Real-time clocking of bimolecular reactions: Application to H+CO2

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‡→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‡ complex are characterized by time constants τ1 and τ2; τ2 spanned the range 0.4–4.7 ps, varying with the available energy. The dynamics of the HOCO‡ decay are discussed.