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Modeling the Rovibrationally Excited C2H4OH Radicals from the Photodissociation of 2-Bromoethanol at 193 nm

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journal contribution
posted on 15.04.2010, 00:00 by B. J. Ratliff, C. C. Womack, X. N. Tang, W. M. Landau, L. J. Butler, D. E. Szpunar
This study photolytically generates, from 2-bromoethanol photodissociation, the 2-hydroxyethyl radical intermediate of the OH + ethene reaction and measures the velocity distribution of the stable radicals. We introduce an impulsive model to characterize the partitioning of internal energy in the C2H4OH fragment. It accounts for zero-point and thermal vibrational motion to determine the vibrational energy distribution of the nascent C2H4OH radicals and the distribution of total angular momentum, J, as a function of the total recoil kinetic energy imparted in the photodissociation. We render this system useful for the study of the subsequent dissociation of the 2-hydroxyethyl radical to the possible asymptotic channels of the OH + ethene reaction. The competition between these channels depends on the internal energy and the J distribution of the radicals. First, we use velocity map imaging to separately resolve the C2H4OH + Br(2P3/2) and C2H4OH + Br(2P1/2) photodissociation channels, allowing us to account for the 10.54 kcal/mol partitioned to the Br(2P1/2) cofragment. We determine an improved resonance enhanced multiphoton ionization (REMPI) line strength for the Br transitions at 233.681 nm (5p 4P1/2 ← 4p 2P3/2) and 234.021 nm (5p 2S1/2 ← 4p 2P1/2) and obtain a spin−orbit branching ratio for Br(2P1/2):Br(2P3/2) of 0.26 ± 0.03:1. Energy and momentum conservation give the distribution of total internal energy, rotational and vibrational, in the C2H4OH radicals. Then, using 10.5 eV photoionization, we measure the velocity distribution of the radicals that are stable to subsequent dissociation. The onset of dissociation occurs at internal energies much higher than those predicted by theoretical methods and reflects the significant amount of rotational energy imparted to the C2H4OH photofragment. Instead of estimating the mean rotational energy with an impulsive model from the equilibrium geometry of 2-bromoethanol, our model explicitly includes weighting over geometries across the quantum wave function with zero, one, and two quanta in the harmonic mode that most strongly alters the exit impact parameter. The model gives a nearly perfect prediction of the measured velocity distribution of stable radicals near the dissociation onset using a G4 prediction of the C−Br bond energy and the dissociation barrier for the OH + ethene channel calculated by Senosiain et al. (J. Phys. Chem. A 2006, 110, 6960). The model also indicates that the excited state dissociation proceeds primarily from a conformer of 2-bromoethanol that is trans across the C−C bond. We discuss the possible extensions of our model and the effect of the radical intermediate’s J-distribution on the branching between the OH + ethene product channels.