thermal equilibrium at room temperature, where there is only 1 vinyl alcohol molecule for every 3.3 million acetaldehyde molecules. Experiments showed that ultraviolet light of the wavelengths that reach the Earth’s surface could alter this equilibrium ratio by more than a factor of 100,000, converting acetaldehyde to vinyl alcohol, and therefore enabling this distinct chemical outcome. In addition to its fundamental value for understanding reaction mechanism control, the research discovered that sunlight can convert acetaldehyde in Earth’s atmosphere into the oxidation product formic acid, which plays an important role in rainwater acidity and is under-predicted by current atmospheric models.

Key Contribution

This far-from-equilibrium process generates up to 60% of total modeled formic acid over mid-latitude oceans on Earth.


  • Balint Szatary, University of the Pacific
  • Dwayne Heard, University of Leeds
  • Dylan B. Millet, University of Minnesota
  • Meredith J. T. Jordan, University of Sydney
  • Scott H. Kable, University of New South Wales

PIs: David L. Osborn, Laura McCaslin, David W. Chandler

Complex chemical reaction networks like those that occur in combustion and in atmospheric chemistry can contain thousands of interlocked chemical transformations. Frequently, important aspects of the overall chemistry depend on a few key reactions, for example, the chain-branching reactions that increase the number of reactive free radicals and drive hydrocarbon oxidation toward autoignition. The reactions in the networks are interrelated—the products of one reaction go on as the reactants in a subsequent chemical step—and often the critical reactions occur between species that are highly transient, reacting away almost as soon as they are formed. These species are sometimes referred to as intermediates and characterizing their reactions directly can be very challenging. At the CRF we work to understand the details of even complicated chemical systems, and so we have devised ways to synthesize these elusive molecules and measure their reactions.

The class of Creigee intermediates is important in tropospheric oxidation of hydrocarbons are carbonyl oxides. These molecules form in ozonolysis (the reaction of ozone with carbon-carbon double bonds)—a relatively slow reaction—and rapidly react away, making their direct detection in ozonolysis very difficult. CRF researchers discovered a way to form Criegee intermediates photolytically, allowing their reaction kinetics to be measured directly for the first time. Since this discovery, several research groups around the world have used this technique to produce and study reactions of these atmospheric chemistry intermediates. CRF researchers showed first that the reactions of Criegee intermediates with sulfur dioxide are fast enough to play a possible role in sulfate aerosol formation and, second, that their reactions with organic acids are fast enough to be a potential source of highly oxygenated molecules that are precursors to secondary organic aerosols in the troposphere. The first direct Criegee intermediate measurements used photoionization mass spectrometry. Subsequent work at the CRF also exploited developments in long-path absorption methods.

Schematic potential energy surface for the ozonolysis of an alkene to produce carbonyl oxide Criegee intermediates. Source:


Rebecca Caravan, CRF postdoctoral associate, now an assistant chemist at Argonne National Laboratory, working on the Sandia multiplexed photoionization mass spectrometry machine at the Advanced Light Source, where direct measurements of Criegee intermediate kinetics were taken.


Time-resolved photoionization mass spectrometry of the simplest Criegee intermediate (at m/z=46), formed in a photolytic reaction at time = 0 and disappearing by reaction with other gas phase molecules. Source:


Key Contributions


  • Carl Percival, Jet Propulsion Laboratory
  • Andrew Orr-Ewing, Anwar Khan, and Dudley Shallcross, Bristol University
  • Rebecca Caravan and Stephen Klippenstein, Argonne National Laboratory
  • Marsha Lester, University of Pennsylvania
  • Daniel Stone, Leeds University
  • Yiguang Ju, Princeton University

PIs: Nils Hansen, David L. Osborn, Leonid Sheps, Craig A. Taatjes

A collaborative effort between the Gas Phase Chemical Physics program at the Combustion Research Facility and the Chemical Dynamics in the Gas Phase group at Argonne National Laboratory, the consortium program focuses forefront research in gas phase chemical physics on the critical national energy mission of combustion. The program provides the framework for rigorous predictions of combustion chemistry through the design and implementation of novel experiments, theory, and modeling to characterize high-pressure combustion kinetics from elementary reactions, to submechanisms, to flames.

The core of the program’s strategy is to coordinate modeling, experiment, and theory (M-E-T) to accelerate progress in the fundamental understanding of the chemical physics of coupled reactions such as those that drive combustion:

Modeling: Detailed models characterize interrelationships of exothermic reactions that cause emergence of non-equilibrium phenomena

Experiment: High sensitivity measurements quantify kinetics and probe intermediates in elementary reactions and reaction systems.

Theory: First-principles kinetics rigorously characterizes the interplay of energy transfer and reaction.

A unique photolysis reactor, specially designed for sensitive photoionization mass spectrometric characterization of molecules sampled from extremely dilute high-pressure reacting mixtures, allows detailed quantitative high- fidelity time profiles of all reacting species simultaneously. Source:

Key Contributions


Stephen Klippenstein, Robert Tranter, Raghu Sivaramakrishnan, and Ahren Jasper, Argonne National Laboratory

PIs: Leonid Sheps, Craig Taatjes, Nils Hansen, Judit Zador

The CRF research into the fundamentals of chemical reactivity characterizes the building blocks important to all chemistry, thus contributing to a broad range of DOE energy missions. The work spans system complexity from dynamics experiments on single collision scattering or photodissociation of small molecules to detailed kinetics investigations of complex processes, e.g., autoxidation, described by a network of chemical reactions. The research employs multiplexed measurements to probe multiple observables simultaneously and is strengthened by close connections between experimental and theoretical investigations. Scientific themes include interactions on complex multi-well potential energy surfaces, reactants with unusual electronic character, characterization of elusive intermediates, reactions that couple multiple electronic surfaces, and effects of non-thermal reactant energy distributions.