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.