Imaging the nonadiabatic dynamics of the CH3 + HCl reaction

Abstract

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH(3) radicals with HCl have been measured for both the ground Cl ((2)P(3/2)) and excited Cl* ((2)P(1/2)) spin-orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH(3) radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (E(coll) = 22.3 kcal mol(-1)), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

“…The CH 3 + HCl reaction images were previously analyzed by using a photoloc method. 14 The Legendre moment fitting procedure of Bass et al 35,36 when compared to the photoloc analysis incorporates an improved allowance for the effects of internal energy of the CH 4 ͑or CD 3 H͒ coproduct in the interpretation of laboratory-frame speeds and spatial anisotropies of the Cl/ Cl * atoms. We also report branching ratios for Cl * products of reaction ͑2͒, measured by ͑2+1͒ REMPI spectroscopy, which are found to be significantly different from the equivalent branching ratio for reaction ͑1͒.…”

“…14,33 Reactions were initiated by photodissociation of CH 3 I ͑Ald-rich, 99.5%͒ or CD 3 I ͑CK Gas, 98%, 99.5% of D atoms͒, which was diluted to 10% in He or Ar and coexpanded through a late mixing nozzle with a 15% mixture of HCl ͑Aldrich, 99+ %͒ in He or Ar. The flows of the dilute CH 3 I ͑or CD 3 I͒ and HCl gas mixtures were regulated by two separate pulsed nozzles ͑General Valve series 9͒ mounted at 90°t o one another on a triangular polytetrafluoroethylene ͑PTFE͒ block that contained a Y-shaped channel with internal diameter of 0.8 mm to merge the two gas pulses into a single flow just prior to expansion into high vacuum.…”

“…This suggests a balance between dynamical and kinematic constraints on product formation, as was discussed previously. 14 Figure 3 shows an image for the Cl products of reaction ͑2͒ of CD 3 radicals with HCl, together with the experimental Legendre moments of the image and the results of fits following the procedures described above. Basis functions were modified to allow for the different masses and kinetic energies of the reagents, and the effects of zero-point energy changes on the reaction exothermicity.…”

“…The discrepancies in the fit to the L = 0 moment at low Cl * speeds are a consequence of subtraction of a background signal caused by HCl photodissociation. Guided by the past experience from our photoloc analysis of such Cl * images, 14 we calculated the basis functions used to fit the image moments for two different cases. In case ͑a͒, equal nonadiabatic reactivity was assumed for all CH 3 radicals; for case ͑b͒, however, only the subset of the faster CH 3 radicals produced in association with ground-state I atoms from CH 3 I photodissociation was considered.…”

“…7 In the case of reactions of polyatomic molecules, the available dynamical information is sparse, but nonadiabatic transitions to low-lying excited electronic states separated from the ground adiabatic state by SO interactions have been reported for a few reactions of triatomic and larger molecules. [8][9][10][11][12][13][14] We recently reported nonadiabatic production of SO excited Cl͑ 2 P 1/2 ͒ atoms ͑henceforth denoted as Cl * ͒ in the reaction CH 3 + HCl → CH 4 + Cl͑ 2 P J ͒ ͑ 1͒…”