The product rovibrational and spin-orbit state dependent dynamics of the complex reaction H+CO2 -> OH((2)Pi;nu,N,Omega, f)+CO: Memories of a lifetime

The product-state-resolved dynamics of the reaction H + CO2--> OH((2)Pi;nu, N,Omega,f) + CO have been explored in the gas phase at 298 K and center-of- mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol (-1)), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differentia l cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin-orbit, Lambda-doub let, rotational and vibrational state selection in the scattered OH. The ne w measurements reveal a rich dynamical picture. The channels leading to OH( Omega,N similar to 1) are remarkably sensitive to the choice of spin-orbit state: Those accessing the lower state, Omega = 3/2, display near-symmetric forward-backward DCSs consistent with the intermediacy of a short-lived, r otating HOCO ((X) over tilde (2)A') collision complex, but those accessing the excited spin-orbit state, Omega = 1/2, are strongly focused backwards a t the higher collision energy, indicating an alternative, near-direct micro scopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier u ltrafast kinetic studies. At the higher collision energy, the state-resolve d DCSs for the channels leading to OH(Omega,N similar to 5-11) shift from f orward-backward symmetric toward sideways-forward scattering, a behavior wh ich resembles that found for the analogous reaction of fast H atoms with N2 O. The correlated product kinetic energy distributions also bear a similari ty to the H/N2O reaction; on average, 40% of the available energy is concen trated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision ener gy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynam ical model involving short-lived collision complexes with mean lifetimes co mparable with or somewhat shorter than their mean rotational periods. The a nalysis suggests a potential new stereodynamical strategy, "freeze-frame im aging," through which the "chemical shape" of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature envir onment of a supersonic molecular beam. (C) 2000 American Institute of Physi cs. [S0021-9606(00)01010-2].