State-selected differential cross sections (DCSs) have been measured for the OH radicals produced from the reactions of O(3P) with saturated hydrocarbons by utilizing Doppler-resolved polarization spectroscopy. Stereodynamics in the reactions of secondary (c-C6H12) and tertiary (i-C4H10) hydrogen atoms are discussed based on the dependences of the DCSs on the collision energy and the structure of these hydrocarbons. For the c-C6H12 reaction, the DCS of the OH(2Π3/2,v′=1,j′=3.5,A′) shows predominant intensities in the backward hemisphere with reference to the incident O(3P) atom at a mean collision energy of 〈Ecoll〉=12 kJ/mol. When the collision energy is raised to 〈Ecoll〉=33 kJ/mol, the OH radicals scattered in the forward hemisphere grow almost to match those in the backward hemisphere. The observed increase in the forward scattering implies that the collision energy makes the large impact parameter collisions contribute to the reactive scattering. At a similar collision energy of 〈Ecoll〉=31 kJ/mol the forward scattering component in the DCS of the i-C4H10 reaction does not exceed that of the c-C6H12. This shows that the cone of acceptance is not enlarged in the i-C4H10 reaction from that in the c-C6H12 reaction, as opposed to the expectation based on the height of activation barrier. The absence of the enlargement of the cone of acceptance can be attributed to a large steric hindrance caused by the three bulky methyl groups surrounding the reactive tertiary C–H bond of i-C4H10. The difference in the steric hindrance can explain the difference in the temperature-dependent pre-exponential factors of the macroscopic reaction rates between the abstraction of the secondary and tertiary C–H bonds. The collision energy dependence of the DCS as well as the internal excitation of alkyl radical products reveal that the O(3P)+alkane reactions are not always dominated by the simple rebound mechanism, which has long been believed.
We have measured the product state-selected differential cross-section ͑DCS͒, and the rotational angular momentum polarization, together with the energy distributions for the reaction O( 1 D) ϩN 2 O→NOϩNO by utilizing Doppler-resolved polarization spectroscopy. The reaction dynamics of the vibrational channel forming the product NO(vЈϭ0) is discussed based on both the scalar and vector properties. The product rotational and center-of-mass translational energy distributions are described as Boltzmann distributions with T rot Ϸ10 000 K and T tr Ϸ13 000 K, respectively. These energy distributions are close to statistical predictions. The product DCS has substantial intensities over the whole angular range with a slight preference for backward scattering. The product rotational angular momentum vector jЈ does not have a noticeable angular correlation with either k or kЈ ͑the relative velocity vectors of the reactant and product, respectively͒. This nearly isotropic angular distribution of jЈ indicates that both in-plane and out-of-plane motions of the collisional ONNO complex contribute to the product rotation to almost the same degree. Considering that this reaction has no potential well deep enough for the formation of a long-lived complex, these nearly statistical scalar and isotropic vector properties suggest that the energy redistribution among the internal modes of the collisional ONNO complex efficiently takes place. It implies that there are strong couplings among the internal modes.
We have studied the kinetic mechanism of the adsorption-induced-desorption (AID) reaction, H+D/Si(100) --> D2. Using a modulated atomic hydrogen beam, two different types of AID reaction are revealed: one is the fast AID reaction occurring only at the beam on-cycles and the other the slow AID reaction occurring even at the beam off-cycles. Both the fast and slow AID reactions show the different dependence on surface temperature Ts, suggesting that their kinetic mechanisms are different. The fast AID reaction overwhelms the slow one in the desorption yield for 300 K < or = Ts < or = 650 K. It proceeds along a first-order kinetics with respect to the incident H flux. Based on the experimental results, both two AID reactions are suggested to occur only on the 3x1 dihydride phase accumulated during surface exposure to H atoms. Possible mechanisms for the AID reactions are discussed.
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