Real-time clocking of bimolecular reactions: Application to H+CO_2
An experimental methodology is described for the real-time clocking of elementary bimolecular reactions, i.e., timing the process of formation and decay of the collision complex. The method takes advantage of the propinquity of the potential reagents in a binary van der Waals (vdW) ``precursor'' molecule. An ultrashort pump laser pulse initiates the reaction, establishing the zero-of-time (e.g., by photodissociating one of the component molecules in the vdW precursor, liberating a ``hot'' atom that attacks the nearby coreagent). A second ultrashort, suitably tuned, variably delayed probe laser pulse detects either the intermediate complex or the newly born product. From an analysis of this temporal data as a function of pump and probe wavelengths, the real-time dynamics of such a ``van der Waals-impacted bimolecular (VIB)'' reaction can be determined. Chosen as a demonstration example is the VIB reaction H+CO2-->HOCO[double-dagger]-->HO+CO, using the HI·CO2 vdW precursor. The pump laser wavelength was varied over the range 231–263 nm; the probe laser detected OH in two different quantum states. The measured rates of formation and decay of the HOCO[double-dagger] complex are characterized by time constants tau1 and tau2; tau2 spanned the range 0.4–4.7 ps, varying with the available energy. The dynamics of the HOCO[double-dagger] decay are discussed.
Additional Information© 1990 American Institute of Physics. (Received 4 December 1989; accepted 22 January 1990) The experimental work is supported (AHZ) by the National Science Foundation (NSF). Some support (RBB) was also provided by the National Science Foundation under grant No. CHE-86-15286. We wish to acknowledge helpful discussions and correspondence with G. Schatz, C. Dykstra, and C. Wittig. NFS also wishes to thank R. Scherer for creative assistance in the preparation of some of the figures. Finally, the authors wish to thank L. Khhundkar for his important contributions in the early stages of development of real-time clocking of biomolecular reactions. Contribution No. 8071 from the California Institute of Technology.
Published - SCHEjcp90.pdf