Indirect channel

Butler D A and Hayden B E 1995 The indirect channel to hydrogen dissociation on W(100)c(2 2)Cu—evidence for a dynamical precursor Chem. Phys. Lett. 232 542... 

In PCL systems care must be taken to ensure that the signal received directly from the transmitter does not compete with and swamp that from the target. Typically this will be the case unless measures are taken to suppress the direct signal occurring in the indirect channel. We can formulate a simple expression for the amount of direct signal suppression required by calculating the ratio of the indirect received sig-... 

One has to distinguish between a direct creation of reactant correlations due to disappearance of some AB pairs and an indirect channel of correlation formation when one of partners of any AA, BB or AB pair recombines with surrounding particle. The former mechanism is taken into account through the... 

The performed analysis permits to assume that an indirect channel of the correlation emergence results in the correlation function Y(77) = z(rj) whose shape is a smoothed step, besides near 77 = 1 because of the continuity reasons one gets z (77) = 0 as 77 1... 

The question arises how does one distinguish experimentally between these two types of photodissociation This question can be answered from consideration of the absorption spectrum. The predissociative state is bound, and, therefore, is characterized by a set of discrete levels. The indirect channel implies the appearance of resonant structure in the photodissociation cross section as a function of the frequency of the incident radiation. Hence, discrete structure in the absorption spectrum indicates the indirect nature of the photodissociation. For example, analysis of the absorption spectrum of C2N2 leads to the conclusion that the process C2N2 (C- -IIu)+ hv -+ CNCX rtj +CN(A II) at V = 164 nm is an indirect photodissociation process (8). 

Another possibility for distinguishing between direct and indirect channels arises from an analysis of the angular dependence of the fragments. If the photodissociation is direct, there will be a correlation between the angular distribution and and direction of the radiation. For indirect photodissociation the correlation vanishes, because the appearance of the photofragments is a result of the radiationless transition. 

Direct and indirect channels to molecular dissociation at metal and metal alloy surfaces... 

Further evidence of an indirect channel at low Ex came from the dependence of S on hydrogen coverage 0H [164], and this result is summarised in Fig. 32 which shows S(0H) for Ex = 63 meV ( ) (Ts = 300 K) and E = 13meV ( ) (Ts = 150 K). These correspond to conditions where the direct and indirect channel will dominate dissociation. Indeed precursor type behaviour is observed under the latter conditions, and a linear decrease (characteristic of direct dissociation of hydrogen on many metals) is evident under the former conditions. An observation which was more difficult to reconcile with the suggestion of an indirect channel at low ) on W(10 0) came from the observed absence of any significant surface temperature dependence in Sq. Figure 33 shows ( ) the dependence Sq(T ) on the... 

---