Direct reaction angular distribution

Angular resolved measurements of the COz formed in the reaction have yielded rather surprising results. Palmer and Smith (196) have reported that the C02 formed on a Pt(l 11) surface is emitted in a peaked distribution that can be described by a cos6 a function where a is the emission direction measured from the surface normal. Becker et al. (190) have reported similar results for a polycrystalline Pt surface. They obtain angular distributions of... 

The peak of the P(ET) curve occurs at 140 and <20 kJ/mol for reactions 77 and 78, respectively. Angular distributions of the Cl atom signal in reaction 77 indicate that the distribution is anisotropic, and that the dissociation is direct with a lifetime of the order of a few picoseconds. Reaction 78 is thought to occur via a singlet-triplet intersystem crossing, followed by the dissociation of the resultant triplet state. 

There have been a large number of measurements of angular distributions in desorption which show sharply peaked distributions and these have recently been reviewed by Kislyuk [44]. In some cases the products of reaction on fcc(l 1 0) surfaces are found to be peaked at an angle to the surface normal along the [001] azimuth (Fig. 9) notably for N2 produced by NO [77, 78] or N20 [79] decomposition, C02 formed by CO oxidation [80, 81] and CO formed by C + O recombination [82]. Sharply peaked distributions indicate a repulsive energy release which lies at an angle to the surface normal [83]. This occurs either because reaction takes place on (111) facets on the reconstructed (1x2) missing row surface (e.g., CO formation on Pt(l 1 0)-(l x 2) surface [82]) or, as in the case of N20 decomposition, because the symmetry of the transition state creates a repulsion which is directed away from the surface normal [84, 85]. 

Figure 13 Angular distributions for N2 formed by recombination at Ag(l 1 1) ( ) [93], Cu(l 1 1) ( ) [86] and Ru(000 1) (dotted line) [95], Recombination of N2 during reaction of NO and H2 on Pd(l 1 0) [96, 127] follows a statistical P(0) = cos 0 distribution (outer circle) while the fits for Ag, Cu and Ru show a P(0) = cos" 0 distribution with n = 75, 28 and 7, respectively. In order to fit on the same scale the angular distributions have been normalised at 0 = 0 and need to be scaled by a factor of (n + 1) to compare the distributions directly.

The reactions M + N02 have large reaction cross sections ( 100— 200 A2) and are consistent with a direct reaction proceeding by means of an electron jump. The MO product is scattered forward with low recoil energy [345, 346]. Visible chemiluminescence is observed from BaO (A Z) and this channel constitutes 0.18% of the total reaction [347]. The BaO (A) vibrational state distribution is found to be inverted with v = 8, 9 and 10 being preferentially populated. It is not expected that the NO fragment is appreciably excited. Polarisation measurements [344] of the BaO product in the chemiluminescent channel indicate a reasonable amount of the total angular momentum of the reactive collision appears as rotational angular momentum of the BaO product. 

---