Absolute reaction cross section

Chemical dynamics experiments in which OH product quantum state distributions and an absolute reaction cross section for reaction (1) could be measured were reported in 1984. Subsequent experiments revealed additional details about the reaction dynamics, including nascent OH( H) spin-orbit and A-doublet rotational fine structure state distributions, Oi P) product fine structure state distributions, and OH angular momentum polarization distributions,as well as differential cross sections. The experimental results indicate that depending on the reagent collision energy... 

All absolute reaction cross section measurements reported in Refs. 33, 36, 40 were based on OH product detection via laser-induced fluorescence (LIE) employing the diagonal bands of the OH(A S+—A H) UV transition. However, due to predissociation of the state only the OH(u" = 0)... 

Fig. 5. Absolute reaction cross section for the H + O2 OH + O elmentary reaction as a function of collision energy, (o) from Ref. 40 ( ) and solid line from Ref. 39 (I) from Ref. 43(b) ( ) 0( P) VUV(LIF) work dashed lines represent theoretical results from QCT/QM (Ref. 56) and QCT-US [Ref. 26(e)] calculations.

Nowadays, femtosecond laser "pump-and-probe" techniques - as pioneered by Ahmed Zewail and his group at the California Institute of Technology [11] -can provide us with fascinating series of snap shots of how reactive systems behave while passing over the transition state [12] and nanosecond laser "pump-and-probe" techniques allow to measure asymptotic scalar and vectorial quantities of the reactive collision e.g. nascent quantum state distributions of reaction products [13], absolute reaction cross sections [14], state-specific rates [15], and stereodynamical correlations [16], respectively. 

Relative and absolute reaction cross section measurements for reaction (1) were carried out in the energy range 1.0 eV < Ec.m. < 2.6 eV. The experimental results as depicted in Fig. 4 show a pronounced maximum at Ec.m. -1.7 eV, a feature which could not be reproduced by earlier QCT calculations on different ab initio PESs [47a-e]. In quantum scattering calculations, where reaction probabilities for total angular momentum equals zero (J = 0) were calculated, it was observed that reactivity is not enhanced by either initial O2 vibrational or rotational excitation [48a]. On the other hand, it was found that reactivity markedly increases with increasing collision energy, in particular at the opening of the OH(v = 1) product channel at Ec.m. - 1.25 eV [48a]. Recently a new extrapolation mefliod was devel-... 

Figure 4 Absolute reaction cross section for H + O2 — OH + O as a function of collision energy. Filled circles [46g] and open squares [46f] are experimental results. Dashed line represent QCT [47a], solid line QCT/QM-QJ [52] results. 

Figure 6. Absolute reaction cross sections for D2 + OH -to results from 5D quantum scattering calculations. 

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