Real-time clocking of bimolecular reactions: Application to H+CO_2

Abstract

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.