John Hershberger
Experimental Physical Chemistry, Laser Kinetics
The detailed kinetics and dynamics of elementary gas-phase chemical reactions is a topic of great interest in chemistry, with applications in atmospheric chemistry, combustion science, and modeling of chemical vapor deposition processes. A major research area in our group is the elucidation of mechanisms in the combustion chemistry of nitrogen. Many nitrogen-containing radicals such as CN, NCO, NO, etc. are formed in combustion environments and ultimately lead to nitrogen oxide emissions, a major source of air pollution. Strategies to control nitrogen oxide (NOx) emissions include several aftertreatment processes such as the Thermal-De-NOx and NO-reburning mechanisms. Our understanding of these processes relies on detailed kinetic models, which in turn rely on elementary kinetic information such as that obtained in our laboratory. We use Nd:YAG and excimer lasers to initiate chemical reactions, and high-resolution infrared diode lasers to probe reactants and products with an extremely high degree of specificity. Many reactions of importance in combustion chemistry, such as NH2 + NO, NCO + NO, CH + NO, and CN + NO2, have several possible product channels, some involving non-intuitive molecular rearrangements via cyclic transition states. Major goals in our laboratory include the quantitative determination of product branching ratios as well as total rate constants.
Another project involves infrared detection and kinetic studies of transient main group hydride radicals, such as SiH3 and GeH3. These species play a crucial role in the combustion of silanes and germanes, and the chemical vapor deposition of silicon and germanium thin films, which have applications as microelectronic and photovoltaic devices. Reactions with a variety of gas-phase species are currently under investigation.
We are also interested in the detailed quantum state dynamics of chemical reactions and photodissociation processes. The resolution of our diode lasers is sufficient to resolve individual rotation-vibration quantum states of numerous small molecules. This has permitted measurements of the disposal of energy into vibrational degrees of freedom for triatomic products of several reactions, including O+CS2 and NCO+NO. Future plans include the extension of these studies to the determination of rotational energy distributions. Experiments such as these, when combined with high-level calculations, can provide a high level of detailed understanding about potential energy surfaces.
Selected Publications
“Reactions of NCO and NCS Radicals,” J.F. Hershberger, book chapter in “N-Centered Radicals”, Z.B. Alfassi, ed., Wiley, p.529-548 (1998).
“A Diode Laser Study of the Product Branching Ratios of the CH+NO2 Reaction,” K.T. Rim and J.F. Hershberger, J. Phys. Chem. 102, 4592-4595 (1998).
“Product Branching Ratios of the HCO+NO2 Reaction,” K.T. Rim and J.F. Hershberger, J. Phys. Chem. 102, 5898-5902 (1998).
“Kinetics of the CN+CH2CO and NCO+CH2CO Reactions,” M.A. Edwards and J.F. Hershberger, Chem. Phys. 234, 231-237 (1998).
“Kinetics of the CN+OCS Reaction”, J. Park and J.F. Hershberger, Chem. Phys. Lett. 295, 89-94 (1998).
“Temperature Dependence of the Product Branching Ratio of the CN+O2 Reaction”, K.T. Rim and J.F. Hershberger, J. Phys. Chem. A 103, 3721-3725 (1999).
“Kinetics of the NCS Radical”, R.E. Baren and J.F. Hershberger, J. Phys. Chem. A 103, 11340-11344 (1999).
“Product Branching Ratio of the HCCO+NO Reaction”, K.T. Rim and J.F. Hershberger, J. Phys. Chem. A 104, 293-296 (2000).
“Recent Progress in Infrared Absorption techniques for Elementary Gas Phase Reaction Kinetics”, C. Taatjes and J.F. Hershberger, Ann. Rev. Phys. Chem. 52, 41-70 (2001).
“Kinetics of HCCl + NOx Reactions”, R.E. Baren, M. Erickson, and J.F. Hershberger, Int. J. Chem. Kinet. 34, 12-17 (2002).
“Kinetics of NCN Reactions”, R.E. Baren and J.F. Hershberger, J. Phys. Chem. A. 106, 11093 (2002).
“Kinetics of the SiH3+O2 and SiH3+H2O2 Reactions”, J.P. Meyer and J.F. Hershberger, J. Phys. Chem. A 107, 5963 (2003)
“Kinetics of the CCO+NO and CCO+NO2 Reactions”, W.D. Thweatt, M.A. Erickson, and J.F. Hershberger, J. Phys. Chem. A 108, 74 (2004).
“Product Channels of the HCCO+NO Reaction”, J.P. Meyer and J.F. Hershberger, J. Phys. Chem. B 109, 8363 (2005).
“Kinetics of The HCCO+NO2 Reaction”, J.P. Meyer and J.F. Hershberger, J. Phys. Chem. A 109, 4772 (2005).
“Ab Initio Study of the HCCO+NO2 Reaction”, J.P. Meyer and J.F. Hershberger, Chem. Phys., accepted for publication.
“Kinetics of the OH+HCNO Reaction”, W. Feng, J.P. Meyer, and J.F. Hershberger, J. Phys. Chem. A., submitted for publication.
